Electrical design guide to St P Solar film project

Electrical design guide to St P Solar film project
Draft Electrical design guidance document 1.0, written by Ken Gale- updated 7th October 2025 – next step to be peer reviewed for buildability and losses etc
This design guidance document supports the wider team's understanding of the planned St P rooftop solar film installation.
For the worked example, BiPVco Flextron panels are integrated end-to-end with the AC power delivery, and cable sizing is developed around Sungrow SG125CX-P2 inverters.
The presentation walks the reader through each technical stage: from thin-film panel specification, to inverter selection, through cable sizing and containment, and on to switchgear interface.
The document seeks provides clear explanations, supporting calculations, and practical build notes to aid design, installation, and commissioning.
A non-technical glossary for this wave roof solar PV project:
Wave roof
The long roof made of repeating "waves" (convex curved ridges and valleys). The waves run the width of the building.
Solar film panel
A thin, flexible sheet that sticks to the roof and turns sunlight into electricity. It's lighter and lower-profile than a framed glass panel.
1 wave
One full ridge/valley unit from the top of the roof to the bottom. In our scheme, each wave is a repeatable building block for laying panels.
Wave section
A wave split into thirds by the big span beams. Each section (one-third of a wave) can fit up to 29 solar film panels side-by-side. (Sections A, B & C)
BiPVco solar panel
One of the brand/types we plan to use is (BiPVco "Flextron"). Each panel is roughly 2.6 m long × ~1 m wide and produces about 360 W in strong sun. It's designed to bond directly to a roof surface.
A Solar string
A small group of panels wired end-to-end so they act together. In this project most strings have 8 panels (called 8S); each wave also has one 7-panel string (7S).
Max AC power
The most electricity the system can deliver to the building at once (after conversion from DC to AC by the inverters). For this design it's about 1.6 megawatts (MW)—roughly like running 1,600 one-kilowatt appliances at the same time.
Cable losses
A small amount of energy that turns into heat as electricity travels through cables—especially over long distances or in thin cables. We reduce this by using short routes and thicker cables, aiming to keep losses around 1–3%
Solar Power Project – Technical Definitions
Primary DC runs – The cable containment that carry all the panel strings from several waves to an inverter. These runs utilize H1Z2Z2-K or PV1-F double-insulated cables, ensuring protection through reinforced insulation rather than a separate earthing conductor.
Primary AC runs – The cable containment trays that carry each inverter's AC cables back to the building's switchgear.
Cowling (architectural) – A neat, weather-resistant cover on the outside of the building used to hide and protect external cable routes.
Rainscreen vs waterproofing – The visible metal cladding is a rainscreen (not watertight). The air/vapour barrier (AVB) or membrane behind it does the real waterproofing; any cable entry must be sealed at the AVB, not just the cladding.
Sarnafil / Triflex – Trade names for roof waterproofing systems. We avoid puncturing them; where we must cross, we use approved details.
Switchgear / Chiller switchgear room – The room with main electrical panels and breakers for big building loads (like chillers). This is where the inverters connect.
Earthing / bonding – For AC circuits, these are safety connections to ground and between metal parts to ensure faults clear quickly and prevent shock. For DC solar circuits, protection relies on a double-insulated cable approach (e.g., H1Z2Z2-K or PV1-F type cables) where the conductors have double or reinforced insulation, eliminating the need for a separate earth conductor.
LPS (Lightning Protection System) – The building's lightning system; PV cables and trays must maintain safe distances or be bonded as the design dictates.
ILR (DC:AC ratio) – The ratio of total soar panel power (DC) to total inverter capacity (AC). Around 1.05–1.2 is common; we're ~1.08.
kWp / MWp – Kilowatt-peak / Megawatt-peak: the rated DC power of the panels in perfect test conditions.
kWac / MWac – The rated AC output at the inverters (what the building can actually use).
kWh / MWh (energy) – Units of energy produced over time (e.g., per day or per year), not instantaneous power.
Vmp / Voc / Imp / Isc – Panel electrical points:
Vmp / Imp – Voltage & current at maximum power.
Voc / Isc – Open-circuit voltage & short-circuit current (design limits).
H1Z2Z2-K or PV1-F 6 mm² – These are double-insulated, single-core, UV-resistant DC cables used from panels to inverters. Protection is provided through reinforced insulation rather than a separate earth conductor, simplifying cable counts and containment sizing.
0.6/1 kV LSZH single-core (e.g., 6491B / H07Z-K or equivalent) – These are single-core LSZH cables for AC wiring, run as 3 × phase + 1 × CPC (earth) in trefoil or flat formation on ladder tray. Typical OD (95 mm²) is roughly 18–20 mm each; these cables carry about 185 A per inverter in our design.
OD / bend radius – Outside diameter of a cable and the minimum curve it can safely make. Bigger cables need larger bend radii and wider trays.
Step-over – A small bridge for people to safely cross over trays on walkways.
Parapet – The short wall at the roof edge.
Gulley – A shallow channel that carries rainwater off the roof; we keep service space clear here for maintenance
BMS – Building Management System; where inverter status and metering can be monitored.
Combiner / Y-branch – Small connectors (or boxes) used to parallel strings near the inverter.
Tray cover – Clip-on cover over a tray to protect cables and keep walkways
Lightning Protection System (LPS) The whole setup that protects a building from lightning: roof rods/tapes, down-conductors, earthing, bonding, and surge devices.
Air-termination (roof rods/tapes) The metal parts on the roof that "intercept" a strike so it hits them—not the building or plant.
Down-conductor The metal strip/cable that carries lightning safely from the roof to the ground.
Earth-termination (earth rods / ring earth) Buried conductors and rods that spread the lightning energy into the soil.
Separation distance (s) The minimum gap you should keep between the LPS and other metalwork/cables to avoid dangerous arcing.
Equipotential bonding Intentionally connecting metal parts together (and to earth) so there's no dangerous voltage difference during a strike.
Bonding bar / PE bar A local metal bar where earthing/bonding connections are made neatly (e.g., at an inverter plinth).
Lightning Protection Zones (LPZ 0/1/2 …) Conceptual zones showing how "harsh" the lightning environment is—outside (exposed) vs. inside (protected). Helps decide where to put surge devices.
Surge Protective Device (SPD) A protective component that diverts brief voltage spikes to earth so equipment isn't damaged.
Type 1 SPD Heavy-duty surge device used where lightning current may enter (e.g., at the service entrance or when bonded to LPS).
Type 2 SPD Mid-level surge device used inside the building (e.g., at inverters and distribution boards) to clamp switching/lightning-induced surges.
Type 3 SPD Point-of-use protection near sensitive kit (e.g., electronics), used in addition to upstream Type 1/2.
Earth protection on DC side
If due to NRHS rules, we cannot run separate earth conductors with DC cables and we want GRP/FRP (non-metal) cable trays which are to be located on the rear of the bull nose wave.
The following is a compliant path that still keeps the design safe and within the spirit of BS 7671 / IEC 60364-7-712.
The short answer:
DC strings on GRP/FRP trays: Use double-insulated PV cable (H1Z2Z2-K / PV1-F) and remove exposed conductive parts along the run. No separate earth conductor is required along the DC route, as the protective measure is double/reinforced insulation.
Inverter earthing (on the AC side): Give the inverter its PE via the AC feeder using single-core LSZH cables in tray. This involves running 0.6/1 kV LSZH single-core (e.g., 6491B / H07Z-K or equivalent) with 3 × phase + 1 × CPC (earth) in trefoil or flat formation on a ladder. The CPC will be a separate conductor run alongside the phases. Typical OD (95 mm²): ~18–20 mm each.
If we insist on single-core AC (for flexibility): This is the preferred approach as described above, ensuring a dedicated CPC is always included. The use of separate single-core LSZH cables allows for flexibility in installation and routing.
Protection on DC Side
How to implement the double-insulated cable approach:
DC side (roof → inverter plinth)
Containment: Use GRP/FRP tray/ladder (non-conductive).
Cables: Use double-insulated PV cable (H1Z2Z2-K 6 mm² or as designed), run +/− tightly paired. No separate earth conductor is required.
Protective measure: Rely on double/reinforced insulation.
Metalwork policy: Avoid metal near the run. If any metal bracket/stand-off is used, ensure it is isolated (with non-conductive spacers) or not accessible. Otherwise, it becomes an exposed-conductive-part and would need bonding (which is not provided on the DC side).
*At the inverter : The inverter's PE terminal bonds to the building earthing system via the AC feeder (as described for the AC side). No DC earth conductor is run.
Lightning protection options:
This guide helps implement lightning protection and surge protection for our wave roof solar PV system. It clarifies where devices should be placed on the DC and AC sides of each inverter, and how to choose between two schemes based on whether sufficient separation from the building's Lightning Protection System (LPS) can be maintained.
Decide which scheme applies:
Scheme A — "Separated": You can maintain the required separation distance s (per BS EN 62305) between PV cabling/metalwork and LPS down-conductors/air terminals along the entire route.
→ Use Type 2 SPDs (surge protective devices) at inverter DC and AC. No deliberate bonding of the DC side to the LPS, relying instead on the inherent double/reinforced insulation of DC cabling.
Scheme B — "Bonded/Inside LPS zone": We cannot maintain separation (e.g., parapet crossings, close to towers).
→ Deliberately bond at a defined point and upgrade to Type 1+2 (or Type 1 on AC service entrance) so devices can handle partial lightning current.
(As our project has parapet/tower interfaces—assume Scheme B for those sections unless the LPS designer certifies you can keep separation everywhere.)
Lightning protection options :
Primary DC SPD location: At the inverter plinth, as close as possible to the inverter DC inputs.
Primary AC SPD location:
At the inverter AC side (panel next to the inverter), and
At the main LV switchboard that aggregates all inverters.
Keep SPD leads short and straight (ideally ≤ 0.5 m per lead).
Scheme A — Separated (no bonding to LPS along DC)
In this scheme, the DC side relies on a double-insulated cable approach where the protective measure is double/reinforced insulation, rather than earthing of DC conductors. 
Use H1Z2Z2-K or PV1-F double-insulated cables without separate earth conductors for all DC cabling. 
Update cable containment sizing to reflect the removal of separate DC earth conductors.
DC side (at the inverter)
Fit Type 2 PV SPDs (Ucpv ≥ Voc,max(cold) of your 8S strings).
Connection mode: common-mode + differential preferred:
PV+ → SPD → Earth, PV− → SPD → Earth, and PV+ ↔ PV− (if in one cartridge).
Note: While DC conductors are double-insulated, SPDs require an earth reference for surge diversion. This local earthing point at the inverter plinth does not constitute earthing of the entire DC array structure or conductors in the traditional sense, but provides a path for surge energy.
Provide SPD upstream gPV backup fuse or breaker if required by SPD manufacturer.
Mount on a small bonding bar/plate at the plinth; connect to the inverter's local Protective Earth (PE) terminal (your AC armour-as-CPC).
Lightning protection options :
DC side (at the inverter plinth)
Fit Type 1+2 PV SPDs (or Type 1 if specified), rated for lightning impulse (I_imp) and with Ucpv ≥ Voc,max(cold).
Same connection mode as above (PV+→PE, PV−→PE, plus PV+↔PV−), ensuring connection to the protective earth (PE) for surge diversion.
Keep all SPD conductors short, routed together with the DC pairs to minimise loop area.
DC cabling should utilize double-insulated cables (e.g., H1Z2Z2-K or PV1-F), where the protective measure is double or reinforced insulation rather than a separate earth conductor. Cable counts and containment sizing should reflect 2 x DC conductors per string.
AC side
At the main LV switchboard that interfaces with LPS/service entrance: Type 1 (or 1+2) AC SPD.
At each inverter AC panel: Type 2 AC SPD (or 1+2 if specified to simplify SKUs).
Earthing
Your AC feeder will now use single-core LSZH cables in tray (0.6/1 kV LSZH single-core, e.g., 6491B / H07Z-K or equivalent). This will consist of 3 × phase + 1 × CPC (earth) run in trefoil or flat formation on ladder. Typical OD for 95 mm² is ~18–20 mm each.
The plinth PE bar bonds to the building earthing system and to any local bonded metal (per the LPS designer).
Mark this location on drawings as "PV LPS Bond Point".
Lightning protection options :
Device selection and ratings (both schemes)
DC SPD voltage (Ucpv): choose ≥ Voc,max at minimum site temperature (use your cold-Voc). For 8S strings you calculated ~995 V Voc(cold) → select Ucpv ≥ 1000 Vdc (PV) class device.
DC SPD polarity: use PV-rated SPDs (with DC long-duration capability and thermal disconnects).
AC SPD system: 400/230 V, 3-phase; choose TN-S/TN-C-S compatible modules; Up low enough for inverter electronics.
Energy coordination: If multiple SPDs exist in series (main LV ↔ local inverter panel ↔ inverter), ensure upstream device has ≥ equal class and adequate discharge capacity.
Back-up protection: Many SPDs need gG/gL fuse or MCB upstream; size per datasheet and prospective short-circuit current (PSC).
Wiring details that matter
Lead lengths: "a+b+c" (both SPD leads + earth) as short as possible (target each leg ≤ 0.5 m).
Routing: keep SPD conductors parallel to the protected cables; avoid loops and sharp bends.
DC cable geometry: For PV strings, use H1Z2Z2-K or PV1-F double-insulated cables without separate earth conductors. Always run +/− together (tied) to minimise inductive pickup; never separate them around obstacles.
AC geometry: prefer trefoil or tight grouping; non-magnetic cleats.
Lightning protection options :
Extra protections (recommended)
Comms SPD: Add Type 2 data/RS485/Ethernet surge protectors at the inverter comms ports and at the network entry to the BMS/Metering room.
Metalwork bonding: Bond architectural cowlings, parapet cable carriers, and any metal support frames to PE at the nearest bonding node.
Labeling: Label SPDs with type, Uc, Up, I_n/I_imp, date fitted, and inspection interval.
What to put on your drawings (ready text)
DC SPD (Scheme A): "At each inverter plinth fit Type 2 PV SPD, Ucpv ≥ Voc,cold, connected PV+→PE, PV−→PE (and PV+↔PV− where provided). Leads ≤0.5 m. Provide manufacturer-approved upstream protection."
DC SPD (Scheme B): "Where LPS separation not achievable, bond at plinth and fit Type 1+2 PV SPD as above."
AC SPD (local): "At each inverter AC panel fit Type 2 AC SPD (3-phase 400/230 V), coordinated with main LV SPD."
AC SPD (main LV): "Fit Type 1 (or 1+2) AC SPD at main LV switchboard; coordinate with downstream devices."
DC cabling note: "PV DC cabling shall use double-insulated H1Z2Z2-K or PV1-F cables. No separate earth conductors are required for DC PV strings. Ensure appropriate containment sizing reflecting the absence of separate DC earth conductors."
AC earthing note: "Inverter PE via 0.6/1 kV LSZH single-core cable (e.g., 6491B / H07Z-K or equivalent) as CPC, run in trefoil or flat formation on ladder. Plinth PE bar bonds to building earthing and any adjacent metalwork. Maintain LPS separation; where not possible, use Scheme B bonding."
Comms SPD: "Provide Type 2 surge protection on inverter comms (RS485/Ethernet) at inverter and at BMS point."
Lightning protection options :
Quick checklist (before sign-off)
Confirm with our LPS designer where separation s is met vs. not met.
1.For each inverter, mark Scheme A or B on the GA.
2.Size Ucpv from your Voc,cold; verify back-up fuse/MCB ratings.
3.Draw SPD symbols at inverter DC and AC, and at main LV.
4.Detail bond point (Scheme B) and ensure short leads.
5.Add comms SPDs on the drawings.
6.Add inspection/maintenance note (check indicators yearly; replace on flag).
7.
Plan view of solar film installation
The illustration shows a solar film array that is arranged as four waves of PV strings per inverter (total 14 Sungrow units), shown in green
The primary containment route (indicated in purple) traverses the two span beams onto the metal work of the glass roof
The main AC containment passes around the West Tower and drops to the Midland Road lower roof for building entry to the Energy Centre - switchgear room.
Solar PV DC cabling system - cable losses
To quantify the DC ohmic losses per string - simple sizing rule + schedule for the 4-waves-per-inverter layout.
How far is each string from the inverter?
With 4 waves feeding one inverter, the DC strings home-run along the span beam. A tidy planning set is to consider four representative one-way distances to the inverter plinth (add ~10 m allowance for local drops, over-nose routing, and verticals):
Nearest wave: ~30 m + 10 m = 40 m
Next wave: ~60 m + 10 m = 70 m
3rd wave: ~90 m + 10 m = 100 m
Farthest wave: ~120 m + 10 m = 130 m
(If the actual set-out differs, we just plug the real lengths into the formula.)
Solar PV DC cabling system - cable losses
To quantify the DC ohmic losses per string - simple sizing rule + schedule for the 4-waves-per-inverter layout.
Strings per wave: 10 × 8S + 1 × 7S = 11 strings
String electricals (Flextron):
8S: ,
7S: , same
Cable type: PV1-F double-insulated single-core Cu, 0.6/1 kV (paired +/-), no separate earth conductor, installed hot (~70 °C)
DC resistance at ~70 °C (used for drop):
6 mm²: ~3.96 Ω/km
10 mm²: ~2.20 Ω/km
16 mm²: ~1.38 Ω/km
Voltage-drop criterion: aim ≤1.5% per string (good practice for MV-range strings).
Formula (per string):
where = one-way length (m), factor 2 = out
Solar PV System and cable losses
Recommended cable size (simple rule):
All strings (most cases): 6 mm² PV1-F double-insulated (black UV-resistant), + and − routed together as a pair, without separate earth conductors.
Exception: For >150 m one-way (rare here), upsize to 10 mm² PV1-F double-insulated.
What about the other losses:
Typical whole-system loss budget (annualised order-of-magnitude):
Module temperature: −6…−10% (climate/roof temp dependent)
Soiling: −1…−3% (maintenance dependent)
Mismatch & wiring (DC): −0.5…−1.5% (your string drops sit in here and are tiny)
MPPT tracking: −0.2…−0.7%
Diodes & connectors: −0.2…−0.5%
Inverter conversion: −2…−3% (efficiency 97–98%)
AC cabling/transformer: −0.3…−1.0%
Clipping (ILR ≈ 1.08): ~0–1% (site/sky-temp dependent)
Your DC ohmic loss per string at our recommended sizes is ~0.2–0.6%, i.e., small compared with temperature/inverter effects.
Flexible cable options - LSZH (AC Side)
We are focusing on flexible cabling alternatives that offer significant advantages over traditional stiff armoured cables due to their smaller overall diameter and improved bend radius. Specifically, we will detail the characteristics and benefits of using single-core LSZH cables in tray for AC applications, relying on a Circuit Protective Conductor (CPC) for earthing.
Here are the practical options, with when/why you'd pick them:
Use single-core LSZH cables in tray (most flexible, common on roofs for AC runs)
1.Type: 0.6/1 kV LSZH single-core (e.g., 6491B / H07Z-K or equivalent).
Make-up: 3 × phase + 1 × CPC (earth) run in trefoil or flat formation on ladder.
Typical OD (95 mm²): ~18–20 mm each (compared to ~48–52 mm for an equivalent 4-core armoured cable).
Min bend radius: ~4–6 × OD (≈ 75–120 mm) → far "bendier" than armoured cables.
Pros: light; easy to dress around tight geometry; smaller bend radius; simpler glanding.
Cons / notes: no armour, so rely on metal tray/ladder for mechanical protection; use non-magnetic cleats and trefoil layout to control magnetic forces/eddy heating (BS 7671 good practice); keep separation from data/LPS per study.
Flexible cable options
1
Use single-core LSZH with a copper wire screen (CWS/Copper tape)
Type: 0.6/1 kV LSZH single-core (e.g., 6491B / H07Z-K or equivalent) with copper screen.
Make-up: 3 × phase + 1 × CPC (earth) run in trefoil or flat formation on ladder.
Typical OD (95 mm²): ~18–20 mm each, offering enhanced flexibility.
Flexibility: Significantly more flexible than traditional 4-core armoured cables; adds EMC shielding and a parallel earth path.
When useful: Ideal for areas with EMC sensitivity or where a screened feeder is required while prioritizing cable flexibility.
2
Consider a more pliable multicore build (with limitations)
Some manufacturers offer LSZH armoured multicore cables with slightly lower OD and improved bending performance compared to standard 4-core 95 mm² XLPE/SWA/LSZH. However, the bend radius will still be ≈ 12×OD (so ~0.6 m on a 50 mm cable). This represents only a minor improvement in flexibility when compared to the superior bend radius of single-core LSZH cables.
3
Parallel smaller double-insulated DC single-core LSZH (engineering option for higher current)
e.g., two runs of H1Z2Z2-K or PV1-F double-insulated 50 mm² cables per inverter instead of a single larger run.
Effect: Each single-core cable has a smaller OD (~18-20 mm for 95mm² equivalent, even smaller for 50mm²) and is significantly easier to route and bend; currents share (~92 A each). This approach offers enhanced flexibility by using multiple runs of these double-insulated DC single-core cables.
Caveats: Paralleling needs identical lengths/paths and terminations; fewer glands/cleats as no separate earth conductor needed; check grouping/derating and protection coordination.
Calculation for PV system cable losses per wave
To quantify the DC ohmic losses per string - simple sizing rule + schedule for the 4-waves-per-inverter layout, utilizing the more flexible double-insulated DC cables (H1Z2Z2-K or PV1-F) in tray.
Schedule of PV system cable losses per string
Gaining better efficiency utilising the 4 waves per inverter (rather than 2) layout design
We gain better efficiency and fewer AC feeders, with the "4 waves → 1 inverter" scheme and the DC:AC ratio is into the sweet spot without changing string length
What one inverter collects (from 4 waves):
Per strict 4-wave block:
Each wave = 10×8S + 1×7S = 11 strings
Four waves = 40×8S + 4×7S = 44 strings
Convert strings → modules:
40 strings × 8 modules/string = 320 solar panel modules
4 strings × 7 modules/string = 28 solar panel modules
Total = 320 + 28 = 348 solar panel modules (per4 waves)
So one inverter serving a strict 4-wave block sees 348 solar panels.
(But ,if we use the spare MPPT to take an extra 4×8S from the next block, the inverters could carry 320 + 28 + 32 = 380 modules.)
If we keep it strictly "4 waves/inverter", each inverter uses 11 of 12 MPPTs (one spare). If you let the spare MPPT take four extra 8S strings from the next block, we can run the plant on 13 inverters (our optimised plan); otherwise, it's 14 inverters exactly (one per 4-wave block).
PART 1: Specify the solar film panels- full wave profile coverage
Solar Film Panels
The solar film panels produces DC (Direct Current) power, which then needs to be converted to AC (Alternating Current) for use in the Station
Panel Model: Flextron F33F-360BM1
In this example design, we are using the Flextron F33F-360BM1 that are part of the 990 mm-wide family from BIPVco. Key specs:
These panels are designed to minimise power loss when partially shaded, thanks to built-in bypass diodes.
PART 2: Maximising the solar film on roof layout
Each wave on the roof is divided into 3 sections ( A, B & C) by two concrete span columns, where the DC peak capacity is shown below:
Wave roof definitions
Each wave: 3 sections ( A, B & C) → 87 Solar Film panels total.
Each section: 30 metres wide, holds a maximum of 29 solar film panels side by side.( 29 x 3 = 87 solar panels)
Whole Roof: 56 waves → 56 waves × 87 solar panels= 4,872 solar panels
Max DC Power Output Calculations
Per Section( e.g. section A): 29 solar panels × 360 W = 10.44 kWp DC
Per Wave( Sections A,B &C ) 87 solar panels × 360 W = 31.32 kWp DC
Whole Wave Roof: 4,872 solar panels × 360 W (max size) = 1,753,920 W = 1.75 MWp DC
Rule of thumb: DC:AC (inverter loading) ratio:
≈ 0.8–0.9 × DC Power
Estimated AC power output of 1.46 MWp AC
PART 3: Design for connecting solar films into Strings
Solar film 'string' definition
A string is a group of solar film panels connected in a line (in series). Their voltages add up but the current stays the same
String length Matters
But, if too many solar film panels are connected in one string, this can exceed voltage limits, especially in cold weather when voltage rises
Cold weather warning
At -10°C, voltage increases by ~10.5%
A panel with Voc = 116 V becomes ~128 V
If we have 9 x panels in series = 1,152 V → Too high for a 1,000 V inverter system
Safe string lengths
To stay within the 1,000 V module rating required for each inverter:
8 panels per string (8S) → Voc(cold) ≈ 963 V → Safe
7 panels per string (7S) → Voc(cold) ≈ 842 V → Very safe
PART 4: To specify the number of Inverters
Function of the Inverter
Inverters convert DC electricity from the panels into AC electricity for use in the station.
The use of MPPT
Each inverter has multiple MPPTs (Maximum Power Point Trackers), these are smart inputs that optimize power from each string.
Analogy:
Think of an inverter as a stereo with 12 volume knobs (MPPTs). Each knob controls a group of speakers (strings). MPPTs adjust each group to get the best sound (power), even if some are shaded or tilted differently.
PART 4: Approach B selected - Per-Wave String Plan and number of inverters
Approach A (2 strings per MPPT): 308 MPPT rows across 26 inverters
Approach B (4 strings per MPPT, Y-paralleled): 154 MPPT rows across 13 inverters
PART 4: Approach B selected - Per-Wave String Plan and number of inverters
Approach B (4 strings per MPPT, Y-paralleled): 154 MPPT rows across 13 inverters:
Whole roof top array is allocated to 154 individual MPPTs, distributed over 13 inverters.
Each Sungrow SG125CX-P2 unit has 12 MPPTs → 13 inverters give 156 MPPTs total.
Our design (Approach B) uses 4 strings per MPPT (Y-paralleled).
Total strings = 56 waves × 11 strings/wave = 616 strings.
MPPTs required = 616 strings ÷ 4 strings/MPPT = 154 MPPTs.
So, the schedule has 154 "MPPT rows" (one row per MPPT showing its four strings), spread across the 13 inverters which leaves 2 MPPTs spare for contingency/future use.
PART 4: Approach B selected - Per-Wave String Plan and number of inverters
Approach B (4 strings per MPPT, Y-paralleled): 154 MPPT rows across 13 inverters:
Y-paralleled means two identical PV strings are connected in parallel using a Y-branch so they share one inverter input.
How it's wired: An MC4 Y-connector combines the + of String 1 with the + of String 2 (and likewise the − with −). That single pair then lands on one DC input of an MPPT, utilizing double-insulated PV cable specifications (H1Z2Z2-K or PV1-F) rather than separate earth conductors.
What changes electrically:
Voltage stays the same as one string (Vmp, Voc unchanged).
Current adds (Imp_total = Imp₁ + Imp₂).
PART 4: Approach B selected - Per-Wave String Plan and number of inverters
Approach B (4 strings per MPPT, Y-paralleled): 154 MPPT rows across 13 inverters
In our design: one string ≈ 3.84 A → two in parallel ≈ 7.7 A per input. The Sungrow SG125CX-P2 allows ~20 A per input / 30 A per MPPT, so you can run two strings per input and hence four strings per MPPT (two per input).
Rules of thumb:
The two strings must be identical (same module count, orientation, shading conditions).
Keep within the inverter's current limits; add string fuses only when required by the module's series-fuse rating (typically when ≥3 strings are paralleled at one point).
Place Y-branches near the inverter (short parallel tails) and keep the two input pairs separate for each MPPT.
PART 4: To specify the Inverters- Approach B
A PV string has a power–voltage (P–V) curve with a single "sweet spot" where P = V × I is maximised. That point shifts all day with irradiance and temperature.
1.An MPPT (Maximum Power Point Tracker) is a DC-DC control stage inside the inverter that continuously adjusts the string's operating voltage so it sits at that sweet spot.
2.It samples string V and I, nudges the setpoint up or down (algorithms like perturb-and-observe or incremental conductance), and locks onto the voltage/current where power is highest—typically updated many times per second.
3.Each MPPT behaves like an independent optimiser. If you wire one or more strings to MPPT-A and different strings to MPPT-B, each group is tracked separately.
4.That means mismatch (different orientations, shading, soiling, temperatures, module makes/ages) on one MPPT doesn't drag down the others.
5.
PART 5: Choosing the right Inverter — Sungrow SG125CX-P2
Sungrow SG125CX-P2, a 1000 V-class inverter with excellent string handling.
🔹 Key Specs
PART 6: Solar film DC Cable quantification
For the first third of a wave (Section A) and using our chosen Approach B (4 strings per MPPT):
Cables that occupy in the first 1/3 (Section A) of a wave:
Strings in Section A: 3 × 8 solar film module (8S) strings (labels: A1, A2, A3)
Solar film panels per string: 8
Total solar film panels in this 1/3: 3 × 8 = 24 (the "leftover" 7-panel string sits in Section C, not here)
How many cables in this 1/3 wave:
Per string: 2 conductors ( + and − )
Total DC conductors: 3 strings × 2 = 6 DC cables (H1Z2Z2-K or PV1-F double-insulated)
6 DC cables in this 1/3's spur, installed as H1Z2Z2-K or PV1-F double-insulated cables in tray.
Hence, in our design for Section A, each string uses 3 x pairs of DC conductors, where we are using six H1Z2Z2-K or PV1-F double-insulated runs. These will be installed in tray with an approximate Typical OD (95 mm²): ~18–20 mm each.
PART 7: Solar film cable management per 1/3 wave
Cable route and fixing on the cladding face (outside the cavity) on the rear of the bull nose of wave:
Use clips or free cable for the grey DC connector cables from the solar film panels to the rear of the bull nose of the wave, as shown in the photo
Fix appropriate containment above the top of the existing cowling clips, as shown in the photo- to accommodate the six H1Z2Z2-K or PV1-F double-insulated DC cables per 1/3 wave
Consider using self tapping screws/ rivets to fix this containment
PART 7: Primary Cable Containment
Primary Cable Containment — Fixed to top of span beams (Approach B: 4 strings/MPPT, 13 inverters)
A. One full wave spur containment (each wave)
Duty: Collect 11 PV strings (10×8S, 1×7S) off the wave.
Cable containment sizing: TBA (adjusted for double-insulated cables)
Cables: 11 string pairs (PV1-F 6 mm² Cu +/− double-insulated, no separate earth conductor).
Primary DC Cable Type (where applicable): Double-insulated, single-core LSZH cables in tray, using 0.6/1 kV LSZH single-core (e.g., H1Z2Z2-K or PV1-F equivalent).
Primary DC Cable Formation: Double-insulated cables run in trefoil or flat formation on ladder, eliminating the need for a separate earth. Typical OD (95 mm² equivalent): ~18–20 mm each.
Supports: To steel span beam at 1.2–1.5 m centres; fixings
PART 7: Primary Cable Containment (conti)
B. Primary DC runs (on span beams) — four-wave blocks to each inverter
Duty: Carry 44 string pairs from 4 waves to one inverter which is bolted using face plate to side of span beam
Cabling: H1Z2Z2-K or PV1-F 6 mm² double-insulated cables +/− per string; TBD depending on length. No separate earth conductors are required due to the double-insulated cable approach.
Fan-out at plinth: 11 MPPT bays; each MPPT gets 4 identical strings (two per input via MC4 Y-branches). Typical 22 Y-branches/inverter (24 if the spare MPPT is used).
What "44 string pairs" means:
Per wave: 10 × 8S strings + 1 × 7S string = 11 strings.
Four waves → one inverter: 4 × 11 = 44 strings.
Each string has two conductors (+ and −), i.e., 44 string pairs = 88 single-core DC cables
Cable + containment design for that duty:
Cable type: PV1-F or H1Z2Z2-K 6 mm² Cu double-insulated cables (one + and one − per string), routed as tight +/− pairs, without separate earth conductors.
Lane allowance: ~15 mm per string pair → 44 × 15 mm = 660 mm of single-layer width.
Containment (primary DC to inverter): TBD ( contractor)
Fan-out at the inverter:
Total from 4 waves: 40×8S + 4×7S.
Use 11 MPPTs per inverter: 10 MPPTs take 4×8S each; 1 MPPT takes 4×7S.
Wiring is Y-paralleled (two strings per input) → 4 strings/MPPT.
Y-branches needed: ~22 per inverter (24 if the spare MPPT is also populated).
String fuses typically not required at 2-in-parallel per input (confirm module series-fuse rating).
Current context:
Per string Imp: ~3.8–4.0 A.
Per MPPT (4 strings): ~15–16 A (below the Sungrow MPPT limit).
So the job of that primary DC run is exactly to carry 44 string pairs (88 single-core cables) from the four waves along the span beam to the inverter plinth, using the tray sizes above
PART 7: Primary Cable Containment (conti)
C. Secondary DC span beam wall containment into inverters
Cable Type: H1Z2Z2-K or PV1-F double-insulated cables for all DC runs, ensuring reinforced insulation as the protective measure.
Tray: short 200–300 mm drop ladder from primary run to inverter gland/entry.
Glanding: Transition rail/box for labeling, strain relief, and segregation to the inverter DC inputs.
Protection: No large combiners (not required); fit gPV fuses only if module series-fuse rating demands (≥3 in parallel at one point).
D. Primary AC runs (on span beams) — inverters to switchgear
Inverter feeder: 95 mm² 0.6/1 kV LSZH single-core (e.g., 6491B / H07Z-K or equivalent) - design ~185 A @ 230 V. Run as 3 × phase + 1 × CPC (earth) per inverter in trefoil or flat formation. Typical OD (95 mm²): ~18–20 mm each.
Containment: 2 × 400 mm × 100 mm ladders along the two span-beam corridors carry all 13 inverter sets (each set is 4 single-core cables) with spacing; segregate from DC (parallel separation, 90° crossings).
Earthing: Bond each ladder; (lightning protection to do) main protective conductor sized per fault levels/LPS coordination.
PART 7: Primary Cable Containment (conti)
E. Materials & compliance (all containment)
Material: Hot-dip galvanized ladder (≥90 µm) or 316 stainless in exposure zones; stainless fasteners.
Supports: Straight ≤1.5 m, bends/tees/drop-outs ≤0.9 m.
Fire/penetrations: Tested fire-stops to required EI rating; maintain bonding continuity across seals.
LPS: Maintain separation distance or apply bonding in accordance with the lightning study; ensure DC cable pairs (e.g., H1Z2Z2-K or PV1-F double-insulated) are kept tight to minimise loop area, leveraging double insulation as the primary protective measure.
Identification: Tray IDs; string IDs (e.g., W12-A1-8S) and MPPT/Inverter IDs at both ends; direction arrows where helpful.
PART 8: Headline numbers
For whole solar film array, end-to-end, for Approach B (4 strings/MPPT, 13 inverters)
Headline numbers:
Array DC nameplate: 1.754 MWp DC (4,872 × 360 W)
Inverters: 13 × 125 kWac Sungrow SG125CX-P2 → 1.625 MWac total nameplate
Peak output (power): ≈1.625 MW AC (clipped at inverter rating)
Annual energy (context): London 900–1,000 kWh/kWp ⇒ ~1.6–1.75 GWh/yr –TBC by modelling & losses at each point
PART 9: AC cabling on the two beams
Primary AC cables (per inverter):
Cable type: 0.6/1 kV LSZH single-core (e.g., 6491B / H07Z-K or equivalent).
Configuration: 3 × phase + 1 × CPC (earth) run per inverter.
Design current: ~185 A per inverter (125 kW ÷ (√3·400 V·η)) – *this current applies across the three phase conductors.*
Typical OD (95 mm²): ~18–20 mm each.
Fault/earthing: CPC (earth) conductor run alongside phases; all cables glanded at each end.
How many AC cables, containment estimation and where:
Total inverter connections: 13 (one set of 4 single-core cables per inverter).
Total single-core cables: 13 inverters × 4 cables/inverter = 52 single-core cables.
Two span beams: split for good balance and smaller bundles:
Beam A: 7 inverter connections (28 single-core cables) → ≈7 × 80 mm lane = ~560 mm.
1.Beam B: 6 inverter connections (24 single-core cables) → ≈6 × 80 mm lane = ~480 mm.
2.Lane rule-of-thumb = space for 4 single-core cables (~20mm each) in trefoil/flat formation + spacing → use 80 mm per inverter connection for tray sizing.
3.
PART 9: AC Cable specification data sheet: 95 mm² LSZH Single-Core, 0.6/1 kV
Cable type: 0.6/1 kV LSZH single-core (e.g., 6491B / H07Z-K or equivalent)
Configuration: 3 × phase + 1 × CPC (earth) run in trefoil or flat formation on ladder
Overall diameter (OD) per single-core cable: ~18–20 mm each
Minimum bend radius (per single-core cable): ≥ 12×OD installed (i.e., ~0.22–0.24 m); use 15×OD during pulling. Minimum bend radius is approx. 24 cm
Cleat spacing (for single-core cables): ~300–450 mm on straight runs; closer at bends/tees, and ensuring formation integrity.
PART 9: Cable size: 95 mm² Cu LSZH single-core, 0.6/1 kV
Electrical loading:
Normal design load per inverter feeder: ≈ 185 A (125 kW ÷ (√3·400 V·η))
This is well within the clipped-direct rating of three 95 mm² Cu LSZH single-core (e.g., 6491B / H07Z-K or equivalent) cables run in trefoil or flat formation on ladder (typically ≥ 230–260 A, depending on install method/ambient).
Conductor resistance (90 °C): = 0.206 Ω/km
Reactance: = 0.08 Ω/km
For the longest run ~140 m at 185 A, 3-phase voltage drop is about:
→ ≈
9.8 V drop → ≈ 2.4 % of 400 V (with cosφ=0.98)
This is inside a common ≤3 % design target.
PART 9: Cable size: 3x95mm² + 1x95mm² LSZH single-core, 0.6/1 kV
Overall diameter (OD): ≈ 18–20 mm each (for 95 mm² single-core)
Summary callouts we can place on drawings:
Cable: "3x95mm² + 1x95mm² LSZH single-core (e.g., 6491B / H07Z-K or equivalent) 0.6/1 kV, run in trefoil or flat formation on ladder/tray; OD ~18–20 mm each; design load 185 A."
Containment sizing already set:
Use 2 × 400 mm × 100 mm ladders on the primary AC corridor (13 feeders total), or one 400 mm × 100 mm per span-beam bundle where they run separately.
PART 9: Primary Cable Containment (conti)
Cable management / containment sizes for single-core LSZH cables
On each span beam (primary AC runs)
Cable type: 0.6/1 kV LSZH single-core (e.g., 6491B / H07Z-K or equivalent). Each feeder consists of 3 × phase + 1 × CPC (earth) run in trefoil or flat formation on ladder. Typical OD (95 mm²): ~18–20 mm each.
Containment: ladder tray, hot-dip galv. or 316 SS where exposure is high.
Beam A (7 feeders): Each feeder now consists of 4 single-core cables (~18-20 mm OD each). This will require significantly more width than the previous 4-core cable. For 7 feeders (28 single-core cables), use 2 × 400 mm × 100 mm ladders in parallel, possibly with an additional 100 mm width ladder depending on exact spacing and formation chosen (trefoil vs. flat).
Beam B (6 feeders): For 6 feeders (24 single-core cables), 2 × 300 mm × 100 mm ladders in parallel or 1 × 600 mm × 100 mm ladder with generous spacing would be more appropriate than a single 400 mm ladder.
Supports: ≤1.5 m centres (≤0.9 m at bends/tees/drop-outs).
Separation: keep AC away from DC; cross at 90° where necessary.
Crossing the glass roof / past the West Tower
Keep ladders fixed to steel (span-beam steel or bracketry) — never to glazing.
Where feeders from the two beams converge near the West Tower:
Option 1 (single corridor): combine all 13 feeders (52 single-core cables total) → lane width required is substantial, likely 2 x 600 mm x 100 mm ladders or more (approx. 1.2m combined width).
Option 2 (tidier): keep them in two bundles (7-feeder (28 single-core cables) and 6-feeder (24 single-core cables)) all the way to the parapet; each bundle would require separate dedicated containment (e.g., 1 x 400 mm x 100 mm for the 6-feeder bundle and 1 x 600 mm x 100 mm for the 7-feeder bundle, or 2 x 300 mm x 100 mm ladders for each bundle, depending on spacing needs).
PART 10: Primary Cable Containment (conti)
Fixed along the wall of West Tower, over the parapet and down the façade (architectural cowling) – to be detailed by others
Best practice: keep the two bundles separate, each in its own cowling, considering bundles of 0.6/1 kV LSZH single-core cables (e.g., 6491B / H07Z-K or equivalent) with 3 × phase + 1 × CPC (earth) per feeder, each cable having a typical OD of ~18–20 mm:
Cowling A (7 feeders, 28 single-core cables): clear internal width ≥580–620 mm, depth ≥150 mm (for flat formation).
Cowling B (6 feeders, 24 single-core cables): clear internal width ≥500–540 mm, depth ≥150 mm (for flat formation).
Construction: colour-matched aluminium/SS cowling with drip at head and weeps at base; mounted to structure/rainscreen rails, not the sheet alone.
Thermal & movement: allow expansion joints; cleated spacing ~400–600 mm; rubber isolators on brackets.
Fire & façade: maintain cavity barriers, fire-stop at building entry per the strategy.
Building entry & route to Chiller Switchgear Room (lower roof level)
Penetration: through the primary air/water barrier (not just rainscreen) via a gland plate/box, sleeved and fully flashed to the AVB; EI-rated fire-seal on the warm side.
Internal riser/duct: baskets or ladders sized for the bundles of single-core LSZH cables (7-feeder and 6-feeder trunks).
Terminations: land to the LV switchgear that backs the Chiller Switchgear Room; breakers and protection per inverter manufacturer's G99/onsite study.
PART 8: Primary Cable Containment (conti)
Peak at the "end of the 56th wave"
Electrically, the maximum AC power arriving at switchgear equals the sum of all 13 feeders: 1.625 MW AC.
Each feeder carries up to ~185 A; the containment at the final corridor must comfortably hold all 52 single-core LSZH cables (hence the 2 × 600 × 100 mm ladders, or two separate 600-mm ladders if you keep the bundles split).
Routing summary you can put on the GA
Span beams (two runs):
Beam A: 7 feeders (28 single-core LSZH cables) on 1 × 600 × 100 mm ladder.
Beam B: 6 feeders (24 single-core LSZH cables) on 1 × 600 × 100 mm ladder.
Glass-roof zone / West Tower: either merge to 2 × 600 × 100 mm ladders (all 13 feeders), or continue as two separate 600-mm ladders.
Parapet crossing: step-over + edge guards; then into two architectural cowlings sized above.
Façade drop: down to the lower roof level entry; sleeved, flashed, and fire-stopped.
Internal: trays/baskets to switchgear; identify each feeder (INV-01…-INV-13) both ends.
If you'd like, I can annotate the drawing with exact tray sizes, bundle counts at each segment, and a small bill of materials (ladder metres, bends, tees, supports, cowlings, glands).
PART 9: Containment across glass roof to West tower