☀ SolarCalculatorHQ

Solar PV Earthing & CPC Size Calculator (BS 7671)

Size the circuit protective conductor (CPC) and module-frame bonding for a UK domestic PV array per BS 7671:2018+A2 Table 54.7 and MCS 005. Free copper / aluminium lookup.

Solar PV Earthing / CPC Size Calculator

Minimum CPC size
6 mm²
Code reference: BS 7671:2018+A2 Tab. 54.7
Terminal torque
2.3 N·m
Module frame bonding jumper: 6 mm²
Notes: BS 7671:2018+A2 Table 54.7 / adiabatic equation. MCS 005 also requires a continuous earth path with all module frames bonded back to the main earth terminal.

How to use this calculator

Enter the live (phase) conductor cross-section of your PV DC string in mm², select copper or aluminium, and the calculator returns the minimum CPC size per BS 7671:2018+A2:2022 Table 54.7. The same value applies to the bonding jumper between module frames and racking.

Inputs you provide:

  1. Live conductor cross-section (mm²) — the cross-section of the DC string conductor. A typical UK 4-7 kWp residential rooftop install uses 6 mm² H1Z2Z2-K solar cable. Larger 10-15 kWp arrays may step up to 10 mm² for VD on long roof-to-inverter runs.
  2. DC overcurrent protection (A) — the fuse rating of the touch-safe DC fuse holder in the array isolator. Used for the lug torque guidance.
  3. CPC material — copper for nearly all UK installs. Aluminium is permitted but rarely used because of the corrosion concern at outdoor terminations.
  4. Mechanically protected run — if the CPC is run inside conduit, trunking, or the cable sheath, the minimum cross-section drops to 2.5 mm². Unprotected runs require a minimum of 4 mm² per BS 7671 Regulation 543.1.

How the math works (BS 7671 Table 54.7)

The CPC is sized from the live conductor cross-section using Table 54.7:

S (live)  ≤ 16 mm²   → CPC = S (same as live)
16 < S    ≤ 35 mm²   → CPC = 16 mm²
S         > 35 mm²   → CPC = S / 2

Aluminium conductors are uprated by approximately 1.5× to match the conductivity of copper.

Worked example. A 5.5 kWp domestic rooftop array on the south coast uses two strings of 10 modules at 9.6 A Imp each, brought into a touch-safe combiner. The DC live conductors are 6 mm² H1Z2Z2-K (rated 70 A free air / 41 A in conduit). Table 54.7 says CPC = 6 mm² copper. MCS 005 minimum bonding conductor is 6 mm² copper, so 6 mm² satisfies both. The 6 mm² green/yellow CPC runs alongside the DC pair into the inverter and on to the consumer-unit earth bar.

If the same array used 10 mm² conductors for a long 30 m roof-to-inverter run (VD upsize), Table 54.7 still gives 10 mm² for the CPC because 10 ≤ 16 mm². No separate adiabatic calculation needed unless the fault-loop impedance is borderline.

BS 7671 Section 712 — PV-specific earthing

Section 712 (Solar photovoltaic power supply systems) covers PV earthing in detail. The clauses you actually use:

  • 712.411.3.2.1.2 — exposed conductive parts of the DC side bonded via a CPC; the CPC must be separate from any functional ground.
  • 712.413 — automatic disconnection of supply on the AC side uses the existing consumer-unit RCD; no extra device required on the DC side if the inverter is non-isolating with built-in residual-current monitoring (most string inverters since 2018).
  • 712.444.4.4 — overvoltage protection (SPD type II) required at AC and DC sides for arrays on TN earthing systems with cable runs over 10 m.

The 18th edition (2018) plus Amendment 2 (March 2022) is the current document for new installations. Older 17th edition installations don’t need a retrofit but any modification has to bring the changed section up to current standards.

MCS 005 grounding requirements

The MCS 005 product standard for PV mounting systems and MCS 015 for installation both add specific grounding requirements beyond BS 7671:

  • Minimum 6 mm² copper bonding conductor (regardless of Table 54.7 result for smaller arrays).
  • Every metallic part of the mounting structure earthed back to the main earthing terminal of the building.
  • Earthing clips, lugs and fasteners must be marked / certified for outdoor UV-exposed use.
  • A continuity test must read ≤ 1.0 Ω from the farthest module frame to the main earth bar.

The MCS Installer Compliance audit specifically checks for the 6 mm² minimum and the < 1 Ω continuity test. Failing either is an immediate non-compliance and grounds for MCS certificate suspension.

Module-frame bonding hardware

UK installers typically use one of three approaches:

  1. Stainless toothed washers / WEEB clips — Wiley Electronics WEEB-9.5 for 40 mm anodised frames is the most common worldwide. UK distributors include Segen and BayWa r.e.
  2. Integrated earthing rail — K2 SingleRail with K2 Earthing Pin, Schletter FixZ-15, IronRidge XR1000 (US-spec but UK-distributed). The rail itself provides a UL 2703 (or EN 50438) listed grounding path through serrated pins.
  3. Discrete M6 earth lugs and bare 6 mm² copper jumper — older approach still acceptable. Use stainless lugs (e.g. ILSCO GBL-4DBT) with toothed washers. Requires a continuous jumper between every panel and the main earth.

Whatever you pick, the manufacturer’s compatibility matrix must list the specific module and rail combination — JA Solar, LONGi, JinkoSolar, REC and Q-Cells all publish compatibility tables for the major bonding-clip brands.

When voltage drop forces a CPC upsize

BS 7671 Regulation 525 keeps domestic VD ≤ 3% on lighting and 5% on power circuits. For PV the practical limit is the same 3% from array to inverter so DC losses stay under 1% of yield. If the calculated VD pushes you to upsize from 6 mm² to 10 mm², your CPC moves to 10 mm² too — same as the live conductor under Table 54.7.

Use the solar panel voltage calculator on this site to check the VD math.

Common UK installation faults

The most common earthing faults flagged by MCS Compliance audits and DNO inspections:

  1. CPC terminated under a non-UV-rated grommet on a rooftop — degrades within 5 years.
  2. No bonding between separate rail sections (each rail section has to be earthed if joined by a sliding joint).
  3. M6 earth lug installed without a toothed washer biting into the anodised frame — looks fine but reads > 5 Ω in a continuity test.
  4. CPC dropped to 4 mm² inside a buried gravel section without conduit — fails mechanical protection.
  5. AC-side SPD missing on a TN-C-S install with > 10 m roof-to-inverter run.

A pre-energisation continuity test from the farthest module frame to the consumer-unit earth bar catches all five — the test should read ≤ 1.0 Ω.

Sources

Frequently asked questions

What size CPC do I need for a domestic solar PV installation in the UK?
Per BS 7671:2018+A2:2022 Table 54.7 the circuit protective conductor is sized from the live (phase) conductor cross-section. For a typical UK domestic PV string with 6 mm² live conductors the CPC is also 6 mm². For 10 mm² live conductors the CPC is 10 mm² (still ≤ 16 mm² so equal to phase). Above 16 mm² the CPC drops to 16 mm², then to half the phase area at 35 mm² and above. MCS 005 also imposes a minimum 6 mm² copper bonding conductor between PV array and main earthing terminal regardless of the table result.
Does the CPC need to be upsized for long cable runs?
BS 7671 doesn't apply an automatic voltage-drop upsize to the CPC like the US NEC does. If you uprate the live conductor for VD reasons (typical on long rooftop-to-CU runs), the adiabatic equation in regulation 543.1.3 still has to be satisfied for the new fault-loop impedance. In practice running the CPC at the same upsized cross-section is the simplest way to stay compliant — and it matches the MCS 005 'continuous low-impedance earth path' wording.
How do I bond solar PV module frames in the UK?
MCS 005 and the IET Code of Practice for Grid-Connected Solar PV Systems (3rd edition) require module frames and racks to be bonded back to the main earthing terminal of the building via the CPC. Use a stainless steel earthing clip or M6/M8 earth lug rated for outdoor use. WEEB-style toothed washers and the K2 / Schletter integrated earthing rails are both standard. The bond is verified with a continuity test ≤ 1.0 Ω from the farthest module frame to the consumer-unit earth bar.
Do I need an additional earth rod at the PV array?
Not normally. A typical domestic UK installation under TN-C-S or TN-S earthing relies on the existing PEN or earth conductor and adds no separate array-side electrode. For TT systems an earth electrode is already mandatory at the building; the PV CPC bonds back to it via the main earthing terminal. Section 712 of BS 7671 (PV-specific) explicitly notes that a separate auxiliary electrode for the PV array is not required unless the array is on a remote outbuilding with its own earthing arrangement.
Can I use single-insulated green/yellow PVC as the CPC in the array?
Yes for indoor runs in conduit or trunking. On the rooftop the CPC must be UV-resistant — single-insulated green/yellow inside flexible conduit is acceptable, or a double-insulated H1Z2Z2-K solar cable in green/yellow run alongside the DC pair. The exposed runs from frame to frame are typically bare 6 mm² copper or stainless braid where mechanical protection allows.
What's the difference between bonding and earthing in a UK PV install?
Earthing connects exposed conductive parts (module frames, inverter chassis, metallic enclosures) to the main earthing terminal so a fault drives enough current through the protective device to disconnect. Bonding is the equipotential connection between separately earthed conductive parts (e.g. the PV array frame and a nearby gas pipe) to keep them at the same potential during a fault. MCS 005 requires both: every metallic part of the PV install is earthed, and where it meets other building services it is bonded to them via supplementary equipotential bonding under regulation 415.2.

Related calculators