EV Charger Cable Sizing Calculator

Summary

EV charger circuits are a common source of undersized cable on domestic jobs because the 32A design current is higher than most electricians are used to sizing for on a single circuit, and the outdoor cable run to a driveway or car port is often longer than a typical indoor circuit. Getting the cable size wrong shows up as excessive voltage drop under sustained full-load charging (an 8+ hour continuous load, unlike most domestic loads which cycle) or, in the worst case, an overheating cable run underground or in a wall void.

This calculator covers the standard method: find the design current, select a trial cable size and installation method, look up the mV/A/m value from BS 7671 Appendix 4, apply the voltage drop formula, and check the result against the 5% limit. It also covers the three things that most often catch installers out on EV circuits specifically: the continuous-load nature of the circuit (unlike an intermittent load, an EV charger runs at or near full design current for hours at a time, so there's no diversity to lean on), the longer typical cable runs to outdoor charging points, and the interaction between cable sizing and the separate PME/PEN earthing requirement under BS 7671 Regulation 722.411.4.

For the full regulatory and installation picture — DNO notification, RCD type, PME earthing options, consumer unit capacity — see ev charger. This article focuses specifically on the cable sizing calculation. For general cable current-carrying capacity and derating, see cable sizing current capacity, and for three-phase voltage drop method (relevant to 22kW chargers), see cable voltage drop 3phase.

Key Facts

• Standard home charger — 7kW, 32A, single phase, 230V
• Fast charger (rare domestically) — 22kW, 32A, three phase, 400V
• BS 7671 voltage drop limit — 5% of nominal voltage, measured from the origin of the installation to the load
• 5% of 230V (single phase) — 11.5V maximum voltage drop
• 5% of 400V (three phase, line voltage) — 20V maximum voltage drop
• Design current for a 7kW charger — 32A (this is also the continuous rated current — treat as a sustained full-load circuit, not a diversified one)
• Minimum cable size for a 32A EV circuit — 6mm² copper, subject to installation method and run length
• Voltage drop formula — Vd (V) = (mV/A/m × Ib × L) / 1000
• mV/A/m source — BS 7671:2018+A4:2026 Appendix 4 tables, matched to cable size, conductor material and installation method
• RCD requirement — Type A or Type B RCD (some chargers require Type B if they cannot detect DC fault currents internally); never Type AC — check the specific charger's installation manual
• MCB/RCBO rating — 32A Type B or Type C to match the charger's inrush characteristics as specified by the manufacturer
• PME/PEN fault protection — a separate requirement (BS 7671 Regulation 722.411.4) that cable sizing does not satisfy; most PME (TN-C-S) supplies need a PEN fault detection device or a dedicated earth electrode
• Underground SWA cable depth — typically 750mm under a driveway, 500mm in a garden border
• Continuous load derating — some designers apply an additional margin for the sustained nature of EV charging load beyond the standard current-carrying capacity tables; check your scheme provider's current guidance

Quick Reference Table: mV/A/m Values (Single-Phase, Copper, PVC/XLPE, Approximate)

Values are approximate reference points; always use the current BS 7671:2018+A4:2026 Appendix 4 tables for the exact installation method, grouping and ambient temperature applicable to the specific run. Current-carrying capacity figures are indicative for Method C, unenclosed clipped direct — apply the correct derating factors for buried/enclosed runs.

Detailed Guidance

Step-by-step method

Step 1 — Establish design current (Ib). For a standard single-phase 7kW home charger this is 32A. Some chargers can be configured to a lower current (e.g. 16A/3.68kW) — always use the charger's actual configured or maximum rated current, not an assumed figure.

Step 2 — Select a trial cable size and installation method. Start with 6mm² for typical domestic runs. Confirm the installation method (clipped direct, in conduit, buried direct, in duct) because both the current-carrying capacity and the mV/A/m value change with method.

Step 3 — Look up mV/A/m. Use the single-phase (2-wire) column of BS 7671:2018+A4:2026 Appendix 4 for the chosen cable size and method.

Step 4 — Calculate voltage drop. Apply Vd = (mV/A/m × Ib × L) / 1000, where L is the one-way cable run length in metres.

Step 5 — Check against the limit. Compare the result to 5% of the nominal supply voltage (11.5V for 230V single phase). If it fails, move up a cable size and repeat.

Step 6 — Check current-carrying capacity separately. Voltage drop compliance does not automatically mean the cable can carry 32A continuously for the installation method and any grouping/thermal insulation involved — check the current-carrying capacity table and apply any relevant derating factors (grouping, ambient temperature, thermal insulation contact) independently of the voltage drop check.

Worked example 1 — 6mm² cable, 15m run, clipped direct

• Cable: 6mm² copper, clipped direct (Method C)
• mV/A/m (single phase): 7.3
• Ib: 32A
• L: 15m

Vd = (7.3 × 32 × 15) / 1000 = 3,504 / 1000 = 3.5V

3.5V ÷ 230V × 100 = 1.5% — well within the 5% (11.5V) limit. 6mm² is compliant for voltage drop at this length.

Worked example 2 — 6mm² cable, 28m run, clipped direct

• Cable: 6mm² copper, clipped direct
• mV/A/m: 7.3
• Ib: 32A
• L: 28m

Vd = (7.3 × 32 × 28) / 1000 = 6,540.8 / 1000 = 6.54V

6.54V ÷ 230V × 100 = 2.84% — still within the 5% limit, but getting closer to the ceiling. At this length, many installers move to 10mm² to leave headroom for any future extension of the run, or where the installation method (e.g. buried, or grouped with other cables) will apply a derating factor to current-carrying capacity.

Worked example 3 — 6mm² cable, 45m run, clipped direct (fails)

• Cable: 6mm² copper, clipped direct
• mV/A/m: 7.3
• Ib: 32A
• L: 45m

Vd = (7.3 × 32 × 45) / 1000 = 10,512 / 1000 = 10.5V

10.5V ÷ 230V × 100 = 4.57% — technically still under 5%, but only just, and this doesn't yet account for any additional drop elsewhere in the installation (from the origin/meter position to the consumer unit). Combine this with any upstream voltage drop already present in the installation before confirming compliance — the 5% limit applies end-to-end from the origin of supply, not just for this final circuit in isolation.

Worked example 4 — 10mm² cable, 45m run, clipped direct

• Cable: 10mm² copper, clipped direct
• mV/A/m: 4.4
• Ib: 32A
• L: 45m

Vd = (4.4 × 32 × 45) / 1000 = 6,336 / 1000 = 6.34V

6.34V ÷ 230V × 100 = 2.76% — comfortably compliant and leaves headroom for any upstream drop. For long runs (30m+), 10mm² is the standard practical choice even though the voltage drop calculation on 6mm² might technically pass — the extra margin protects against upstream drop, future load changes and installation-method derating.

Three-phase (22kW) chargers

Three-phase 22kW domestic chargers are uncommon (most UK homes have a single-phase supply) but occur in some new-build and rural three-phase properties. Use the three-phase mV/A/m values and the 400V line voltage limit (5% = 20V) rather than the single-phase method above — see cable voltage drop 3phase for the full three-phase method, worked examples and the mV/A/m table.

Why the continuous-load nature of EV charging matters

Unlike a cooker or shower circuit, which draws its design current only intermittently, an EV charger commonly runs at or close to its full rated current for 6-10 hours in a single overnight session. This sustained loading is exactly the scenario that current-carrying capacity tables are designed around (they assume continuous full-load operation, not diversified intermittent use), so there's no diversity factor to apply when sizing an EV charging circuit — size for the full design current as a continuous load, and check ambient temperature and grouping derating carefully, especially for cables run in insulated walls, ducts, or grouped with other circuits.

Voltage drop is not the only check — PME earthing

Passing the voltage drop calculation does not mean the installation is compliant. Most UK domestic supplies use PME (TN-C-S) earthing, and BS 7671 Regulation 722.411.4 requires either a PEN fault detection device or a dedicated earth electrode for EV charger installations on a PME supply, independent of the cable sizing calculation. See ev charger for the full PME/PEN detail — don't treat a compliant voltage drop calculation as the end of the design process.

Frequently Asked Questions

What's the minimum cable size for a 7kW EV charger?

6mm² copper is the practical minimum for a standard 32A, 7kW single-phase EV charger circuit, suitable for runs up to roughly 20-25m depending on installation method and any derating factors. Below 6mm² (e.g. 4mm²), voltage drop and current-carrying capacity both become marginal or non-compliant for a 32A continuous load at typical domestic run lengths.

At what cable run length should I move from 6mm² to 10mm²?

There's no single fixed distance because it depends on installation method, ambient temperature and grouping — but as a practical guide, runs beyond roughly 25-30m commonly need 10mm² to stay comfortably within the 5% voltage drop limit once upstream drop elsewhere in the installation is accounted for. Always run the calculation for the specific job rather than relying on a rule of thumb alone.

Do I need to apply a diversity factor when sizing an EV charger circuit?

No. Unlike many domestic circuits, an EV charger is a continuous, sustained load for the duration of a charging session, so size the circuit for its full design current (typically 32A for a 7kW charger) with no diversity applied.

Does passing the voltage drop calculation mean the installation is BS 7671 compliant?

No — voltage drop is one of several separate checks. You also need to confirm current-carrying capacity with the correct derating factors for the installation method, and satisfy the PME/PEN earthing requirement under Regulation 722.411.4, which is an entirely separate compliance check from cable sizing. See ev charger for the full installation requirements beyond cable sizing.

Can I use aluminium cable for an EV charger circuit to save cost?

Aluminium cable has different mV/A/m and current-carrying capacity values from copper for the same cross-sectional area (aluminium requires a larger CSA for equivalent performance) and is uncommon on domestic EV circuits. If used, always source the aluminium-specific tables from BS 7671 Appendix 4 rather than adjusting copper figures by an approximate factor.

Regulations & Standards

• BS 7671:2018+A4:2026 — IET Wiring Regulations; Section 722 covers electric vehicle charging installations, including the earthing requirements at Regulation 722.411.4; Appendix 4 provides the mV/A/m and current-carrying capacity tables used in this calculator.

• BS 7671 Regulation 525 — voltage drop limits generally (5% from origin to load in most installations).

• Engineering Recommendation G100 — DNO notification requirements for EV chargers as a demand-side load; see ev charger for thresholds.

• Part P of the Building Regulations — new EV charger circuits are notifiable electrical work.

• IET BS 7671 Wiring Regulations — 18th Edition, including Appendix 4 tables and Section 722.

• IET Guidance Notes series — Guidance Note 1 covers voltage drop calculation method in detail.

• Energy Networks Association G100 — DNO connection/demand assessment for EV chargers.

• ev charger — full EV charger installation guide: DNO notification, RCD type, PME earthing, OZEV grant detail.

• cable voltage drop 3phase — three-phase voltage drop method, relevant to 22kW three-phase chargers.

• cable sizing current capacity — general current-carrying capacity and derating factors referenced in the current-capacity check above.

• cable derating and grouping factors — derating for grouped cables and thermal insulation, relevant when the EV circuit shares a route with other cables.