Can I use 4 mm² cable for a 7 kW EV charger?

4 mm² twin-and-earth (Iz around 36 A in the most favourable reference method) might just satisfy the current-carrying capacity check for a 32 A charger on a very short run, but voltage drop typically fails beyond ~10-15 m. Most professional UK installers default to 6 mm² for 7 kW chargers because it provides headroom for voltage drop and grouping factors. 4 mm² may be acceptable for a 16 A charger (3.6 kW) on a short run.

What protective device do I install for a 7 kW EV charger?

A 32 A Type A 30 mA RCBO is the most common choice. Type A is preferred over Type AC because modern EV chargers can produce DC fault current components — Type AC RCDs may not detect these reliably. Some installers specify Type F (or higher) for chargers with frequency-converter electronics. Confirm the charger's installation manual — many manufacturers (e.g. Wallbox) require Type A as a minimum; some (e.g. certain Tesla Wall Connectors) require Type B for full DC fault detection.

Does the EV charger need a separate consumer unit?

Not required by BS 7671, but increasingly common in practice. Reasons to use a dedicated EV charging consumer unit: simpler load management, dedicated 32 A RCBO + isolator, clear separation for fault diagnosis, easier upgrade to three-phase later. Reasons against: extra cost, additional point of failure, more cable terminations. For most domestic 7 kW installations, fitting a dedicated RCBO in the existing consumer unit (if it has a spare way) is the simpler approach.

How does cable size change for a 22 kW three-phase charger?

A 22 kW three-phase charger draws 32 A per phase. The three-phase cable carries this current per core in a four-core (3 phase + neutral) configuration. 10 mm² four-core SWA handles up to ~50 A per core in free air and is the typical starting point for runs up to 20 m. For longer runs, voltage drop usually governs and 16 mm² becomes the practical choice. For very long runs (>50 m) at full 22 kW, 25 mm² or 35 mm² may be needed.

What about voltage drop on a long run?

BS 7671 Regulation 525 caps voltage drop at 3% for lighting and 5% for "other" circuits. EV charging is "other" — so 5% is the technical maximum — but designing to 3% leaves headroom and matches the lighting limit (sensible if the same supply also feeds lighting). On a long run (say 50 m), 6 mm² 7 kW would exceed 3% — step up to 10 mm² and recheck. Calculator below does the maths.

Do I need to consider grouping if the EV cable shares a trench with other cables?

Yes. BS 7671 Appendix 4 Table 4C1 (and equivalents for buried installations) gives grouping factors Cg that reduce Iz when multiple loaded cables share a trench, conduit, tray or trunking. A grouping factor of 0.8 means the cable's effective Iz is 80% of the un-grouped value. Apply Cg in your sizing calculation; the practical effect is that you often step up one size when the EV cable is grouped with other heavily-loaded cables.

Should I install conduit / trunking even where not strictly required?

For SWA buried direct, no — the armouring provides the mechanical protection BS 7671 requires. For 6242Y twin-and-earth running through a roof void or down through an external wall to an outdoor charger, conduit is sensible mechanical protection even where not strictly mandated. Confirm against the IET Code of Practice for EV Charging Equipment Installation for the latest practical guidance on cable management.

CABLE SIZING

Cable Size for an EV Charger (7 kW / 22 kW)

The single most-asked cable-sizing question for UK domestic electricians. The short answer: 6 mm² twin-and-earth or 6 mm² SWA for a typical 32 A 7 kW single-phase home charger; 10 mm² or 16 mm² for a 22 kW three-phase unit. This guide walks the full sizing process including Section 722 PME mitigation requirements.

Start 7-day free trial

Free for 7 days · No charge until day 8 · Cancel anytime · Used by 1,000+ UK electricians

11 min readUpdated 2026-06-10Andrew Moore, Founder of Elec-Mate

Written and reviewed by Andrew Moore, founder of Elec-Mate, against BS 7671:2018+A4:2026, IET Guidance Note 3 and the IET On-Site Guide.

ShareX in W

1,000+

UK electricians

“Replaced three separate apps with Elec-Mate. Certs, quotes, and scheduling all in one place.”

Daniel Palmer — DP Electrical

Key Takeaways

1Typical 7 kW single-phase domestic EV charger: 32 A rated, 6 mm² twin-and-earth or 6 mm² SWA on most short runs — verify with full BS 7671 Appendix 4 calculation for your specific cable route, reference method and length.
2Typical 22 kW three-phase EV charger: 32 A per phase, 10 mm² four-core SWA for most short runs, stepping to 16 mm² for longer runs (typically >20-30 m depending on derating).
3BS 7671 Section 722 EV Charging governs the installation. Regulation 722.411.4.1 addresses PME open-PEN risk mitigation — a real concern on PME supplies feeding outdoor EV chargers.
4Three sizing checks matter: current-carrying capacity (Iz after derating), voltage drop (≤ 5% from origin to charge point per Regulation 525 / Table 4Ab; design to ≤ 3% to leave headroom), and disconnection time at the calculated Zs.
5Reference method matters: cable in trunking vs buried direct vs free air all give different Iz values. The A4:2026 EICR/EIC Schedule of Circuit Details has a dedicated "Reference method" column to record the chosen method.
6Cable type matters: 6242Y (twin-and-earth) for clipped-direct routes through habitable spaces is fine; 6241X (SWA) for outdoor / buried / exposed runs; 6491X (single core) only inside conduit or trunking.

01 · Cable Sizing

The Short Answer (Most Common Scenarios)

For the typical UK domestic EV charger installation, the cable size most often specified is:

**7 kW single-phase (32 A) charger, short run (under ~25 m), clipped-direct or in trunking through habitable space** → 6 mm² twin-and-earth (6242Y) or 6 mm² SWA (6241X) for buried/outdoor sections.
**7 kW single-phase (32 A) charger, longer run (25-50 m)** → 10 mm² typically required to meet voltage-drop and disconnection-time targets. Calculate to confirm.
**22 kW three-phase (32 A per phase) charger, short run (under ~20 m)** → 10 mm² four-core SWA.
**22 kW three-phase (32 A per phase) charger, longer run (20-50 m)** → 16 mm² four-core SWA.
**Three-phase 22 kW on very long runs (>50 m)** → 25 mm² or 35 mm² four-core SWA — voltage drop usually governs at this scale.

These are starting points, not final answers

Every EV charger installation must have its cable size calculated against BS 7671 Appendix 4 for the specific route, reference method, ambient temperature, grouping factor and load. Use the calculator linked at the end of this guide or perform the full design calculation yourself.

Free download

Get the BS 7671 A4:2026 Cheat Sheet — free

Every key change in the 2026 amendment on one page. AFDDs, TN-C-S protection, new schedule columns, model forms. Pinned on your van dash.

Every regulation change summarised
New model forms (EIC + MEIWC)
Free PDF — no subscription

02 · Cable Sizing

The Three Cable Sizing Checks

Cable sizing for an EV charger (or any other load) is a three-stage check. The cable size you select must satisfy all three:

**Current-carrying capacity (Iz)** — the cable must carry the design current Ib continuously without exceeding its maximum permitted conductor temperature. Iz is read from BS 7671 Appendix 4 tables (4D1A / 4D2A / 4D4A / 4E4A etc.) for the chosen cable type and reference method, then adjusted for ambient temperature (Ca) and grouping (Cg).
**Voltage drop** — total voltage drop from the origin of the installation to the load must not exceed 3% for lighting circuits or 5% for other circuits (Regulation 525 / Appendix 4 Table 4Ab). An EV charger is an "other" circuit so the technical limit is 5%, but designing to 3% leaves headroom for the addition of other loads.
**Disconnection time at calculated Zs** — the protective device must disconnect within the time specified in Tables 41.2 to 41.6 for the relevant disconnection time (0.4 s for fixed equipment ≤32 A in TN systems). This means Zs at the charger end must be below the maximum permitted Zs for the device.

03 · Cable Sizing

Worked Example — 7 kW Single-Phase Charger, 20 m Run

A typical UK domestic EV charger installation: 7 kW, 32 A, 230 V, single-phase. Cable route 20 m from consumer unit to outdoor wall-mounted charge point on a TN-C-S (PME) supply.

**Design current (Ib)** = 7,000 W / 230 V ≈ 30.4 A. Round up to 32 A for the protective device (Type B MCB or 30 mA Type A RCBO).
**Cable selection trial** — 6 mm² twin-and-earth (6242Y), reference method C (clipped direct to a non-metallic surface) — Iz from BS 7671 Appendix 4 Table 4D1A = 41 A (single-phase, 70°C PVC T+E, 30°C ambient, no grouping). Note: the 47 A figure applies to 3-core XLPE SWA (Table 4D4A) — do not confuse the two.
**Derating** — assume Ca = 1.0 (typical UK 30°C ambient), Cg = 1.0 (single circuit). Adjusted Iz = 41 A. Ib (32 A) < Iz (41 A) — passes current capacity check.
**Voltage drop** — 6 mm² 6242Y voltage drop per Appendix 4 ≈ 7.3 mV/A/m for two-core cable. For 20 m at 32 A: 7.3 × 32 × 20 / 1000 = 4.67 V ≈ 2.0% of 230 V. Passes voltage drop check (target ≤3%).
**Disconnection time** — verify Zs at the charge point against the maximum permitted Zs for the chosen RCBO. Typical 32 A Type A 30 mA RCBO has a maximum permitted Zs around 1.37 Ω (Table 41.3 for Type B MCB equivalent, with RCD ensuring 0.4 s disconnection). Measured Zs typically well below this on a short PME run — pass. (A4:2026 note: maximum Zs tables previously in Appendix 14 have been moved to Appendix 3 in BS 7671:2018+A4:2026; Appendix 14 now covers prospective fault current determination.)

Conclusion for this scenario

6 mm² twin-and-earth, 32 A Type A 30 mA RCBO, satisfies all three checks for a 7 kW EV charger on a 20 m run.

04 · Cable Sizing

Worked Example — 22 kW Three-Phase Charger, 30 m Run

A 22 kW three-phase charger drawing 32 A per phase, 30 m cable route to an external location, on a TN-C-S (PME) supply.

**Design current per phase (Ib)** = 22,000 / (√3 × 400) ≈ 32 A. Protective device: 32 A Type C MCB or 32 A 30 mA Type A RCBO.
**Cable selection trial** — 10 mm² four-core SWA, reference method E (in free air or on cable tray) — Iz from Appendix 4 typically around 51 A for four-core 10 mm² 70°C PVC SWA.
**Derating** — for buried portions (Method D), apply the relevant table; for fully buried 10 mm² four-core SWA, Iz is around 47 A. Ib (32 A) < Iz (47 A) — passes.
**Voltage drop** — 10 mm² three-phase voltage drop per Appendix 4 ≈ 4.4 mV/A/m for three-phase + neutral SWA. For 30 m at 32 A: 4.4 × 32 × 30 / 1000 = 4.22 V ≈ 1.06% of 400 V. Passes comfortably.
**Disconnection time** — verify Zs at the charger. Three-phase 32 A device on a TN-C-S supply with proper bonding typically meets the 0.4 s requirement easily on 30 m of 10 mm² SWA — confirm with measurement.

Try Elec-Mate free for 7 days

16 certificate types, 70+ calculators, RAMS, quoting, invoicing, AI agents, and 46+ training courses — from £6.99/mo.

Start free trial

05 · Cable Sizing

PME Open-PEN Conductor Mitigation (Regulation 722.411.4.1)

BS 7671 Section 722 — EV Charging — addresses a specific safety concern with outdoor EV chargers on PME supplies. If the distributor's combined PEN conductor breaks upstream of the installation ("open-PEN"), the metal casing of an outdoor charger and any connected EV bodywork could become live at supply potential — a fatal shock hazard.

Regulation 722.411.4.1 (as amended by A4:2026) lists four mitigation options (indents (i)–(iv)). Note that indent (a) has been deleted in the A4:2026 revision — consult the current edition. The four options are:

**Open-PEN detection device** — a charger with onboard voltage monitoring that disconnects the charging output if the PEN conductor fails. Many modern UK charge points have this built-in (e.g. Wallbox Pulsar Plus, Project EV, certain Ohme units). Corresponds to indent (i).
**TT earthing arrangement for the EV charging circuit** — installing a dedicated earth electrode for the EV charger circuit, electrically separated from the PME earthing system. Requires the charger to be on a TT-style RCD with appropriate disconnection arrangements. Corresponds to indent (ii).
**Earth-mat or earth-rod with sufficient resistance** — less commonly used now that open-PEN detection chargers are widely available. Corresponds to indent (iii).
**Alternative solution per indent (iv) (A4:2026 addition)** — Regulation 722.411.4.1 was amended in A4:2026 to add indent (iv), which provides an additional alternative solution for charging installations. Consult the full regulation text and the redrafted Annex to Part 722 for the specific requirements of this option.

TN-C-S (PNB) supplies have different exposure

The open-PEN risk addressed by 722.411.4.1 is specific to distributor-owned PME supplies. TN-C-S (PNB) installations downstream of a privately-owned transformer have different broken-neutral exposure and the mitigation requirements may differ — consult the latest IET Code of Practice for EV charging.

V2G and smart charging — prosumer scope in A4:2026

BS 7671:2018+A4:2026 Section 722 now explicitly references prosumer's electrical installations — premises that both consume and generate electricity (e.g. with PV, battery storage, or vehicle-to-grid (V2G) export capability). Where a V2G-capable charger is installed, the design must account for the interaction between export and import, protective device co-ordination, and earthing implications. A common documentation omission is failing to record V2G export permissions and export limits on the EV charger installation certificate — include these where applicable.

06 · Cable Sizing

Cable Type — Twin-and-Earth, SWA, or Single Core?

Choose cable type by the route. For an EV charger feed:

**6242Y twin-and-earth** — fine for routes entirely inside the dwelling (e.g. consumer unit to a charger on the inside of the garage wall). Not suitable for outdoor exposed runs, buried direct, or where cable abuse is foreseeable.
**6241X SWA (Steel Wire Armoured)** — the workhorse for EV chargers. Suitable for buried direct (most common for external charger feeds), exterior wall surface mounting, and routes through outbuildings. Available as two-core for single-phase, four-core for three-phase.
**6491X single core** — only inside conduit or trunking. Sometimes used for the final tail from a junction to the charger inside a metal enclosure, but rarely the main feed.
**6491B single core for armouring earth** — where SWA is used and the armour serves as the CPC, ensure the design verifies that the armour's resistance meets BS 7671 requirements for the protective conductor.

07 · Cable Sizing

Common Cable Sizing Mistakes

**Skipping the disconnection-time check** — many installers verify Iz and voltage drop, but forget to verify Zs against the maximum permitted Zs for the protective device. A cable that passes the first two checks can still fail the third on a long run or high-resistance termination.
**Ignoring grouping factor** — when the EV charger cable runs alongside other loaded circuits (consumer unit to outbuilding feeding lights + sockets + charger), the grouping factor reduces Iz. Apply Cg from Table 4C1.
**Using PVC tables for thermosetting cable (or vice versa)** — XLPE/EPR (90°C thermosetting) cables have higher Iz than PVC (70°C thermoplastic) of the same CSA. Match the cable specification to the table.
**Forgetting the PME open-PEN mitigation** — installing a generic 32 A RCBO on a PME outdoor charger circuit without addressing 722.411.4.1 is a C2 observation on the next EICR.
**Sizing on rated power only, not actual continuous current** — Ib is the actual current the load draws. A 7 kW charger nameplate-rated at 32 A draws very close to 32 A continuously when charging. Don't use the historical 80% rule that's appropriate for socket-outlet loading — EV charging is a steady-state load.
**Not notifying the DNO when required** — a 7 kW or 22 kW EV charger is a significant additional load. Depending on the existing supply capacity and the distributor's requirements, notification under the distributor's connection agreement (or G99/G100 for generation/storage) may be required. Failure to notify is a common compliance oversight and can affect the validity of the installation certificate.

Frequently Asked Questions

What electricians say

Verified reviews from the UK App Store.

One App for Everything!

Elec-Mate is my go to app for business and electrical work. It's feature rich without feeling cluttered. A true all in one app for quotes, certs, calculations, RAMS, EICRs, and more. I use it every day without fail, and it makes my workflow much smoother since I'm not jumping between apps anymore. The price-to-feature ratio is excellent. Any issues I've had, the developer responds within the hour and usually fixes them the same day. 100% recommend.

Fantastic app for electricians

I've used the app and the web based version for a while now and it's well worth the investment. If you're an apprentice or experienced Spark give it a go, you won't be disappointed.

Absolutely amazing

I've been using Elec-Mate for a while now, and honestly, it's one of the best apps I've ever downloaded. Every aspect of it feels thoughtfully designed, from the clean and intuitive interface to the powerful features that make everything so easy to manage. It's clear that a lot of care and attention went into building this app, and it shows in every detail.

Trusted by electricians across the UK

Real feedback from real sparks

“Replaced three separate apps with Elec-Mate. Certs, quotes, and scheduling all in one place.”

Daniel Palmer

Sole Trader · DP Electrical

“I've won two contracts this month because I could turn quotes around same-day with the AI cost engineer.”

Nathan Perry

Electrician · NP Electrical Services

“The study centre got me through my AM2. Mock exams and flashcards are brilliant.”

Jake Pizey

3rd Year Apprentice · Apprentice

7-Day Free Trial — Cancel Anytime, No Hassle

Size EV charger cables in seconds

Elec-Mate's Cable Sizing Calculator runs all three BS 7671 checks (Iz / voltage drop / Zs) and the EV-specific Section 722 mitigation prompts for you. 7-day free trial.

“Replaced three separate apps with Elec-Mate. Certs, quotes, and scheduling all in one place.”

Daniel Palmer, DP Electrical

From £6.99/mo after trial — less than a coffee a week

or download the app

7 days free, then from £6.99/moCancel in one tap — no calls, no hassleiOS, Android & WebBS 7671 compliant

Cable Size for an EV Charger (7 kW / 22 kW)

Key Takeaways

The Short Answer (Most Common Scenarios)

These are starting points, not final answers

Get the BS 7671 A4:2026 Cheat Sheet — free

The Three Cable Sizing Checks

Worked Example — 7 kW Single-Phase Charger, 20 m Run

Conclusion for this scenario

Worked Example — 22 kW Three-Phase Charger, 30 m Run

Try Elec-Mate free for 7 days

PME Open-PEN Conductor Mitigation (Regulation 722.411.4.1)

TN-C-S (PNB) supplies have different exposure

V2G and smart charging — prosumer scope in A4:2026

Cable Type — Twin-and-Earth, SWA, or Single Core?

Common Cable Sizing Mistakes

Frequently Asked Questions

Can I use 4 mm² cable for a 7 kW EV charger?

What protective device do I install for a 7 kW EV charger?

Does the EV charger need a separate consumer unit?

How does cable size change for a 22 kW three-phase charger?

What about voltage drop on a long run?

Do I need to consider grouping if the EV cable shares a trench with other cables?

Should I install conduit / trunking even where not strictly required?

Continue reading

Cable Sizing Calculator (BS 7671 Appendix 4)

Voltage Drop Calculator

Earth Loop Impedance (Zs) Calculator

TN-C-S (PME) vs (PNB) Earthing

AFDD Changes A4:2026 — EV Charging Exemption

EV Charger Certificate

What electricians say

One App for Everything!

Fantastic app for electricians

Absolutely amazing

Trusted by electricians across the UK

Size EV charger cables in seconds