EV charger cable size calculator
Get the right EV charger cable size in seconds: pick a 3.6, 7.4, 11 or 22 kW unit, enter the run, and it checks current-carrying capacity and voltage drop as a proper continuous load. Free, no sign-up.
- Charger current–
- Suggested device–
- Voltage drop–
Guidance only, not a design. EV circuits fall under Section 722 of BS 7671 with their own earthing, RCD and device requirements, and DNO notification applies. Use this to get in the right ballpark, then design it properly.
What cable for an EV charger? How to size it properly
An EV charger is not a shower or an oven. It's a continuous load: the car pulls full rated current for hours on end, so there's no diversity to lean on and no "it'll never actually draw that" to hide behind. You design at full whack. A 7.4 kW charger is 32 A at 230 V, and it will happily sit at 32 A from evening plug-in until the car is full.
That one fact drives everything else. The cable needs the current-carrying capacity for the whole load after the installation method has taken its cut, and it has to pass voltage drop over the full run. Those are the same two checks my general cable size calculator runs, this tool just does them pre-loaded with real charger currents.
The short answer: for a 7.4 kW home charger it's 6 mm² on shorter runs and 10 mm² once the run gets long, and it's nearly always voltage drop, not current-carrying capacity, that forces the step up. Here's the full picture.
Typical answers, and why you should still run the numbers
Here's where most installs land in practice. Treat every cell as a starting point, because the installation method and the route decide the final answer, which is exactly what the calculator above is for.
| Charger | Short run (under ~15 m, favourable) | Longer run |
|---|---|---|
| 3.6 kW | 2.5 mm² T&E often passes, 4 mm² common | 4-6 mm² |
| 7.4 kW | 6 mm² | 10 mm² (voltage drop usually decides) |
| 11 kW three-phase | 2.5-4 mm² 5-core SWA | 4-6 mm² |
| 22 kW three-phase | 6 mm² 5-core SWA | 10 mm² |
Worked example: 7.4 kW charger, 20 m run, clipped direct
Here's the whole method on a real job, the same sums the calculator runs:
- Design current. 7.4 kW ÷ 230 V = 32.2 A, and it's continuous, so that's the number. No diversity.
- Protective device. The device has to sit at or above the load, which is why the calculator suggests 40 A for a 7.4 kW unit rather than parking a 32 A breaker at its full rating for hours. Plenty of 7 kW chargers are current-limited to 32 A and specify a 32 A device instead, the manufacturer's instructions win.
- Current-carrying capacity. The cable must carry the device rating after the installation method takes its cut. 6 mm² T&E clipped direct is rated 47 A, comfortable. The same cable buried in a stud wall full of insulation is rated 23.5 A, a fail. Same copper, different route, different answer.
- Voltage drop. 6 mm² drops 7.3 mV/A/m, so 7.3 × 32.2 × 20 ÷ 1000 = 4.7 V, inside the 11.5 V (5%) limit with room to spare.
- Verdict. 6 mm² T&E clipped direct, done. Stretch the same job to 35 m and the drop is 8.2 V, technically a pass but skinny once the rest of the installation takes its share, which is when 10 mm² (4.4 mV/A/m, 5.0 V at 35 m) earns its keep.
Voltage drop is what bites on driveway runs
Charger circuits are long more often than they're hot. The board is under the stairs, the charger is on the far gatepost, and suddenly you're 25 m out. At 32 A, 6 mm² T&E drops 7.3 mV/A/m, so a 25 m run eats 5.8 V of your 11.5 V budget before the rest of the installation takes its share. That's why longer 7.4 kW runs land in 10 mm² even though 6 mm² carries the current comfortably. Sanity-check any borderline run with the voltage drop calculator, and if you're quoting an odd-rated unit, convert it with the kW to amps calculator first.
What Section 722 actually asks for
EV circuits get their own section of BS 7671, Section 722, and it stacks a few extra requirements on top of the usual sums:
- A dedicated final circuit for each charge point. No spurring one off the garage ring, ever.
- RCD protection with DC fault detection. That means a Type A RCD plus 6 mA DC detection, which is often built into the charger itself, or a Type B RCD. The manufacturer's instructions tell you which, so read them before you buy the board gear.
- PME supplies need the open-PEN risk managed. On a TN-C-S supply a broken PEN conductor can put the car body at a dangerous potential. Most domestic chargers now ship with built-in O-PEN protection, and if yours doesn't, you're into an earth electrode or one of the other solutions in 722.411.4.1.
- The DNO needs to know. Charge points are notified through the ENA process, and above 60 A of total demand it becomes an application before you connect, not a notification after.
Twin and earth or SWA?
Inside the house, T&E is fine, and it's what the single-phase options above assume. The moment the run goes outside, underground or along a fence line, SWA is the standard answer: mechanical protection, direct burial, and it glands straight into the charger or an adaptable box. Buried runs sit outside this calculator's installation methods though, ducting, depth and soil all move the rating, so take buried ratings from the BS 7671 tables or the On-Site Guide rather than winging it. Three-phase jobs are 5-core SWA territory pretty much by default, and singles in conduit remain an option where there's containment the whole way.
Three quick wins from jobs I hear about constantly:
- Check the charger's manufacturer instructions for the required device and RCD type before you board it out, not after the van's loaded.
- Feeding a garage consumer unit? The submain needs its own sums on top of the charger circuit, size the whole chain, not just the last leg.
- On long rural runs, a 22 kW three-phase quote can work out cheaper per kW than fighting voltage drop on single-phase. Worth pricing both before you commit to a trench full of copper.
EV charger cable size FAQs
What cable do I need for a 7kW EV charger?
Usually 6 mm², stepping up to 10 mm² on longer runs. A 7.4 kW charger pulls 32 A continuously, so the cable has to carry that full load after installation-method factors, then pass voltage drop on top. The method and the route decide, which is why "what cable for a 7kW charger" has no one-word answer. Run the calculator with your actual run length.
Can I use 4mm cable for a 7kW EV charger?
Rarely, and I wouldn't plan a job around it. Only very short runs in favourable installation methods squeeze 4 mm² past both the current-carrying and voltage drop checks at a continuous 32 A. The moment there's insulation involved or a few extra metres, it fails. Run the numbers, and expect the answer to be 6 mm² or bigger.
Does an EV charger need its own circuit?
Yes, no arguments. Section 722 of BS 7671 requires a dedicated final circuit for each charge point, with RCD protection that includes DC fault detection. Spurring a charger off an existing ring is a straight non-compliance, and given the load runs flat out for hours, a daft idea even without the regulation.
What breaker and RCD does a 7kW EV charger need?
A 7.4 kW unit draws just over 32 A continuously, which is why the calculator above suggests a 40 A device rather than parking a 32 A breaker at its full rating for hours. Plenty of 7 kW chargers are current-limited to 32 A and their instructions call for a 32 A device instead, so the manufacturer's instructions always win. On the RCD side, Section 722 wants DC fault detection: a Type A RCD plus 6 mA DC detection, often built into the charger itself, or a Type B RCD. Check before you buy the board gear, not after.
Does an EV charger need its own consumer unit?
Not as a rule. Section 722 demands a dedicated circuit, not a dedicated board. That said, a small separate consumer unit is often the tidy answer in practice: the main board is full, the existing RCD type doesn't suit, or you're feeding the charger from the meter tails via a switch fuse. If there's a spare way in the existing board and the RCD arrangement works, use it and save the customer the cost.
What cable does a 22kW EV charger need?
That's a three-phase job, 32 A per phase, typically wired in 6 to 10 mm² 5-core SWA depending on the run. The catch is the supply: most UK homes are single-phase, so a domestic 22 kW install usually means a DNO application for a three-phase upgrade before you touch a cable. On commercial sites with three-phase already in, it's much more straightforward.
Do I need an earth rod for an EV charger?
It depends on the supply and the charger. On PME (TN-C-S) supplies the open-PEN risk has to be managed, because a broken PEN can make the car live. Most modern domestic chargers handle this internally with built-in O-PEN protection, so no rod needed. If the charger doesn't have it, then yes, an earth electrode or one of the other 722.411.4.1 solutions. The manufacturer's instructions are your first stop.
Do I need to tell the DNO about an EV charger?
Yes, every time. Charge points get notified to the DNO through the ENA process. If the total demand goes above 60 A, it flips from a notification to an application, meaning you apply and wait for approval before connecting. Build it into the job timeline, not as an afterthought once the charger's on the wall.
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