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Current first, then derating, then voltage drop. In that order.
The rating plate gives you kilowatts or amps. If it gives kilowatts, divide by 220 V for single phase or by 1.732 × 400 for three phase to get the current in each conductor. Choose a cable whose tabulated capacity clears that current after the corrections for installation method, ambient temperature and grouping have been applied — not before. Then check the run length separately, because a cable can carry the current comfortably and still drop too many volts. Both checks have to pass, and the more demanding one wins.
Everything starts here, and it is worth writing the numbers down because the rest of the job checks against them.
| Charger | Supply | Current drawn | Arithmetic |
|---|---|---|---|
| 3.6 kW | Single phase 220 V | 16.4 A | 3600 ÷ 220 |
| 7 kW | Single phase 220 V | 31.8 A | 7000 ÷ 220 |
| 11 kW | Three phase 400 V | 15.9 A per phase | 11000 ÷ (1.732 × 400) |
| 22 kW | Three phase 400 V | 31.8 A per phase | 22000 ÷ (1.732 × 400) |
Note what the table says about 11 kW. Spread across three phases, an 11 kW charger asks each conductor for 15.9 A — less than half what a 7 kW single-phase unit asks of its two. Three-phase charging is easier on cable, not harder. What it needs is a three-phase supply, which is the actual constraint.
Single-phase 220 V, so the charger draws 7000 ÷ 220 = 31.8 A. Take a route of 18 m from the distribution board to the charge point, clipped along a wall and through a short length of conduit where it passes outside.
The current check. 4 mm² carries 36 A clipped direct at 30°C, so on paper it clears 31.8 A by 4.2 A — about 13%. Apply a conduit correction and a Lahore-summer ambient correction to that tabulated 36 A and the margin can disappear entirely. 6 mm² starts at 46 A and has 14.2 A of headroom to spend on the same corrections. Which one survives your route is arithmetic your electrician does with the actual factors, and it is the reason this page will not simply say “use 6 mm”.
The voltage drop check. Drop is current × length × the tabulated mV/A/m ÷ 1000. On 4 mm²: 31.8 × 18 × 11 ÷ 1000 = 6.3 V, which is 2.9% of 220 V and inside the 5% BS 7671 allows, 11 V here. On 6 mm²: 31.8 × 18 × 7.3 ÷ 1000 = 4.2 V, or 1.9%. Both pass at 18 m, so on this route the current check decides, not the distance.
The order matters.
Run the current check first and the voltage drop check second, then take whichever size is larger. People who do it the other way round often settle on a size that carries the load fine over a short run and then extend the route later without redoing the first check.
One more thing this example hides: the 18 m is the cable's route, not the distance across the driveway. Walk it along the path the cable will really take, adding the climbs over doorways and the drop into the board, before either check is worth anything. The difference between the straight line and the real route is routinely a third.
Three-phase 400 V, so each phase carries 11000 ÷ (1.732 × 400) = 15.9 A. Take the same 18 m route, this time on a four-core cable carrying three phases and a neutral.
The current check. With three loaded conductors, 4 mm² carries 32 A and 2.5 mm² carries 24 A — both comfortably above 15.9 A before derating, which is the point made earlier about three-phase being easier on copper. The voltage drop check. On 4 mm²: 15.9 × 18 × 11 ÷ 1000 = 3.1 V against a 20 V budget on 400 V, which is 0.8%. There is a great deal of room here.
Which is why, on a three-phase supply, the cable is rarely the difficult part of an EV installation. The difficult parts are whether the supply exists, whether the board has three spare ways, and what the charger's own instructions require of the protective device. Those are covered on what your supply needs before a charger.
Charger specifications are not written for electricians, and the number on the box is often the least useful one in the manual. Three things are worth finding before any cable is priced.
Many units are specified by their current setting - 16 A, 32 A - which is the number you want. Where only kilowatts are given, convert them and keep the working.
A lot of chargers can be commissioned at a lower current than their maximum. A 32 A unit set to 20 A is a 20 A load, and that setting belongs in the record alongside the cable size.
The same model is often sold in both. They draw completely different currents per conductor, so the variant decides the cable long before the kilowatt figure does.
The adjustable limit is worth dwelling on, because it cuts both ways. Commissioning a charger below its maximum is a legitimate way to fit one to a supply that could not otherwise carry it — but the cable, the device and the setting then form a set, and changing the setting later without revisiting the other two is how a compliant installation quietly stops being one. Write the commissioned current on the board schedule.
They get conflated constantly, and the distinction is simple. The protective device exists to protect the cable. So the cable's capacity, after every derating factor has been applied, has to be at least the rating of the device in front of it. Size the cable to the load, then choose a device that both carries the load and stays within what the cable can take — not the other way round.
Fitting a larger breaker because the charger “trips sometimes” is the exact failure this ordering prevents: it removes the protection the cable was relying on while leaving the cable unchanged. If a charge point trips, the reason is worth finding rather than turning up. And because EV chargers can produce fault currents that ordinary residual current devices are not designed to detect, most units specify in their own manual what protection they require. That instruction is the authority.