Factory-direct from Lahore · Nationwide delivery in 2–4 days
+92 319 4262451
WhatsApp
A cable can carry the current comfortably and still be the wrong size, because the volts ran out first.
Voltage drop is current × route length × the tabulated mV/A/m, divided by 1000, and BS 7671 allows 5% on a power circuit — 11 V on a 220 V supply, 20 V on 400 V. At the 31.8 A a 7 kW single-phase charger draws, 4 mm² spends that 11 V at about 31 m and 6 mm² at about 47 m. Garages, gates and back walls are often further from the board than people estimate, which is why distance decides the cable on EV jobs more often than the load does.
Two numbers and one multiplication. The current is the charger's draw. The mV/A/m figure is a property of the cable size, tabulated per BS 7671. Multiply them by the one-way route length in metres, divide by 1000, and you have the volts lost in the run.
The budget is 5% of the supply voltage on a power circuit: 11 V on 220 V single phase, 20 V on 400 V three phase. Note that single-phase circuits get roughly half the budget in absolute volts, which is why long single-phase charger runs are the ones that bite. Measure the route rather than the straight line — cable follows walls, goes up and over doorways, and takes the long way round a car port.
| Size | Current capacity | Voltage drop | Reach at 31.8 A |
|---|---|---|---|
| 2.5 mm² | 27 A | 18 mV/A/m | 19 m |
| 4 mm² | 36 A | 11 mV/A/m | 31 m |
| 6 mm² | 46 A | 7.3 mV/A/m | 47 m |
| 10 mm² | 63 A | 4.4 mV/A/m | 79 m |
| 16 mm² | 85 A | 2.8 mV/A/m | 123 m |
The 2.5 mm² row is there for completeness rather than as an option — its 27 A capacity is already below the 31.8 A the charger draws, so it fails the current check before distance is even considered.
An 11 kW three-phase charger draws 15.9 A per phase against a 20 V budget, and both of those work in its favour. Half the current and nearly double the budget means roughly four times the reach for the same cable size.
| Size | Current capacity | Voltage drop | Reach at 15.9 A |
|---|---|---|---|
| 2.5 mm² | 24 A | 18 mV/A/m | 70 m |
| 4 mm² | 32 A | 11 mV/A/m | 114 m |
| 6 mm² | 41 A | 7.3 mV/A/m | 172 m |
| 10 mm² | 57 A | 4.4 mV/A/m | 286 m |
A 22 kW three-phase charger draws 31.8 A per phase — the same current as the 7 kW single-phase unit, but against the larger 20 V budget. On 6 mm² that gives 20 ÷ (31.8 × 7.3 ÷ 1000) = 86 m of reach, against 47 m for the same cable and the same current on a 220 V supply. The supply voltage, not the charger, is doing most of the work.
Every figure on this page is driven by one input people habitually underestimate. Voltage drop is calculated from the metres of cable installed, not from how far apart the two ends look. Cable does not travel diagonally across a lawn: it follows walls, climbs to go over a doorway, drops to enter the board, and goes the long way round anything it cannot pass through.
Walk the route with a tape and add the verticals. A charge point that sits 15 m from the board across open ground is regularly 25 m of installed cable once the path is real, and at 31.8 A on 4 mm² that is the difference between 5.2 V and 8.7 V of drop. Add a little for the slack at each end and for the drop into the enclosure. It costs nothing to measure generously at this stage and a great deal to discover the shortfall after the cable is clipped up.
Outdoors is a different question from long.
If the route crosses a yard, a driveway or anywhere a shovel or a trolley can reach it, the size of the conductor is not the protection that matters. That is a case for armoured cable, where steel wire armour does the protecting rather than the PVC sheath. Buried and outdoor runs also sit in different thermal conditions from a wall-clipped cable, which changes the derating again.
Detached garages, farm gates, boundary walls and plot-corner parking all produce runs that make the tables above look optimistic. Three approaches, in the order worth considering them.
A sub-board nearer the charge point shortens the high-current run and often costs less than the copper it saves, especially above 40 m.
Each step up roughly halves the volts lost per metre. It is the simplest answer and, on a single run, frequently the cheapest.
Most chargers can be set below their maximum. Less current is less drop, and an overnight charge rarely needs the full rate.
What does not work is accepting the drop. A charger fed below its expected voltage does not simply charge more slowly in a tidy way — the cable is dissipating the difference as heat along its whole length, continuously, for the hours the session lasts. That is the failure mode worth designing out rather than living with.