Voltage drop calculator

Pick the size you plan to run, the load on it and the length of the run. You get the drop in volts, the drop as a percentage of the supply, and a pass or fail against the 2.5% design target — 5.75 V at 230 V, 10.00 V at 400 V. If the size does not hold it, the calculator names the size that does.

Cable size

Copper conductors. The figure beside each size is its single-phase volt drop per amp per metre; three-phase uses that figure × 0.866.

Load on the circuit

Watts Amps

Everything the cable feeds, not just the biggest item. If you know the running current — motors, inverters, anything with a low power factor — enter amps instead.

Supply

Single-phase230 V Three-phase400 V

Run length

One way, from the distribution board to the load, along the route the cable takes. The mV/A/m figure already accounts for the return path.

Result

Enter the load. The drop in volts, the drop as a percentage, the voltage left at the load and the longest run this size can take all appear here.

Reading the result

Measure the route

One way from the board to the load, along the path the cable actually takes — up the wall, across the ceiling, down again. Not the straight line across the room.

A failing run has two fixes

A larger conductor, or a shorter route. The readout gives the longest run this size can take at your load, so you can see which one costs less.

Starting current is not running current

Motors, pumps and compressors draw several times their running current at start. Enter the running current here; have the start condition checked by a licensed electrician.

How the drop is worked out

Volt drop is the conductor’s own resistance eating the supply on its way to the load: the current, the length and the size decide it. The calculator multiplies the size’s mV/A/m figure by the design current and the run length, then compares the result with 2.5% of the supply voltage. It also checks the load against the size’s current capacity, because a run that passes on volt drop can still be over the conductor.

The calculator reads the copper rows of our published spec tables: volt drop and current capacity per BS 7671 (IEC 60364-5-52), reference method C, PVC insulated copper at 70°C. Verify sizing with a licensed electrician for your installation.
Assumption	Value used
Supply voltage	230 V single-phase · 400 V three-phase
Power factor	1.0 when the load is entered in watts
Volt drop figures	mV/A/m from the spec table — the single-phase 2-core column
Three-phase drop	single-phase figure × 0.866 (√3 / 2)
Volt drop target	2.5% — 5.75 V at 230 V, 10.00 V at 400 V
Current capacity	BS 7671 method C (clipped direct), 30°C ambient, no derating applied
Loaded conductors	2 single-phase · 3 three-phase
Conductor temperature	70°C PVC — no allowance for a conductor running below its rating
Range covered	0.75 – 120 mm² copper
Not covered	aluminium conductors · correction factors for grouping, ambient and thermal insulation · DC strings and battery runs — for a solar DC run see the panel to inverter cable guide

Volt drop is one check of several. It says nothing about earth fault loop impedance, disconnection time, or the derating a hot roof space or a bunched conduit imposes — a cable that passes here can still be the wrong cable for that route. Have a licensed electrician confirm the size, the breaker and the earthing before you buy.

Volt drop by size

The same table the calculator reads. Multiply the figure by the current in amps and the run in metres, then divide by 1000, to get the drop in volts.

Copper conductors, PVC insulated, 70°C rating. Volt drop and current capacity per BS 7671 reference method C (clipped direct), 30°C ambient. The three-phase column is the single-phase figure × 0.866. Derate the capacity for conduit, bunching and heat. Verify sizing with a licensed electrician for your installation.
Size	Drop, single-phase	Drop, three-phase	Capacity, single-phase	Capacity, three-phase
0.75 mm²	62 mV/A/m	53.7 mV/A/m	6 A	6 A
1 mm²	44 mV/A/m	38.1 mV/A/m	10 A	10 A
1.5 mm²	29 mV/A/m	25.1 mV/A/m	19.5 A	17.5 A
2.5 mm²	18 mV/A/m	15.6 mV/A/m	27 A	24 A
4 mm²	11 mV/A/m	9.53 mV/A/m	36 A	32 A
6 mm²	7.3 mV/A/m	6.32 mV/A/m	46 A	41 A
10 mm²	4.4 mV/A/m	3.81 mV/A/m	63 A	57 A
16 mm²	2.8 mV/A/m	2.42 mV/A/m	85 A	76 A
25 mm²	1.75 mV/A/m	1.52 mV/A/m	112 A	96 A
35 mm²	1.25 mV/A/m	1.08 mV/A/m	138 A	119 A
50 mm²	0.93 mV/A/m	0.81 mV/A/m	168 A	144 A
70 mm²	0.63 mV/A/m	0.55 mV/A/m	213 A	184 A
95 mm²	0.46 mV/A/m	0.40 mV/A/m	258 A	223 A
120 mm²	0.36 mV/A/m	0.31 mV/A/m	299 A	259 A

Got the size that holds the run?

Send the size, the length in metres and your city. We quote the factory rate for that day and dispatch — Punjab in 1–2 days, rest of Pakistan in 2–4.

WhatsApp for today's rate