Why is the length doubled?

Because the current has to return. The conductor the current travels through is twice the distance to the load, and forgetting that halves the calculated drop.

My cable is rated for the current, so why is it too small?

Current rating and voltage drop are separate limits. On a long run the drop binds first, and a cable that will not overheat can still leave the load undervolted.

Construction & HomeelectricalUnited States only

Voltage Drop Calculator

Voltage drop over a cable run. The factor most often missed is that current flows out and back, so the conductor length is twice the run, and long runs fail on drop well before they fail on current rating.

Also called: cable size calculator, volt drop calculator.

Voltage drop

5.04

5.04 V over 30 m, which is 2.19% of 230 V. That is within the three percent commonly allowed for a final circuit. The minimum conductor for a 3% limit is 2.92 mm², and 100.8 W is lost as heat in the cable.

Drop as a percentage

2.19

Voltage at the load

224.96

Minimum conductor for 3%

2.92

Power lost in the cable

100.8

Cable resistance

0.25

Verdict

That is within the three percent commonly allowed for a final circuit.

On regulation

This is a sizing estimate. The applicable wiring or building regulation governs and overrides any figure here.

How this is calculated

Method and background

How this is calculated

Resistance is resistivity times length over cross section, and the length is twice the one-way run because the current returns. Three-phase uses root three rather than two, since the return is shared. The practical point is that a cable rated for the current can still be inadequate: on a long run the drop makes motors run hot and lights dim, and three percent for a final circuit is the usual limit. Aluminium has about 1.6 times the resistivity of copper, so it needs a larger section for the same drop.

the factor of two is the return path: current flows out and back

rho: Resistivity
L: One-way length
A: Cross section

Worked examples

Each of these is asserted on every build. If a change to the engine ever moved one of these answers, the build would fail before the page could print it.

20 A over 30 metres in 4 mm²

Current: 20 A
One-way run length: 30 m
Conductor cross section: 4 mm²
System voltage: 230 V
Conductor: Copper
System: Single phase

Voltage drop5.04

1.68e-8 x 30 x 2 / 4e-6

Open this example

aluminium needs more section

Current: 20 A
One-way run length: 30 m
Conductor cross section: 4 mm²
System voltage: 230 V
Conductor: Aluminium
System: Single phase

Voltage drop8.46

boundary: 1.68 times the copper resistivity

Open this example

Method and limits

What it assumes

Conductor at 20 degrees Celsius and a resistive load.

What it deliberately does not model

Resistance rises with temperature, so a loaded cable drops more than this.
Reactance matters on large conductors and long runs and is not included.
The applicable wiring regulation sets the permitted drop and overrides any rule of thumb.

Formula version 1.0.0 · definition 1.0.0 · United States · Report a problem with this calculator

Frequently asked questions

Why is the length doubled?: Because the current has to return. The conductor the current travels through is twice the distance to the load, and forgetting that halves the calculated drop.
My cable is rated for the current, so why is it too small?: Current rating and voltage drop are separate limits. On a long run the drop binds first, and a cable that will not overheat can still leave the load undervolted.