Skip to main content
Tool Belt Calc

Electrical

Voltage Drop Calculator

Voltage drop over a run, checked against the 3% target.

Your measurementsEvery assumption editable01

Distance to the load, not the round trip — the formula doubles it.

Resistivity in ohm-circular-mils per foot at 75 °C.

Voltage dropCalculated live02

Result

2.48% drop

5.96 V lost — within 3%

Conductor areacomputed from the AWG geometric definition
10383 circular mils
Run length
120 ft one way
Load
20 A at 240 V
Voltage drop
5.96 V
As a percentage
2.48%
Voltage at the load
234 V
Assessment
Within 3%

Three per cent on a branch circuit and five per cent overall is the widely used design target. It is a recommendation about performance rather than a hard limit — but a motor fed at low voltage draws more current, runs hotter and dies early, so it is worth respecting.

Enter the one-way distance. The formula already accounts for current flowing out and back, which is where the factor of two comes from on single phase and √3 on three phase.

Voltage drop is a separate question from ampacity. A conductor can be large enough to carry the current safely and still deliver unusable voltage at the far end of a long run — long runs are usually sized by drop, not by ampacity.

Enter the run length, load and conductor size. Conductor area is computed from the AWG geometric definition rather than looked up, so the result is exact for any gauge. Three per cent on a branch circuit is the widely used design target.

Why use this tool?What it does differently03

Why use this tool?

Area computed, not looked up

AWG is a geometric progression, so circular mils are derived from the gauge definition.

Assessed against 3%

States plainly whether the run falls inside the usual design target.

Copper or aluminium

Both resistivity constants, since aluminium drops roughly 64% more over the same run.

Single or three phase

The multiplier changes from 2 to √3, which is a substantial difference.

How this worksThe method04

How this voltage drop calculator works

Voltage drop for single phase is two times K times length times current, divided by conductor area in circular mils. K is resistivity — about 12.9 for copper and 21.2 for aluminium at typical operating temperature. Three phase substitutes √3 for the 2.

The factor of two exists because current travels out to the load and back. Enter the one-way distance; the formula already accounts for the return path.

Conductor area comes from the AWG definition rather than a table. Each gauge step changes diameter by a factor of 92^(1/39), so circular mils can be computed exactly for any size — which also means nothing here is reproduced from a copyrighted standard.

How to use itStep by step05

How to use it

  1. Step 1: Enter the system voltage

    The nominal supply voltage. Percentage drop is relative to this.

  2. Step 2: Enter the load current

    In amps, at the far end of the run.

  3. Step 3: Measure the one-way length

    Distance to the load, not the round trip.

  4. Step 4: Pick conductor size and material

    Then adjust until the percentage falls inside your target.

Example usageWorked figures06

Example usage

A long branch circuit
20 A at 240 V over 120 ft of 10 AWG copper drops 5.96 V — 2.48%, inside the 3% target but not by much.
The same run in aluminium
Identical conditions in aluminium drop 9.80 V, or 4.08% — now outside target. Aluminium's higher resistivity costs about 64% more drop for the same gauge.
An undersized long run
20 A at 120 V over 200 ft of 14 AWG copper drops 25.13 V — 20.9%, far outside target. Long runs are usually sized by drop rather than by ampacity.
Frequently asked questionsCommon questions07

Frequently asked questions

How do I calculate voltage drop?

For single phase: 2 × K × length × current ÷ circular mils, where K is 12.9 for copper and 21.2 for aluminium. Three phase uses √3 in place of the 2.

What is an acceptable voltage drop?

Three per cent on a branch circuit and five per cent overall is the widely used design target. It is a performance recommendation, but motors fed low voltage draw more current and fail early.

Do I enter one-way or round-trip length?

One-way. The factor of two in the formula already accounts for current flowing out and back.

Why does aluminium drop more than copper?

Higher resistivity — about 21.2 against copper's 12.9. Over the same run and gauge, aluminium drops roughly 64% more, which is why aluminium feeders are usually sized larger.

Is voltage drop the same as ampacity?

No, and conflating them is the commonest sizing error. Ampacity is a safety limit about heat; voltage drop is a performance limit about delivered voltage. On long runs, drop usually governs first.

Related toolsElsewhere on the site08

Browse every tool in Electrical Calculators.