Electrical

Electrical Voltage Drop Calculator

Estimate voltage loss on a fixed conductor and inspect the longest run indicated by the tool's comparison threshold.

Result

Voltage drop 7.90%

Volts lost 9.5

Voltage at the load 110.5

Longest run inside 3% 46 Feet

Drop is 7.90%, so 110.5 volts reach the load. That is beyond the 3% a branch circuit should hold to; the longest acceptable run at this size is about 46 feet.

A planning estimate, not a code calculation. Conductor sizing, overcurrent protection and load calculations are governed by the NEC as adopted and amended where you are, and by the listing of the equipment involved. Have the design confirmed by a licensed electrician and inspected. The 3% branch-circuit and 5% total figures are recommendations in NEC informational notes rather than hard requirements, though many local rules and specifications make them mandatory.

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What moves the number

Route distance should follow the proposed cable path.
The actual load current is more informative than a protective-device rating.
The conductor selection fixes the area used by the resistance estimate.

Where this one goes wrong

Longest run inside 3% describes this simplified model, not an approved cable length. Upstream losses and project-specific requirements may leave a different allowance for the branch. The form does not model motor-starting transients.

Questions

About this calculator

What can the longest-run output help decide?

It can flag whether relocating a supply point or reviewing conductor size deserves attention. Compare alternative routes using the same current assumption.

Does choosing 480 V model a three-phase circuit?

No. This tool keeps its fixed two-conductor drop equation; selecting voltage only changes the percentage calculation. Use a phase-aware model when required.

Does passing the threshold establish compliance?

This remains a planning check. Local electrical code, specified voltage performance and manufacturer limits govern the design, alongside independent ampacity verification.

How useful was this calculator?

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The formula

drop = 2 x K x amps x distance / circular mils, where K is 12.9 for copper and 21.2 for aluminium

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How to use this calculator

Load (amps). Enter your own figure; the page starts at 20.
One-way run (ft). Enter your own figure; the page starts at 120.
Wire size. Enter your own figure; the page starts at 12 AWG.
Conductor. Enter your own figure; the page starts at Copper.
Voltage. Enter your own figure; the page starts at 120 V.

The result updates as you type, so you can try a figure, see what it does to voltage drop, and change it back. Nothing you enter is sent anywhere or saved.

A worked example

These are the figures the calculator opens with. Every number in the results column below is produced by the same calculation the page runs, so you can follow it through before putting your own figures in.

What goes in

Input	Value
Load (amps)	20
One-way run (ft)	120
Wire size	12 AWG
Conductor	Copper
Voltage	120 V

What comes out

Result	Value
Voltage drop main result	7.90%
Volts lost	9.5
Voltage at the load	110.5
Longest run inside 3%	46

Drop is 7.90%, so 110.5 volts reach the load. That is beyond the 3% a branch circuit should hold to; the longest acceptable run at this size is about 46 feet.

How sensitive the answer is

Holding everything else steady and changing only load (amps), this is what happens to voltage drop. It is worth knowing which of your inputs the answer actually hinges on, and which barely move it.

Load (amps)	Voltage drop
10	3.95%
15	5.93%
20 example above	7.90%
30	11.85%
40	15.80%

Every other input is held at the example value while this one changes, which is not how a real job behaves. Treat it as a guide to which number matters most, not as five separate quotes.

What each figure means

Load (amps): A number, between 0 and 1000, starting at 20.
One-way run (ft): A number, between 0 and 5000, starting at 120.
Wire size: Choose from 14 AWG, 12 AWG, 10 AWG, 8 AWG, 6 AWG, 4 AWG, 3 AWG, 2 AWG, 1 AWG, 1/0 AWG, 2/0 AWG, 3/0 AWG, 4/0 AWG.
Conductor: Choose from Copper, Aluminium.
Voltage: Choose from 120 V, 240 V, 208 V, 480 V.

What the results tell you

Result	What it is
Voltage drop	The headline result. A percentage.
Volts lost	A decimal figure.
Voltage at the load	A decimal figure.
Longest run inside 3%	A number.

Before you rely on this number

Conductor sizing, voltage drop and load figures are governed by code, and the code takes precedence over any calculator. Ambient temperature, conduit fill, continuous load factors and the specific installation all change the answer. Treat this as a first pass, not a design.

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