Is the length one-way or round trip?

One-way. Enter the distance from the source to the load. The formula already doubles it for single-phase circuits.

Does the NEC require 3% voltage drop?

Generally no. The 3% branch and 5% total figures are in informational notes, which are advisory. Some sections, local amendments and project specs do make them mandatory.

Why is aluminum's K higher than copper's?

Aluminum has about 61% of copper's conductivity, so the same circular-mil area has more resistance. That's why aluminum is usually upsized two AWG sizes compared with copper.

Should I use the breaker size or the actual load for current?

Use the actual expected load current. Using the breaker rating gives a worst-case answer that often oversizes the wire.

Voltage Drop Calculator (NEC) for Copper and Aluminum Wire

How the calculator works

Every conductor has resistance, so current flowing through it loses some voltage before it reaches the load. This calculator uses the approximate method from the NEC Handbook, based on the conductor's circular-mil area and a resistivity constant K.

Single-phase: VD = 2 × K × I × L ÷ CM Three-phase: VD = 1.732 × K × I × L ÷ CM K = 12.9 Ω·cmil/ft for copper, 21.2 for aluminum I = load current (A) L = one-way length (ft) CM = conductor area in circular mils (Chapter 9, Table 8)

The single-phase formula uses 2 because current travels out on one conductor and back on the other. Three-phase uses √3 (1.732) because of the phase relationship between conductors.

Worked example

A 30 A, 240 V single-phase load sits 150 ft from the panel on 10 AWG copper (10,380 cmil):

VD = 2 × 12.9 × 30 × 150 ÷ 10,380 = 11.2 V 11.2 ÷ 240 = 4.7% drop

That's over the 3% branch-circuit guideline. Moving up to 8 AWG (16,510 cmil) brings it to 7.0 V, or 2.9%.

What's an acceptable voltage drop?

The NEC's informational notes recommend no more than 3% on a branch circuit and 5% total for feeder plus branch circuit. These are recommendations, not requirements, except in a few places such as fire pumps (695.7) and sensitive electronic equipment (647.4(D)). Many specs and energy codes make the 3%/5% limits mandatory, so check your project documents.

When this method is less accurate

Large conductors (roughly 1/0 and up) on AC, where reactance starts to matter. Use NEC Chapter 9 Table 9 impedances for critical feeders.
Conductors running much hotter or cooler than 75 °C.
Motor starting, where inrush current can be six times running current.

Frequently asked questions

Is the length one-way or round trip?: One-way. Enter the distance from the source to the load. The formula already doubles it for single-phase circuits.
Does the NEC require 3% voltage drop?: Generally no. The 3% branch and 5% total figures are in informational notes, which are advisory. Some sections, local amendments and project specs do make them mandatory.
Why is aluminum's K higher than copper's?: Aluminum has about 61% of copper's conductivity, so the same circular-mil area has more resistance. That's why aluminum is usually upsized two AWG sizes compared with copper.
Should I use the breaker size or the actual load for current?: Use the actual expected load current. Using the breaker rating gives a worst-case answer that often oversizes the wire.

Quick answers

Results are estimates for planning and education. Verify against the current code edition your jurisdiction has adopted, manufacturer data and a licensed professional before you build or install.

Voltage drop calculator

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