What voltage drop is acceptable?

The NEC recommends a maximum of 3% on a branch circuit and 5% combined for feeder plus branch. These are recommendations rather than hard requirements, but exceeding them causes dim lighting and motor overheating.

Why does a long run need thicker wire than the breaker size suggests?

Ampacity governs safety at the panel, but resistance accumulates over distance. On long runs the voltage drop limit, not the breaker, dictates conductor size.

Can I use aluminium wire with these numbers?

Not directly. Aluminium has roughly 61% the conductivity of copper, so use K of about 21.2 and expect to go one to two sizes larger for the same load and distance.

What is a continuous load?

A load expected to run for three hours or more. Size conductors and overcurrent protection at 125% of the continuous current before using this calculator.

Is this a substitute for an electrician?

No. It is a planning and estimating tool. Final conductor sizing must follow your local code, the installation method, ambient temperature and any required derating.

Trade Calculators

Wire Gauge / Electrical Load Calculator

Recommended AWG, load in watts and voltage drop.

Inputs

Given

System voltage

Load current

One-way run length

Phase

Power factor

Max voltage drop

Results

Computed

Recommended copper conductor

8 AWG

Ampacity 50 A at 75°C · 16,510 circular mils

Minimum size by ampacity10 AWG

Voltage drop on this run4.69 V

Voltage drop percentage1.95%

Load7,200 W

Apparent power7.20 kVA

Voltage at the load235.3 V

Sizes assume copper conductors at 75°C in a normal ambient with no more than three current-carrying conductors in a raceway. Derate for heat, bundling, or aluminium.

8 AWG · 1.95% drop · 7.20 kW

FIG. 5 — VOLTAGE DROP OVER A RUN

How this is calculated

Conductor selection has to satisfy two separate limits. The first is ampacity: the conductor must safely carry the current without overheating, which comes from published tables for copper at a given insulation temperature rating. The second is voltage drop over the length of the run.

Vdrop = (2 × K × I × L) ÷ circular mils (single phase)

Vdrop = (√3 × K × I × L) ÷ circular mils (three phase)

K is the resistivity constant, about 12.9 ohm-circular-mils per foot for copper at 75°C. L is the one-way run length; the factor of 2 on a single-phase circuit accounts for current travelling out and back.

Load is calculated from voltage, current and power factor:

watts = V × I × PF (single phase) | watts = √3 × V × I × PF (three phase)

The recommendation shown is the smallest conductor that meets both the ampacity requirement and your voltage drop target. Always confirm against the current edition of your local electrical code, and derate for high ambient temperature, conduit fill, or aluminium conductors, which need roughly one to two sizes larger than copper for the same load.

Worked example

Feeding a detached garage subpanel

A 60 A, 240 V single-phase subpanel sits 100 ft from the main panel.
Load = 240 V × 60 A = 14.4 kW available at the panel.
Copper 4 AWG carries 60 A comfortably at typical insulation ratings.
Over 200 ft of round-trip conductor, 4 AWG drops roughly 4.9 V at full load — about 2.1% of 240 V.
That sits under the common 3% branch-circuit guideline, so 4 AWG works; at 150 ft you would step up to 2 AWG.

Common mistakes

Using one-way length
Voltage drop happens over the full circuit — out and back. Entering the one-way distance instead of the round-trip length understates the drop by half.
Confusing ampacity with voltage drop
A wire can be thick enough to carry the current safely yet still drop too much voltage on a long run. Ampacity protects against heat; drop protects equipment. Long runs are usually drop-limited.
Ignoring aluminum vs copper
Aluminum has about 61% of copper's conductivity, so the same load needs roughly two AWG sizes larger. Feeder runs often use aluminum for cost — check the gauge against the right material.