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Wire Size Calculator

Find the minimum conductor size for a load based on ampacity, temperature, bundling and distance.

Load

Load current

System voltage

Continuous loadRuns three hours or more. NEC 210.19(A) requires sizing at 125% of the load.

Conductor

Material

CopperAluminum

Termination temperature rating

NEC 110.14(C) limits you to the lowest-rated termination in the circuit.

Current-carrying conductors in the raceway

Neutrals on a balanced multiwire circuit and grounds do not count.

Installation

Ambient temperature

Phase

Single-phaseThree-phase

One-way run length

Measure the actual routed distance, not the straight line.

Voltage drop limit

Minimum conductor size

8 AWGcopper

Sized by corrected ampacity; voltage drop is within your limit.

Design currentLoad as entered: 50.0 A
Overcurrent deviceNext standard size, NEC 240.6(A): 50 A
Voltage drop at this size5.8 V lost over 75 ft: 2.43%
Derating appliedAmbient 100% × bundling 100%: 100%

Ampacity governs this size

The conductor is sized by corrected ampacity, and voltage drop over 75 ft is within your 3% limit at 2.43%.

Candidate sizes under these conditions
Size	Table ampacity	After derating	Voltage drop	Status
14 AWG	20 A	20.0 A	9.81%	Ampacity short
12 AWG	25 A	25.0 A	6.19%	Ampacity short
10 AWG	35 A	35.0 A	3.88%	Ampacity short
8 AWG	50 A	50.0 A	2.43%	Compliant
6 AWG	65 A	65.0 A	1.53%	Compliant
4 AWG	85 A	85.0 A	0.96%	Compliant
3 AWG	100 A	100.0 A	0.77%	Compliant
2 AWG	115 A	115.0 A	0.61%	Compliant
1 AWG	130 A	130.0 A	0.48%	Compliant
1/0 AWG	150 A	150.0 A	0.38%	Compliant
2/0 AWG	175 A	175.0 A	0.30%	Compliant
3/0 AWG	200 A	200.0 A	0.24%	Compliant
4/0 AWG	230 A	230.0 A	0.19%	Compliant
250 AWG	255 A	255.0 A	0.16%	Compliant

What this assumes

Ampacity values come from NEC Table 310.16 for not more than three current-carrying conductors in a raceway, at 30°C ambient.
Temperature correction uses NEC Table 310.15(B)(1); bundling adjustment uses Table 310.15(C)(1).
Termination temperature is limited to 60°C for circuits of 100A or less unless equipment is listed for 75°C, per NEC 110.14(C).

Important

This is a planning aid, not a design. Conductor sizing depends on the installation method, the specific equipment listing and locally adopted amendments. A licensed electrician must verify any installed circuit.

Understanding the result

Three constraints, not one

A conductor has to satisfy three separate limits at once: the table ampacity after correction and adjustment, the temperature rating of the terminations at both ends, and an acceptable voltage drop over the run. The governing size is whichever of the three is largest — which is why a short 50A run and a 200-foot 50A run need different wire.

Why terminations often decide the answer

NEC 110.14(C) limits you to the lowest temperature rating in the circuit. Most breakers and equipment 100A and under are listed for 60°C terminations, so even though a 90°C conductor has higher table ampacity, you must size from the 60°C column. The 90°C column is used only as the starting point for derating.

Copper versus aluminum

Aluminum carries roughly 61% of copper's conductivity, so an aluminum conductor is typically two AWG sizes larger for the same ampacity. It costs substantially less on large feeders, which is why service entrance and feeder conductors are commonly aluminum while branch circuits are copper.

Common questions

What size wire do I need for a 50 amp circuit?

For a typical 50A branch circuit with 60°C terminations, 6 AWG copper is the common answer for short runs. Longer runs, high ambient temperature, or more than three current-carrying conductors in the raceway can push that to 4 AWG or larger.

Does conduit fill change the wire size?

Yes. Once you have more than three current-carrying conductors in a raceway, NEC Table 310.15(C)(1) reduces allowable ampacity — 80% for four to six conductors, 70% for seven to nine, and lower beyond that.

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