Inputs
Describe the circuit
Most equipment terminals are listed 60°C or 75°C.
Table basis is 30°C (86°F).
Four or more in one raceway triggers adjustment.
3% is the usual branch-circuit target.
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What size wire do I need?
Most wire-size answers get one half of the problem right. This one does both: the conductor has to carry the current after derating, and it has to deliver usable voltage at the far end. AmpSizer sizes for each and shows you which one governs.
Result
Recommended conductor
6 AWGcopper
- Required ampacity50 A
- Smallest conductor by ampacity8 AWG
- Smallest conductor by voltage drop6 AWG
- Voltage drop at 6 AWG2.05% (4.92 V)
- Next standard breaker50 A
- Equipment grounding conductor10 AWG
Voltage drop governs. Ampacity alone would allow 8 AWG, but over 100 ft that lands above your 3% limit. Voltage-drop limits are recommendations in the 2023 NEC, not requirements — your local jurisdiction may treat them differently.
Need the breaker first? See wire size for a 50 A breaker.
Source: NFPA 70, National Electrical Code, Table 310.16; NFPA 70, National Electrical Code, 210.19(A) and 215.2(A) informational notes. Published by NFPA.NEC 2023
How this calculation works
Two independent requirements decide a conductor, and the larger one wins.
Ampacity. Every conductor has a published current-carrying capacity for its size, material, and insulation temperature rating, listed at a 30°C ambient with no more than three current-carrying conductors bundled together. Real installations rarely match that basis, so the published figure gets corrected for the actual ambient temperature and adjusted for the number of conductors sharing the raceway. The smallest conductors carry an additional overcurrent ceiling that applies no matter what the ampacity table says — which is why 14 AWG copper is a 15 A conductor even though the 90°C column reads 25 A.
Voltage drop. Resistance turns some of your voltage into heat on the way to the load. Over a short run that loss is trivial; over a few hundred feet it can leave a motor stalling or a heater underperforming. The estimate here uses the circular-mil method, scaling the drop by conductor area, current, and one-way distance. Unlike ampacity, voltage-drop targets are performance recommendations rather than requirements in the 2023 NEC — though specifications and some jurisdictions do enforce them.
What this calculator does not model
Motor and air-conditioning circuits, welders, and other loads with their own sizing articles. Paralleled conductor sets. Conductors in free air or direct burial rather than a raceway. Voltage-drop effects from conductor reactance and power factor on large AC feeders. Rooftop, ampacity-limitation, and temperature-limitation provisions. Local amendments, which are common and can be stricter than the base code.
Use the result as a starting point for a conversation with your electrician and inspector, not as the final word. See also the ampacity lookup, the voltage drop checker, and the reference tables.