Wire Size by Amps

Short answer: a 15 A circuit uses 14 AWG copper, 20 A uses 12 AWG, 30 A uses 10 AWG, 40 A uses 8 AWG, 50 A uses 6 AWG in NM-B cable (8 AWG THHN in conduit with 75°C terminations), and 60 A uses 4 AWG in NM-B (6 AWG THHN). A 200 A dwelling service uses 2/0 copper or 4/0 aluminum. Pick your breaker below for the full answer, including distance, grounding and conduit size.

Breaker-to-wire chart

Sizes from NEC 2023 Table 310.16 (60°C column for NM-B and UF cable, 75°C for conductors in conduit with 75°C-rated terminations) and Table 310.12 for single-phase dwelling services and main feeders. Before voltage drop, ambient temperature or bundling corrections.

Why the same breaker can need two different wire sizes

Three rules decide the answer, and they explain most of the "it depends" you'll hear:

1. The cable or wire type sets the temperature column. NM-B (Romex) and UF cable are limited to the 60°C ampacity column (334.80, 340.80). Individual THHN/THWN-2 conductors in conduit can use the 75°C column when every termination is rated 75°C (110.14(C)). That's why a 50 A circuit is 6 AWG in NM-B but can be 8 AWG THHN.
2. Small conductors have their own breaker limits. Under 240.4(D), 14 AWG copper is limited to 15 A, 12 AWG to 20 A and 10 AWG to 30 A, even though the table lists higher ampacities.
3. Dwelling services and main feeders get Table 310.12. For a single-phase 120/240 V service or feeder carrying the whole dwelling load, conductors may be sized at 83% of the rating. A subpanel feeder doesn't qualify, which is why a 100 A subpanel needs 3 AWG copper while a 100 A service can use 4 AWG.

Distance changes the answer

The chart gives the minimum size for ampacity. On long runs, voltage drop often forces a larger wire. As a rule of thumb, check any branch circuit longer than about 50–75 feet and any feeder longer than about 100 feet. Each page linked above includes a table of wire sizes by distance, or use the voltage drop calculator for your exact run.

Aluminum or copper?

Aluminum is lighter and costs less per foot, and it is common for services, feeders and SER cable to subpanels. It needs a larger size for the same current, terminations listed for aluminum, and correct torque. Branch circuits of 15 and 20 A are almost always copper.

Related references

• Ampacity chart: the full Table 310.16 by size and temperature column.
• Wire gauge chart: diameters, circular mils and areas by AWG.
• Wire size calculator: ampacity and voltage drop together.

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Based on NEC 2023. Always confirm with the code edition your jurisdiction has adopted and your inspector; electrical work should be done by qualified people. Last updated: October 6, 2026.