Wire size for a breaker · explained

How to pick the right wire size for a breaker

Why one-size-fits-all wire charts disagree, and the seven-step method that gets you the code answer every time — terminations, continuous loads, derating, and 240.4.

Need the actual numbers?

Wire-to-breaker sizing comes from NEC Table 310.16 plus the rules around it. Look up Table 310.16 and sections 110.14(C) and 240.4 in the free NFPA 70 (NEC) online viewer (free NFPA account, read-only), or your printed code book.

Section numbers and values vary by NEC edition, and your jurisdiction may adopt an older edition or local amendments. The edition your AHJ enforces governs.

The method: wire size for a breaker

Step	What you do
1. Start from the load	Determine the load current. If it is continuous (expected to run 3 hours or more), multiply it by 125% for both the breaker and the conductor (210.19(A), 210.20(A), 215.2, 215.3).
2. Pick the breaker	Choose a standard breaker size (240.6) that is at least the value from step 1 — or the size the equipment nameplate specifies (e.g., a maximum overcurrent protection value on HVAC equipment).
3. Find the termination rating	Check the breaker and equipment lugs. Under 110.14(C), circuits of 100 A or less generally use the 60°C column unless everything is listed/marked for 75°C; larger circuits use 75°C. NM cable is limited to the 60°C column regardless (334.80).
4. Look up the conductor	In Table 310.16, read the column for your termination rating and material. Take the smallest size whose ampacity covers the breaker (or covers the load, with 240.4(B) next-size-up if ≤ 800 A and not a multi-outlet receptacle branch circuit).
5. Derate if needed	High ambient temperature or more than three current-carrying conductors in the raceway reduce ampacity (310.15(B) and (C)(1)). If the derated value falls below what you need, go up a size.
6. Apply the small-conductor limits	240.4(D) caps the breaker on 14, 12, and 10 AWG copper (and small aluminum) no matter what the table cell says.
7. Check voltage drop	Long runs may need an upsize for voltage drop — and if you upsize the phase conductors, 250.122(B) requires the equipment ground to grow proportionally.

Worked examples

Example	Inputs	Steps	Result
A. 50 A range circuit, NM cable	NM-B cable, 50 A breaker, non-continuous load.	NM is limited to the 60°C column (334.80). In that column 6 AWG Cu is 55 A per Table 310.16, which covers the 50 A breaker.	6 AWG Cu NM
B. Same 50 A, THHN in EMT, all 75°C	THHN/THWN-2 in conduit; breaker, lugs, and appliance terminals all listed for 75°C.	Read the 75°C column: 8 AWG Cu is 50 A per Table 310.16, which matches the breaker. If any termination is only 60°C, go back to example A.	8 AWG Cu (only if everything is 75°C)
C. 40 A continuous EV charger	EVSE nameplate 40 A continuous; 75°C terminations.	Continuous load × 125% = 50 A, so a 50 A breaker and conductors good for at least 50 A. The conductor lookup is then the same as example B.	50 A breaker, conductor per B

Values are quoted only to illustrate the calculation. Look up your own conductor in the official table for your adopted edition.

Common field mistakes

Mistake	What to do instead
Sizing by the breaker alone	The breaker is picked first, but the wire must be checked against the termination column, derating, and continuous-load rules — not just "matched" to the breaker.
Using 75°C ampacity on 60°C terminations	Many small breakers, receptacles, and older equipment are 60°C-rated. Read the lugs.
Forgetting the 125% continuous-load factor	Water heaters, EV chargers, and lighting that runs all night are continuous loads.
Treating NM like THHN	NM-B contains 90°C conductors but the cable ampacity is limited to the 60°C column.
Applying 310.12 to a sub-panel	The 83% dwelling allowance only covers the service and the main feeder carrying the whole dwelling load.
Upsizing for voltage drop but not the ground	250.122(B) requires the EGC to be increased by the same area ratio.

What changed between editions

Edition	What to know
2017 NEC	Ampacities were in Table 310.15(B)(16), and dwelling services/feeders had their own size table in 310.15(B)(7).
2020 NEC	The ampacity table became Table 310.16. The dwelling size table was dropped in favor of the 83% method in 310.12, so older "100 A dwelling = X AWG" charts may no longer match.
2023 NEC	Kept the 2020 numbering for the sections used here. Check the edition’s change markers for revisions to 110.14(C), 240.4, and 310.12.
2026 NEC and later	Being adopted state by state — confirm section numbers in the edition your AHJ enforces.

Summary of numbering changes for orientation only — confirm in the edition your jurisdiction has adopted.

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Why generic wire/breaker charts disagree

Search "what size wire for a 50 amp breaker" and you will find two different answers. Both can be right: the size depends on the termination temperature rating, the wiring method (NM cable vs. conductors in conduit), whether the load is continuous, and whether the run needs derating or a voltage-drop upsize. A single lookup chart hides all of that.

The breaker protects the wire, so the conductor's ampacity — from Table 310.16, in the right column, after derating — must support the breaker under 240.4. Work the steps above and you get an answer you can defend to an inspector.

For the full ampacity walk-through see how to read NEC Table 310.16 and standard breaker sizes. Check a long run with the voltage drop calculator, and size the raceway with the conduit fill calculator. Field PM keeps panel schedules and inspection records together so the install matches the design.

FAQ

What size wire for a 50 amp breaker?+

It depends on the termination rating and wiring method, which is why different charts disagree. With 60°C terminations or NM cable, 6 AWG copper (55 A in the 60°C column of Table 310.16) is the usual answer. Where the wire, breaker, and equipment terminals are all rated 75°C, 8 AWG copper (50 A in the 75°C column) can qualify. Continuous loads, derating, and voltage drop can push the size up.

What size wire for a 100 amp sub-panel?+

Find the smallest conductor whose ampacity in the termination-temperature column of Table 310.16 (after any derating) is at least 100 A — or, under 240.4(B), close enough that 100 A is the next standard size up. The dwelling 83% shortcut in 310.12 does NOT apply to an ordinary sub-panel; it only covers the service or the main feeder carrying the entire dwelling load. Look up the column in the free NFPA viewer at nfpa.org/freeaccess.

Can I just use a wire size vs. breaker chart?+

Only as a starting point. Generic charts assume one termination rating, no continuous load, no derating, and short runs. The code answer comes from Table 310.16 plus 110.14(C), 210.19/215.2, 240.4, and the correction/adjustment factors in 310.15.

Why does aluminum need to be larger?+

Aluminum has roughly 61% of copper's conductivity, so for the same ampacity it is typically about two AWG sizes larger. Read the aluminum half of Table 310.16 and make sure the lugs are rated AL/CU.

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