What Size Wire for a 50 Amp Breaker?

The rule behind the answer

Sizing a conductor to a breaker is a 240.4 problem: the overcurrent device protects the conductor, so the conductor's ampacity has to be at least the breaker rating (with two narrow exceptions covered further down). Ampacity comes from Table 310.16, but you do not get to read it in the 90°C column. NEC 110.14(C) limits the ampacity you may use to the temperature rating of the weakest termination in the circuit, which for most breakers and lugs on a 50 A circuit is 75°C, and defaults to 60°C for equipment not listed for anything higher. The 90°C column exists for adjustment and correction math, never as the number a terminal sees.

So the question "what size wire for a 50 A breaker" is really "what is the smallest conductor whose Table 310.16 ampacity, read in the terminal's temperature column, is 50 A or more."

50 A, copper, worked out

Take the candidate copper sizes from Table 310.16 and compare each against a 50 A breaker:

Reach for 6 AWG copper for a general-use 50 A branch circuit and you are covered regardless of whether the terminals are 60°C or 75°C rated. The 8 AWG "it works at 75°C" argument is technically true and practically brittle: the moment one end of the circuit is only listed for 60°C, the conductor is undersized.

50 A, aluminum

Aluminum runs one to two sizes larger for the same current. From Table 310.16, 6 AWG aluminum is 40 A at 60°C and 50 A at 75°C, so it has the same 75°C-only, zero-margin problem 8 AWG copper has. 4 AWG aluminum is 55 A at 60°C and 65 A at 75°C, clearing 50 A in both columns. Use 4 AWG aluminum for a 50 A circuit, which is also why aluminum SER and URD feeders for 50 A subpanels show up as 4 AWG.

The chart, common breaker sizes

Every conductor below is read from Table 310.16 and paired to the breaker by 240.4. The copper column assumes 75°C terminations, the common modern case, and the aluminum column likewise, since aluminum lugs and breakers are listed for 75°C. Where the 60°C column would force a larger copper wire, it is noted.

*6 AWG copper carries 60 A only in the 75°C column; its 60°C ampacity is 55 A, so a circuit landing on 60°C terminals needs 4 AWG copper. This is the same 75°C-versus-60°C fork that trips up the 50 A case, one size up.

Two exceptions that change the pairing

240.4(B), next standard size up. When a conductor's ampacity falls between two standard breaker ratings, you may protect it at the next size up, provided the breaker is 800 A or less and the circuit does not feed multiple cord-and-plug receptacles. Example: 4 AWG copper is 85 A at 75°C, and 85 A is not a standard rating, so a 4 AWG feeder may be protected at 90 A rather than dropped to 80 A. This allowance is about protecting an already-chosen larger conductor at a higher breaker, not about shrinking the wire below the breaker rating.

240.4(D), small-conductor limits. Regardless of computed ampacity, 14, 12, and 10 AWG copper are capped at 15, 20, and 30 A, and 12 and 10 AWG aluminum at 15 and 25 A. Next-size-up never overrides these caps. It is why 10 AWG copper, whose 90°C ampacity is 40 A, still cannot sit on anything larger than a 30 A breaker, and why 10 AWG aluminum tops out at 25 A and cannot serve a 30 A circuit at all.

Before you finalize: length and load

The 240.4 pairing sizes the conductor to the breaker. Two more checks can push it larger. First, the conductor still has to carry the actual load after any ampacity adjustment for conductor count or ambient correction, taken in the 90°C column and then held to the termination limit. Second, on a long run, voltage drop governs before ampacity does: a 50 A circuit past roughly 100 feet often wants 4 AWG copper to hold under the 3 percent branch-circuit target, even though 6 AWG is ampacity-legal. Run the distance on the Voltage Drop Calculator and the full conductor decision, ampacity and drop together, on the Wire Size Calculator.

Continuous loads size the breaker first

This page starts from a known breaker and works down to the wire. If you are starting from a load instead, and especially a continuous one like an EV charger or water heater, the breaker itself comes from the 125 percent rule before you ever open Table 310.16. Sizing a breaker for a continuous load works that direction step by step, then hands off to the conductor pairing on this page.

NEC reference

NEC 2020. Conductor-to-overcurrent-device pairing from 240.4, including the next-size-up allowance in 240.4(B) and the small-conductor limits in 240.4(D). Ampacities from Table 310.16, 60°C and 75°C columns. Termination temperature limits from 110.14(C). Standard breaker ratings from 240.6(A). Values are identical in the 2017, 2020, and 2023 editions; the table was numbered 310.15(B)(16) before 2020.

Related

• Breaker Size Calculator: load in, breaker and minimum conductor out, with the 240.4 interactions flagged.
• How to Size a Breaker for a Continuous Load: the 125 percent rule that sets the breaker before you pick the wire.
• Wire Size Calculator: size a conductor for ampacity and voltage drop together.