125 Amp Service Wire Size

Wire size for a 125 amp service

This page applies 310.12 to a 120/240 V single-phase dwelling service that carries the entire load of a dwelling unit. That rule sizes the conductors at 83% of the 125 A rating, 103.75 A, using the 75 °C column of Table 310.16. It does not apply to commercial, three-phase or multifamily services.

What this answer leaves out

• No ambient (310.15(B)(1)) or conductor-count (310.15(C)(1)) correction is applied. Hot locations or bundled conductors need their own calculation.
• The 310.12 result assumes the service and meter terminations are listed or marked for 75 °C (110.14(C)). If any terminal is rated 60 °C, the conductor may need to be larger; for example #4 copper is 70 A at 60 °C against 85 A at 75 °C.
• Grounded (neutral) and grounding conductors have their own sizing rules; this is not an answer for them.
• Voltage drop is not included; long runs may need a larger conductor.

125 amp wire size questions

What size wire is needed for a 125 amp service?

For a 120/240 V single-phase dwelling service under 310.12, #2 AWG copper or 1/0 AWG aluminum (or copper-clad aluminum). 310.12 allows 83% of the 125 A rating, 103.75 A, in the 75 °C column of Table 310.16. Voltage drop is not included; long runs may need a larger conductor.

Why is that smaller than Table 310.16 gives for 125 amps?

Without the 310.12 allowance, 125 A in the 75 °C column needs #1 AWG copper or 2/0 AWG aluminum. 310.12 lets the service conductors of a one-family dwelling be sized at 83% of the rating.

Does this apply to commercial, three-phase or multifamily services?

No. This page covers only 120/240 V single-phase dwelling services that carry the entire load of a dwelling unit, 100 A to 400 A. Anything else is sized from Table 310.16 with the wire size calculator.

Does the 125 amp service answer assume 75 °C terminations?

Yes. The 310.12 result is read from the 75 °C column, so it assumes the service and meter terminations are listed or marked for 75 °C (110.14(C)). If any terminal is rated 60 °C, the conductor may need to be larger: for example #4 copper carries 85 A at 75 °C but only 70 A at 60 °C. Check the equipment markings.

Does the answer include ambient or conductor-count corrections?

No. The 310.12 answer applies no ambient (310.15(B)(1)) or adjustment (310.15(C)(1)) factors, and it is not a rule for the grounded or grounding conductors, which have their own rules. Use the wire size calculator for hot locations or bundled conductors.

Other sizes