Can I use 2 AWG copper for a 125 amp service?

Yes, on a single-family dwelling. Section 310.12 allows the main power feeder to be sized at 83 percent of the service rating, which is 103.75 amperes, and 2 AWG copper is 115 amperes at 75 degrees C.

What aluminum wire size is used for a 125 amp service?

1/0 aluminum on a dwelling under the 310.12 83 percent rule, or 2/0 aluminum where the full 125 amperes must be carried.

What size grounding electrode conductor goes with a 125 amp service?

Table 250.66 gives 8 AWG copper for 2 AWG copper service conductors. If the only electrodes are driven ground rods, 250.66(A) limits the required size to 6 AWG copper.

What size wire do you need for a 125 amp service?

For a single-family dwelling, 2 AWG copper or 1/0 aluminum. For a service or feeder that does not get the dwelling allowance, 1 AWG copper or 2/0 aluminum. The difference comes from Section 310.12, which lets dwelling service and feeder conductors be sized at 83 percent of the service rating when the conductor is the main power feeder to the entire dwelling.

Running the 83 percent math

125 amperes times 0.83 equals 103.75 amperes. Now go to Table 310.16 and use the 75 degree C column, which is what 110.14(C)(1)(b) allows for conductors larger than 1 AWG or circuits over 100 amperes when the equipment is rated 75 degrees C. Copper 2 AWG at 75 degrees C is 115 amperes, which covers 103.75. Aluminum 1/0 at 75 degrees C is 120 amperes, which also covers it. Section 310.12(A) also states that if the 83 percent result does not correspond to a standard rating in 240.6(A), the next higher standard rating is permitted.

Without the dwelling allowance, you need the full 125 amperes. Copper 1 AWG is 130 amperes at 75 degrees C. Aluminum 2/0 is 135 amperes at 75 degrees C. Copper 2 AWG at 115 amperes is short of 125 and would need to be protected at 125 amperes only if 240.4(B) applied, and 240.4(B) does not apply to services because 230.90 requires the overcurrent device to be rated not greater than the ampacity of the conductors, with limited exceptions.

Neutral, ground, and the rest of the package

The grounded conductor is sized per 220.61 from the maximum unbalanced load, and 250.24(C) sets a floor: it cannot be smaller than the value from Table 250.102(C)(1) based on the ungrounded conductor size. For 2 AWG copper ungrounded conductors, that floor is 8 AWG copper.

The grounding electrode conductor comes from Table 250.66. For 2 AWG copper service conductors, the GEC is 8 AWG copper. Section 250.66(A) caps the GEC at 6 AWG copper where the sole connection is to a made electrode such as ground rods, and 250.66(B) caps it at 4 AWG copper where the sole connection is to a concrete-encased electrode.

If you are running the service underground in conduit, remember the raceway is a wet location. Use THWN-2 or XHHW-2 and check Chapter 9 Table 1 fill. Three 2 AWG THWN-2 plus an 8 AWG neutral fits comfortably in 1 1/4 inch PVC.

Field takeaway

125 amp dwelling service: 2 AWG copper or 1/0 aluminum, 8 AWG copper neutral minimum, 8 AWG copper GEC or 6 AWG to rods only. Anything other than a dwelling main power feeder: step to 1 AWG copper or 2/0 aluminum.