What Size Wire for a 225 Amp Service to a Dwelling Unit?

A user asked the app what conductor size a 225 amp dwelling service requires. Under NEC 2023, the answer is 3/0 copper or 250 kcmil aluminum, straight out of Table 310.12(A). The 225 amp rating sits between the common 200 and the heavy 400, and it gets the same dwelling unit allowance the popular sizes do.

How 310.12 gets you there

Section 310.12 applies to single phase, 120/240 volt services and feeders rated 100 through 400 amps that supply the entire load of a dwelling unit. It permits conductors with an ampacity of at least 83 percent of the service rating. For 225 amps, that is 225 times 0.83, which comes to 186.75 amps. Check the 75 degree C column of Table 310.16: 3/0 copper is rated 200 amps and 250 kcmil aluminum is rated 205 amps, so both clear the requirement. Table 310.12(A) saves you the arithmetic by listing those exact sizes next to the 225 amp rating.

Remember the boundary conditions. The 83 percent allowance only applies when the conductors carry the entire dwelling load. A 225 amp feeder to a barn, a shop, or a subpanel serving part of the house must be sized from Table 310.16 directly, which pushes copper to 4/0 at 230 amps, since 3/0 at 200 amps falls short of 225. Three phase services never qualify for 310.12 regardless of the building type.

Grounding and practical notes

The grounding electrode conductor for 3/0 copper service conductors is 4 AWG copper per Table 250.66, and for 250 kcmil aluminum service conductors it is 2 AWG copper. Most installers run 250 kcmil aluminum SER or XHHW for cost, and at this size the termination lugs on a 225 amp main are sized to accept it. Watch voltage drop on long laterals: past roughly 150 feet, stepping aluminum up to 300 kcmil keeps the far end healthy. And confirm the meter socket is rated for a 225 amp continuous class; some 200 amp sockets pass inspection on paper but are not listed for the larger main.