What Size Wire for a 100 Amp Service in a Commercial Setting?
A user asked the app what wire size a 100 amp service needs in a commercial building. The answer is 3 AWG copper or 1 AWG aluminum, taken straight from the 75 degree C column of NEC 2023 Table 310.16. The number surprises residential electricians because a 100 amp house service commonly runs 4 AWG copper. The difference is that the shortcut they are used to does not exist outside dwellings.
No 83 percent rule here
Section 310.12 grants its 83 percent sizing allowance only to single phase services and feeders that supply the entire load of a dwelling unit. A store, office, shop, or any commercial occupancy sizes service entrance conductors at full value. That means the conductors need a table ampacity of at least 100 amps at the termination temperature rating. In the 75 degree column, 3 AWG copper is rated exactly 100 amps and 1 AWG aluminum is rated 100 amps as well. 4 AWG copper at 85 amps falls short, even though it passes on a house under the dwelling allowance.
If the service is three phase, which is common in commercial work, 310.12 would not apply regardless of occupancy, so the sizing is identical: each ungrounded conductor at 100 amps minimum from Table 310.16, adjusted for conditions. Apply ampacity correction if the conductors pass through high ambient spaces like a rooftop or boiler room, and adjustment factors if more than three current carrying conductors share a raceway, which can happen with two services or parallel runs in one conduit.
The rest of the package
Continuous loads matter more in commercial buildings. Section 230.42(A) requires service conductors sized for the noncontinuous load plus 125 percent of the continuous load, so a building with heavy lighting or equipment running three hours or more may need conductors larger than the bare 100 amp minimum. The grounding electrode conductor for 3 AWG copper service conductors is 8 AWG copper per Table 250.66, and most inspectors want it protected in raceway at that size. Bonding, working clearance under 110.26, and a properly listed 100 amp rated disconnect finish the checklist. When in doubt, run the load calculation under Article 220 first; the service size should come from the calc, not from the panel that happened to be on the truck.