Wire size for 100 amp service
Most 100 A residential services use 4 AWG copper or 2 AWG aluminum, sized at 83% of the rating under NEC 310.12. Anywhere that rule doesn't apply, Table 310.16 at 75 °C calls for 3 AWG Cu or 1 AWG Al.
Standard conditions only: 30 °C ambient, no more than three current-carrying conductors together, short run. Check your conditions with the wire size calculator. Local codes vary; a licensed electrician should size and install the conductors.
Why 3 AWG?
NEC 310.12(A) lets service conductors for a one-family dwelling (or an individual dwelling unit) supplied at 120/240 V single-phase, carrying the entire load of the dwelling, have an ampacity of at least 83% of the service rating, for services from 100 A to 400 A. 100 × 0.83 = 83 A; at 75 °C that's 4 AWG copper (85 A) or 2 AWG aluminum (90 A).
The same allowance covers a feeder that carries the whole dwelling load (for example, from an outdoor meter-main to the indoor panel). It does not cover feeders to subpanels, detached buildings or commercial services; those use Table 310.16 at full rating: 3 AWG Cu (100 A) or 1 AWG Al (100 A). If temperature or bundling corrections apply, the 83% table shortcut can't be used.
Maximum distance for 3% voltage drop
One-way length in feet, copper, at 80% of 100 A (80 A), from NEC Chapter 9 Table 8 resistance.
| Copper size | 240 V |
|---|---|
| 3 AWG | 184 ft |
| 2 AWG | 232 ft |
| 1 AWG | 292 ft |
| 1/0 AWG | 369 ft |
| 2/0 AWG | 465 ft |
For your exact load and length, use the voltage drop calculator.
Questions
What size wire for 100 amp service?
For a single-family home fed at 120/240 V where the service conductors carry the entire dwelling load, NEC 310.12 allows conductors with ampacity of at least 83% of the service rating (83 A): 4 AWG copper or 2 AWG aluminum at 75 °C. Otherwise size from Table 310.16 at 75 °C: 3 AWG copper or 1 AWG aluminum.
Does the 83% rule apply to subpanels?
Only to a feeder that carries the entire load of the dwelling unit. A feeder to a detached garage or a subpanel serving part of the house is sized from Table 310.16 at full rating.
Copper or aluminum for service entrance?
Aluminum SE and SER cable is widely used for services because it costs less. Terminals must be rated for aluminum, lugs torqued to the manufacturer’s spec, and antioxidant compound used where the lug maker calls for it.