A subpanel is fed by a feeder from the main panel. Sizing it correctly means matching the breaker, the conductors and the equipment grounding conductor.
Start with the load
Calculate the load the subpanel will serve and select a breaker that covers it, with continuous loads at 125%. Common feeder breaker sizes are 60, 100 and 125 A.
Choose the feeder conductors
Using the 75°C column of NEC Table 310.16 for copper: 6 AWG is 65 A, 4 AWG is 85 A, 3 AWG is 100 A and 1 AWG is 130 A. For aluminum: 4 AWG is 65 A, 2 AWG is 90 A and 1/0 is 120 A.
- 60 A subpanel: 6 AWG copper or 4 AWG aluminum
- 100 A subpanel: 3 AWG copper or 1 AWG aluminum (100 A)
The 83% reduction in NEC 310.12 applies only to feeders carrying the entire load of a dwelling, not to a subpanel feeder serving part of it. Use the standard table ampacity.
Grounding and neutral
Use a four-wire feeder: two hots, a neutral and an equipment grounding conductor. NEC Table 250.122 sets the minimum copper grounding conductor: 10 AWG for 60 A, 8 AWG for 100 A, 6 AWG for 200 A. In the subpanel, keep the neutral and ground bars separate and do not install the main bonding jumper (250.24(A)(5) and 408.40). Detached structures follow 250.32.
Check voltage drop
A 100 A feeder 100 ft long on 3 AWG copper at 240 V drops 2 × 12.9 × 100 × 100 ÷ 52,620 = 4.9 V (2.0%). Longer runs may need a size up.
Other checks
- Working space and clearances around the panel (110.26).
- Panel and breaker ratings and labeling.
- Permits and inspection for new feeders.
Try the calculators
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