What Size SER Cable for a 100 Amp Subpanel?

Feeding a subpanel breakered at 100 amps from the main panel in a house is one of the most common feeder jobs there is, and this exact question came in through the Ask BONBON app this week. Here is the sizing under NEC 2023.

The standard answer

Using the 75 degree column of Table 310.16, which is what standard breaker and lug terminations allow per 110.14(C), you need 3 AWG copper at 100 amps or 1 AWG aluminum at 100 amps. SER is almost always aluminum in the field, so 1 AWG aluminum SER is the stock answer for a 100 amp interior subpanel feeder.

You will hear electricians say 2 AWG aluminum is fine for 100 amps. That comes from NEC 310.12, which permits 83 percent sizing, but that allowance applies only to service conductors and to feeders that supply the entire load of a dwelling unit. A subpanel that serves part of the house, a garage, or a shop does not qualify, so a 100 amp branch subpanel gets full Table 310.16 sizing: 1 AWG aluminum.

Watch the insulation and routing rules

SER is Type SE cable, and 338.10(B)(4) says interior installations follow the rules for NM cable where run inside a dwelling. The important one: where SER is run in thermal insulation, its ampacity is limited to the 60 degree column. At 60 degrees, 1 AWG aluminum is only good for 85 amps, so a run buried in attic insulation would need to jump to 1/0 aluminum. Keep the cable out of insulation where you can, or size up where you cannot.

Four wires, always

A subpanel feeder needs two ungrounded conductors, a neutral, and a separate equipment grounding conductor, which standard SER provides with its bare fourth conductor. At the subpanel, the neutral bar floats on its insulator and the bonding screw stays out. Grounds land on a bar bonded to the can. The only place neutral and ground meet is the service disconnect per 250.24(A)(5).

Protection and terminations

Torque the aluminum lugs to the label value per 110.14(D), and use a breaker or lugs rated for aluminum, which nearly all modern equipment is. The 100 amp breaker at the main panel protects the feeder, so the subpanel main can be a breaker or main lug only, though a main breaker adds a convenient local disconnect.

Need the numbers for a different amperage or a long run with voltage drop? Ask BONBON runs the table lookup and cites the section in seconds.