What Size Wire for a 100 Amp Subpanel?

Table 310.16 is the ampacity table for insulated conductors run in a raceway, cable, or earth, with not more than three current-carrying conductors, at a 30°C (86°F) ambient. Every figure in it is a starting point, not a finished answer.

The table has three temperature columns: 60°C, 75°C, and 90°C. Which one you may size from is not decided by the wire. It is decided by the terminations at both ends, under NEC 110.14(C). Unless the equipment at both ends is listed and marked for something higher, the 75°C column is what you get on a feeder of this size. THHN is 90°C insulation, but 90°C wire landed on a 75°C lug is still a 75°C circuit.

The 90°C rating still matters: you may begin derating calculations from the 90°C ampacity, as long as the final corrected value does not exceed the termination rating.

• Termination temperature rating. NEC 110.14(C) ties you to the lowest-rated termination in the circuit. Most modern breakers and panel lugs are marked 75°C, but the marking is on the equipment — go look at it.
• More than three current-carrying conductors. Additional ones in the same raceway or bundle trigger the adjustment factors in NEC 310.15. On a 120/240-volt, three-wire feeder the neutral carries only unbalanced current and is generally not counted.
• Ambient temperature. Table 310.16 assumes 30°C. An attic, a rooftop raceway in the Valley, or a hot mechanical room requires the temperature correction factors, which can be severe.
• The wiring method itself. NM-B (Romex) is limited to the 60°C column by NEC 334.80, which is why NM is rarely the vehicle for a 100-amp feeder. Type SE/SER used as interior wiring has its own rules in 338.10(B)(4) and can be pushed to the 60°C column where it passes through thermal insulation.
• Voltage drop and the dwelling feeder allowance. The NEC's voltage-drop figures live in informational notes, so they are guidance rather than an enforceable rule — but on a long run to a detached garage they are the reason to upsize. Separately, NEC 310.12 has a reduced-size allowance for conductors carrying the entire load of a dwelling unit; it is conditional and often misapplied, so read it in your adopted edition rather than assuming it covers your subpanel.
• The equipment at the far end. The feeder breaker has to protect the conductors, and the subpanel's busbar rating must be at least the rating of that overcurrent device (NEC 408.36).

A subpanel gets a four-wire feeder: two ungrounded conductors, a grounded (neutral) conductor, and an equipment grounding conductor. The neutral is sized for the maximum unbalanced load per NEC 220.61, but it cannot drop below the minimum size required for bonding purposes in NEC 250.102(C) and its table — look both up, because a reduced neutral that ignores the second rule is a common correction.

Size the equipment grounding conductor from NEC Table 250.122, using the rating of the overcurrent device ahead of the feeder. Read the row in your adopted edition rather than working from a number someone told you.

Keep neutrals and grounds separated at the subpanel. The neutral bar must be isolated from the enclosure — the main bonding jumper or bonding screw stays out — and the equipment grounding conductors land on a separate ground bar bonded to the can. The system is bonded once, at the service, and this is the most common thing an inspector red-tags on a subpanel. The old allowance for a three-wire feeder to a separate structure was removed for new installations in the 2008 NEC; on an existing three-wire feed, that is a conversation with your AHJ.

We stock what this takes — see our wire, cable, and conduit inventory for THHN/THWN-2, aluminum feeder wire, EMT and PVC, and our load centers, panelboards, and breakers for the subpanel, feeder breaker, and ground bar. We cut wire to length at the counter — send us the run length and breaker size for a same-day quote.

Reference only. This page is general reference information, not a substitute for the currently adopted code or the ruling of the authority having jurisdiction. California adopts the NEC as the California Electrical Code (Title 24, Part 3) with state amendments, and your city or county may amend it further. Requirements change from one code cycle to the next — verify every value against the edition in force for your permit, and confirm the details with your inspector before you buy material or energize anything. Ora Electric Lighting & Supply is a wholesale distributor. We sell materials. We do not perform electrical work, and we do not provide design or engineering services.