For this 100-amp subpanel example, use 3 AWG copper or 1 AWG aluminum. Both sizes are rated for 100 amps in the 75°C column of 2020 NEC Table 310.16.
This sizing assumes conductors and terminals rated at least 75°C, a wiring method that permits 75°C sizing, and no conditions that reduce conductor ampacity. The smaller wire sizes allowed for certain whole-dwelling feeders do not automatically apply to a garage subpanel.
Use 2020 NEC Article 310 for this example. Check the load, wiring method, terminal ratings and installation conditions before selecting the conductor.
Start with the conditions in the question
Assume a 100-amp feeder from a dwelling’s 200-amp service supplies a garage panel. It does not carry the dwelling’s entire load. The conductors have insulation rated at least 75°C and are installed in a raceway. All relevant terminals permit 75°C sizing, ambient temperature is 30°C, and there are no more than three current-carrying conductors requiring adjustment.
Also assume the load requires 100 amps of conductor ampacity after applying the required continuous-load treatment. A 100-amp panel label tells you the equipment rating. Calculate the connected loads separately.
Read the 75°C column
| Conductor | 75°C ampacity | Result for this example |
|---|---|---|
| 4 AWG copper | 85 A | Below 100 A |
| 3 AWG copper | 100 A | Meets the stated ampacity |
| 2 AWG aluminum | 90 A | Below 100 A |
| 1 AWG aluminum | 100 A | Meets the stated ampacity |
Check the conductor material and column heading before reading across the row. A 90°C insulation rating does not override a 75°C terminal limit. Other wiring methods can impose additional limits.
Watch: why terminal ratings matter
Victor Ring explains 2020 NEC 110.14(C). Follow the terminal-temperature rule before choosing a column in the ampacity table.
How the 100-Amp Rule Works - NEC 110.14(C) (2020)Watch JCR’s lesson on YouTube ↗
Practice conductor sizing and NEC tables
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See CE courses and class datesWhy do some charts say 4 copper or 2 aluminum?
Table 310.12 contains dwelling service and feeder allowances. Its 100-amp row lists 4 AWG copper and 2 AWG aluminum when the section’s conditions are met.
The allowance in 310.12(B) applies when the feeder supplies the dwelling unit’s entire load. A garage feeder carrying only part of that load does not qualify on that basis. Also check 310.12(C): it can limit the feeder size required relative to the dwelling’s qualifying service or feeder. Read all the conditions before using the dwelling table.
In this example, the 100-amp garage feeder from a 200-amp service doesn’t get a blanket 83% reduction simply because it’s at a house. See our dwelling service sizing guide for the separate whole-load rule.
What about neutral and ground?
Size the neutral and equipment grounding conductor separately. Size the neutral for its calculated load. A wire-type equipment grounding conductor is generally sized from the feeder’s protective device under Table 250.122.
For a 100-amp protective device, that table starts at 8 AWG copper or 6 AWG aluminum for the wire-type equipment grounding conductor. Other requirements can affect the installation or require a larger conductor. See the EGC sizing examples.
In a typical new feeder-supplied panel, keep the neutral isolated from the enclosure and equipment grounding bar. A separate building’s grounding electrode requirements are another step.
Check what could change the answer
- A lower terminal or wiring-method temperature limit.
- Higher ambient temperature or additional current-carrying conductors.
- A larger calculated load.
- A long run where voltage drop affects the design.
- Local amendments and equipment instructions.
Work through ampacity correction and adjustment before assuming the table value is your final allowable ampacity.
Common questions
Can I use 2 AWG aluminum for any 100-amp subpanel?
No. It is 90 amps in the 75°C column of Table 310.16. A different applicable permission must justify its use. The dwelling allowance isn’t automatic.
Does a 100-amp panel need a 100-amp supply?
Its rating is a maximum equipment rating, not a required supply current. The load, protective device and conductors must be coordinated.
Examples use the 2020 NEC. References checked October 25, 2026.
