Most 100 amp service upgrades that fail inspection fail because the cable was sized from the breaker instead of the load. Skip the allowance and you overbuy copper; stretch it to a subpanel and the job never passes.
Load diversity drives the split. A house rarely pulls its full 100 amps at once, while a feeder carrying a defined load has no such cushion. A 100 amp breaker label cannot tell you which side of that line your job sits on.
What Size Service Entrance Cable For A 100 Amp Dwelling Service?
NEC 310.12 carries the allowance for services rated 100 through 400 amps.
Section numbers have shifted: older references cite 310.15(B)(7) for this dwelling allowance; current editions list it as 310.12. Check which edition your inspector works from before committing to a size.
Ampacity alone changes the size here. Before ordering, confirm whether the conductors feed the entire dwelling or a smaller load—the main breaker label and meter base will tell you which.
| Installation | Copper | Aluminum |
|---|---|---|
| 100 amp dwelling service entrance (one-family or individual unit) | 4 AWG | 2 AWG |
| 100 amp feeder, subpanel, or non-dwelling service | 3 AWG | 1 AWG |
| Equipment grounding conductor, 100 amp circuit | 8 AWG | 6 AWG |
Why The Dwelling Allowance Stops At The Service
The 83% figure applies only where conductors carry the whole load of the qualifying dwelling unit. Feeder conductors must carry the calculated downstream load, so the dwelling break does not reach them.
Voltage drop is a separate question from code minimums; a long run to a detached garage or shop can push the size up a step or two. Aluminum keeps cost down on longer pulls if the lugs and breaker terminals are listed for it.
Mistakes And Limits That Change The Final Size
- Sizing from the breaker. A 100 amp breaker says nothing about whether the conductors serve a whole dwelling or just a subpanel.
- Treating a feeder as a service. The dwelling allowance covers service conductors only, never the feeder leaving the main panel.
- Reading the wrong temperature column. The cited guidance works from the 75°C column, not the 90°C column.
- Skipping the grounding conductor.
The allowance assumes no adjustment or correction factors apply to the run, and meter base limits, service equipment ratings, and local utility rules can push the final size either way. Your local authority has the last word, so a size that passes one town over can be rejected in yours.
Pre-cut lengths matched to your own math save a second supply-house trip. Our tested picks for 100 amp service entrance cable lay out type, gauge, and run length for each option.
Three checks before ordering: the conductors feed the entire dwelling load, no correction factors apply, and the meter lugs and breaker terminals are listed for the size and metal you chose.
FAQs
Can A 100 Amp Subpanel Feed Use 4 AWG Copper?
No. The dwelling allowance applies to conductors carrying a whole home’s load, not feeders. Inspectors flag this often because the two gauges sit side by side on a supply house shelf.
Is 2 AWG Aluminum Really Enough For A 100 Amp House?
For a qualifying dwelling service, yes. Commercial and non-dwelling services get no such allowance.
What Size Ground Wire Comes With A 100 Amp Circuit?
It is calculated separately and is the item most often left off a wire list. Match it to the overcurrent device and confirm against your local code edition.
References & Sources
- Electrical License Renewal. “NEC 310.12 Dwelling Service Conductors” Explains the 83% dwelling allowance and the 75°C column selection that yields 4 AWG copper and 2 AWG aluminum.
- Electrical License Renewal. “NEC 310.15(B)(7) Dwelling Service Conductors” Covers the earlier code location for the same dwelling service sizing rule.
- LearnMetrics. “NEC 310.12 Table” Reference table showing service conductor sizes for 100 amp through 400 amp dwelling services.
