What Size Wire for a 100 Amp Sub Panel?
A 100 A feeder is a standard size, but the conductor that carries it depends on three things people usually forget: the termination temperature rating, the ambient temperature, and how many current-carrying conductors share the raceway.
Quick answer
| Material | Minimum size | Ampacity |
|---|---|---|
| Copper | 3 AWG/kcmil | 100 A |
| Aluminum | 1 AWG/kcmil | 100 A |
Computed by this site's calculation engine for a 100 A load under 75°C terminations, 30°C ambient, three current-carrying conductors in the raceway. Table values are pending review against the current NEC edition — confirm with the wire size calculator before ordering cable.
Why there is no single answer
The conductor has to satisfy two separate limits. First, its corrected ampacity must cover the load. Second, the termination it lands on caps how much of that ampacity you may actually use.
Because terminations on equipment rated 100 A or less are commonly limited to the 60°C column unless listed otherwise, the conductor often ends up larger than the ampacity table alone would suggest.
Work through it in order
Run the steps in this order every time:
- Start from the load: a 100 A feeder with no continuous load is 100 A; add 25% for the continuous portion.
- Pick an insulation temperature rating (THHN is typically 90°C, but you seldom get to use the 90°C column).
- Apply the ambient correction factor for the actual temperature where the raceway runs.
- Apply the conductor-count adjustment if more than three current-carrying conductors share the raceway.
- Take the lower of the corrected ampacity and the termination-limited ampacity.
- Choose the smallest standard size that still covers the load, then check voltage drop on the actual run length.
Do not forget voltage drop
A sub panel feeder is often long. Even a conductor that passes the ampacity check can exceed the recommended 3% branch-circuit voltage drop, which pushes you one or two sizes up.
Check the run with the voltage drop calculator before you buy cable — upsizing after the fact is expensive.
Frequently asked
- Does the sub panel need a grounding conductor?
- A feeder to a separate structure is normally a four-wire feeder: two ungrounded conductors, one grounded conductor and one equipment grounding conductor. Grounding and bonding rules differ for separate structures, so confirm the arrangement with your local inspector.
- Can I use aluminum for a 100 A feeder?
- Aluminum is commonly used for feeders and terminations are listed for it, but aluminum carries less current than copper at the same size, so the required size is larger. Run the calculator for both and compare.
- Is 100 A the breaker size or the calculated load?
- They are different. Size the conductor for the calculated load and the overcurrent device for the circuit. A standard 100 A device is a common choice, but the conductor still has to carry the load after corrections.
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Educational information only — not professional engineering advice. Reference table values are pending review; see the methodology page.