Wire Size & Ampacity Chart (NEC 310.16)
NEC 310.16 · Ampacity · Updated 2026-07-10
"What size wire for 100 amps?" has two honest answers, and inspectors care which one applies. The chart below is NEC Table 310.16 at the 75°C column - the column that governs almost every real termination - with the small-conductor caps of 240.4(D) noted.
Copper THHN/THWN-2 in conduit, not more than three current-carrying conductors, 30°C ambient. More conductors or hotter spaces? That's derating.
Copper ampacity chart (75°C terminations)
| Size | Ampacity (75°C) | Max breaker |
|---|---|---|
| #14 | 20 A | 15 A (240.4(D)) |
| #12 | 25 A | 20 A (240.4(D)) |
| #10 | 35 A | 30 A (240.4(D)) |
| #8 | 50 A | 50 A |
| #6 | 65 A | 65 A (70 A next-up) |
| #4 | 85 A | 85 A (90 A next-up) |
| #3 | 100 A | 100 A |
| #2 | 115 A | 115 A (125 A next-up) |
| #1 | 130 A | 130 A |
| 1/0 | 150 A | 150 A |
| 2/0 | 175 A | 175 A |
| 3/0 | 200 A | 200 A |
The 100-amp and 200-amp answers
- 100 A sub-panel or feeder: #3 copper or #1 aluminum at 75°C.
- *100 A dwelling service*** (or the feeder carrying the whole dwelling load): #4 copper or #2 aluminum - NEC 310.12 allows 83% sizing for single-family dwelling services.
- 200 A dwelling service: 2/0 copper or 4/0 aluminum (via 310.12). A 200 A feeder that isn't the whole-dwelling load needs 3/0 copper.
- Common branch circuits: 15 A → #14 Cu, 20 A → #12 Cu, 30 A dryer → #10 Cu, 50 A range → #6 Cu (per 240.4(D) and 310.16).
Why the 75°C column (and when 90°C matters)
THHN is a 90°C wire, but breaker and lug terminations are listed at 75°C (110.14(C)), so the circuit's usable ampacity is capped at the 75°C column. The 90°C column still earns its keep as the starting point for derating - you take adjustment and correction factors off the 90°C number, then cap the result at 75°C. Continuous loads add one more step: conductors (and OCPD) at 125% of the continuous current per 210.19 and 215.2.
Wire size with the citations attached. Stop second-guessing the columns. Conduit Fill & Bending Calc sizes conductors through the full NEC pipeline and exports inspector-ready PDFs. Free on the App Store. Download Conduit Fill & Bending Calc on the App Store.
Frequently asked questions
What size wire do I need for a 100 amp sub-panel?
#3 copper or #1 aluminum at 75°C terminations (NEC 310.16). The #4-copper answer you'll hear applies only to whole-dwelling services and main feeders under the 83% rule of 310.12 - not to a garage or shop sub-panel fed as a normal feeder.
What size wire for a 200 amp service?
For a single-family dwelling service: 2/0 copper or 4/0 aluminum under NEC 310.12. For a 200 A feeder that doesn't carry the entire dwelling load: 3/0 copper (200 A at 75°C in 310.16).
Why is #12 wire limited to a 20 amp breaker if it's rated 25 A?
NEC 240.4(D) caps overcurrent protection for small conductors: #14 at 15 A, #12 at 20 A, #10 at 30 A, regardless of the table ampacity. The headroom above the cap is what derating eats before it costs you a breaker size.
Can I use the 90°C column for THHN?
Only as the starting number for derating calculations. The final ampacity can't exceed the 75°C column (or 60°C for some older or NM-cable terminations) because that's what the terminations are listed for - NEC 110.14(C).