Wire Ampacity Calculator
What copper or aluminum can a load ride on? Smallest conductor rated for your amps at the right termination temp.
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Wire ampacity is the maximum current a conductor can carry continuously without exceeding its insulation's temperature rating (NEC Table 310.16). Size a wire by choosing the column that matches your equipment's terminal rating — 60°C, 75°C, or 90°C per 110.14(C) — then the smallest conductor whose ampacity meets the load. For continuous loads, size at 125% (load ≤ 80% of ampacity).
The wire ampacity formula
Required conductor ampacity ≥ 125% × continuous load + 100% × noncontinuous load. Adjusted ampacity = Table 310.16 ampacity × ambient-temperature correction × conductor-bundling adjustment (NEC 310.15(B) and (C)). Use only the column allowed by the terminal temperature rating (110.14(C)).
Worked example
Enter a 50 A load on 75°C-rated terminals: Fieldwatt returns #6 AWG copper, rated 65 A in Table 310.16. Its 80% continuous rating is 65 × 0.8 = 52 A, so #6 copper carries a 50 A continuous load with headroom. In aluminum you'd step up to #4 AWG (65 A at 75°C), since #6 aluminum is rated only 50 A.
NEC Table 310.16 ampacity (amps) for common conductor sizes
| Size | Cu 60°C | Cu 75°C | Cu 90°C | Al 75°C |
|---|---|---|---|---|
| #14 | 15 | 20 | 25 | — |
| #12 | 20 | 25 | 30 | 20 |
| #10 | 30 | 35 | 40 | 30 |
| #8 | 40 | 50 | 55 | 40 |
| #6 | 55 | 65 | 75 | 50 |
| #4 | 70 | 85 | 95 | 65 |
| #2 | 95 | 115 | 130 | 90 |
| 1/0 | 125 | 150 | 170 | 120 |
| 2/0 | 145 | 175 | 195 | 135 |
| 3/0 | 165 | 200 | 225 | 155 |
| 4/0 | 195 | 230 | 260 | 180 |
NEC references
- NEC Table 310.16 — allowable ampacities of insulated conductors at 60/75/90°C
- NEC 110.14(C) — the terminal temperature rating limits which ampacity column you may use
- NEC 240.4(D) — small-conductor overcurrent limits: #14 = 15 A, #12 = 20 A, #10 = 30 A
- NEC 310.15(B) and (C) — ambient-temperature correction and bundling adjustment factors
How to size a wire by ampacity
- Determine the load in amps; use 125% of any continuous (3-hour) load.
- Find your equipment's terminal temperature rating — 60°C, 75°C, or 90°C (110.14(C)).
- In that column of NEC Table 310.16, pick the smallest conductor whose ampacity meets or exceeds the load.
- Apply ambient and conduit-fill adjustment factors (310.15) when it's above 86°F or more than three conductors share the raceway, and respect the 240.4(D) limits on #14–#10.
Frequently asked questions
What size wire do I need for a 50 amp circuit?
For a 50 A load on 75°C terminations, #6 AWG copper (rated 65 A in NEC Table 310.16) or #4 AWG aluminum (65 A) is the smallest conductor that qualifies. At 60°C terminations you stay on #6 copper as well. Always confirm the breaker and terminal ratings before sizing.
What is the 80% rule for wire ampacity?
A conductor supplying a continuous load must be rated for at least 125% of that load, which is the same as loading it to no more than 80% of its ampacity (NEC 210.19(A) and 215.2(A)). So a 65 A conductor is good for a 52 A continuous load.
Why are there 60, 75, and 90°C columns in the ampacity table?
Each column is the conductor's ampacity at that insulation temperature. NEC 110.14(C) limits you to the column matching the lowest-rated terminal in the circuit — usually 75°C for equipment over 100 A and often 60°C for 100 A and under — even if the wire's insulation is rated 90°C.
Can I use the 90°C column to size my wire?
Only for derating. The 90°C ampacity is the starting point for ambient and bundling adjustments (310.15), but the final conductor still can't exceed the ampacity of its terminal's temperature rating (110.14(C)), which is typically 60°C or 75°C.
Built for the field
- Live results as you type — every value recomputes instantly.
- Works fully offline once the app has loaded.
- Cites the governing NEC table for every result.
- Save calculations to a job and build a material list (Pro).
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For guidance only. Always verify against the current National Electrical Code and your local amendments. Fieldwatt does not replace an engineer of record.