Ampacity Calculator
Size your conductors right the first time. NEC Table 310.16 with temperature correction, conductor count derating, and terminal limits—all in one calculation.
Switch to CEC1Load Information
Enter your load in amps. Continuous loads (3+ hours) are multiplied by 125% per NEC 210.20(A).
Loads running 3+ hours
Intermittent loads
2Conductor Properties
Common insulation types
3Installation Conditions
Standard: 30°C (86°F)
Current-carrying only (not EGC)
4Terminal Temperature
Per NEC 110.14(C), equipment terminals limit final ampacity. Default: 60°C for ≤100A, 75°C for >100A.
Enter load information to calculate ampacity
How We Calculate Ampacity
This calculator applies derating factors in the correct order per NEC methodology. We start with Table 310.16 ampacity, apply 310.15(B)(1) ambient temperature correction and 310.15(C)(1) conductor count adjustment, then verify against 110.14(C) terminal limits.
Key NEC references:
- 210.20(A) — 125% continuous load factor
- 310.16 — Base ampacity values at 30°C ambient
- 310.15(B)(1) — Ambient temperature correction factors
- 310.15(C)(1) — Conductor count adjustment (4+ conductors)
- 110.14(C) — Terminal temperature limitations
Ampacity: What Every Electrician Should Know
What's the difference between ampacity and load?
Load is the actual current your circuit will carry. Ampacity is the maximum current a conductor can safely carry continuously.
Your conductor's ampacity must exceed your load—plus a 25% margin for continuous loads. For example, a 40A continuous load requires a conductor with at least 50A ampacity (40 × 1.25 = 50A).
Why does the 125% continuous load rule exist?
The 125% factor from NEC 210.20(A) accounts for heat buildup over time. When current flows continuously for 3+ hours, conductors and terminations heat up significantly.
The extra 25% ensures the conductor doesn't operate at its thermal limit—extending its life and preventing insulation breakdown. Some breakers are rated for 100% continuous duty, but the conductor still needs the margin.
What's the terminal temperature rule (110.14(C))?
This is the rule that trips up a lot of electricians. Even if you use 90°C rated wire like THHN, your final ampacity is limited by the equipment terminals.
Typically: 60°C for circuits 100A and under, 75°C for larger circuits. You can use the 90°C column for derating calculations, but the final result can't exceed what the 60°C or 75°C column allows for that wire size.
When do I need to derate for conductor count?
When you have more than 3 current-carrying conductors in a raceway or cable, you must derate per NEC 310.15(C)(1):
- 4-6 conductors: 80%
- 7-9 conductors: 70%
- 10-20 conductors: 50%
- 21-30 conductors: 45%
- 31-40 conductors: 40%
- 41+ conductors: 35%
Equipment grounding conductors and neutrals carrying only unbalanced current don't count toward this total.
How does ambient temperature affect ampacity?
Ampacity values in Table 310.16 assume 30°C (86°F) ambient temperature. Higher temperatures reduce the conductor's ability to dissipate heat, so you must apply correction factors from Table 310.15(B)(1).
At 40°C ambient with 90°C insulation, you only get 91% of the table ampacity. The calculator handles this automatically—just enter your actual ambient temperature.
What about rooftop installations?
Per NEC 310.15(B)(2), conduits on rooftops exposed to sunlight require an additional temperature adder before applying the ambient correction factor:
- Directly on roof: +33°C (60°F)
- 0-13mm above: +33°C
- 13-90mm above: +22°C
- 90-300mm above: +17°C
- 300-900mm above: +14°C
This can significantly impact conductor sizing for rooftop equipment.
Do I multiply or add the derating factors?
Multiply them. Per NEC 310.15(A)(2), when you have multiple conditions requiring adjustment (like high ambient temperature AND more than 3 conductors), you multiply the factors together.
For example: 91% ambient factor × 80% conductor count factor = 72.8% combined. This combined factor is applied to the table ampacity.
Why use 90°C wire if terminals are limited to 75°C?
Here's the practical advantage: you can start with the higher 90°C ampacity for derating calculations. If multiple factors reduce your ampacity significantly, you might still end up with adequate capacity that wouldn't work starting from the 75°C column.
The NEC allows this approach per 110.14(C)(1)(a)(4)—as long as your final derated ampacity doesn't exceed the 75°C (or 60°C) column value.