How it works
NEC Table 310.15(B)(16) lists the allowable ampacity of each wire size for insulation rated at 60°C, 75°C, and 90°C. The temperature column to use is set by the lowest-rated part of the circuit, including the equipment terminals.
Continuous loads (three hours or more) are sized at 125% of the load. Ambient temperatures above 30°C and more than three current-carrying conductors in a raceway reduce the table ampacity; enter the combined factor and the calculator raises the required table ampacity by the same amount.
Worked example
A 40 A non-continuous load on copper wire in the 75°C column with no derating:
- Required ampacity = 40 A ÷ 1 = 40 A
- Smallest copper size at 75°C with at least 40 A: 8 AWG (50 A)
- Wire size: 8 AWG
| Input | Value |
|---|---|
| Load current | 40 A |
| Load type | Non-continuous (100%) |
| Conductor | Copper |
| Temperature column | 75°C column |
| Correction and adjustment factor | 1 × |
| Result | Value |
|---|---|
| Smallest wire size | 8 AWG |
| Ampacity required from the table | 40 A |
| Table ampacity of that size | 50 A |
NEC Table 310.15(B)(16) allowable ampacity (amps)
| AWG | Cu 60°C | Cu 75°C | Cu 90°C | Al 60°C | Al 75°C | Al 90°C |
|---|---|---|---|---|---|---|
| 14 | 15 | 20 | 25 | — | — | — |
| 12 | 20 | 25 | 30 | 15 | 20 | 25 |
| 10 | 30 | 35 | 40 | 25 | 30 | 35 |
| 8 | 40 | 50 | 55 | 35 | 40 | 45 |
| 6 | 55 | 65 | 75 | 40 | 50 | 55 |
| 4 | 70 | 85 | 95 | 55 | 65 | 75 |
| 3 | 85 | 100 | 115 | 65 | 75 | 85 |
| 2 | 95 | 115 | 130 | 75 | 90 | 100 |
| 1 | 110 | 130 | 145 | 85 | 100 | 115 |
| 1/0 | 125 | 150 | 170 | 100 | 120 | 135 |
| 2/0 | 145 | 175 | 195 | 115 | 135 | 150 |
| 3/0 | 165 | 200 | 225 | 130 | 155 | 175 |
| 4/0 | 195 | 230 | 260 | 150 | 180 | 205 |
Show the nec table 310.15(b)(16) allowable ampacity (amps) table
| AWG | Cu 60°C | Cu 75°C | Cu 90°C | Al 60°C | Al 75°C | Al 90°C |
|---|---|---|---|---|---|---|
| 14 | 15 | 20 | 25 | — | — | — |
| 12 | 20 | 25 | 30 | 15 | 20 | 25 |
| 10 | 30 | 35 | 40 | 25 | 30 | 35 |
| 8 | 40 | 50 | 55 | 35 | 40 | 45 |
| 6 | 55 | 65 | 75 | 40 | 50 | 55 |
| 4 | 70 | 85 | 95 | 55 | 65 | 75 |
| 3 | 85 | 100 | 115 | 65 | 75 | 85 |
| 2 | 95 | 115 | 130 | 75 | 90 | 100 |
| 1 | 110 | 130 | 145 | 85 | 100 | 115 |
| 1/0 | 125 | 150 | 170 | 100 | 120 | 135 |
| 2/0 | 145 | 175 | 195 | 115 | 135 | 150 |
| 3/0 | 165 | 200 | 225 | 130 | 155 | 175 |
| 4/0 | 195 | 230 | 260 | 150 | 180 | 205 |
Not more than three current-carrying conductors, 30°C (86°F) ambient. Check the edition your jurisdiction has adopted.
Assumptions and limits
- Values are transcribed from the table as printed in the 2011 NEC. Check the edition your jurisdiction uses and the terminal ratings on your equipment.
- Voltage drop, short-circuit rating, motor and other special rules, and cable-type limits are not evaluated. See the voltage drop calculator for drop.
- Sizes above 4/0 AWG are not listed here.
Common questions
What size wire do I need for 40 amps?
In the 75°C column, 8 AWG copper (50 A) carries 40 A; in the 60°C column you need 8 AWG copper (40 A) exactly. Check terminal ratings and the local code.
Which temperature column should I use?
The lowest temperature rating among the wire, its insulation, and the terminals at both ends. Many devices under 100 A are limited to the 60°C column unless listed otherwise.
What is the 125% rule?
For a continuous load, the conductor and overcurrent device are sized for 125% of the load. Multiply the load by 1.25 first.
Does this include voltage drop?
No. A long run can need a larger wire even if the ampacity is enough; use the voltage drop calculator.
Sources
- NEC Table 310.15(B)(16) (formerly 310.16), as printed in the 2011 edition; NEC 210.19(A)(1) and 215.2(A)(1) for the 125% continuous-load rule; NEC 240.4(D) for small conductors.
- Read them: NEC Table 310.15(B)(16) as printed (2011 NEC, PDF)
Updated 2026-09-30