Cable sizing according to UL / NEC
How to choose the AWG size of a copper cable and its protection for panels and installations in the USA and Canada, according to the NEC (NFPA 70) and UL standards.
Standards this guide follows
UL / NEC. NFPA 70, National Electrical Code (NEC): ampacity table 310.16, correction factors 310.15 and overcurrent protection from article 240. UL 489: circuit breakers for branch-circuit protection. UL 508A: industrial control panels.
Values for copper conductors. National regulations may be stricter (in Spain the REBT, in Germany DIN VDE 0298-4, in Italy CEI 64-8, in the USA the local authority having jurisdiction). This guide is for preliminary sizing: always check the calculation against the standard and regulations that apply to your installation.
Installation in Europe? Use the cable sizing guide according to IEC, with calculator.
What changes in North America
In the USA (and, with its own code, in Canada) installations follow the NEC (NFPA 70), conductor sizes are given in AWG and equipment must be UL listed. These are the differences that matter most when designing a panel for that market:
- Cable type. The most common are THHN/THWN-2 (90 °C) in raceways and MTW (machine tool wire) inside panels and machines.
- Temperature column. Even with 90 °C cable, the ampacity is limited by the temperature rating of the equipment terminals (NEC 110.14(C)): up to 100 A or sizes 14 to 1 AWG, use the 60 °C column unless the terminals are marked for 75 °C; above 100 A, the 75 °C column. The 90 °C column is the starting point for applying correction factors.
- Where it is installed. Table 310.16 applies to not more than three current-carrying conductors in raceway, cable or earth. Conductors in free air use table 310.17, and cable trays follow article 392.
- Upstream protection. Branch-circuit protection requires a circuit breaker listed to UL 489. Small UL 1077 devices (supplementary protectors) cannot be used as branch-circuit protection, even though they look like an IEC miniature circuit breaker.
- Small conductors. Even if the table gives more, the overcurrent protection may not exceed 15 A for 14 AWG, 20 A for 12 AWG and 30 A for 10 AWG (NEC 240.4(D)).
- Continuous loads. If the load runs for three hours or more, the conductor and the protection are sized at 125 % of its current (NEC 210.19 and 210.20).
NEC table of AWG sizes
Ampacity in amps, copper conductors, not more than three current-carrying conductors, 30 °C (NEC table 310.16):
NEC table 310.16 · copper · 30 °C
| AWG | mm² | IEC equivalent (mm²) | 60 °C | 75 °C | 90 °C | Maximum protection (240.4(D)) |
|---|---|---|---|---|---|---|
| 14 | 2.08 | 2.5 | 15 | 20 | 25 | 15 A |
| 12 | 3.31 | 4 | 20 | 25 | 30 | 20 A |
| 10 | 5.26 | 6 | 30 | 35 | 40 | 30 A |
| 8 | 8.37 | 10 | 40 | 50 | 55 | – |
| 6 | 13.3 | 16 | 55 | 65 | 75 | – |
| 4 | 21.2 | 25 | 70 | 85 | 95 | – |
| 3 | 26.7 | 35 | 85 | 100 | 115 | – |
| 2 | 33.6 | 35 | 95 | 115 | 130 | – |
| 1 | 42.4 | 50 | 110 | 130 | 145 | – |
| 1/0 | 53.5 | 70 | 125 | 150 | 170 | – |
| 2/0 | 67.4 | 70 | 145 | 175 | 195 | – |
| 3/0 | 85.0 | 95 | 165 | 200 | 225 | – |
| 4/0 | 107.2 | 120 | 195 | 230 | 260 | – |
The “IEC equivalent” column gives the equal or next larger standard metric size; it is a guide, not a reason to replace a listed cable with another.
NEC correction factors
Ambient temperature (table 310.15(B)(1))
| Ambient (°C) | 60 °C | 75 °C | 90 °C |
|---|---|---|---|
| 21–25 | 1.08 | 1.05 | 1.04 |
| 26–30 | 1.00 | 1.00 | 1.00 |
| 31–35 | 0.91 | 0.94 | 0.96 |
| 36–40 | 0.82 | 0.88 | 0.91 |
| 41–45 | 0.71 | 0.82 | 0.87 |
| 46–50 | 0.58 | 0.75 | 0.82 |
More than 3 current-carrying conductors in the same raceway or cable (310.15(C)(1))
| Current-carrying conductors | Factor |
|---|---|
| 4–6 | 80 % |
| 7–9 | 70 % |
| 10–20 | 50 % |
| 21–30 | 45 % |
| 31–40 | 40 % |
| 41+ | 35 % |
Standard overcurrent device ratings (NEC 240.6(A)): 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 110, 125, 150, 175 and 200 A. If the conductor ampacity does not match a standard rating, the NEC allows the next higher rating up to 800 A (240.4(B)). For voltage drop, the NEC recommends (in an informational note) not exceeding 3 % in the branch circuit and 5 % overall.
Common questions
Can I use an IEC circuit breaker in a panel for the USA?
Only if it is listed to UL 489 for that application. Devices with only UL 1077 recognition are supplementary protectors and are not valid as branch-circuit protection.
How many mm² is 12 AWG?
3.31 mm². The nearest larger standard metric size is 4 mm².
Links
- NFPA 70 (NEC) · National Electrical Code (official NFPA page)
- Cable sizing according to IEC · FaseCAD guide
- kW to amps calculator · FaseCAD tool