Minimum size by temperature column
| Conductor | 60°C | 75°C | 90°C |
|---|---|---|---|
| Copper | 14 AWG15 A | 14 AWG20 A | 14 AWG25 A |
| Aluminum | 12 AWG15 A | 12 AWG20 A | 12 AWG25 A |
Equipment grounding conductor
A circuit protected at 15 A takes a 14 AWG copper or 12 AWG aluminum equipment grounding conductor, from the 15 A row of NEC Table 250.122. Raise the ground with the ungrounded conductors when you increase them for voltage drop or any reason other than 310.15(B) or (C) derating, because 250.122(B) requires the same proportion. The ground is never required to be larger than the circuit conductors.
Longest run at 3% voltage drop
| Conductor | 120 V | 240 V |
|---|---|---|
| 14 AWG copper | 35 ft | 75 ft |
| 12 AWG aluminum | 35 ft | 70 ft |
NEC 210.19(A)(1) Informational Note 4 recommends keeping branch-circuit drop at or below 3%. These lengths come from the Table 8 resistance at 75°C for a stranded conductor at unity power factor, rounded to the nearest 5 ft. Reactance is not included, so a large conductor in a steel raceway falls short of the figure. A three-phase run reaches about 15% farther than these single-phase figures.
Small-conductor limit under 240.4(D)
NEC 240.4(D) caps overcurrent protection at 15 A for 14 AWG copper, 20 A for 12 AWG copper, 30 A for 10 AWG copper, 15 A for 12 AWG aluminum, and 25 A for 10 AWG aluminum. On a 15 A circuit that cap removes a size the ampacity table would otherwise allow.
- 16 AWG copper carries 18 A at 90°C, but cannot be protected above 10 A.
The caps apply after ambient correction and conductor adjustment, so derating the load does not raise them.
What a 15 A circuit feeds
- General lighting and receptacle branch circuits
- Bedroom, hallway, and living area outlets
Fifteen amps is the standard 120 V lighting and receptacle circuit and the smallest rating the code recognizes. The receptacle is a NEMA 5-15R, and 14 AWG copper is both the size that reaches the rating at 60°C and the largest conductor 240.4(D)(3) allows on a 15 A breaker. Nothing prevents 12 AWG copper on a 15 A breaker; the cap only forbids the reverse.
Treat the circuit as 12 A when a load runs for three hours or more, because 210.20(A) sizes the overcurrent device at 125% of a continuous load. Dwelling circuits at this rating also need arc-fault protection under 210.12, and the locations listed in 210.8(A) need ground-fault protection, which is what catches a 15 A circuit extended into a bathroom or a garage.
This is the rating that knob-and-tube wiring was built on, and a 15 A circuit cannot serve a modern kitchen countertop: 210.11(C)(1) requires two or more 20 A small-appliance branch circuits for those receptacles. Running a new 20 A circuit is usually less work than extending an old 15 A one, and it is the reason kitchen remodels start at the panel.
15 A wire size questions
- What size wire do I need for a 15 amp circuit?
- Use 14 AWG copper or 12 AWG aluminum at the 60°C column, which sets the default under 110.14(C)(1)(a) for equipment rated 100 A or less, and 334.80 and 340.80 hold NM-B and UF-B cable to that column regardless of the terminals. With 75°C terminations, 14 AWG copper or 12 AWG aluminum carries the circuit.
- What size ground wire for a 15 amp circuit?
- 14 AWG copper or 12 AWG aluminum, taken from the 15 A row of NEC Table 250.122.
- Can a higher temperature column let me use a smaller conductor on a 15 A circuit?
- No. 16 AWG copper reaches 15 A in the 90°C column but NEC 240.4(D) caps its overcurrent protection at 10 A, so the 90°C column cannot carry this circuit on a smaller conductor than the 60°C column does.
- What size aluminum wire for 15 amps?
- 12 AWG aluminum at the 60°C column carries 15 A. That equals the circuit rating, so any ambient correction, bundling adjustment, or voltage drop pushes the size up.