Quick check: what size copper NM-B do I need for a standard 20A receptacle circuit, and for a 15A lighting circuit?

20A receptacle circuit: Use 12 AWG copper NM-B (minimum). NEC 240.4(D)(3) requires 12 AWG for 20A overcurrent protection.

15A lighting circuit: Use 14 AWG copper NM-B (minimum). NEC 240.4(D)(3) allows 14 AWG for 15A protection.

Both assume standard residential 120V, 60°C terminations per 110.14(C). NM-B cable sheath must remain intact and properly secured. If any termination is rated only 60°C, 12 AWG on 20A is still compliant since 12 AWG exceeds 20A ampacity at 60°C (20A vs. 20A minimum).

Important: If your circuit includes any 14 AWG wire, you must use a 15A breaker — never mix 12 and 14 AWG on the same circuit.

For new installations, always check local amendments — some jurisdictions require AFCI protection (NEC 210.12) for bedrooms and living areas.

20A receptacle circuit: 12 AWG copper NM-B (NEC 210.19(A)(1), 240.4(D)(5)). Breaker must be 20A. 14 AWG is not permitted on a 20A breaker.

15A lighting circuit: 14 AWG copper NM-B (NEC 210.19(A)(1), 240.4(D)(3)). Breaker must be 15A.

Critical reminders:

• Derating: If you run more than 3 current-carrying conductors in a raceway/cable bundle >24", apply 310.15(C)(1) adjustment factors (may force 12 AWG for 15A, or 10 AWG for 20A).
• Voltage Drop: For runs >100 ft (20A) or >130 ft (15A) at 120V, upsize one gauge (10 AWG / 12 AWG) to keep drop ≤3% (NEC 210.19(A) FPN No. 4).
• GFCI/AFCI: 20A receptacles in kitchens, baths, garages, outdoors, basements require GFCI (210.8). Most dwelling areas require AFCI (210.12).
• NM-B Rating: Use 90°C column for derating calculations, but final ampacity limited to 60°C column (334.80) — 14 AWG = 15A, 12 AWG = 20A, 10 AWG = 30A.