Most homes use both 15 A and 20 A branch circuits. The difference comes down to wire size, the breaker rating and the loads the circuit is meant to serve.
Wire size and breaker
NEC 240.4(D) limits overcurrent protection for small copper conductors: 14 AWG copper to 15 A and 12 AWG copper to 20 A. Never put a 20 A breaker on 14 AWG wire, even though the 90°C ampacity in Table 310.16 is higher.
Receptacles
A 15 A circuit takes 15 A receptacles. A 20 A circuit with more than one outlet may use either 15 A or 20 A receptacles (NEC 210.21(B)(3)). The 20 A receptacle has a T-shaped slot for 20 A plugs.
Continuous loads
A continuous load (three hours or more) may use no more than 80% of the breaker rating: 12 A on a 15 A circuit and 16 A on a 20 A circuit.
Where 20 A circuits are required
- Kitchen countertop small-appliance circuits (210.11(C)(1) and 210.52(B)).
- Laundry receptacle circuit (210.11(C)(2)).
- Bathroom receptacle circuit (210.11(C)(3)).
- Garage and many outdoor receptacle circuits depend on local adoption; confirm with your inspector.
Voltage drop on long runs
A 12 A load 50 ft from the panel on 14 AWG copper drops 2 × 12.9 × 12 × 50 ÷ 4,110 = 3.8 V, which is 3.1% at 120 V. The same run on 12 AWG drops 2.4 V (2.0%), so upsizing the wire is a cheap fix on long runs.
Which should you choose?
Use 15 A with 14 AWG for general lighting and light loads. Use 20 A with 12 AWG where heavier plug-in loads are expected, such as kitchens, workshops and garages, and wherever the code requires it. The extra cost of 12 AWG is small compared with the flexibility.
Try the calculators
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