Can 12-Gauge Wire Be Spliced With 10-Gauge in a House?

Wire Ampacity

• Ampacity is the current-carrying ability of a wire. A 12-gauge copper wire may carry up to 20 amperes and a 10-gauge copper wire may carry up to 30 amperes, according to the National Electrical Code table 310-16 for sizing wires in house wiring.

  A circuit that will carry up to 20 amperes of current is correctly wired with either 10-gauge or 12-gauge copper wire, since both wire sizes have an ampacity of at least 20 amperes.

Circuit Breaker Sizing

• The ampacity of the wires used in any circuit determines the circuit breaker or fuse size. The circuit breaker or fuse rating must not exceed the wire ampacity of any wire used in the circuit.

  Circuit breakers and fuses protect the wiring from overheating caused by excessive electrical current. The circuit breaker or fuse for a circuit wired with 12-gauge wire must be rated at 20 amperes or less, since 20 amperes is the ampacity of the 12-gauge wire.

Mixing Wire Gauges

• Splicing 12-gauge wire to 10-gauge is allowed if the circuit breaker or fuse is sized at 20 amperes or less. If the circuit breaker is 25 or 30 amperes, only 10-gauge or larger wire is allowed.

  This is true even if the 12-gauge wire will only carry 20 amperes or less on that portion of the circuit. A partial short along the 12-gauge wire could cause the current flow to exceed 20 amperes, which results in the wire overheating if the breaker or fuse is greater than 20 amperes.

Best Practice

• Even though the electrical codes allow for mixing wire gauges if the circuit breaker or fuse is sized correctly, it is a poor practice to follow.

  Someone making modifications to the wiring in the future might not realize there is 12-gauge wire on the circuit. If they increase the circuit breaker to 25 or 30 amperes, the danger exists for the 12-gauge wire to overheat and cause a fire.

  Because of this danger, local building inspectors may fail any inspection that finds different wire sizes spliced together.