Planning a Manual Transfer Switch Installation: Materials

Make a list of the materials you intend to use for your manual transfer switch installation. You will need the list to apply for the building permit, and having a list will limit returning to the store for more parts while performing the installation. Read Planning A Manual Transfer Switch Installation if you haven’t done so already.

Transfer Switch Choices

The manual transfer switch that you select will depend on several factors. If some of the circuits you plan to operate during an outage use special circuit breakers like Ground Fault Circuit Interrupter (GFCI), Arc Fault Circuit Interrupter (AFCI) or Surge Protecting Circuit Breakers (SPCB), select a transfer switch that is compatible with those types of breakers.

Transfer switches handle neutral wiring in two ways. Some switch the neutral wire between the generator supply and the utility supply, and some do not. If GFCI, AFCI or PSCB breakers are used, a switched neutral transfer switch will simplify the wiring for those breakers.

Transfer Switch Circuit Breaker

The manual transfer switch installation guide will usually specify the current rating of the transfer switch circuit breaker installed in the main service panel—usually a double-pole, 240-volt breaker. This breaker protects the wiring between switch and the main panel from overloads. The current rating of this breaker is matched to the current-carrying capacity of the wires between the switch and main panel.

Utility Supply Wires

National Electrical Code (NEC) rules specify that wires that carry current must be protected by a circuit breaker matched to the current-carrying capacity of the wires. It is acceptable for the wire ampacity to exceed the breaker current rating, but the breaker current rating must never exceed the wire current-carrying capacity.

The larger the wire gauge, the smaller the wire and lower the current carrying capacity.

For example, 10-gauge copper wire may carry up to the 30 amperes and must be protected by a 30-ampere or smaller circuit breaker. A 12-gauge wire may only carry 20 amperes. You can use 10-gauge wire with a 20-ampere breaker, but never use 12-gauge wire with a 30-ampere breaker.

Common transfer switch breaker and copper wires sizes include:

30-amp breaker: 10-gauge wire

50-amp breaker: 6-gauge wire

60-amp breaker: 6-gauge wire

100-amp breaker: 2-gauge wire

Generator Supply Wires and Sizes

The wires that connect the inlet box to the transfer switch are protected by a circuit breaker on the generator, and must follow the same rules for the wires that connect the transfer switch to the service panel. The circuit breaker also protects the generator from overloads and will be sized accordingly to the maximum output the generator can handle continuously.

Circuit Breaker Wire Gauge (copper)

20-amp breaker: 12-gauge wire

30-amp breaker: 10-gauge wire

50-amp breaker: 6-gauge wire

60-amp breaker: 6-gauge wire

Note: Most inlet boxes have a receptacle rated at 30 amp, which would allow connection to a generator that supplies 30 amps. Even if your generator supplies less than 30 amperes, safety and NEC codes still require the use of 10 AWG wire or larger wire.

Note: Branch circuit wires follow the same rules. The smallest size wire allowed is 14 gauge wire with a maximum circuit breaker size of 15 amps.

Conduit

PVC Conduit is easy to install and suitable for use in most single-family homes less than three stories high.

Electrical codes limit the number of wires inside a conduit, depending on the wire gauge and conduit diameter. The wires may only use up to 30 percent of the space inside the conduit. For example: The wires between the transfer switch and main distribution panel include four 6-gauge wires, four 12-gauge wires, four 15-gauge wires. The minimum PVC conduit size will be 1 1/4 inches.

To run four, 10 gauge wires from the inlet box to the transfer switch requires 3/4-inch conduit, but four 6-gauge wires requires a minimum of 1-inch conduit.

Each installation is different. Consult the NEC conduit fill charts in a wiring guide to help you choose the correct size conduit.

Continue with Manual Transfer Switch Insallation

*To help make sense of your power situation, consult a licensed electrician before beginning any work.

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