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.

A portable generator connected to an inlet box.

A manual transfer switch is the safest way to power your home with a portable generator

Transfer Switch Choices

The manual transfer switch that you select will depend on several factors. The primary consideration is the amount of power the portable generator can supply and the available power outlets on the generator. Typically, a portable generator that supplies a house will have a single 240-volt receptacle rated at 50-amperes or 30-amperes.

Some transfer switches only supply a single load. These control a sub-panel that distributes power to selected circuits by switching the sub-panel from utility power fed from the main service panel to portable generator power from the inlet box.

Other transfer switches have the sub-panel built into them. The concept is almost the same however. Power fed from the main service panel is switched to power from the generator.

Yet another possibility is a manual transfer switch at the service entrance that allows a portable generator to feed the main service panel.

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 transfer switch and main panel.

Utility Supply Wires

Manual Transfer Switch Installed

Manual Transfer Switch Installed in Garage

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, and 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 Installation

*To avoid wiring errors, consult a licensed electrician before beginning any work.

Updated June 22, 2017

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