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

Graphic depicting a portable generator connected to a home through an inlet box and manual transfer switch

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. Very large portable generators like the Generac GP17500E watt may have multiple 30-Amp and 50-Amp receptacles. If that is the case, you may need more than one manual transfer switch for the home to make full use of the generator’s power capability.

Portable Generator Manual Transfer Switch Basics

Some manual 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. Another single-load option is a critical appliance like a furnace or air conditioner.

Other transfer switches have the sub-panel built into them. The concept is the same. When the power goes out, the generator supplies power through the manual transfer switch to the circuits selected at installation.

Yet another possibility is a manual transfer switch at the service entrance that allows a portable generator to feed the main service panel. Generally, unless the homeowner is a certified electrician, local building departments may not permit the homeowner to install a whole house transfer switch without the help of a certified or licensed electrician.

Transfer Switch Importance in Portable and Standby Generator Systems

Many Portable Generators have a 30-Amp or 50-Amp 120/240-Volt Outlet, or both. These outlets require a 120/240-Volt Plug on a Generator Cord for connection to the inlet box. It’s a good idea to pair the largest outlet on the generator to the inlet box and transfer switch. For example, install a 50-Amp transfer switch and inlet box if your generator has a 50-Amp outlet and use a 50-Amp Generator Cord.

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.

A Transfer Switch for a Home Generator

Transfer Switch Wire Size

Manual Transfer Switch Installed

Manual Transfer Switch Installation in a garage next to the main breaker panel.

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 (ampacity.) 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.

 

Wire Size for Main Panel to Transfer Switch Wiring

  • 20-amp breaker: 12-gauge wire
  • 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 Wire Size

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.

FEMA Recommends a Home Backup Generator

Generator Wire Size by Circuit Breaker

  • 15-amp breaker: 14-gauge wire
  • 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 30-Amp or 50-Amp Receptacle. The wires from the inlet box to the transfer switch much meet or exceed the minimum wire size for a 30-Amp or 50-Amp breaker depending on the receptacle rating. If the inlet is rated at 50-Amps, you must use wire rated for 50-amps.  For example, even if your generator supplies less than 30 amperes, safety and NEC codes still require the use of 10-gauge wire or larger wire for a 30-amp inlet box.

Note: A 50-amp outlet on a generator will have a 50-amp breaker. All wiring between the generator 50-amp outlet and the transfer switch must be rated for 50 amps and no less, even if the load connected to the transfer switch is less than 50 amps.

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

How to Use a Portable Generator for Power During an Outage

Wire Size for a 240-Volt Circuit

In the USA and North America, a 240-Volt Circuit uses two 120-Volt supplies to make 240 volts. Each supply wire may carry the maximum rated capacity of the circuit breaker. 

The 240-volt circuit supply is a double circuit breaker. A 30-Amp Double Circuit Breaker has two breakers, each rated at 30-amps, tied together so if one trips, they both trip. The wire size for a 30 Amp 240 volt circuit is 10-gauge or larger because each wire and each circuit breaker may carry up to 30 amps. 50 Amp 240 Volt Wire Size is 6-gauge or larger. Other wire sizes follow the same rules.

Portable Generator Safety Rules

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 may 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 September 29, 2020