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Generators and Transfer Switches: A Comprehensive Guide

Generators and Transfer Switches: A Comprehensive Guide

A transfer switch is one of the most vital components of any home-backup generator setup. It allows you to safely connect your generator to your home's electrical system, so you can breeze through power outages with minimal stress. In this article, we'll explain the various types of transfer switches, discuss how to select a transfer switch, and address installing and using a transfer switch with your generator.

Types of transfer switches

To begin with, a transfer switch is a permanently-installed device that allows you to safely connect your generator to your home's electrical system. It isolates your generator from the power grid, so that your home is only ever energized by one power source at a time - either the generator or the grid.

WARNING! Never connect your generator directly to your home's electrical system without using a transfer switch or interlock kit, as required by local electrical codes. Directly connecting your generator like this is known as "backfeeding". It is a dangerous practice that could cause an electrical fire, as well as lead to severe shocks or electrocution for any utility linemen working to restore power to the grid. Always use a transfer switch that has been installed in accordance with all local electrical codes and the National Electrical Code by a licensed professional electrician. 

Diagram showing the interconnections between the grid, generator, and transfer switch
This diagram shows how a transfer switch is used. It connects to your home's electrical panel, and is used to isolate and manually switch your home's power back and forth between the generator and the power grid. 

Transfer switches generally come in two flavors: automatic transfer switches and manual transfer switches.

Automatic transfer switches

Automatic transfer switches (abbreviated ATS) are most often used with whole-home standby generators. These large generators are permanently installed near a home, usually on a concrete pad, and usually turn themselves on automatically when the power goes out. They're often sized between 5,000 - 20,000 watts (5 - 20 kW). When power is restored, the ATS switches power from the generator back to the grid. Usually, an ATS is used to provide power to every circuit in the home. If power outages are very common in your area, it's a good idea to consider investing in an ATS and standby generator.

Manual transfer switches

Manual transfer switches are less expensive than their automatic cousins and are better suited for use with portable generators. Generally speaking, you should choose a generator rated for at least 3,600 watts to connect to a transfer switch.

When the power goes out, the generator is started up, then connected to the transfer switch. You can then manually switch the home's power - or, more precisely, the specific circuits connected to the transfer switch - from grid power to generator power. Once power is restored, you must shut down and disconnect the generator, then manually switch power back over to the grid.

For the rest of this article, we'll focus on manual transfer switches.

How to choose a transfer switch

1. Decide what you want to power.

The first step in choosing a transfer switch is understanding your energy needs. Figure out which appliances or devices you want to power in the event of a power outage. For example, it's a good idea to be able to power a refrigerator or freezer (if needed for medical supplies or food), some lights and fans, the furnace and/or air conditioner, and maybe a TV or computer. 

2. Calculate the voltage and wattage you need.

Once you have a complete list of the devices you want to be able to run in the event of an outage, take note of each device's voltage. Most household devices that plug into wall outlets run on standard 120-volt AC power. Some larger plug-in appliances, such as washers, dryers, and ovens, may run on 240-volt AC power. HVAC equipment (such as air conditioners and furnaces) is often hardwired into the electrical system; "hardwired" means it doesn't plug into a wall outlet. 

Determine the amount of power (in watts) each device requires. Usually, this is printed on the device's nameplate label. If it isn't, you can get a reasonable estimate by multiplying the device's rated voltage by its amperage. For example, a device rated for 120 volts and 5 amps usually requires approximately 600 watts (120 x 5). 

Certain devices, especially those with onboard motors or compressors, need extra bursts of power to start up. This includes refrigerators, air compressors, power tools, and others. Make sure to account for this in your calculations.

Estimated wattage chart for common household devices

An estimated wattage chart for common household devices. All electronics and appliances are built differently, so always check the wattage listed on your electrical devices before relying on this chart.

Once you know all your devices' voltage and wattage requirements, make sure to choose a transfer switch that is capable of handling all voltages required, as well as at least the total amount of wattage needed. For example, if all your devices together require 5000 watts, and you have a mix of 120-volt and 240-volt devices, make sure to choose a transfer switch that can be used with both 120-volt and 240-volt devices, and that is rated for at least 5000 watts.

NOTE: Some transfer switches are rated in amps, rather than watts, so double check before purchasing. It's a good idea to err on the side of caution if you have both 120-volt and 240-volt devices to power; for example, a 240-volt device drawing 5 amps uses the same amount of electrical energy as a 120-volt device drawing 10 amps. In this case, you would use the 10-amp figure in your calculation of how much amperage you need.

Also make sure that your generator is capable of providing at least the wattage you need, if not more. For more information, you may want to read our helpful article on choosing a generator.

3. Know what features to look for. 

Below are some features you may want to look for when purchasing a transfer switch. This isn't necessarily a complete list - there may be other features that are important to you. 

  • Onboard power meters. These meters will tell you, in real time, how much power is being consumed. This is important for load balancing, a concept we'll discuss more later.
  • Tie bars. Tie bars are used to convert two single-pole breakers (usually used with 120-volt circuits) to one double-pole breaker (usually used with 240-volt circuits). If you don't plan to power anything connected to a 240-volt circuit, you probably don't need this feature.
  • A weather-resistant or weatherproof inlet box. Make sure the inlet box (more on this later) used with your transfer switch is capable of handling the rigors of outdoor use. Make sure its enclosure has a NEMA 3R (weather-resistant), 4 (weatherproof), or 4X (weatherproof and corrosion resistant) rating. Most have a 3R rating. If you live in a coastal area with lots of salt air or salt spray, you may want to consider one with a 4X rating.
  • UL 1008 certification. Look for a transfer switch that has been certified to the UL 1008 standard for electrical safety.
  • A long extension cord to connect the generator and inlet box. Ideally, this will be at least 20 feet. Generators must only be used outdoors, at least 20 feet away from the home, as well as away from doors and windows, to reduce the chance of carbon monoxide (CO) entering the home. Make sure the extension cord uses the correct plug and receptacle type (usually, it's a 30-amp NEMA L14-30) to mate with your generator's outlet and the inlet box. Also make sure the cord is the right gauge to handle the loads that will be required of it.

NEMA L14-30R outlet on a WEN DF875iX generator

The NEMA L14-30R outlet on the control panel of a WEN DF875iX dual-fuel open-frame remote-start inverter generator.

Installing and using a transfer switch

Installation

A typical manual transfer switch installation involves 5 components: a generator, an extension (connection) cord, an inlet box, the transfer switch, and your home's electrical panel. 

A diagram showing the typical setup for a generator and transfer switch.

A diagram showing the typical connection of a generator to a home's electrical panel using a manual transfer switch. 

The generator (which should always be positioned at least 20 feet from the house, as well as away from doors, windows, and vents) is connected via an extension cord to the inlet box, which is permanently installed on the outside of the house, preferably close to the transfer switch and electrical panel. The inlet box is hardwired through the wall, indoors to the transfer switch. The switch itself is usually mounted on the wall near the electrical panel, and is hardwired into certain circuits in the panel.

NOTE: Some transfer switches do not require an inlet box. These transfer switches have NEMA 3R, 4 or 4X enclosures, and are mounted on an exterior wall, as close to the electrical panel as possible.

The transfer switch should always be installed by a licensed professional electrician in accordance with the manufacturer's instructions and all local electrical codes, as well as the National Electrical Code. The electrician will handle most of the following, but it's good to be aware of them as well: 

  • Load balancing. Have your device wattage calculations on hand to help the electrician. Usually, transfer switches have their circuits split into two blocks. For example, an 8-circuit switch might split its circuits like this: 1-2-3-4 and 5-6-7-8, or 1-3-5-7 and 2-4-6-8. Try to assign roughly equal wattage requirements to each block, so you're not putting lots of electrical load on one block and little on the other.
  • Neutral-ground bonding. Per the National Electrical Code, there must be one and only one point in the system where the neutral and ground wires are bonded. This point is often located in the main electrical panel. If the neutral on your generator is floating (may be labeled "neutral floating" or "floating neutral"), you may not need to do anything. If the neutral on your generator is bonded (may be labeled "bonded neutral" or "neutral bonded"), you may need to have the electrician disconnect the neutral-ground bond on your generator. Talk to your electrician and consult your generator's owner's manual and/or wiring diagram for more details.
  • Proper grounding. Your portable generator's owner's manual may require or advise you to connect the generator's grounding lug to a grounding rod driven into the Earth. Per OSHA and Article 250 of the National Electrical Code, this must always be done when the generator is supplying electric power to a structure by connection via a transfer switch. Consult your generator's owner's manual for instructions on grounding your generator to Earth, as well as grounding rod and grounding wire recommendations.

Usage

A diagram showing the typical setup for a generator and transfer switch.

For easy reference, here's the diagram again, showing a typical generator and transfer switch setup.

Operate the transfer switch as instructed in your owner's manual. Generally, this goes as follows: 

  1. The power goes out. (Obviously - otherwise you wouldn't be using a transfer switch!)
  2. You walk over to your transfer switch and make sure all switches are in the LINE position (or whatever position indicates connection to the grid). 
  3. You pull your generator out of storage, set it up, and connect the generator via the cord to the inlet box (or switch itself, if it's installed outdoors). 
  4. You start your generator and let it warm up for a few minutes, according to the instructions in your generator's owner's manual.
  5. You switch the switches on your transfer switch from the LINE position to the GENERATOR position (or whatever position indicates connection to the generator), one at a time. That way, you're not putting a large amount of load on the generator all at once. 
  6. Enjoy using your devices during the outage. 

When power comes back on, you'll generally: 

  1. Shut down the generator and disconnect it. 
  2. Switch the transfer switch back over to the LINE position.
  3. Store the generator properly.

Summary

Thanks for reading! We hope this has been a helpful resource as you search for a transfer switch. If you have any questions about using WEN generators with your transfer switch, consult your owner's manual, or please feel free to give us a call at 1-847-429-9263 (M – F, 8 – 5 CST), or drop us a message here to talk to our friendly and knowledgeable technical support team.

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