Transfer switch 101.
Have you ever wanted to run more than just a few devices from your generator? Maybe you want the convenience and safety of connecting your existing home AC wiring to an off grid source? Maybe you don’t like the idea of running extension cords all over the house? Most gas generators can do much more than power a few radios. A transfer switch is the safest and best way to get power to where you need it.
A transfer switch is permanently installed in a home or other structure. It allows you to switch between commercial grid AC power and a backup source. AC outlets, lights and appliances connected back to the transfer switch will then work normally. You do not have to run temporary cords or physically disconnect and then reconnect individual devices.
What happens inside the box.
The internal functioning of a transfer switch is fairly simple. A switch allows the load to be connected to either commercial AC power, or the backup source. It will never allow both at the same time. Some versions are actually several small transfer switches that move individual circuits between power sources. Retailers also offer transfer switches that automatically move the load from main to backup power when the main power fails.
Choosing a transfer switch.
Wow, there are a lot of choices out there! Very basic manually operated switches sell for under $150. Automatic switches with internet connectivity can go well past $1000. For my own home, I went with a Reliance Controls 30216A. It can transfer individual circuits and includes watt meters for load balancing. Reliance Controls is probably the most popular switch on the market; they are available almost anywhere. I’ve had my switch in service for about ten years with excellent performance.
The Reliance 30216A and others like it come with an added bonus. Because it switches individual circuits, there is no need to disconnect the main feed to your house. This design feature makes installation barely more complex than changing a breaker.
Another factor to consider is materials needed to install your transfer switch. Reliance Controls sells their products as a kit. You get everything you need, including a cable to connect your generator. If you go with the cheapie $139 switch, you’ll have to buy extra items to make everything work. When you add everything up, expect to spend at the low end $300-$500.
The 240 volt question.
Most mid-sized and up generators have a 240 volt outlet that will connect to your transfer switch. So what do you do if your generator does not have a 240 volt outlet? This is common on smaller generators, such as the immensely popular Honda eu2000i. I located adapter cables for sale on line, but they are rare. Finding one that’s compatible with your application might be difficult. The other option is to make your own cable.
Using a converter cord I made myself, if needed I can connect any 120 volt generator to my 240 volt transfer switch. Making your own adapter is actually super-simple but dangerous if you mess it up. Since the safety consequences of doing it wrong are so serious, I decline to give instructions on this blog.
The photo below is my converter cord. The two standard male 120 volt connectors plug into the small generator. On the other end is a female L-14-30 plug that goes to the transfer switch. Keep in mind that you still cannot exceed the capacity of the generator.
To say the least, installing a transfer switch is not for beginners. Having a thorough knowledge of home electric systems and being confidant working near exposed live conductors is an absolute must. There are many instructional YouTube videos, but not all of them give good advice. If you have even the slightest doubt about your abilities, listen to that inner voice and get competent help even if you have to hire a pro. I did my own work, with a skilled assistant, and it took a weekend. This included cutting a wall open, cosmetic finishes, and running 40 feet (12 meters) of wire to a new generator tap on the outside of my house. Simpler installs can probably be done in one day.
When operating your generator, always have a fire extinguisher nearby. Always run the generator outdoors (not in the garage, even with the door open). Always have working carbon monoxide detectors in your home.
The internet is full of dubious tricks to connect a generator to your home wiring. Some hacks involve clamping automotive jumper cables in your breaker box. Others employ a “cheater cord”: a cable with a male AC plug on both ends. All of these lame ideas are dangerous and illegal. Don’t play games; do it right. If a dumb shortcut results in property damage or personal injury, you could find yourself on the wrong end of a lawsuit. Your insurance carrier may not help you either.
Code compliance & permits.
Building codes and ordinances regulating transfer switches vary by locality. You may need a permit, depending on where you live. The National Fire Protection Association document NFPA-70 is the nationwide standard for all electrical work. Many state and local governments have codes that are stricter than what is in NFPA-70. Do your due diligence!
In general, you need a permit if your proposed off grid system meets one or more of the following criteria:
- The backup generator or power source is greater than 15 kilowatts.
- The generator starts and transfers the load automatically.
- Your generator is permanently wired to the transfer switch.
- The generator is fueled by commercial natural gas or a non-portable fuel tank (this would include large propane tanks).
- The generator itself is permanently mounted/not portable.
Also, generators greater than 15 kW require a manual disconnect “easily visible and accessible from the generating device”. In some situations more than one disconnect is required (NFPA-70 445.18, 445.18D, 702.12A). Generators less than 15 kW do not require a manual disconnect (NFPA-70 702.12B). Wiring to the transfer switch and all related conductors must be rated for 115% of the maximum power capacity of the generator (NFPA-70 445.13).
In most situations, generators less than 15 kW that are not permanently wired and are started/transferred manually do not require a permit. This would cover most applications an off grid ham would encounter.
I must stress again that these are general guidelines. Your locality can and probably does have different and/or more rigorous standards.
If your generator has a 240 volt output, keep in mind that it’s two 120 volt AC sources combined. The generator achieves this by spinning two separate coils that produce 120 volts each. The off grid ham must be careful not to overload one side of the generator while placing little or no demand on the other.
On most consumer-grade generators, the total rated output equals the sum of the two 120 volt sections. In other words, your “7000 watt generator” is really two 3500 watt generators integrated into one physical unit. Therefore, you cannot pull more than half of the total capacity from either side of the generator.
With a transfer switch load balancing is somewhat simplified. Each half of the 240 volts is directed to its own side of the switch. All you need to do is assign your loads evenly between the two sides. In the photo of my Reliance switch above, notice that it is divided into two banks of three circuits each, A-B-C and D-E-F, with corresponding watt meters. When I’m on my 5000 watt generator, I must not exceed 2500 watts on either side.
The 120 volt standard outlet on the front of your 240 volt generator is probably split internally so each plug is wired to a different coil. This allows you to tap both coils for load balancing, but do not pull more than 50% of the total from either plug.
When installing your transfer switch, be careful not to assign all your high power demand circuits to the same side of the switch. Distribute them as equally as possible between both sides so you’ll get the most benefit from your generator.
Here is a link to the complete NFPA-70 document (free registration required).
Reliance Controls makes some excellent transfer switches. Their documentation is top notch and they even have instructional videos with accurate information.
This very cool website gives a concise listing and specifications for numerous AC plugs used with generators and transfer switches.
This Off Grid Ham article from April 2018 discusses NFPA-70 in detail.