I have 200A service, but I want to add a whole house generator (GENERAC 17k) that has a 100A transfer switch. I plan to use a seperate 100A load center after the transfer switch for the backed up circuits.

My question is what breaker do I use out of the 200A load center to feed the 100A transfer switch? Do I use a 100A dipole (100A/100A) or a 50A dipole (50A/50A)?

If i use the 50A dipole (50A/50A) what size copper conducter do I use and still fit it into the breaker?

2 Answers 2


That is the right way to do it! A transfer switch to an ordinary subpanel. Remember to separate neutral and ground at all points.

You would use a 100A breaker. The 100A is at 240V, so you could put 80A of 240V heaters onto that breaker/sub. (because you must derate heater loads by 20%).

At 120V it is good for both legs. That is, with 120V loads, you can put 100A onto pole L1, and another 100A onto pole L2. (assuming they are loads that do not need derate).

Make sure to get a Rather Large panel. You will find your generator can support a lot more circuits than you might think. Also, plan for a whole space per circuit: use of "twin/duplex" double-stuff breakers cannot be relied on, because most circuits will in the future need AFCI, GFCI or both, and those specialty breakers are not made in double-stuff.

The subpanel can have any amp rating 100A or larger - 225A is fine. That panel does not need a main breaker, and if it has one, the size doesn't matter. The breaker in the main panel protects it, the wire to it, and the transfer switch.

If you don't need an automatic transfer switch, some panel makes (Siemens) allow very cheap transfer switches - using a factory kit that straps two backfeed breakers together, and interlocks them so they can't both be on at once. For Code compliance reasons, (well for safety reasons),

  • the breakers must be physically bolted/strapped down so they can't come out like a normal breaker.
  • The interlock (that keeps both breakers turning on at once) should not be disabled if you remove the panel cover. That precludes many aftermarket kits that bolt to the panel cover.

I get a little confused when I read:

whole house generator (GENERAC 17k)

The generator is designed for 17K or a 70 amp 240v maximum load. So if you are trying to serve a WHOLE HOUSE with a 200A service, then you would may need a larger generator to do so.

Getting back to the rest of your question. Your transfer switch is sized to carry 100A and is capable of doing that under normal conditions (utility power). So your Breaker from your main panel to your transfer switch can be as large as 100A. However, as I have pointed out your generator is only sized for 70A. So the maximum amount of power you could possibly pull in an emergency situation would be 70A. That means that the demand load of the subpanel from the transfer switch should be 70A or less.

That being said it may be wiser to feed the transfer switch with a 70A breaker and save yourself some money in feeder size.

Good luck.

  • Thank you. I was using "whole house" to describe a mounted generator as opposed to a mobile unit. The Generator is breaker at 65A so I know I wont get more than that. I am using a 200A panel before the transfer switch and a 100A panel after, dividing the circuits to which I want backed up and which I do not need backed up. If I use 100A breaker out of the main to feed the transfer switch I will get 100A to the subpanel unless it is on generator then I will get 65A? Sound about right?
    – Jay Hay Ga
    May 20, 2019 at 15:13

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