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Question DetailsAsked on 9/23/2017

Can i install a 100 amp power transfer switch for a standby generator on my 200 amp panel?

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I presume you mean BEFORE the panel, not on / at it - the standby generator disconnect has to disconnect the power BEFORE any demand point - so definitely before the breaker panel at least. I have heard confusion about whether it should be before or after the main household breaker (if separate from the breaker panel as I believe it always should be for safety - because you don't want to be tripping out a main breaker in a distribution/breaker panel if the panel itself has a short or fire in it). If generator does not have built-in main breaker to protect the circuit (which it should have) then should be BEFORE (on meter side) of the main breaker so that will cut out in the event of an overload - though in your case if putting in 100A generator capacity then a 200A breaker is pretty poor protection for it. Others say it should be (assuming it has built in main breaker) AFTER (on household side) of the household main breaker. I would put it BEFORE the main breaker, right after the meter panel - between that and the main breaker, and as close to the meter panel as possible so as much of the wiring is protected by the breakers as possible.

The transfer switch absolutely has to be rated for the maximum demand (load) on it - so the switch itself has to be rated at least as high as the main breaker - so 200A in your case if your main breaker and household service is 200A, because when on normal power it could be pulling (theoretically) up to 200A. But as to whether you could use a 100A generator and put a 100A breaker in the generator side of the transfer switch (if a comnbined breaker and transfer switch) - that depends.

Be sure to check with utility on their requirements too - especially as I understand some digital meters do funny things when you cut out the public power supply with a disconnect - some of the meters freak out evidently. You also need to find out their grounding requirements - most require using a separate generator ground, but interconnecting the ground rods (public power and generator) and the bonding wire size requirement seems to vary a lot by area (I have seen numbers as small as #10 and as large as 4/0 on normal 150-200A services), so a certified master or journeyman electrician or an electrical engineer working for an architect should be designing this system change.

You would have to ask your electrician or local building code inspector for local specifics on the amperage - if the generator is going to produce less than full capacity for the household, some areas require that you have the generator feeding a separate breaker panel which then feeds only those items it can adequately support. Other areas allow you to undersize the generator and just trust that you will not overload it by using too much of the built-in household breaker panel capacity. Of course, a 200A panel (if that is the panel rating on the door/label) is only a maximum allowable load - you might not have that kind of connected load (and the connected load is a lot less than the sum total of the breaker amperages, because not all will be operating to capacity at any time). So you definitely need professional design on this to protect not only the household system but also the generator itself - I have seen a few emergency generators totally smoked because of improper connection, incorrect grounding, lack of independent lightning protection (required in many areas by code), etc.

Answered 3 years ago by LCD

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