How to size a ATS

[ckesey]

I just purchased a MEP-003A at goverment auction that was used by previous owners in emergency managment department. Is like to use this generator for a whole home backup and have started looking at Automatic Transfer Switches. Ultimately I want it to automatically start, transfer, transfer back after utility is restored, then shutoff the gen after a cool down period. This is how our gens operate at work.

The one question I'm unsure of is how to size the ATS. Does it need to be sized baised on the service entrence coming into our home (400 amp) or baised on the size of the gen itself?

We have a 400 amp main panel/meter base located outside that has our HVAC breakers and a 200 amp breaker feeding an interior sub panel with the remainder of the house loads.

 

[Scoobyshep]

First of all, how big is the house to have a 400 amp service? Second the short and easy answer is it has to carry the largest current potentially applied to it. So you need a 400 amp switch. You can go smaller if and only if you setup an emergency sub panel. Put all the loads you want to run on that panel.

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[ckesey]

First of all, how big is the house to have a 400 amp service? Second the short and easy answer is it has to carry the largest current potentially applied to it. So you need a 400 amp switch. You can go smaller if and only if you setup an emergency sub panel. Put all the loads you want to run on that panel.

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We just built our house last year and our electrician debated if to install a 200 amp or 400 amp service. After we discussed the possibility of a large barn/shop he opted for the 400 amp. Looking back on it with all appliances using natural gas, we could have probally got by with a 200 amp service (150 for inside sub panel)

Thanks for the feedback. I was afraid of that. A 400 amp ATS is going to cost me 3x what I purchased.

 

[Scoobyshep]

Do me a favor. Get me a schedule of each panel. I might be able to help you figure out a different solution. Pics help too.

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[ckesey]

Do me a favor. Get me a schedule of each panel. I might be able to help you figure out a different solution. Pics help too.

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Schedule? How many positions are used?

 

[Scoobyshep]

List of panels and what is fed from each and size of breakers

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[Coug]

So you said 400 amp service, but a 150 amp inside sub panel (fed off a 200 amp breaker in the main panel)

If you want to transfer the entire 400 amp service, yes, you need a 400 amp transfer.

However if you just want to transfer the 150 amp sub panel, then a 200 amp could be used instead.

Or even still you could add another sub panel with just the loads you really want powered.

Or you could replace the inside panel with something like Generac Genready switch, which replaces your panel and has the transfer mechanism in the middle of the panel, so the bottom half gets transferred when the gen is on and the top does not. This allows you to only transfer what you really want/need, and you drop off unnecessary loads to prevent overloading the generator if you aren't home when it comes on or something like that.

 

[Daybreak]

Howdy,
Not gonna happen.
You will need a sub-panel setup for using a ATS. There is no way by code for any other way. Anything behind the ATS has to be fully supported by the generator.
ATS = Automatic Transfer Switch.
Even if you have all gas appliance's, and have city water and sewer, You have the potential service of 400 amps. Even if on a daily basis you actually use a max of say 4kw-8kw (3000-8000 watts) the code say's no.

 

[LEOK]

Here is a way around your problem, put a 200 transfer switch that has 200 amp main breaker in it, in front of the 400amp panel. You will be downgrading the house service to 200 amp. This is a dumb generac ATS controlled by a Deep Sea Controller line monitor on the generator.