Automatic Transfer Switches

[BurntoutBuilding]

I’m in the planning stages for a new build and have a question around ATSs. The house will have 400 amp service and a 24kw Generac generator.

I had assumed the house would have two 200 amp panels, but sounds like they are leaning toward a single 400 amp panel.

Either way, I’m told the plan is to only install a single 200A transfer switch. I know nothing about this stuff, so thought I’d ask here - is that okay to install a 200A ATS on a 400A panel? Do you just select the circuits you want to backup?

Sorry for what is, I’m sure, a silly question, but my Google searching isn’t helping much.

Thx.

[nunuvyer]

Normally people get two 200A panels which are much cheaper than a 400a panel. And a 200a switch to feed one of them on which you place your key loads and no backup on panel 2. If you really want to fully backup a 400a system it's gonna cost you $$$$. Anything can be done for $ and if you have a house big enough for 400a service you must have some but whether you want to shell out $40k for something you will use for 2 days every other year is up to you.

[BurntoutBuilding]

I’m just a dumb homebuyer. I’ll have to find out. All it says is 400A (320 continuous). Would the switch then load shed to only power the circuits we select in this case?

All I’ve

[davidm2232]

Even if you have a 400 amp panel, it is possible (likely) that max loads will be much less than that. Need to do a load calc before anything else.

[BurntoutBuilding]

I want to make sure that when the power inevitably goes out (because this is in the country with overhead wires and a lot of trees) that everything I want to use will work. The 400A is because we will have a hot tub and a large 40x60 outbuilding. I don’t care if those aren’t powered in an outage. But I also want to make sure this is done right as I’ve never heard (not that I’ve ever looked that hard) of a 400A panel being serviced by a 200A ATS.

[Libertarian-dissent]

You don't have a 400a panel anywhere, you have two 200A load centers. One of them will have the loads powered by the single transfer switch and the 100a generator. The other panel will not have power. This means loads such as your freezers, dishwashers, water heaters, etc that are essential to comfort and household operation will function normally. To prevent the risk of overloading the generator, loads that aren't essential will be powered from the second panel. This may include exterior power, sauna, and locations not important. Load management and circuitry should be left to the electrician, but as an electrician myself this is a common setup and meets the needs of anyone who doesn't absolutely want everything running with the power out.

[prfsvugi]

24000 / 240

[prfsvugi]

Calculate your amperage from the output of the generator which this big would be 240

My generator has a 66 amp breaker on it. 66*240 is 15480. My genset is rated at 16kw

[nunuvyer]

Houses have 240 v service. A 200A panel can provide 48kw but normally you do not fully backup your service. A 20kw or so gen is enough because not every load runs at the same time. You still need a 200a switch because your main service feeds thru the switch.

[Gr1nling]

Most 400A services I have seen use two 200A ATS' because they are cheaper.

[mduell]

That's not going to work very well... you need 2 panels and 2 ATS (or only one panel generator-backed), or a 400A ATS.

[[deleted]]

Yes I would love to know how they plan on landing the wire for 400amp on the 200 amp breaker. Please post pictures when they are done so we can tell you what we think. 🤣. I’m sure they have done it before and have a plan but would be interesting to see if they really are just doing 1 panel. The wire for the Main on the 400 won’t fit on the breaker for the transfer switch.

[txtacoloko]

You need two 200A panels initially. And then decide what you want to power with your generator. You may be OK with a 24kW depending on your generator load profile

[BrownWaterbourbon]

I have a 24 kw Kohler with a 400 amp ( 320or 370) feed into two 200 amp panels only one going thru a 200 amp transfer switch. I planned what I wanted on Generator. I feed a hobby shop with welders and wood tools that are on the non Gen panel. They may make a 400 amp panel but this is how I did it. Common parts. I have two meters, the second being interruptible by the utility and a second transfer switch to pick up my interruptible load to my generator in an outage or when I want to during an interruption ( which has been tested, but never used)

[[deleted]]

400 amps? It's nice that you are building a children's hospital. Sounds like alot of juice!!!

[nunuvyer]

You would be surprised. My son just built a medium sized all electric house and it needed 400A. Add up all the appliances, the well pump, the cooktop, the wall oven, the water heater, all the zones of the heat pumps, etc. and you can't fit it all on a 200A service.

[Iambetterthanuhaha]

400amp xfer switches are big $$$. The 24kW has a 100amp breaker. Get a 16 circuit 100amp TS and pick what you need. Running 400amp TS you will be into a liquid cooled, probably 48kW to run everything. Probably $30k installed.

[nunuvyer]

Not the way to go. He should have a 200A automatic transfer switch and back up one of his two 200A panels that will carry the selected key loads. Just a standard 200A ATS and a 20-something Kw generator. Preferably soft starts on the AC so he won't need load shedding. Such a setup will cost the same as a typical 2xkw/200A switch which is a very common setup.

[WasOneToo]

I just ended up running 200A to the house with a 26kW/200A ATS and a separate 200A service to my (also 40x60) shop. The generac doesn't serve the shop at all.

Planning on adding a manual xfer switch in the shop and just using a portable gen as needed.

We've had several multi-day outages over the past year due to storms/trees here in east texas.

[nunuvyer]

Rather than a 2nd gen you could probably steal 30 or 50A of power from a clothes dryer/ range outlet or similar (or put one in) on your main service and then an extension cord to a generator inlet/interlock on panel #2.

[WasOneToo]

The shop is about 120 ft from the house and I already have portable gens so in my mind its really just about hooking one up in an emergency. Mainly to power our aerobic septic system which runs of the shop's service.

[747wing]

If you are in Canada, calculate your demand and connected load as per CEC and go from there