Determining generator loads

Question

See related question here: Using portable generator with transfer switch

I live on a property with 600 AMP service (2x200AMP service to home, 1x200AMP service to workshop / barn). The home and barn are new construction, and due to the rural location, standby generator transfer switch for the property was provisioned (Without the generator being purchased).

The home has 2 200amp panels, and the barn has a single 200amp panel.

Due to the property being serviced with 600amps, but my transfer switch equipment is only rated for 200amps, I assume that transfer switch is only setup to service partial loads within my home.

How can I determine which loads will be serviced by the generator (Once I have one setup). With my existing setup, is there any possibility to service the full property (Home and barn)?

Answer 1

Easy. The bog-standard way of provisioning 400A service to a residence is to fit dual 200A panels. Because 200A gear is readily available at sane cost (e.g. $120/panel) in the consumer space, whereas true 400A gear requires you go into industrial-tier arcana, with industrial-tier pricing (i.e. nobody blinks at $3000/panel).

In your case, from the equipment and wiring used, and the breakers, it's obvious that they ran two feeders to your house. One of the feeders comes straight off utility and feeds one 200A panel. The other feeder comes via the transfer switch and feeds the other 200A panel.

If the installers knew what they are doing, they were careful to put loads you'd likely to want on generator on the generator-enabled panel.

400A service to houses is usually for things like 140A of heat pump emergency heat or 180A of on-demand water heater. Those few huge loads would be in the non-generator panel, since powering them is hopeless with a generator that isn't Cat yellow.

So you should be "all set"; just tie the generator in at the ATS and all the right circuits will have power available.

Answer 2

Impressive setup. And Kohler is a great brand these days. I just installed that transfer switch at my new son's house for a 15KW propane generator. OK, down to business, you can't use a 200 amp transfer switch to switch up to 600 amps.

Personally I have 2 200 amp panels for my home and one panel is on the generator transfer switch and the other is not. I put all the important loads on the one connected to the transfer switch.

One possibility, and it would depend upon the topology of your system would be to use MTS (Manual Transfer Switch) with appropriate lockouts from the panel fed by the generator to the other two panels. With that setup, if your power went out, the generator would start and transfer load to the panel fed by the transfer switch. Then you could manually switch the other panels via the MTS to power up the 2nd home panel and barn panel. Generator sizing would be your most important consideration. What loads to you plan on running? Just convenience circuits (lighting and outlets), or major A/C loads, heat pumps, etc. This will require some significant planning and additional wiring. Not sure if my answer is completely code compatible, but I think it is. Perhaps one of the "big 3" here could also comment.

Answer 3

To be honest servicing the entire property would cost tens of thousands for the switch gear and wiring alone I did a 400 amp set up a few years back and to provide full power we had to get a 110kva diesel generator. As this was a machine shop And a home the actual load was ~85kva depending on the loads you actually need to run you may have a much smaller system but code requires that the generator to be sized for the connected load. if you load shed or turn some things off a smaller generator and transfer switch can be used. As 600 amp residential is quite unusual you would need to find what you need to power, in some cases I recommend an E power panel this is the panel you want to power with a generator splitting multiple feeders can be done but is almost as expensive as a larger single transfer switch. We don’t know what your actual demand is or what needs to be powered but I go with MFG recommendations of 70-80% capacity of the generator to allow for motor start. Or bringing a step down transformer on line in the case of the machine shop 480v then we had a 45kva for the house single phase transformer. Big motors and transformers act like a short or close for a few seconds so a larger generator is needed in these cases.

Depending on your loads you may be able to get by with a smaller generator I gave the above example where the house had 100a service and the machine shop could still fully function this was on a true 400 amp 3 phase closed delta wye supply. You have 240 single phase you can power everything but it will become very expensive to do it and the larger generators I find diesel to be more efficient and less restrictions on the tanks since has much lower code standards than gasoline. Or natural gas.