Generator - Automatic Transfer Switch Sizing 1

[tldr]

Would you please help clarify a question regarding a standby generator application I want to put in the home I am building.

We will have a 600 amp service from the utility company.
I want to install a generator and be able to run everything in the house (not all at the same time).

The electrical contractor is saying that a 600 amp automatic transfer switch is required. I don't understand this as we plan for a 45kw generator which comes with a 200 amp automatic transfer switch.

I realize that the generator cannot run everything in the house at the same time, but want to be able to run everything at some time by turning off breakers to stay within the capacity of the generator. this would only be in an at home extended power outage scenario.

If we are away or absent on vacation I was thinking I could just simply turn off all the breakers in the main panel leaving only the essential equipment breakers on which wouldn't exceed the generator capacity.

The size and expense of the larger transfer switch is a significant factor.

My question is two-fold
1. Can a 200 amp automatic transfer switch be safely interconnected to a 600 amp main service panel without damaging the generator or equipment in the house?

2. Is there a way to accomplish the flexibility of being able to run everything at some point without the expensive and bulky 600 amp switch?

Any insight or advice would be greatly appreciated

Best regards,

Terry Elder

 

[rbulsara]

You can not connect 200A ATS to a 600A feed, both by Code and by engineering principles.

A proper and economical way would be to have a seperate 200A sub panelboard which serves all the loads that you want to keep running during a power failure. Feed that panel through the 200A ATS. Normal source of the ATS will be fed by a 200A breaker in the 600A main panel.
Feed non-essential loads from the 600A main panel.

Other way will be to have the main panelboard (or a separate enclosure) with a 200A breaker, mechanically interlocked (kirk keys) with the 600A main CB such that only one breaker can be closed at the same time. Connect the generator to the 200A breaker. This may or may not be acceptable to utiltiy co. or the inspector.
But this will permit you to run any load you want within 200A limit.

 

[dpc]

The method described by rbulsara is the normal approach - use a subpanel to feed the loads that require standby power. This is necessary if you want the generator to start and pick up load automatically in the event of a power outage.

If you can live with simply a manual transfer of power, you can reverse feed the main panelboard through a 200A breaker. This will allow the generator to serve the entire panelboard. However, the utility will (and should) require this to be key-interlocked to eliminate any chance of you backfeeding into the utility system with your generator. This can be extremely hazardous.

600A service - must be a big house...

 

[jbartos]

Suggestion: Check the local Utility requirements, what will be inspected by the electrical inspector without any deficiency.
The 200A transfer switch should have at most 200A rated Circuit Breakers on the Normal supply side and on the emergency supply side.
Also, verify how much of the load, the 200A transfer switch may transfer according to industry standards and transfer switch manufacturer recommendations. It may be as little as 30% of the transfer switch rated ampere rating. This is because of motor load (if applicable) that requires the transfer to be derated.