My first 400 amp service. Need help!

And if you want to supply both panels, 702.4(A)(2) requires that the generator be able to supply the full load that is connected or big enough to feed an EMS set limit. The actual load versus the calculated connected load can greatly differ, but if this is a 400A service we can assume the calculated load is well over 200A. An ATS puts you on the path to oversizing the generator unless you have automatic load shedding equipment. Supplying only one 200A panel somewhat forces a homeowner decision as to what can be permanently shed. But even a 200A panel typically needs a large generator if it has no load shedding.

An ATS may have some simplistic load shedding capability (e.g. a few relays that cut out some specific circuits, like car chargers). Or you may be able to save and buy a smaller generator but then have to buy a pricey energy management system.

You can also go with two manual transfer switches in which case the homeowner can manually load shed if that becomes necessary.
 
suemarkp said:
And if you want to supply both panels, 702.4(A)(2) requires that the generator be able to supply the full load that is connected or big enough to feed an EMS set limit. The actual load versus the calculated connected load can greatly differ, but if this is a 400A service we can assume the calculated load is well over 200A. An ATS puts you on the path to oversizing the generator unless you have automatic load shedding equipment. Supplying only one 200A panel somewhat forces a homeowner decision as to what can be permanently shed. But even a 200A panel typically needs a large generator if it has no load shedding.

An ATS may have some simplistic load shedding capability (e.g. a few relays that cut out some specific circuits, like car chargers). Or you may be able to save and buy a smaller generator but then have to buy a pricey energy management system.
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Excellent points, and its important to note common Generac load shed relay + ATS do not qualify as NEC article 750 Energy Management System (EMS), Generac and others now offer a more expensive product that they do say is a EMS.
when that section of code changed here a few cycles back it all but killed off the common method of a whole house 200A ATS and load shed relays, now I typically do a 100A 'critical loads' sub panel and no load shed relays. I did not really ever like those relays anyways they were problematic.
 
A 400A ATS for a residential home would be way oversized. Many of our commercial customers don't even need an ATS that large.

This entire system needs to start with building loads and start from there. An oversized generator will run very poorly under load.
 
jst270 said:
An oversized generator will run very poorly under load.
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Why? There can be wet stacking issues with lightly loaded diesel engines but it will run just fine until that issue gets bad. Fuel efficiency can be poor when engine is under loaded yet running at a higher RPM.
 
Yes, a wet stacked engine will still idle okay. But when you put it under full load you'll really miss the extra power. Most people figure out they have a misfire on the interstate.

A common issue we see is customers getting a generator, putting it on a monthly exercise timer and running it without hardly any load for years. Soot/fuel will collect in the cylinders and exhaust system; it might even leak into the crankcase via the potentially unseated piston rings. Until the day comes where they actually need to run their generator for day(s) under full load and it'll underperform.
 
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