back up generator calculations

[laketime]

I have a customer that is looking at buying a generator/transfer switch to back up their 200amp service. Would I multiple the 200ampsX220v=44kw generator?

 

[charlie b]

You could use that method, except that the nominal voltage these days is 240. That would give you the minimum size generator that can handle the same total load that the service panel can handle. But the loads inside the building are likely to be much lower than that. And the loads that are needed, in the event of a loss of utility power, are lower still. So I would do a load calculation, and I expect that would tell me that a smaller generator would suffice.

That said, if your customer is willing to pay any price for the assurance that 100% of the building will have backup power, then a 50kW generator would certainly give the customer that assurance.

 

[laketime]

The only issue I had with the load calcs is that can we back the system up at the service disconnect and not provide 200 amps? The service disconnect is in the garage and the panel is the the house.

 

[steve66]

If they are using an automatic transfer switch, and all the loads will be transfered to the generator when the power fails, then you should do a load calculation and size that generator for the result.

If they are using a manual transfer switch, then they can shut off any circuits they want before starting the generator. In that case, you can size the generator for the loads they want to run.

One word of caution (especially whith the second method), a large AC unit or a large motor load can take a lot of starting current. Sometimes its necessary to verify the generator is capable of starting these loads.

 

[laketime]

they would be using an automatic transfer switch

 

[JES2727]

702.5(B) requires that you calculate the load according to Article 220, and if you're using an automatic transfer switch then you have to size the generator for the full calculated load. This means you can't install an 18kw (for example), and just keep most of the lights off while it's running.

 

[laketime]

So automatic transfer switch you have to back up the entire house. If the use a manual transfer switch then they can go with a smaller generator?

 

[roger]

So automatic transfer switch you have to back up the entire house. If the use a manual transfer switch then they can go with a smaller generator?

This is correct however, you can still use an automatic switch with a selected number of circuits that would not overload the generator, this would mean more expense in the way of an additional panel or a means to shed selected loads.

Roger

 

[sinnadurai sripadmanaban]

back up generator

back up generator

You should take into account the type or characteristics of the connected load.If motors of large sizes are involved the genset should withstand the starting surge.

 

[M4gery]

You can always add a relay to keep the AC condensing units off during a power outage. That's a cheap way to load shed one of the biggest loads.

 

[tufts46argled]

Ats

Ats

It's usually not that hard to add a sub panel and have the generator only run the circuits that are wanted. We install on average 2 to 3 generators a month all have ATS and managed loads! We can do a 20kw air cooled unit with a ATS for around $9000.00. So we sell a lot in our area.