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What size standby generator would typically be used for an 800A 277-480V 3 phase electrical service?

This is for a cafeteria/seminar center.

To supply FULL LOAD potential at 480V (assuming power factor 1.00 of all connected loads), one would need a 384 kw generator set (480 x 800 = 384,000).

This scenario would be easiest to connect as well, as you could just install a 800A transfer switch right before the current 800A service panel(s), and connect the generator set to that. This way, all possible loads that are currently serviced by the utility can be serviced by the generator set...

Devices with reactive loads (motors, compressors and such) would have a different power factor, and may require additional power generation capability. All resistive loads are power factor 1.00. Fortunately, most generator heads/sets are rated at power factor .80 - which should take all of those loads into account, so you really shouldn't need to worry about it (unless you have something VERY unusual!) - but you should still verify.

Now, you may not want to get a 384kw generator set and 800A transfer switch, since you will most likely never need that full capacity, so you should first determine the major loads that will be used. If you want select loads operational, such as lighting, PA, HVAC - you should determine the power consumption of those loads, and move them to a subpanel connected to a transfer switch, connected to the generator set and already existing panels. You may be able to get by with a substantially smaller generator set if this is the case...

To determine the generator set load potential needed, find all of the devices you will be moving to the 'standby power' panel, and calculate their combined amperage draw, and do the same calculation: Volts x Amps = Watts (at power factor 1.00). So, say you wanted to power an HVAC that draws 100A (480v), some refrigeration equipment that draws 50A (480v), and lighting that draws 40A (480v) - then you can say: 190A x 480v = 91200 watts. To be safe, IN THIS EXAMPLE, you would probably go with a 100kw genset (rated at power factor .80 - more than likely)...

I personally like to be able to power all loads connected, so I'd personally go with the full-load option.

Hint: Online auctions can be a real money saver here - I've personally purchased a few generators and transfer switches from them and saved big...

Sorry for being so long-winded here, but there are a lot of factors at play... Simply stating a size would probably not be helpful.

Good luck.

PS: The guy that answered 25kw.... That would only supply a 52 amp load at 480v!!!! There is no way this would be adequate, (unless that's all the load you have - my guess, in a cafeteria, is no way)!

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