Generator Breaker

[Lady Engineer]

Hi all,

I have a new 750KW, 480V/277W, 3ph, 4W diesel generator. The generator breaker will be rated 1200A. The generator is optional standby, as the nursing home already has it's critical and life safety loads on a 600KW generator.

I think I need ground fault on the generator breaker, but the panel that is fed from the generator is MLO and has no breakers rated over 1000A.

Do the ATS that are fed from those breakers need to be 4 pole?

 

[tom baker]

typically 4 pole TS is used with ground fault protection, otherwise the GF relay won't work.
95% of all generator installs do not transfer the neutral.
I would have to draw your application out to see if you need a 4 pole switch.
Cummins has good information on this topic

 

[iwire]

.Cummins has good information on this topic

And here it is http://power.cummins.com/sites/default/files/literature/technicalpapers/PT-6006-GroundingAC-2-en.pdf

 

[ron]

I think I need ground fault on the generator breaker, but the panel that is fed from the generator is MLO and has no breakers rated over 1000A.

Do the ATS that are fed from those breakers need to be 4 pole?

215.10 says you need GFP on the 1200A breaker

Do you need 4W downstream of the ATS? If not, I would bond the N-G at the generator and not bring it downstream to the ATS, then the ATS can be 3 pole.

If you bring the neutral downstream because you need 4W, then you still bond the N-G at the generator and you will need a 4 pole ATS to make the GFP sense properly.

 

[Lady Engineer]

215.10 says you need GFP on the 1200A breaker

Do you need 4W downstream of the ATS? If not, I would bond the N-G at the generator and not bring it downstream to the ATS, then the ATS can be 3 pole.

If you bring the neutral downstream because you need 4W, then you still bond the N-G at the generator and you will need a 4 pole ATS to make the GFP sense properly.

I'm aware of needing the GFP on the 1200A breaker, but the problem lies with trying to use 3 or 4 pole ATSs. I think you hit the nail on the head. I am not utilizing 3W panels, so I'll have to go with 4 pole ATSs.

 

[chris kennedy]

Why 1200A on a 750?

 

[GoldDigger]

Why 1200A on a 750?

Possibly 1200A max with the trip set lower? In most cases the max must be used for this determination.

 

[smoothops10]

A 750kW generator is typically rated with a 0.8 pf. 937.5kVA would provide an FLA output of 1127A. It is not uncommon to size the breaker at 100% or 125% of the FLA. So a 1200AT or even 1600AT would not be unheard of provided the conductor ampacity is equal or greater than the AT of the breaker. Cummins application manual 5-37 to 5-38 explains this. https://powergenapps.cummins.com/www/literature/applicationmanuals/t030-ch-5-en.pdf

Another consideration with the breaker sizing if the breaker is meant to provide protection for the generator is to see if it is possible to set the STD to intersect with the generator decrement curve. See reference here.
http://www.skm.com/applicationguides11.html
http://www.skm.com/applicationguides16.html

If ground fault protection is being provided at the generator and the service it is generally recommended to ground the generator as a separately derived system and use a 4P transfer switch.

I have not seen this approach in my work but 700.26 appears to allow ground fault alarming for the generator without automatic disconnection if it is a legally required system.