Vehicle mounted generator grounding

[rojay]

Our company has been asked to supply back up power for some cell equipment located in the penthouse of an 8 floor building. Specifically there are two rectifier racks each with a max AC rated current of 36.4 amps that need to be fed. They are both single phase loads fed from a 120/240 volt three phase high leg delta panel. The generator is a trailer mounted 50kva setup. This is for temporary power to accomodate some rooftop equipment maintenance that is going on. We were planning on running a #2-3 conductor SO cord up the side of the building and back feeding a 2 pole 100 amp breaker at the panel supplying the rectifiers. The run is approx. 250 ft so I know voltage drop will be a factor. There seems to be some play in the rectifier supply voltage though. The minimum operating voltage is 180 volts, so I think I will be covered even with voltage drop. Utility power will be locked out, so I believe transfer equipment is not necessary. 250.34 B has me kind of confused though. If I connect the generator to the panel via the SO cord ground wire- isnt it now connected to a grounding electrode? Am I missing anything? Any input would be greatly appreciated!

 

[GoldDigger]

When calculating the effects of voltage drop, you need to know the current profile/distortion power factor of the rectifier banks.
The peak current may be much higher than the average.
Just knowing the crest factor of the current waveform would be a good guide.

Tapatalk...

 

[Smart $]

... If I connect the generator to the panel via the SO cord ground wire- isnt it now connected to a grounding electrode? Am I missing anything? Any input would be greatly appreciated!

Yes, it is connected to the building GES. Without a switched neutral it will not be a separately derived system. With a #2 supply-side bonding jumper (SSBJ), you are good to go. The SSBJ is only required to be #8.

However, if you only run 3/C SO, you are only supplying 240. If the generator isn't center grounded (i.e. for 120/240 output), that'd be non-compliant. It'd also be non-compliant if you happen to supply any 120V loads, even if inadvertently. Check every possibility. If there's even a chance you can supply a 120V load, run 4/C SO, and isolate neutral from ground at gennie.

 

[rojay]

However, if you only run 3/C SO, you are only supplying 240. If the generator isn't center grounded (i.e. for 120/240 output), that'd be non-compliant. It'd also be non-compliant if you happen to supply any 120V loads, even if inadvertently. Check every possibility. If there's even a chance you can supply a 120V load, run 4/C SO, and isolate neutral from ground at gennie.[/QU

Thanks for the advise. Generator is center grounded 120/240 volt single phase. Only two 240 volt single phase loads are going to be fed via this setup.