Portable Generator

[harvey828]

I am looking for some advice on a job that has recently came my way.

The local volunteer fire dept. has requested that I install a transfer switch for a portable generator. They have a 55kW, trailer-mounted, diesel generator that they want to use to power the station during power outages. The station is currently powered through a 200A main panel.

I understand the transfer switch portion of the equation, but I am struggling with conductor sizing (for generator connection leads).

I am afraid my calulations are off for the current-carrying conductors. According to the 2002 NEC, I am interpreting that these must be sized to handle 263A, which means 300kcmil @ 75C. Is this correct or have I messed up? Also, I am seeing the neutral as having to be sized for 140A, which means 1/0 @ 75C. At that point, I got frustrated and decided to put the questions to the experts. Could you please point me in the right direction?

P.S. What about grounding? Do I need to tie the generator, when in service, to the building ground, or drive a separate ground rod?

Thanks in advance for all the help. I haven't installed a setup like this before.

Harvey828

 

[ron]

Re: Portable Generator

What size set mounted circuit breaker does the generator have on it? Is it a 200A to match the incoming utility service? What size transfer switch will be used, 200A?
55kW at 208/120V with a PF = 0.8, equals 191 FLA. Depending on the set mounted breaker, if 200A you could go with 3/0AWG copper conductors.

[ January 13, 2006, 09:37 PM: Message edited by: ron ]

 

[harvey828]

Re: Portable Generator

What size set mounted circuit breaker does the generator have on it? Is it a 200A to match the incoming utility service? What size transfer switch will be used, 200A?
55kW at 208/120V with a PF = 0.8, equals 191 FLA. Depending on the set mounted breaker, if 200A you could go with 3/0AWG copper conductors.

I have not seen the generator as of yet, but will try to get a peek at it tomorrow. I am assuming it is a 200A disconnect on the generator. I can't see any need in using a transfer switch any larger than the 200A they are using in the main panel. Is there something I am missing by making that assumption?

 

[LarryFine]

Re: Portable Generator

Originally posted by harvey828:
Is there something I am missing by making that assumption?

Not if it's correct. Treat the generator like a second 200-amp service.

You do need to find out whether the generator has its neutral bonded to its chassis and grounding conductor before selecting the transfer switch.

 

[iwire]

Re: Portable Generator

Originally posted by LarryFine:
Not if it's correct. Treat the generator like a second 200-amp service.

Larry I have to disagree here, this is a trailer mounted generator (portable) if you treat it like a service you will have to bond the generators neutral to ground, provide an electrode system for it (could be the buildings but you would have to connect to it) and use a transfer switch that switches the neutral (more money).

I would go the other way.

I would make sure the generator neutral is not bonded to the frame and I would use a transfer switch with a solid neutral.

Originally posted by LarryFine:
You do need to find out whether the generator has its neutral bonded to its chassis and grounding conductor before selecting the transfer switch.

That is the key, finding out if the unit is bonded or not and if you can unbond it if you want.

 

[mastergasfitter]

Re: Portable Generator

I am not an electrician but i noticed your calculations are based on 208v and most likely that generator is going to put out 240v

 

[iwire]

Re: Portable Generator

Originally posted by mastergasfitter:
I am not an electrician but i noticed your calculations are based on 208v and most likely that generator is going to put out 240v

That would depend if this is a 3 phase or single phase generator.

 

[LarryFine]

Re: Portable Generator

Originally posted by iwire:

Originally posted by LarryFine:
Not if it's correct. Treat the generator like a second 200-amp service.

Larry I have to disagree here, this is a trailer mounted generator (portable) if you treat it like a service you will have to bond the generators neutral to ground, provide an electrode system for it (could be the buildings but you would have to connect to it) and use a transfer switch that switches the neutral (more money).

I agree. When I read:

Originally posted by harvey828:
I am assuming it is a 200A disconnect on the generator. I can't see any need in using a transfer switch any larger than the 200A they are using in the main panel. Is there something I am missing by making that assumption?

. . . and I responded with:

Originally posted by LarryFine:

Originally posted by harvey828:
Is there something I am missing by making that assumption?

Not if it's correct. Treat the generator like a second 200-amp service.

. . . I meant that I agree that the 200-amp breaker on the generator meant itwas not necessary to go higher on the transfer switch.

 

[harvey828]

Re: Portable Generator

quote:
--------------------------------------------------------------------------------
Originally posted by mastergasfitter:
I am not an electrician but i noticed your calculations are based on 208v and most likely that generator is going to put out 240v
--------------------------------------------------------------------------------

That would depend if this is a 3 phase or single phase generator.

I can't remember or not if the original post included the fact that the generator is currently wired for 240V single phase.