Portable Generator Hookup

Bigjabe

Member
Location
Vancouver BC
Funny how the simplest things can stump you sometimes...

We have a generator we need to hook up to a panel to power some construction trailers. We'll be hooking up at 120/208V 3ph, and the generator is rated for 78kVA at this voltage (~216A). I'd like to hook it up to a 225A panel, w/ a 225A main breaker. The thing is, the generator has a 400A 3P breaker on it where we intend to hook up... do I need to size the conductor to 400A?

I'm thinking we can size to 225A based on the main breaker of the panel, but I'm just not totally sure. This is in Canada btw, but if there's an NEC clause that'd be helpful as well.

Thanks,
Mike
 

Bigjabe

Member
Location
Vancouver BC
It's CEC actually, but they can be aligned on many things. The CEC rule that applies most that I can find is this:

14-100 Overcurrent protection of conductors (see Appendix B)
Each ungrounded conductor shall be protected by an overcurrent device at the point where it receives its supply
of current and at each point where the size of conductor is decreased, except that such protection shall be
permitted to be omitted in each of the following cases:


(a) where the overcurrent device in a larger conductor properly protects the smaller conductor;

(b) where the smaller conductor
(i) has an ampacity not less than the combined computed loads of the circuits supplied by the smaller
conductor and not less than the ampere rating of the switchboard, panelboard, or control device
supplied by the smaller conductor;
(ii) is not over 3 m long;
(iii) does not extend beyond the switchboard, panelboard, or control device that it supplies; and
(iv) is enclosed in non-ventilated raceways, armoured cable, or metal-sheathed cable when not part of
the wiring in the switchboard, panelboard, or other control devices;

We will satisfy clause b), but since there is no "bigger" conductor in this case... just not sure if this truly applies.
 

Bigjabe

Member
Location
Vancouver BC
Can the generator's breaker be replaced or adjusted? Who owns the generator? If a rental, it may be something that they can provide.
Yeah I’ve considered that approach but I’d rather not deal with that headache if not required... yup it’s a rental, but we already have it and the rental company is about 4 hours away!
 

JFletcher

Senior Member
Location
Williamsburg, VA
Yeah I’ve considered that approach but I’d rather not deal with that headache if not required... yup it’s a rental, but we already have it and the rental company is about 4 hours away!
it seems to me a 400 amp breaker would only be installed on that generator to run a single large motor load to account for start-up current... i.e., it may not be the factory breaker... Maybe the rental company has the factory breaker and can Next Day Air it to you.

A 200 amp fused disconnect or breaker installed right at the generator maybe the best option if it is required... Code wise I do not know if you can use the 400..
 
it seems to me a 400 amp breaker would only be installed on that generator to run a single large motor load to account for start-up current... i.e., it may not be the factory breaker... Maybe the rental company has the factory breaker and can Next Day Air it to you.

A 200 amp fused disconnect or breaker installed right at the generator maybe the best option if it is required... Code wise I do not know if you can use the 400..
Correct, a typical 3phase generator at 75 KVA or 60 KW is 208.4 amps @ three phase, the 400 amp breaker as you mentioned sounds like it was used for a special purpose like inrush current, but a 225 a breaker is what should be attached to meet the KVA rating and total capacity of the generator.

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Correct, a typical 3phase generator at 75 KVA or 60 KW is 208.4 amps @ three phase, the 400 amp breaker as you mentioned sounds like it was used for a special purpose like inrush current, but a 225 a breaker is what should be attached to meet the KVA rating and total capacity of the generator.

Sent from my LGMP260 using Tapatalk
It appears that they might have used the generator for 120volt loads, where the amps are up around 360+

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broadgage

Senior Member
Location
London, England
The generator capacity in amps is the same regardless as to whether the load is at 120 volts or at 208 volts.

The generator does not "know" if the 75KVA load consists of three loads each 120 volts and 25KVA or if it is 3 loads each of 25KVA and 208 volts.
75 Kva is the same number of amps in either case.

For this size of generator, a 225 amp breaker sounds reasonable, the 400amp breaker actually fitted sounds like either a mistake, or upsizing to allow for short term overloading when starting a large motor.
 
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