generator size

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Kessler4130

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I have looked through a few past threads and my code book, the generator section is very vague. About the only real thing I got from it was that I have to size my conductors at 115% of the load. My question is, on a 400 amp service with a 400 ATS, do I just size the generator to carry the calculated load ?


Any help would be appreciated
 
The easiest way to be sure you get it right is to use one of the generator sizing software aps available from the generator manufacturers. The one I like is from Cummins. You input a lot of detailed info about your loads and then the software determines the best Generator for your specific application.
 
Kessler4130 said:
... About the only real thing I got from it was that I have to size my conductors at 115% of the load ...
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Don't have my code book in front of me at the moment ... so I'm working from memory ... but I think that it says something more like 115% of the max rated "nameplate" output of the GENSET.

That's not the same as the max "load" (a.k.a. connected load)
 
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NoVA Comms Power said:
Don't have my code book in front of me at the moment ... so I'm working from memory ... but I think that it says something more like 115% of the max rated "nameplate" output of the GENSET.

That's not the same as the max "load" (a.k.a. connected load)
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You may be right, I didn't have mine with me at the time, and I do not now either I just glanced through and saw 115%. But thank you for pointing that out, I will be sure to take another look when they decide what they want.
 
No problem.

As to sizing the GENSET ... as long as your GENSET is "optional" (Art 702) ... you can pretty much size it to carry as much (or as little) connected load as the customer wants / needs / is willing to pay for. (702.5)

Obviously, load-control in those installations where the GENSET can't carry and/or start everything at once becomes an important design consideration.

Edit to add: Check w/ your AHJ re: any possible local sizing requirments which might exist.
 
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Kessler4130 said:
You may be right, I didn't have mine with me at the time, and I do not now either I just glanced through and saw 115%. But thank you for pointing that out, I will be sure to take another look when they decide what they want.
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I think this 115% requirement is for the generator manufacturer to comply with.....NEC 445.13 refers to conductors from the generator "terminals" to the "first" distribution device. This to me means that if there is an output circuit breaker on the generator, then this wiring is on the line side of the this circuit breaker and usually comes from the factory.
 
Dave_PE said:
I ... I think this 115% requirement is for the generator manufacturer to comply with.....NEC 445.13 refers to conductors from the generator "terminals" to the "first" distribution device. This to me means that if there is an output circuit breaker on the generator, then this wiring is on the line side of the this circuit breaker and usually comes from the factory.
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While I understand your rationale, I disagree.

I interpret the term "Generator Terminals" in 445.13 as the Field Connection of the GENSET.

[Edit to add] I'm not "hard-over" on it ... I think I could make an equally convincing argument for your interpretation too.

Looking forward to other's interpretation of this same wording.
 
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NoVA Comms Power said:
Looking forward to other's interpretation of this same wording.
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I agree with David on this issue.

Once you get past the first OCPD beyond the windings (The Generator) regardless of where it is physically located 445.13 is a non-issue.

I interpret the term "Generator Terminals" in 445.13 as the Field Connection of the GENSET.
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I do not.

You are assuming "Genset" but 445 is specifically about genertors not Gensets.

A "Generator" is a generator, it is not a Genset, it is not a overcurrent protective device and an overcurrent protective device is not a generator.

Beyond that it makes no senses electrically to require 115% rated conductors beyond the first over current device.
 
Another item to consider.....if your 100kW\125kVA 120/208V genset has a namplate rating of 348A, but comes with 60A and a 250A output circuit breakers, are you going to run 400A minimum rated wire (115% x 348A) from each circuit breaker.....probably not.
 
iwire said:
I agree with David on this issue.

You are assuming "Genset" but 445 is specifically about genertors not Gensets.
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After re-reading thru all of 445 (it's a pretty short section) I agree.

That differentiation between GENSET and "Generator" is what I was overlooking and getting tripped-up.

The specifying of the conductor size "downstream" of an OCPD (in a GENSET application) never quite properly "meshed" in my mind anyway ... now 445.13 makes perfect sense.

Thanks! :smile:

Jim
 
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NoVA Comms Power said:
After re-reading thru all of 445 (it's a pretty short section) I agree.
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Cool.

Truth be told I think David's post about a Genset with multiple breakers really points out a problem if we had to apply 115% of the generators rating to each of the feeders beyond these breakers.

It is not uncommon for us to install gensets that have two or three output breakers for different purposes.
 
iwire said:
Cool.

Truth be told I think David's post about a Genset with multiple breakers really points out a problem if we had to apply 115% of the generators rating to each of the feeders beyond these breakers.

It is not uncommon for us to install gensets that have two or three output breakers for different purposes.
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In the case of multiple breakers, are the generator set required to be UL2200?? If so, which manufacturer(s) do you use?
 
Kessler4130 said:
I have looked through a few past threads and my code book, the generator section is very vague. About the only real thing I got from it was that I have to size my conductors at 115% of the load. My question is, on a 400 amp service with a 400 ATS, do I just size the generator to carry the calculated load ?


Any help would be appreciated
Click to expand...

We have been told that if you have a 400 amp service and are going to power up the entire connected load then you have to have a 400 amp service entranced rated ATS and a generator sized the entire 400 amp potential using a 400 amp main line circuit breaker. The only way around this right now is to get a 2 year demand load from the electric company. I believe, from what I have been reading on this thread, that the 2008 NEC will not allow demand load calculations. This poses a real issue, more so, in residential applications. Electrically, I believe, it would be more cost effective to power up the entire residence then to install a sub panel. Mainly because of wire prices.

Any input??
 
dlamp67 said:
... We have been told ... Any input??
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By whom and citing what ???

A few comments:

1) The ATS isn't required to be "service rated" if there is a service-rated disconnect between the meter and the ATS (the "disconnect" actually becomes THE service)

IMO this is also a better design as it allows you to easily and completely deenergize the ATS enclosure for maintenance and/or repair.

2) The ATS does need to be rated to carry the full service -- or service disconnet- current (i.e. 400 in this case) as the ATS is "inline" between the service and the loads.

3) As far as arbitrarily sizing an "optional" genset (see Art 702 definition) to carry the "full 400" ... that's just silly. (although I'm not doubting that's what you were told)

How much of that "400" is the customer actually even using?

If a customer wants only enough optional backup power to run a single light bulb, that his perogative (702.5).

4) Lastly, a too-lightly-loaded genset (i.e. oversized) is not a "happy genset" -- all sorts of mechanical problems (mostly prime-mover related) can arise from long-term light-loading

Hopefully the Mike Holt drawing below will help illustrate the electrical points:
image1.PNG
 
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For optional standby, there is no requirement to size the generator, because the user can choose which loads to serve (from none at all to all of them), but either way, a calculation is not required.

However, this will change in the 2008, when Article 702 will require compliance with Article 220. In my opinion, a bad code change. If it is optional standby, why should the NEC care if it will carry the load?
 
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