SIzing Generators

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brian john

Senior Member
Location
Leesburg, VA
We have a customer that has an Emergency System, diesel generator, single ATS, one distribution panel. The Generator is Rated 30 KW 37.5 KVA @. 8pf 480/277 3-phase 4-wire wye 36.1 amps Full Load with a 50 amp main circuit breaker.


They recently added an additional emergency lighting, increasing the load from, 25/27/21 amps per phase to 30/31/29 amps per phase. An electrical contractor did a site review for the building and told the customer that he has violated the NEC and the NFPA 70. That the full load capacity of a diesel generator is limited to 80% of full load KW rating and their generator should be replaced ASAP.

I have worked around generators for years servicing, installing and load testing and while limiting load to 80% is a good practice I can not locate any reference in the NEC/NFPA 70 nor NFPA 110 regarding limiting the load to 80%…………Am I missing something?
 

ron

Senior Member
Re: SIzing Generators

I don't know of a code the requires 80% capacity on the generator either. You are limited to 80% continuous capacity of the circuit breaker (assuming a standard CB). It would seem ok to me.
They are ok with the kVA rating of the Gen. Is the power factor possibly extreamly low, making it over its kW capacity?
The generator really cares about watts.
 

steve66

Senior Member
Re: SIzing Generators

The generator really cares about watts.
I have to disagree with this statement. A generator usually has two ratings: Kilowatts (KW) and Kilovolt-amperes (KVA). The load placed on the generator should not excede either rating (Exception: the generator may be rated to handle higher loads for a very short duration).

KVA is related to KW by the power factor (PF) of the load. A PF of 1.0 means KVA=KW. A PF of 0.8 means KW= 0.8 * KVA. On most generators used for standby operation (it only runs when the POCO fails), the KW rating is 0.8 times the KW rating. Thus, your 30KW generator is also rated at 37.5 KVA.

If you are measuring amps and multiplying that by 480 volts and sqrt(3) for 3 phase, you are calculating KVA. You can only find the KW rating on the generator if you know the PF of the load, or if you use a power meter. If the generator mostly runs emergency lighting, 0.8 would be a reasonable guess at the PF.

At about 30 Amps load, the generator is only putting out 24.9KVA. If the PF is 0.8, the generator is only loaded to 20KW.

Your contractor who did the review compared KVA to KW (apples to oranges). The approximate 20KW load is only 66% of the generator capacity.

Steve
 

ron

Senior Member
Re: SIzing Generators

Steve,
I think the point was gotten across, but I'll defend myself anyway for the benifit of education (mine probably).
The engine on the engine-generator set only cares about Real Power (Watts). The Apparent Power calculation (Volt-Amps)is missing the very important aspect of power factor, which is the characteristic of the load and generally is without tangible benefit to the building owner.
So my point that "The generator [engine-generator set] really cares about watts", I will still stand-by (pun intended) the statement.
Can you come up with an example, where the kVA rating will be exceeded before the kW rating?

[ February 04, 2004, 09:50 AM: Message edited by: ron ]
 

nvcape

Senior Member
Re: SIzing Generators

Possibly there is still more to this, the KW/KVA is not the only limit. The starting load may be the limiting factor, especially if loads can not be step started.
 

steve66

Senior Member
Re: SIzing Generators

Ron:

I like the pun :D

You asked:
Can you come up with an example, where the kVA rating will be exceeded before the kW rating?
Sure I can. Anytime the load's power factor is less than 0.8, the KVA rating will be exceeded before the KW rating is. As a real example, small single phase motors usually have power facors less than 0.8. So if the generator was running a lot of small motors, the KVA rating could be exceeded before the KW rating.

The current from the generator is determined by the KVA of the load. A high KVA equals a high current. IF the PF is very low, the current could be very high even when the KW is low. If you think about it, the KVA (and current) could even be high enough to trip the breaker (again, while the KW is below the generator rating).

I don't think brian john has to worry about any of this. It sounds like his generator is plenty large enough, unless the PF is very low.
(nvscape does has a valid point about the starting currents. I am assuming that this is mostly an emergency lighting generator, and doesn't have any big motor loads on it.)
Steve

[ February 04, 2004, 11:19 AM: Message edited by: steve66 ]
 

scott thompson

Senior Member
Re: SIzing Generators

Time for a few Trivia Questions!!!
(these Qs are guaranteed to generate hot files, as they did in past threads).

Question 1: What happens to the Genny if the connected loads exceed the true power output rating of the Genny?

Question 2: What HP Prime Mover will properly drive that 30KW Genny? (30KW LCL output, 60Hz ? 0.5%).

Question 3: If I use a Prime Mover with 90% the HP required to drive the Generator at full 30KW output LCL, what will happen if I connect 29.9 KW worth of loads to the running Genny?

Question 4: What will happen if I connect the output leads together (bolted L-L-L), then start the Prime Mover?

Question 5: What will happen if I create a Bolted L-L-L fault at the Output terminals, with the 30KW Genny in operation?

I'll stop here, as these items will get some very different responses!

BTW; this is a "Benny" like scenario - as many would know!

Scott35
 

charlie b

Moderator
Staff member
Location
Seattle, WA
Occupation
Electrical Engineer
Re: SIzing Generators

Answer 1: It will slow down, thus producing power at less than 60 hz.

Answer 2: Depends on the mechanical efficiency. If you presume an 80% efficiency, then you need a Prime Mover of at least 30kW/0.8, or 37.5 kW.

Answer 3: This is the same as Question #1. See my Answer #1.

Answer 4: (NOTE: I presume that you mean that you connect the output leads together upstream of the generator breaker. Otherwise, this becomes the same as Question #5.) One of two things will happen; it?s a question as to which will happen first. One is that the generator?s prime mover will stall out (i.e., fail to come up to speed, by virtue of having too great a load during the run up period). The engine would be tripped by one of its protective devices. The other is that the generator breaker will trip first, thus taking all load off the generator, and permitting the prime mover to continue up to full speed. In this case, the generator will run unloaded.

Answer 5: The prime mover will sense a sudden and major increase in load, and will slow down. It might trip on underspeed. At the same time, the generator breaker will receive a command to trip on overcurrent. Here again, I don?t know which will happen first.
 
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