Oversized Generators

B

Ben BVE

Member
Location
California
Occupation
Assistant PM
I have a client that has oversized generators for their systems. For example, they have a 400-amp 200kw generator feeding a 200-amp ATS and 200-amp service. When brought up to the engineering team they said to upgrade the lugs on the ATS to accommodate the 350 kcmil from the generator. However, I feel that this is still violating a few different NEC codes ie. 700.4, 110.3, and 240.4. I am hoping to get the soundest code violation that I can bring to them. Let me know if I am wrong.

I also feel like this is going to cause extra maintenance like load banking.

Let me know what you all think and thanks in advance.
 
don_resqcapt19

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
I don't really see any code concerns other than maybe changing the lugs. If the generator and transfer switch are both outside, there would be no need to change the lugs, just use the outside unlimited length tap rule in 240.21(B) and run 200 amp wire from the generator to the transfer switch.
 
G

garbo

Senior Member
Ben BVE said:
Diesel
Click to expand...
Lucky that you are running on Diesel fuel. We had the generator company give us a free upgrade on a natural gas 600 KW they had laying around. Come time one earlier Sunday morning when they attempted to commission this gen set & 4 or 5 ATS's for this new 23 floor office building they ran into trouble. After the normal 30 minutes of ATS'S running on emergency power they would not transfer back to normal power. After 20 minutes of finger pointing they found the brand new but gas generator output voltage was too high due to very low load on it. They could not resolve the problem until they installed a load bank that would have to be manually turned on every monthly ATS testing. Generator tech said they never have this problem on Diesel generators. We had four 760 KW pig natural gas generators that had problems running 700 KW load bank runs during hot weather even after the hospital spent over a million dollars to have the 4 generators rebuilt and updated controls. Amazing thing about these 760 KW Caterpillar natural gas gen sets they were identical 16 cylinder to 2,000 KW Diesel gen sets only that the gas only ran at 1,200 RPM because they were used every summer to save ton of money load shedding for utility company. Diesels ran great at 1,800 RPM. All of them appeared to have identical 10' high huge radiators. Pig 760 KW gas gen sets had to have coolant expansion mounted above each radiator.
 
Z

zbang

Senior Member
Location
Roughly 5346 miles from Earls Court
I can't see how the fuel type itself would have any effect on the voltage regulation. Could be that the NG gensets have different regulators, but why? (I have seen mechanical governors on NG sets have trouble with harmonics, but not electronic governors.)
 
B

Birken Vogt

Senior Member
Location
Grass Valley, Ca
The problem I see is that you are feeding a 200 amp ATS with a 400 amp feeder. It is not rated for 400 amps and the downstream conductors are not protected to their ampacity when on generator. You could put a smaller breaker in the genset or maybe adjust the one that is in there.

Ben BVE said:
I have a client that has oversized generators for their systems. For example, they have a 400-amp 200kw generator feeding a 200-amp ATS and 200-amp service. When brought up to the engineering team they said to upgrade the lugs on the ATS to accommodate the 350 kcmil from the generator. However, I feel that this is still violating a few different NEC codes ie. 700.4, 110.3, and 240.4. I am hoping to get the soundest code violation that I can bring to them. Let me know if I am wrong.

I also feel like this is going to cause extra maintenance like load banking.

Let me know what you all think and thanks in advance.
Click to expand...
 
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