200 amp transfer switch and 60 KW generator

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NSTech

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The 60 KW 3 phase 208/120 generator and 200 amp service rated ASCO transfer switch have arrived and I am getting ready to install them in a few days. The neutral is not switched. (The ASCO has a 200 amp CB for utility connection). I plan on running four 4/0 copper and a #4 copper equipment grounding conductor.
The small office building has a 208/120 3 phase service with a 2 year maximum demand of about 30 KW. In the future they may put on a small addition and add 10-20 KW of load.

The amperage for the 60 KW generator is: 60 KW/208/1.73=167 amps. Here is my question: After unpacking the transfer switch and generator I see that the circuit breaker on the generator is 225 amps. Because the generator output maximum is 167 amps I am guessing/hoping I would not be in violation of any NEC code because of the 225 amp CB on the generator feeding through a 200 amp transfer switch. It is my understanding that the ASCO switch is rated for 200 amps continuous. I can't "simply" increase the size of the transfer switch. The next size is 400 amps and is a ground mounted cabinet versus a post/wall mounted cabinet.

I would appreciate any thoughts on the situation of using a 200 amp service rated transfer switch with a 60 KW generator that has a built in 225 amp circuit breaker. Thanks!
 
What is the KVA of the generator? To get full load from the generator, you'll need to size based on KVA and not KW.

Tell us why you decided not to switch the neutral? Is the generator separately derived or not? Is the #4 truly an EGC or is it a SSBJ or otherwise? What size is the existing service and does it have ground fault protection that needs to be maintained?

To protect the 200A transfer switch, upstream needs to be not greater than 200A OCPD. Furthermore, you'll need to comply with UL 1008 7th edition for the SCCR based on said upstream breaker.
 
publicgood said:
What is the KVA of the generator? To get full load from the generator, you'll need to size based on KVA and not KW.

Tell us why you decided not to switch the neutral? Is the generator separately derived or not? Is the #4 truly an EGC or is it a SSBJ or otherwise? What size is the existing service and does it have ground fault protection that needs to be maintained?

To protect the 200A transfer switch, upstream needs to be not greater than 200A OCPD. Furthermore, you'll need to comply with UL 1008 7th edition for the SCCR based on said upstream breaker.
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Thanks for the response! I'll try to respond to your questions (to my question). I'm not sure what you mean by "you'll need to size based on KVA and not KW". Although I know how to do PF calculations I assumed the 60 KW takes into account KVA and PF. I do not need ground fault protection and have no reason to switch the nuetral...so I have a solid neutral. With a solid neutral it is not separately derived. Because we are not on the supply side and it is not separately derived I believe the terminology would be EGC rather than SSBJ. The existing general service is not ground fault protected. You mention, "upstream needs to be not greater than 200A OCPD". That is my question and I appreciate your answer. Because the generator cannot sustain that level of current and because the load is well below that level, I was hoping someone might have a different answer. As an analogy, you can have a load (lights) on a 20 amp circuit breaker using 12 gauge wire but switched with a 15 amp switch. The 15 amp switch is ok to use as long as the load doesn't exceed its rating. I was hoping the same "logic" would work for the transfer switch. I may end up inserting a 200 amp OCPD between the generator and the transfer switch. If so, I will do as you suggest and try to use a CB with the new short circuit ratings as described in the 7th edition of UL 1008.
 
JFletcher said:
Judging by this link of a package 60kW generator and transfer switch:

http://www.gegenerators.com/standby-generators/home-generators/60kW-Home-Generator-System-GE-Version

I'd guess that as long as the calculated load doesnt exceed 200A it is okay.

As far as specific code, I dont know; 240.21(B)(5) perhaps?
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I am betting that you are right! The load is well below the 200 amp rating of the ASCO transfer switch but the generator has a 225 amp OCPD. Worse case I might have to install a 200 amp CB between the generator and transfer switch if the inspector requires it. Thanks for responding!
 
NSTech said:
Although I know how to do PF calculations I assumed the 60 KW takes into account KVA and PF.
Click to expand...

You'll note KVA will be larger than generator KW; thus, although the generator is 60KW, it is larger than 60KVA. To utilize the full 60KW, would need to size the feeder for the KVA.

It may be that manufacturers do not provide KVA ratings for these smaller generators and are actually calling the 60KVA generator 60KW, as to make it less complicated. At larger scale, sizing for KW is not an accurate approach.

Example:

https://powersuite.cummins.com/PS5/...Binary_Asset/pdf/Commercial/Diesel/d-3334.pdf

NSTech said:
With a solid neutral it is not separately derived.
Click to expand...

To clarify, you are not bonding N-G anywhere on the generator side of the distribution system, correct? This is the defining characteristic of non-separately derived, not that you decided not to switch the neutral.
 
publicgood said:
You'll note KVA will be larger than generator KW; thus, although the generator is 60KW, it is larger than 60KVA. To utilize the full 60KW, would need to size the feeder for the KVA.

It may be that manufacturers do not provide KVA ratings for these smaller generators and are actually calling the 60KVA generator 60KW, as to make it less complicated. At larger scale, sizing for KW is not an accurate approach.

Example:

https://powersuite.cummins.com/PS5/...Binary_Asset/pdf/Commercial/Diesel/d-3334.pdf



To clarify, you are not bonding N-G anywhere on the generator side of the distribution system, correct? This is the defining characteristic of non-separately derived, not that you decided not to switch the neutral.
Click to expand...

I was at the generator location this afternoon and discovered the published amperage is 215 amps which is higher than the 167 that I initially calculated from the 60 KW. I had thought I knew it all until now.
I am not bonding the neutral at the generator but will bring it and the EGC (5-wire) to the SE transfer switch where they will be bonded together. Then I will run 5-wire to the existing building service and remove the main bonding jumper and separate the grounds and neutrals. Thanks for your help!
 
Clarifying: N-G bond not existing at the generator or first disconnecting means and not switching the neutral defines this as non-separately derived system.

Since the generator has an on-board disconnect, an EGC is sufficient. A SSBJ would have been required otherwise [250.35(B)].

Yes, bonding at the new service entrance rated ATS (for the service) and removing at the existing.

Hope all goes well with the installation.
 
NSTech said:
The 60 KW 3 phase 208/120 generator and 200 amp service rated ASCO transfer switch have arrived and I am getting ready to install them in a few days. The neutral is not switched. (The ASCO has a 200 amp CB for utility connection). I plan on running four 4/0 copper and a #4 copper equipment grounding conductor.
The small office building has a 208/120 3 phase service with a 2 year maximum demand of about 30 KW. In the future they may put on a small addition and add 10-20 KW of load.

The amperage for the 60 KW generator is: 60 KW/208/1.73=167 amps. Here is my question: After unpacking the transfer switch and generator I see that the circuit breaker on the generator is 225 amps. Because the generator output maximum is 167 amps I am guessing/hoping I would not be in violation of any NEC code because of the 225 amp CB on the generator feeding through a 200 amp transfer switch. It is my understanding that the ASCO switch is rated for 200 amps continuous. I can't "simply" increase the size of the transfer switch. The next size is 400 amps and is a ground mounted cabinet versus a post/wall mounted cabinet.

I would appreciate any thoughts on the situation of using a 200 amp service rated transfer switch with a 60 KW generator that has a built in 225 amp circuit breaker. Thanks!
Click to expand...

did you do a proper load calculation
 
I would be included in the group that felt if you had a 225 amp output breaker in the generator that you would need a 225 or better rated Transfer Switch.


JAP>
 
electricallouie said:
did you do a proper load calculation
Click to expand...

Yes, I did a load calculation using VA for square footage and receptacles. There is also a few exhaust fans and a small house style kitchen as part of the calculations. But I have 5 years of historical demand which is pretty close to the calculations. The 60 KW generator will be about double the calculated and historical load.
There is another 600 amp "off peak" service for the water heaters, plenum heaters, and air conditioners. This service is not on the generator. Thanks for your input!
 
jap said:
I would be included in the group that felt if you had a 225 amp output breaker in the generator that you would need a 225 or better rated Transfer Switch.


JAP>
Click to expand...

I am now in that group too! I spoke with a very helpful engineer at the transfer switch factory (starts with an A) who also said the transfer switch rating should be 225 instead of 200.
The generator supplier who also supplied the transfer switch wants me to insert a 200 amp breaker between the generator and transfer switch. I would need to nipple a 3R 200 amp breaker box on the back of the transfer switch. It would not be one of my prouder moments if I am pressured to do this. I have a conference call with them Monday. I will be pushing for a 225 amp transfer switch and we can simply keep the 200 for stock. Thanks for your opinion that the 225 transfer switch is required!!
 
NSTech said:
I am now in that group too! I spoke with a very helpful engineer at the transfer switch factory (starts with an A) who also said the transfer switch rating should be 225 instead of 200.
The generator supplier who also supplied the transfer switch wants me to insert a 200 amp breaker between the generator and transfer switch. I would need to nipple a 3R 200 amp breaker box on the back of the transfer switch. It would not be one of my prouder moments if I am pressured to do this. I have a conference call with them Monday. I will be pushing for a 225 amp transfer switch and we can simply keep the 200 for stock. Thanks for your opinion that the 225 transfer switch is required!!
Click to expand...

UL 1008 7th edition
 
NSTech said:
I am now in that group too! I spoke with a very helpful engineer at the transfer switch factory (starts with an A) who also said the transfer switch rating should be 225 instead of 200.
The generator supplier who also supplied the transfer switch wants me to insert a 200 amp breaker between the generator and transfer switch. I would need to nipple a 3R 200 amp breaker box on the back of the transfer switch. It would not be one of my prouder moments if I am pressured to do this. I have a conference call with them Monday. I will be pushing for a 225 amp transfer switch and we can simply keep the 200 for stock. Thanks for your opinion that the 225 transfer switch is required!!
Click to expand...

Interesting thread, what was the final design that was installed? Has to be done by now. Thanks
 
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