sizing grounding conductors for solid neutral se rated ats and genset

ctaylo360860

Member
Location
colorado
Occupation
ME
hey guys im trying to make sure that i size my grounding conductors correctly. any and all help is greatly appreciated!

genset4.JPG

genset5.JPG
I have a Service entrance rated solid neutral automatic transfer switch fed from the secondary side of a 277y 480v utility transformer.
my automatic transfer switch feeds a panel with a 400a ocpd.
I have 250kw genset with a 300a ocpd feeding to gen side of automatic transfer switch

in my automatic transfer switch I will Bond my Grounded and grounding conductors.
I will isolate grounds and neutral conductors at the 400A panel.
I will also isolate my neutral from the frame of the generator because this is a non separate derived system.

1 G.E.C- should be sized based on 250.66 - runs from bonded neutral bar inside of ATS to the ground rods in the earth
2. supply side bonding jumper based on 250.102(c) runs from neutral bar to the ground bar inside of ATS
3. Main bonding jumper based on 250.102(c) runs from ground bar to the frame of ats
4. equipment grounding conductor sized based on 250.122 runs from frame of ats to the frame of the 400A panel
5. equipment grounding conductor or supply side bonding jumper? sized on 250.122 or 250.102 (c) runs from frame of genset to the groundbar of ATS
 

texie

Senior Member
Location
Fort Collins, Colorado
Occupation
Electrician, Contractor, Inspector
hey guys im trying to make sure that i size my grounding conductors correctly. any and all help is greatly appreciated!

View attachment 2551950

View attachment 2551951
I have a Service entrance rated solid neutral automatic transfer switch fed from the secondary side of a 277y 480v utility transformer.
my automatic transfer switch feeds a panel with a 400a ocpd.
I have 250kw genset with a 300a ocpd feeding to gen side of automatic transfer switch

in my automatic transfer switch I will Bond my Grounded and grounding conductors.
I will isolate grounds and neutral conductors at the 400A panel.
I will also isolate my neutral from the frame of the generator because this is a non separate derived system.

1 G.E.C- should be sized based on 250.66 - runs from bonded neutral bar inside of ATS to the ground rods in the earth
2. supply side bonding jumper based on 250.102(c) runs from neutral bar to the ground bar inside of ATS
3. Main bonding jumper based on 250.102(c) runs from ground bar to the frame of ats
4. equipment grounding conductor sized based on 250.122 runs from frame of ats to the frame of the 400A panel
5. equipment grounding conductor or supply side bonding jumper? sized on 250.122 or 250.102 (c) runs from frame of genset to the groundbar of ATS
1. Good except if you only have rods for an electrode you can just use #6.
2. No jumper is required. The case can be the conductor for a ground bar.
3. Main bonding jumper must connect at the neutral bar to the enclosure. The ATS likely comes with this jumper.
4. Good.
5. This will be an EGC, 250.122.
Make certain that you remove the neutral bonding jumper in the genset if there is one there.
 

ctaylo360860

Member
Location
colorado
Occupation
ME
I would also add that you get an "A" for researching this so thoroughly.
texie I want to genuinely thank you for answering all my questions so thoroughly! I’m a young cautious electrician, and I want to make sure I do my installs as safe and code compliant as possible! So thankful to be able to discuss this type of work with like minded people.
 

texie

Senior Member
Location
Fort Collins, Colorado
Occupation
Electrician, Contractor, Inspector
texie I want to genuinely thank you for answering all my questions so thoroughly! I’m a young cautious electrician, and I want to make sure I do my installs as safe and code compliant as possible! So thankful to be able to discuss this type of work with like minded people.
You are entirely welcome. There are many people here that are very well versed in many ares of the NEC and can be a very valuable resource.
I think I speak for many that we especially like a poster such yourself that is clearly gaining knowledge from being a member and participant.
 

ctaylo360860

Member
Location
colorado
Occupation
ME
1. Good except if you only have rods for an electrode you can just use #6.
2. No jumper is required. The case can be the conductor for a ground bar.
3. Main bonding jumper must connect at the neutral bar to the enclosure. The ATS likely comes with this jumper.
4. Good.
5. This will be an EGC, 250.122.
Make certain that you remove the neutral bonding jumper in the genset if there is one there.
so I was doing a little more investigating and i want to make sure I'm understanding 250.122(b). Does this only apply to underground conductors only? im planning on running seal tight from my ats to my genset. my genset has a 300a ocpd. I am going to run parallel conductors large enough for 400a from genset to automatic transfer switch. do I then need to increase my equipment grounding conductor? so say normally for 300A OCPD I could run paralleled 1/0 cu and be fine with my #4 equipment grounding conductor. I am planning on running parallel 3/0 cu. so if this were the case 1/0 is 105600cmill and 3/0 is 167800cmill. I get my ratio by dividing 167800cmil/105600 cmil=1.58. I would then multiply this by my cmill of my #4 equipment grounding conductor. 41740cmil x 1.58= 65949.2 which would put me at a #2awg cu Equipment grounding conductor....
 

infinity

Moderator
Staff member
Location
New Jersey
Occupation
Journeyman Electrician
250.122(B) it applies to ungrounded conductors not underground conductors. Not sure which one you meant.
 

ctaylo360860

Member
Location
colorado
Occupation
ME
250.122(B) it applies to ungrounded conductors not underground conductors. Not sure which one you meant.
Wow thanks *totally read it wrong... thought it said underground, but it was obviously ungrounded* thanks for that! So that be the case since I upsized the ungrounded conductors-from my gen set to automatic transfer switch, I would also have to upside my equipment grounding conductor?
 

infinity

Moderator
Staff member
Location
New Jersey
Occupation
Journeyman Electrician
Yes generally an increase in your ungrounded conductors would require a proportional increase in the EGC as well.
 

ctaylo360860

Member
Location
colorado
Occupation
ME
You are entirely welcome. There are many people here that are very well versed in many ares of the NEC and can be a very valuable resource.
I think I speak for many that we especially like a poster such yourself that is clearly gaining knowledge from being a member and participant.
Hoping to get some more help on this genset install.... this would have to be with sizing my conductors from my genset to my Automatic transfer Switch...

nameplate on genset motor is:
kw-276 volts-208/416
kva-364 amps 958/479

nameplate on genset alternator:
volts-277/480
amps-381.6
kva-312.5

So my genset has a 300a breaker, I planned on running wires big enough for 400Abetween genset and ats, incase in the future its needed to install a larger 400a breaker. I was looking at running 5 wires in a piece of liquid tight flexible metal conduit between ATS and GENSET. if I do this I will have to derate those conductors to 80% since I'm over four current carrying conductors? so if I were planning to upsize id actually have to run 250kcmill copper paralleled to get 400amps. 255A x 80% = 204amps... 2 paralleled conductors = 408amps... with that said Ill have to have two pieces of 3" liquid tight flexible metal conduit with five conductors in each raceway... my lugs are 1=600kcmill or 2-250kcmill. I'm wondering if I should just size the conductors to the 300A breaker because its going to be unlikely they will add more load to the generator, and the 300A breaker is sufficient to carry the load they have right now.... Also I'm wondering if anyone has any ideas that i can easily install and uninstall the genset. the genset is used to keep tarps for corn bunks on during a power outage, but once the bunks are empty I will be uninstalling and storing generator. I was thinking something like a powertite 400amp pin and sleeve cable connectors and plug.... the only problem is i can only find 4wire, 4 pole. I need to run 5 conductors in each raceway since my genset is a non separately derived system, and I need isolate grounded conductor from equipment groundings conductors and frame of genset... any and all help is greatly appreciated!!!! THANKS IN ADVANCE!
 
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