Genset up sized

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Dbronx

Senior Member
Location
Colorado
1. A 379 KVA genset with 1200 amp OPD replaced a 258 KVA genset adjusted to 410 amps OPD feeding a 400 amp transfer switch and 400
amp MDP. There is parallel 4/0 with # 3 ground conductors pulled frpm gen to Transfer then MDP. Would it be code compliant to up size the ground 3/0 to handle 1200 amps fault current.
Is 445.13 referring to the conductors which are commonly installed by the factory from the terminals to the built-in generator main breaker which could be considered the first distribution device and overcurrent protection? Or does it refer to the conductors from the genset main breaker to an external distribution panel?
 

Smart $

Esteemed Member
Location
Ohio
1. A 379 KVA genset with 1200 amp OPD replaced a 258 KVA genset adjusted to 410 amps OPD feeding a 400 amp transfer switch and 400
amp MDP. There is parallel 4/0 with # 3 ground conductors pulled frpm gen to Transfer then MDP. Would it be code compliant to up size the ground 3/0 to handle 1200 amps fault current.
Is 445.13 referring to the conductors which are commonly installed by the factory from the terminals to the built-in generator main breaker which could be considered the first distribution device and overcurrent protection? Or does it refer to the conductors from the genset main breaker to an external distribution panel?
I've always considered the built-in breaker(s) as the distribution device(s) mentioned... as long as such complies with 445.18.

As for grounding conductor sizing, that's covered in Article 250... not 445. You did not provide enough info to elaborate. See 250.35

Your opening statement wasn't clear. Did you mean the new or old gennie's breaker is/was adjusted to 410A???
 

Dbronx

Senior Member
Location
Colorado
I've always considered the built-in breaker(s) as the distribution device(s) mentioned... as long as such complies with 445.18.

As for grounding conductor sizing, that's covered in Article 250... not 445. You did not provide enough info to elaborate. See 250.35

Your opening statement wasn't clear. Did you mean the new or old gennie's breaker is/was adjusted to 410A???


The 258 KVA is the old genset with a 630 amp adjustableOPD turned down to 410 amps. The new 379 KVA genset has a 1200 OPD that is notadjustable, when conductors are in parallel the EGC has to be rated to handlethe fault current of the highest OPD. With a 1200 OPD at new genset connectedto parallel 4/0 with #3 EGC direct burial cable to a 400 amp transfer switchand then to a 400 amp MDP. Would upsizing the EGC to a 3/0 EGC which is ratedfor 1200 amps be code compliant? The calculated load is 275 amps, although amp readingswere A? 121 amps B? 134 amps C? 104 amps, mild weather today.



 

Smart $

Esteemed Member
Location
Ohio

The 258 KVA is the old genset with a 630 amp adjustableOPD turned down to 410 amps. The new 379 KVA genset has a 1200 OPD that is notadjustable, when conductors are in parallel the EGC has to be rated to handlethe fault current of the highest OPD. With a 1200 OPD at new genset connectedto parallel 4/0 with #3 EGC direct burial cable to a 400 amp transfer switchand then to a 400 amp MDP. Would upsizing the EGC to a 3/0 EGC which is ratedfor 1200 amps be code compliant? The calculated load is 275 amps, although amp readingswere A? 121 amps B? 134 amps C? 104 amps, mild weather today.
With a 1200A ocpd, I believe it is required. As such, your parallel 4/0 feeders are considered taps.
 

Dbronx

Senior Member
Location
Colorado
With a 1200A ocpd, I believe it is required. As such, your parallel 4/0 feeders are considered taps.

A few spacing error corrections. The 258 KVA is the old genset with a 630 amp adjustable OPD turned down to 410 amps. The new 379 KVA genset has a 1200 OPD that is not adjustable, when conductors are in parallel the EGC has to be rated to handle the fault current of the highest OPD. With a 1200 OPD at new genset connected to 2 parallel 4/0 with #3 EGC direct burial cables to a 400 amp transfer switch and then to a 400 amp MDP. Would upsizing the EGC to a 3/0 EGC which is rated for 1200 amps be code compliant? The calculated load is 275 amps, although amp readings were A? 121 amps B? 134 amps C? 104 amps, mild weather today.
The Tap Rule that may apply is 240.21(B) (5), because the cables are about 42 feet long and when emerging from grade is protected in SCH 40 PVC. Could the fact that the conductors are connected at a transfer switch terminals before the 400 amp main in panel make this install non compliant?


 

Smart $

Esteemed Member
Location
Ohio
... Could the fact that the conductors are connected at a transfer switch terminals before the 400 amp main in panel make this install non compliant?
If 240.21(B)(5) does apply, it is non-compliant.

Downsize ocpd at supply end so conductors are feeders, not feeder taps... then no need to upsize the grounding conductor. Otherwise, you'd end up upsizing the grounding conductor AND putting an ocpd somewhere prior to transfer switch anyway.
 

Dbronx

Senior Member
Location
Colorado
If 240.21(B)(5) does apply, it is non-compliant.

Downsize ocpd at supply end so conductors are feeders, not feeder taps... then no need to upsize the grounding conductor. Otherwise, you'd end up upsizing the grounding conductor AND putting an ocpd somewhere prior to transfer switch anyway.

Changing genset OCPD is not a option, finding and adding a extra OCPD will be hard. Is there another way around that.
 

Smart $

Esteemed Member
Location
Ohio
Changing genset OCPD is not a option, finding and adding a extra OCPD will be hard. Is there another way around that.
Didn't ask previously but is the transfer switch service-entrance rated... i.e. does it have integral ocpd on the line sides? If so, then you are just looking at upsizing the grounding conductors.
 

Dbronx

Senior Member
Location
Colorado
Didn't ask previously but is the transfer switch service-entrance rated... i.e. does it have integral ocpd on the line sides? If so, then you are just looking at upsizing the grounding conductors.

No it is a stand alone unit with 4'' PVC nipple to DB
 

Smart $

Esteemed Member
Location
Ohio
No it is a stand alone unit with 4'' PVC nipple to DB
So it amounts to, as it is now, it is non-compliant. How to make it compliant is a design decision. I see adding a feeder-rated ocpd at the generator end as the easiest, most economical alternative for bringing into compliance.
 

Dbronx

Senior Member
Location
Colorado
So it amounts to, as it is now, it is non-compliant. How to make it compliant is a design decision. I see adding a feeder-rated ocpd at the generator end as the easiest, most economical alternative for bringing into compliance.

Thank for the information you have been a big help. One more thing the transfer switch is a manual 4 pole, can you elaborate on why it is not compliant...i.e. dangers, fault current path, and design problems
 

Smart $

Esteemed Member
Location
Ohio
Thank for the information you have been a big help. One more thing the transfer switch is a manual 4 pole, can you elaborate on why it is not compliant...i.e. dangers, fault current path, and design problems
What makes the intallation non-compliant is that conductors, considered feeder taps, are required to terminate on a single ocpd (breaker or single set of fuses) properly rated for the taps ampacity, to provide a measure of ocp. The transfer switch having no integral line-side ocpd does not meet that requirement. If the conductors connected directly to the MDP MCB, the requirement would be met. As is, you have 400A equipment and conductors being [not] protected with a 1200A CB.
 
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Dbronx

Senior Member
Location
Colorado
What makes the intallation non-compliant is that conductors, considered feeder taps, are required to terminate on a single ocpd (breaker or single set of fuses) properly rated for the taps ampacity, to provide a measure of ocp. The transfer switch having no integral line-side ocpd does not meet that requirement. If the conductors connected directly to the MDP MCB, the requirement would be met. As is, you have 400A equipment and conductors being [not] protected with a 1200A CB.

I can live with that, off to the gen yard to see if I can find a breaker to retrofit Thanks
 
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