NEC 250.30 (A)(2) Interpretation

Status
Not open for further replies.
F

FaradayFF

Senior Member
Location
California
Hi Folks,
I have an Separately Derived System installation in which a an alternate source, stand-by generator, feeds a switchboard via a 4-pole transfer switch. Article 250 mentions "first disconnecting means" several times. Does this refer to the transfer switch or a Mains in the switchboard? My understanding is that this refers to the Main breaker in the Switchboard downstream of the transfer switch. If so, a system bonding jumper can also be installed(retained) in the switchboard if existing so that the grounded conductor from the switchboard back to the transfer switch is permitted to serve as ground-fault return path per 250.32(B)(1) Exceptions 1&2.
My understanding further is that the EGC from the Generator to the MTS will act as a supply-side bonding jumper, am I correct here?
Thank you for your feedback.
Regards,
EE
 
FaradayFF said:
Hi Folks,
I have an Separately Derived System installation in which a an alternate source, stand-by generator, feeds a switchboard via a 4-pole transfer switch. Article 250 mentions "first disconnecting means" several times. Does this refer to the transfer switch or a Mains in the switchboard? My understanding is that this refers to the Main breaker in the Switchboard downstream of the transfer switch. If so, a system bonding jumper can also be installed(retained) in the switchboard if existing so that the grounded conductor from the switchboard back to the transfer switch is permitted to serve as ground-fault return path per 250.32(B)(1) Exceptions 1&2.
My understanding further is that the EGC from the Generator to the MTS will act as a supply-side bonding jumper, am I correct here?
Thank you for your feedback.
Regards,
EE
Click to expand...
I don't see why not. However the SSBJ is generally sized larger than the EGC.
 
Am I picturing correctly ? You have a non-fused 4 pole manual transfer switch ahead of your Main Breaker Switchboard?
 
augie47 said:
Am I picturing correctly ? You have a non-fused 4 pole manual transfer switch ahead of your Main Breaker Switchboard?
Click to expand...
Hello,
The 4 pole manual transfer switch assembly is service entrance rated, it utilizes a circuit breaker ahead of the switch assembly for the utility source and when the switch is in alternate source position, it is protected upstream by the stand-by gen breaker.
The load side of this switch feeds a Main breaker switchboard downstream, which is also service entrance rated. The switchboard is existing equipment, I'm just adding a 4 pole manual transfer switch.

Thanks,
EE
 
IMO, with your MTS containing a service disconnect and being service rated, the main bond jumper & GEC connections should be at that MTS and the existing switchboard being treated as a "sub-panel"
 
augie47 said:
IMO, with your MTS containing a service disconnect and being service rated, the main bond jumper & GEC connections should be at that MTS and the existing switchboard being treated as a "sub-panel"
Click to expand...
why would it matter if the MTS is service rated or not? does anyone even make a TS in that size that is not service rated?

you might be right about it being the first point of disconnection though.
 
I think that's the main question here, if the manual transfer switch would be considered the first disconnecting means in this installation. I believe the switch assembly by itself, if not equipped with a breaker in the switch enclosure, would not be considered such.
 
I would consider the switch as the 1st disconnect but I can see the other opinion.
(Due to the low short-circuit rating I almost never see a MTS, alone, being used a service)
 
FaradayFF said:
The 4 pole manual transfer switch assembly is service entrance rated, it utilizes a circuit breaker ahead of the switch assembly for the utility source
Click to expand...
FaradayFF said:
I I believe the switch assembly by itself, if not equipped with a breaker in the switch enclosure, would not be considered such.
Click to expand...
Possibly, but that seems not to be relevant for your current situation, per the earlier description.

Cheers, Wayne
 
Well, exception No. 2 in NEC 250.30(A)(1) allows the system bonding jumper at both the source and the first disconnecting means. My interpretation here is that the source would be the service rated MTS when facility runs on Utility supply and generator when on backup. The POCO supply conductors terminate at the MTS breaker. The first disconnecting means would be the Mains in the switchboard located in a building.

Thanks,
EE
 
IMO, Exception 2 applies to structures supplied by a SDS. In your case, you have an SDS but your structure is also Service Supplied so it is not applicable.
 
augie47 said:
IMO, Exception 2 applies to structures supplied by a SDS. In your case, you have an SDS but your structure is also Service Supplied so it is not applicable.
Click to expand...
Sir, I beg to differ. NEC 250.32(B)(1) Exception No. 1 and 2 allows grounded circuit conductor to be connected to the ground at the switchboard.
Unless I'm missing something here.
Thanks,
EE
 
Not having studied 250 closely, I would expect that with a transfer switch as described, if the feeder from the generator is disconnected, the remaining system would have to comply with 250.24 "Grounding Service-Supplied Alternating-Current Systems." Neither 250.30 "Grounding Separately Derived Alternating-Current Systems," nor "250.32 Buildings or Structures Supplied by a Feeder(s) or Branch Circuit(s)" would apply to the service aspect of things.

Cheers, Wayne
 
Exception No. 2: If a building or structure is supplied by a feeder from an outdoor separately derived system,.......

Remove your Service so your structure is only supplied by a SDS and I'll agree butb that's not what you have.
You asked for "feedback". You got feedback. Do with it what you wish.
 
Thanks for the responses everyone. I got a better understanding now, I think.
Unfortunately, for some reason, NEC seem to allow system bonding jumper downstream of the MTS for SDS, but not for the service derived services..

Regards,
EE
 
Status
Not open for further replies.
Top