I have a 480 volt 3 wire (no neutral) feed between two structures run in PVC. It feeds an MCC (Service) that contains a bucket for a transformer that provides a 120/208 secondary service. The engineer claims that the NEC does not require that an equipment ground be run with this feed between structures. I maintain that with neither a neutral nor a ground between the structures, there is no low impedance path to trip the breaker in a ground fault condition. Is this correct? If so, where does the NEC support this? Additional Information: I don't know whether there is continuous underground water piping between structures. A shielded fire alarm cable was pulled between the structures and the only way to get rid of nuisance alarms was to bond the shield at both structures. I believe the problem was caused by different potentials between the two structures. However, the equipment manufacturer says to insulate the shield everywhere except at the main FACP. If we cannot install a ground in the PVC with the feeder, is it acceptable to install a bond conductor in the fire alarm conduit so that we can isolate the shield per the manufacturer's recommendation?
Grounding Between Structures
- Thread starter jckenner
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ryan_618
Senior Member
- Location
- Salt Lake City, Utah
Re: Grounding Between Structures
In my opinion, the seperate structure is required to have an electrode. (250.32) With that in mind, the manner in which you connect to it is an option. You state that the service is 480...is there GFPE installed on it as required by 230.95? If so, you must pull an EGC and use it. If not, it may be possible to use a grounded conductor as a GEC.
In my opinion, the seperate structure is required to have an electrode. (250.32) With that in mind, the manner in which you connect to it is an option. You state that the service is 480...is there GFPE installed on it as required by 230.95? If so, you must pull an EGC and use it. If not, it may be possible to use a grounded conductor as a GEC.
Re: Grounding Between Structures
Q. The engineer claims that the NEC does not require that an equipment ground be run with this feed between structures. I maintain that with neither a neutral nor a ground between the structures, there is no low impedance path to trip the breaker in a ground fault condition. Is this correct? If so, where does the NEC support this?
A. Your engineer is correct; an EGC is not required per NEC for the condition you describe. However a common GEC system is required per NEC 250.32
Q. If we cannot install a ground in the PVC with the feeder, is it acceptable to install a bond conductor in the fire alarm conduit so that we can isolate the shield per the manufacturer's recommendation?
A. You can run a GEC bonding jumper in the PVC (alarm cable shield is not acceptable), but it is not required. Again a common bonded GEC system is required IMO per 250.32, but not a EGC. A ground electrode system should be installed at both buildings per NEC 250.32, and bonded together IMO. Whether you bond them together via a bonding jumper in PVC, or sub-terrain level is a design issue.
[ November 11, 2003, 10:31 PM: Message edited by: dereckbc ]
Q. The engineer claims that the NEC does not require that an equipment ground be run with this feed between structures. I maintain that with neither a neutral nor a ground between the structures, there is no low impedance path to trip the breaker in a ground fault condition. Is this correct? If so, where does the NEC support this?
A. Your engineer is correct; an EGC is not required per NEC for the condition you describe. However a common GEC system is required per NEC 250.32
Q. If we cannot install a ground in the PVC with the feeder, is it acceptable to install a bond conductor in the fire alarm conduit so that we can isolate the shield per the manufacturer's recommendation?
A. You can run a GEC bonding jumper in the PVC (alarm cable shield is not acceptable), but it is not required. Again a common bonded GEC system is required IMO per 250.32, but not a EGC. A ground electrode system should be installed at both buildings per NEC 250.32, and bonded together IMO. Whether you bond them together via a bonding jumper in PVC, or sub-terrain level is a design issue.
[ November 11, 2003, 10:31 PM: Message edited by: dereckbc ]
Re: Grounding Between Structures
If this is grounded system (480/277V), you either need an equipment ground conductor or a neutral plus the ground electrode. As Ryan stated, if you have GFI then an equipment ground conductor should be used. If a neutral is used, it can be derated. The handbook has a description of this. Earth alone is not an adequate ground path.
If this is grounded system (480/277V), you either need an equipment ground conductor or a neutral plus the ground electrode. As Ryan stated, if you have GFI then an equipment ground conductor should be used. If a neutral is used, it can be derated. The handbook has a description of this. Earth alone is not an adequate ground path.
Re: Grounding Between Structures
jckenner, is this a grounded or ungrounded system? Your post indicated 3-wire so I assumed ungrounded in my first post.
If it is a grounded system in which you are only running the three phase conductors you have to also run an EGC with each feeder to facilitate OCPD operation.
[ November 12, 2003, 11:09 AM: Message edited by: dereckbc ]
jckenner, is this a grounded or ungrounded system? Your post indicated 3-wire so I assumed ungrounded in my first post.
If it is a grounded system in which you are only running the three phase conductors you have to also run an EGC with each feeder to facilitate OCPD operation.
[ November 12, 2003, 11:09 AM: Message edited by: dereckbc ]
Re: Grounding Between Structures
Yes, both services are grounded 480/277Y. I left out that pivotal detail. After reading 250.32 about 6 times, I think I got it. I had been falling back on 250-2d that tells us not to use the earth as the sole fault current path, but 250.32 spells it out specifically. Thank everyone very much (including nvcape). Someone won't be grateful though, trying to install grounds in several hundred feet of parallel conduit past 600MCM.
Yes, both services are grounded 480/277Y. I left out that pivotal detail. After reading 250.32 about 6 times, I think I got it. I had been falling back on 250-2d that tells us not to use the earth as the sole fault current path, but 250.32 spells it out specifically. Thank everyone very much (including nvcape). Someone won't be grateful though, trying to install grounds in several hundred feet of parallel conduit past 600MCM.
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