Separately Derived System - Grounded Conductor to First Disconnect?

[FDSENG]

I need a little help here. I am designing for a primary metered client who currently has an ungrounded 480VAC 3ph 3W delta system installed circa 1985 in one part of the plant. A 1500kva transformer, located outside, feeds two disconnects, grouped in a single room. Currently there are 3x500 ungrounded conductors in metallic conduit feeding one disconnect and 2x500 ungrounded conductors in metallic conduit feeding the other disconnect. There are no grounded conductor connections of course - just feeders motors and lighting transformers. All good here.

We are replacing the 1500 kVA transformer, and I would like to change this system to a grounded system. Because they are primary metered, this is a separately derived system (not a service). The source and the first disconnect(s) will be in separate enclosures. I'd like to install the system bonding jumper in the transformer, outside, and run the supply side bonding jumpers to the disconnects (with the ungrounded conductors).

I remember an AHJ telling me once - probably related to a service - that the grounded conductor had to be brought to the first disconnect.

We are on the 2017 NEC and I do not see any requirement to bring the grounded conductor to the first disconnect for this installation. Am I correct? Besides - what would it do there, be ready in case someone needed it in the future?

Thanks!

 

[don_resqcapt19]

You are correct, you only need to bring the grounded conductor to the service disconnect. That does not apply to a SDS. Your design with the system bonding jumper at the transformer and a supply side bonding jumper to the first disconnect is code compliant.

 

[electrofelon]

Besides - what would it do there, be ready in case someone needed it in the future?

Thanks!

Note that the requirement to bring the grounded conductor to the service, is not about so it's there if someone needs it, it is because that is your grounding/bonding conductor back to the source for fault clearing. For an SDS, you still need a fault clearing conductor at your first disconnect, but for that case they allow you to run an ssbj which then goes back and connects to the grounded conductor at the source of the SDS. I admit I am not completely sure why these requirements are a little different.

 

[wwhitney]

I admit I am not completely sure why these requirements are a little different.

Because with an SDS the property owner has control over what conductors are run from the transformer, while with a service they don't?

Cheers, Wayne

 

[electrofelon]

Because with an SDS the property owner has control over what conductors are run from the transformer, while with a service they don't?

Cheers, Wayne

True, but the NEC could theoretically mandate an isolated grounded conductor past the load side of the service point and they do not.

 

[wwhitney]

True, but the NEC could theoretically mandate an isolated grounded conductor past the load side of the service point and they do not.

If I'm following your idea, it would be to effectively make the EGC originate at the service point, rather than at the service disconnect? That would certainly make the service point location very clear, and would avoid neutral current on a lot of the bonded portions of the service equipment. But for the case of a service supplying only line-line loads, the only difference from the current rules would be in insulation color.

Cheers, Wayne

 

[electrofelon]

If I'm following your idea, it would be to effectively make the EGC originate at the service point, rather than at the service disconnect? That would certainly make the service point location very clear, and would avoid neutral current on a lot of the bonded portions of the service equipment. But for the case of a service supplying only line-line loads, the only difference from the current rules would be in insulation color.

Cheers, Wayne

Right. And just to be clear, I am not necessarily saying that service bonding to the neutral should only happen at the service point, just that it's interesting that service and SDS bonding is a bit different. Yes I agree with no L-N loads, the difference may just in what we call it, with some differences in color and insulation.

 

[FDSENG]

Thank you all. Very insightful.