Bonding two feeders at a separate building

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greenspark1

Senior Member
Location
New England
Hi all. I have an interesting situation where I’m pulling two feeders to a new building, one normal and one UPS/emergency power. Per NEC 250.32 feeders to a separate building need a new ground electrode installed. So I will install two rods per feeder and bond to each incoming EGC. But then I need to bond together the ground rods, cold water piping, bldg steel, and foundation to form a grounding electrode system. Which ultimately means that the normal and UPS feeders will have their equipment grounds connected through the new building steel, water pipe/etc. It seems a bit odd to have this parallel path for ground fault current and is not something I’ve run into before. Of course neither system will have the neutral-ground bond at the separate building.

I don’t see any way to keep the new feeder grounding electrodes separate while still meeting 250.32. Anyone have a good solution/explanation?
 

iwire

Moderator
Staff member
Location
Massachusetts
Having EGCs in parallel is not a problem at all and as you mentioned there is no way to avoid it.

Consider how many parallel EGCs there are in a single steel framed building wired with metallic cable or conduit.
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
Hi all. I have an interesting situation where I’m pulling two feeders to a new building, one normal and one UPS/emergency power. Per NEC 250.32 feeders to a separate building need a new ground electrode installed. So I will install two rods per feeder and bond to each incoming EGC. But then I need to bond together the ground rods, cold water piping, bldg steel, and foundation to form a grounding electrode system. Which ultimately means that the normal and UPS feeders will have their equipment grounds connected through the new building steel, water pipe/etc. It seems a bit odd to have this parallel path for ground fault current and is not something I’ve run into before. Of course neither system will have the neutral-ground bond at the separate building.

I don’t see any way to keep the new feeder grounding electrodes separate while still meeting 250.32. Anyone have a good solution/explanation?

I think the requirement is for a GES at the structure, not for each feeder.

In any case, if it is a new building it may (probably?) has a CEE in which case you would be required to use that and no ground rods would be required at all.
 

greenspark1

Senior Member
Location
New England
I think the requirement is for a GES at the structure, not for each feeder.

In any case, if it is a new building it may (probably?) has a CEE in which case you would be required to use that and no ground rods would be required at all.

Good point, one GES per structure. Yes, we have a CEE so technically no ground rods are required but owners always want to see them for some reason. Misguided peace of mind? One rod will work here.

Re: iwire, Effectively bonding feeder EGCs together on load side just feels wrong :)

Thanks for the good input!
 

greenspark1

Senior Member
Location
New England
:)

You have done it anytime you have run feeders to metallic equipment in a steel framed building.


When you put it that way, yes I did. But it was inadvertent :) Hospitals require parallel ground return paths so I suppose it doesn't do any harm, just feels weird.

Hypothetically the UPS system could be a separately derived system when under emergency power (4 pole transfer switch). Which would then be very strange to connect the EGCs together from two different systems.
 

GoldDigger

Moderator
Staff member
Location
Placerville, CA, USA
Occupation
Retired PV System Designer
When you put it that way, yes I did. But it was inadvertent :) Hospitals require parallel ground return paths so I suppose it doesn't do any harm, just feels weird.

Hypothetically the UPS system could be a separately derived system when under emergency power (4 pole transfer switch). Which would then be very strange to connect the EGCs together from two different systems.
It would not be all that strange. Both EGC networks will be connected to the same building GES. So if the neutral of the SDS is grounded, it will use a fault current path that may have elements in common with the EGC wiring of the service.
But the normal currents in each system will follow only their own neutral wires.
Another example of why it would be wrong to bond neutral and EGC of either system anywhere downstream of the main/system bonding jumpers.
 

Smart $

Esteemed Member
Location
Ohio
I think the requirement is for a GES at the structure, not for each feeder.

In any case, if it is a new building it may (probably?) has a CEE in which case you would be required to use that and no ground rods would be required at all.
Single GES per building or structure is correct.

He'd have to install at least one rod because of the water-pipe electrode, and then another unless he verified less than 25 ohms on the first.
 

Smart $

Esteemed Member
Location
Ohio
Bob P was talking about a CEE, if you have a CEE the 25 ohm rule does not apply.
My bad...

I should have explained better...

Once you have a water pipe electrode in the GES, it requires a supplemental electrode of type 250.52(A)(2) through ((A)(8) [which includes a CEE]. If the supplemental electrode is not a single rod, pipe, plate, [which includes a CEE] no other electrodes are required. If on the other hand, the supplemental... and only other... electrode is a single rod, pipe, plate, it must be 25 ohms to ground or another electrode of the type 250.52(A)(2) through ((A)(8) needs to be added. However, the conundrum arises that if you then install another electrode of the type 250.52(A)(2) through ((A)(8) other than a single rod, pipe, plate, the first rod, pipe, plate would never have had to been installed to begin with.
 

greenspark1

Senior Member
Location
New England
Good conversation, thanks all.

I think it's simpler than you think:

Per 250.50 if you have (A)(1) through (A)(7) (ex: water pipe plus bldg steel plus concrete encased electrode) you don't need a ground rod at all.

Per 250.53(2) if you install a ground rod AND a supplemental electrode ((A)(2) thru (A)(8)), you don't need a second rod or 25ohm measurement.
 
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