Potential parallel paths of neut current for separate bu

[sdbob]

I'm replacing a feeder at a local school, 400 amp, 120/208y, 500' long. This school has a main service freestanding in the middle of campus, with 4-5 separate buildings fed from there.

The contractor that pulled the particular feeder we're replacing did not pull an EGC, but bonded the neutral at both the main service and the building service. Perfectly acceptable, but not the best method , especially in a school imo.

I emailed the district admin and his electician today explaining why I wanted to pull an EGC and why we need to add a ground buss to the building's service and isolate all the building's EGCs from the neutral buss.

Obviously with the neut being used as the EGC there's the potential for parallel paths, which got me thinking.

How many possible paths could exist?

The obvious possibilities:

water service
nat. gas
circuits common to multiple buildings (house lights, ect.)

not so obvious:

cast iron plumbing?
grounded phone, alarm, signaling, ect. conductors common to two or more buildings?

Maybe none, maybe several.

Am I right for asking to install an EGC? When no obvious metal paths connect the building to other buildings fed from the service or the service itself isn't it prudent to assume these paths could exist in many forms?

 

[iwire]

Re: Potential parallel paths of neut current for separate bu

sdbob, In my opinion if there are no parallel paths the day this feeder was installed it meets the NEC. This does not at all mean it was prudent to do it this way.

Here is part of 250.32(B)(2)

.....there are no continuous metallic paths bonded to the grounding system in both buildings or structures involved,........

I doubt there are any parallel paths between the free standing service and the single building that the this feeder serves.

If all the feeders are done in this way IMO there are very likely parallel paths between the buildings.

This is no different than typical power company distribution, the common underground water service between buildings end up creating parallel paths between the buildings.

In the absence of job specifications that spell out a separate EGC to be run with the feeder IMO the contractor had the option of bonding at both ends of this feeder, if no parallel paths where present.

If now someone decides that an EGC would be prudent, that person will have to foot the bill for the new pull.

The feeder would have to be puled out and the conductors would most likely have to be replaced with new just to get this EGC installed.

If this happens do not forget about 250.122(B).

I am surprised a job at this scale did not have engineered drawings to follow. I have never seen engineered drawings that did not include separate EGCs.

Just my opinion, good luck, Bob

[ July 23, 2004, 05:48 AM: Message edited by: iwire ]

 

[dereckbc]

Re: Potential parallel paths of neut current for separate bu

Originally posted by sdbob:
Am I right for asking to install an EGC? When no obvious metal paths connect the building to other buildings fed from the service or the service itself isn't it prudent to assume these paths could exist in many forms?

Yes. The problem you may have is raceway capacity to add an EGC. Are you having to upsize to compensate for voltage drop?

 

[sdbob]

Re: Potential parallel paths of neut current for separate bu

Originally posted by dereckbc:

Originally posted by sdbob:
Am I right for asking to install an EGC? When no obvious metal paths connect the building to other buildings fed from the service or the service itself isn't it prudent to assume these paths could exist in many forms?

Yes. The problem you may have is raceway capacity to add an EGC. Are you having to upsize to compensate for voltage drop?

Yes but I have 2 - 4" ducts, so it's not a problem.

 

[sdbob]

Re: Potential parallel paths of neut current for separate bu

We've finished the job, and I had a chance to look around for potential parallel paths.

There's a steel fence attached to the building connecting other buildings fed off the same service. Ditto for the awning. Connected to the GEC?

There's a common house light circuit between multiple buildings, presumably with an EGC.

Copper water service between buildings.

Galvanized steel gas service between buildings.

Multiple low-voltage (telco, fire alarm, data) connections, some likely grounded, between buildings.

Just when you started to feel comfortable sending your kids to public school, you find the fence holding them inside is searching for more objects to trade electrons with.

trippi

 

[caosesvida]

Re: Potential parallel paths of neut current for separate bu

A lot of what I see in schools is appalling, and some of what is see is exceptional, on the school jobsites around here there rarely is a qualified overseer, so if you have good quality electricains doing the work you get great work, if not,well....