Feeder gounding

[nsutton]

I am working on an existing hospital with an attached assisted living facility. There are two electric services involved, one for the hospital and the other for the assisted living.

The hospital has an existing 277/480V/3 generator system with an unswitched grounded conductor. The generator feeds a step-down transformer with 120/208V/3 secondary. A fused disconnect switch is located in the generator room and is fed from the secondary of the transformer. The disconnect serves emergency power to the assisted living facility.

The feeder from the disconnect to the assisted living is (4) #1/0 in a 2" conduit; the conduit is utilized for equipment grounding conductor (EGC). This feeder is approximately 300 feet long and extends to a manual transfer switch in the assisted living. The normal feed for the transfer switch is from the assisted living facility electric service.

The owner is concerned about reliability of the conduit EGC on the emergency feed. My level of concern is much less than the owner's as NEC 250.118 (2005) states EMT, IMC, or RMC are acceptable EGC's. Of course there is a remote possibility the conduit may separate and the grounding connection will be lost.

In an effort to address the owner's concern, I am considering having the contractor install a separate #6 EGC. To avoid pulling the existing feeder out and reinstalling it in the same conduit with the new EGC, I am considering running the new EGC secured with ty-wraps on the exterior of the feeder conduit.

I have looked at the code for guidance with the following results. 215.6 states if a feeder supplies loads which require an equipment grounding conductor the feeder shall include a grounding means in accordance with 250.134.

250.134(B) states that equipment connected by permanent wiring shall be grounded by an EGC contained within the same raceway, cable, or otherwise run with the circuit conductors. This appears to permit the installation of the EGC on the exterior of the conduit unless I have misinterpreted the phrase "otherwise run with".

Have I overlooked pertinent code sections or is my interpretation correct?

 

[tom baker]

Re: Feeder gounding

The EGC must be run inside the raceway. If its outside then the impedance will be too high and it may not clear a ground fault. This rule is in 2002 NEC section 300.3(B)

 

[petersonra]

Re: Feeder gounding

Originally posted by tom baker:
The EGC must be run inside the raceway. If its outside then the impedance will be too high and it may not clear a ground fault. This rule is in 2002 NEC section 300.3(B)

However, since the conduit is already serving as the EGC, I don't think there is any prohibition against adding another EGC that follows a seperate route. This happens many times, often unintentionally, but would not seem to be an added hazard.

 

[don_resqcapt19]

Re: Feeder gounding

I think that the words "otherwise run with" apply to installations like single conductors in cable tray or trench installations. I agree with Tom that the NEC prohibits your proposed installation.
Don

 

[coulter]

Re: Feeder gounding

My background is 99% industrial. From that point of view, I'm with the owner. I like to see a good, fat, insulated grounding conductor inside of the raceway - especially if it is ferrous conduit.

A ferrous conduit contains the magnetic field, and doesn't let an outside grounding conductor be part of the low impedance path needed to clear a fault quickly.

The NEC is pretty clear, and rightly so, if you use a grounding conductor, it is run right tight with the supply conductors.

Yes, the NEC says the conduit is accetpable for the grounding conductor. In my experience, the conduit joints can get loose/corroded over time and do not provide a good, low impedance ground fault path. (here comes the industrial slant)

Faults generally start out phase to ground. If there is a good low impedance path, the current climbs quickly and the CB trips quickly. If the low impedance path has degraded, the CB does not trip quickly and eventually the fault goes phase to phase. That will definitely trip quickly. It seems the size of the fireball to put out, and the ammount of burned up, melted down equipment to replace, has a lot to do with how quickly the CB trips.

carl