Need help understanding 4-wire feed to RDP

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1

Hello,

I am a bit confused over the NACHI Pre-Licensing Curriculum’s detailed emphasis on the required 4-wire feed for the remote distribution panel.

The 240V 3-wire feeder to the RDP should be all that is required for a detached structure with a stand alone dual grounding electrode circuit.

Bringing the 4th wire (service equipment ground) to the remote structure, and connecting it to the isolated equipment ground bus in the RDP will only re-create the possibility for the phantom circuit which you are trying to avoid. This phantom circuit is due to possibility of difference in potential between the two earth grounds. The 240V 3-wire feeder does not require a neutral ground since the feed is inherently balanced (just as the SEC is) by the 180deg phase difference of the ungrounded lines. Therefore, the earth ground bonded 3rd wire from the service panel is the equipment ground in the feeder circuit, and should be connected to the isolated neutral bus in the RDP.

Maybe I’ve just had too much coffee this morning…but your 4 wire feeder seems to cancel the possibility of true isolation from the isolated equipment ground bus in the RDP. Then again, I am new at this whole residential electrical thing. I learned all my electrical and plumbing theory working on ships in the US Navy.

2

William,

All feeders that are supplying a remote distribution panel would require a 4-conductor setup. The grounding electrode system at the detached structure is to serve lightning needs primarily. The 4th conductor could be a raceway or wire in different cases but is needed to supply a separate low impredance fault current path to the source to allow operation of the overcurrent protective device supplying that feeder. Isolation is not what is trying to be achieved in the sense of a isolated receptacle concept…the separation is to ensure current is not traveling on the equipment ground path as well as the grounded “neutral” conductor path creating parallel path for neutral current which creates a hazardous condition.

Look at Section 250.32 of the National Electrical Code for more clarity as well as 250.4(A) as well for the path requirements. Hope this was helpful as I did not go into alot of the technical details…but remember wiring of SHIPS can be different that with premise type wiring.