4-wire feeder to Remote Distribution Panel???


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. Yes, No, Maybe So?

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.:shock:

The equipment-grounding terminal is not supposed to be isolated. It’s required to be bonded to the enclosure.

When properly installed, the requirement for the fourth conductor ensures isolation of the grounded conductors from the grounding conductors, which in turn prevents the potential for “objectionable” current on exposed metallic components.

The 2008 removed the allowance for a 3 wire feeder to a detached structure, even if there are no other metallic paths between buildings.

I think you are also confusing the purpose of a grounding electrode system. These are for high voltage events like lightning and serve no purpose with the grounding prong on a receptacle or to trip a breaker during a fault.

Always required In Canada 4 wires and only one place to be connected to earth ,Grounding Rods?? at the primary location .

Sorry but the NEC doesn’t give much legitimacy to your theory about a 4-wire feeder to a remote structure. The change in 2008 NEC requiring a 4-wire feeder was to increase safety not the other way around.

Side note to Roy C, Here under the NEC any outbuilding served by more than one circuit would require a grounding electrode system. A MWBC is considered one circuit.

From what I remember and have read this can cause eddie currents big time .Some animal experiences show they will not drink from some places .
The voltage reading were extremely low but cows and especially horses could feel them.
The cure was only one ground at the main supply.

An equipotential bond grid is used in dairy building floors to prevent the step potential that animals can feel. It starts in the ramp into the building.

To go with Jim’s post:


YES. The service panel neutral bus bar is bonded to the equipment ground bus, the enclosure, all plumbing, and everything that may possibly conduct electricity in the structure.

BUT. The equipment ground is to be isolated from the neutral bus bar of the remote distribution panel. (That’s the difference)

Therefore the 3rd and 4th wire of the feeder to the detached structure are at the exact same potential. If you attach the 4th wire to the equipment ground bus in the remote panel, your are now bonding it to the neutral bar.

So are both the grounded conductor and equipment ground feeds supposed to be connected to the neutral bus of the remote distribution panel?

No the neutral and equipment grounds are isolated at non-service panels.

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Thanks for posting the graphic Robert.

No, because doing so will put the EGC and the neutral within the feeder in parallel. That will allow some of the neutral current to flow on the EGC which is not permitted under normal conditions.

Ok. So where is the 4th wire (EGC) supposed to be attached?

I really appreciate the suggested related threads at the bottom of the posts. I read the long historical document which discusses the development of required grounding of the neutral at the transformers in the high voltage distribution network. I know my answer is somewhere in there, it may just take some time to figure it out.

The egc should always be attached (bonded) to the panel. In addition, for separate structures, the egc will be bonded to a separate grounding electrode.

The neutrals will only be grounded at the service equipment and not at the separate structure. At every location other than the service equipment, the neutrals should be isolated from any grounding connection.