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I have checked existing wiring to a pole barn and would like to know if it is correct. There is a heavy 3-wire run between a service disconnect box and pole barn, 2 wires hooked on a double breaker, (1 on each pole) at the service disconnect box and neutral wire to neutral buss.

At the pole barn the 2 hots are connected at the main breaker at top of box with the neutral to the neutral bus bar. There is also a ground wire from the ground bus to a ground rod just outside (about 6 feet ). The distance between the two boxes is about 170 feet.

I would like to know if this a safe hookup. Thank you. Mark

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    What gauge is the wire? What size (amperage) are the breakers? Is the ground rod outside the main panel or the pole barn? – mmathis Jul 11 '17 at 15:37
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    A photo of the interior of the panel would be helpful. – wallyk Jul 11 '17 at 16:09
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    I'm unclear on whether a) this is being fed off the main panel in another building? Or b) this is a separate service specifically for the pole barn with the main disconnect at the meter and that meter serves nothing else? – Harper Jul 11 '17 at 17:30
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If the barn has its own main service

If the power company is supplying an electric meter that powers only this structure, then you're sympatico. This panel is a main service and this is the normal way to wire that.

However since this is the home improvement forum, I assume a home is involved.


If the barn is fed off a main panel in another building

Like the windmill said to the small child: "I'm not a huge fan."

You're in a no-win situation with your grounding.

The principles

Grounding serves at least two purposes: To dissipate ESD and lightning strikes back to the earth, and to return hot-ground fault currents back to source with enough current to trip the breaker (50-100 amps). If the latter doesn't happen, the fault will try to "light up" the grounding system and shock people (0.1 amps kills).

A grounding rod was never even imagined to return fault currents. It just can't. Dirt is an unreliable conductor, which is why they don't use it for wire. Dirt can flow 0.1 amps, but 50 amps is not gonna happen. It's good for grounding ESD and lightning, and for pegging neutral to earth potential to make a non-isolated system.

Second, if the neutral wire breaks, you have a classic "open neutral" problem: the 120V loads are not equal, and they pull neutral toward one pole or the other... the effect of this is to make neutral hot.

Here are your choices, Sophie

One choice is to tie neutral to ground. This will assure that a hot-ground fault trips your local breaker. However, an "open neutral" electrifies all your grounds. Every bit of conduit, all the switch plate cover screws, all equipment chassis, even the subpanel door! The grounding rod will valiantly try to return this current via the dirt, but it can't win.

The other choice is to wire it as an isolated system and intentionally isolate ground from neutral. Except it's not an isolated system, is it? Neutral is pegged to ground back at the main house, and by the power company on the pole. Yes, now an open neutral will not shock you. But hot-ground fault will! It will pull the grounding system back up toward 120V without flowing enough current for a breaker trip... so you're getting shocked again! You can't win.

The right way

It's perfectly safe and legal to retrofit a ground. Hit the hardware store and get some copper (can't be aluminum) and run it back to the house and tie it to the main panel there. It doesn't need to follow the same path, just needs to use a legal wiring method.

I note that #2 ground wire costs nearly a buck a foot and needs to be buried 12". So does Rigid conduit, which can be buried only 6" deep. I also note that the metal shell of rigid conduit is itself a legal grounding path.

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If the wire is protected by a properly sized circuit breaker at the service then the short answer is: yes it is safe.

Back before 2005 or so (exactly when I can't remember) separate buildings were allowed to be wired with 3 wires (2 hot legs and neutral) instead of 4 (2 hot legs, neutral, and equipment ground).

If your sub-panel was installed back then this was legal according to the National Electrical Code. Old installations are "grandfathered" and allowed to continue unchanged. New installations all have to be 4 wire now.

As Harper suggested, there should be a green bonding screw or bond strap installed at the sub-panel tying the neutrals and equipment grounds together. Do not remove that green screw or bond strap.

Rest easy.

  • We're talking about NEC Article 250(B)(1) Exception 1. So The Pole Barn service is within code standards. – Retired Master Electrician Jul 11 '17 at 19:04
  • @RetiredMasterElectrician you didn't put a section number on that so I don't know quite what you are referencing, but I agree it was Code compliant when it was built. – ArchonOSX Jul 11 '17 at 23:02

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