3 vs 4 wire feed to garage

This type of installation (bonded neutral at separate structure) was permitted for many decades but changed in the 2008 NEC. I’m not an HI but as an electrical inspector I was look to see what code cycle it was installed under and then determine if it is code complaint or not. For an HI that would be outside of the scope of their inspection and with many changing codes it would be impracticable.

Chris gave you a simple rule. :sunglasses:

well thanks Chris and Robert!
That is a big distinction to know about, I am wondering why none of the courses address that? They simply say that distribution panels ALWAYS need isolation. I wonder how many other inspectors need to know that?
So I suppose any home pre 2008, we shouldn’t even point out, unless there is no EGC.

Do Home Inspectors use Ground Impedance Testers?

When I was still active in the electrical craft we used them in conducting electrical safety inspections on homes when we were hired to do so. The reasons that we had been called in varied widely. When we applied the clamp on Ground Impedance Tester to the Grounding Electrode Conductor (GEC) at an out building we often found a measurable voltage between the Grounding Electrode System and actual earth ground. That could create a real hazard by raising the touch potential of the non current carrying conductive parts of the electrical system to a dangerous voltage. That was caused by the voltage drop on the neutral conductor of the feeder. That was especially dangerous if the feeder was overloaded; above 80% of it’s actual ampacity; rather long, and the outbuilding floor was earth or concrete on earth without any moisture barrier.

What I had never anticipated was how hard the elevated voltage was on livestock, especially dairy cows. In one case the voltage which was hurting the dairy cows was multi-grounded neutral current from the utility’s lines flowing out onto all of the available ground connections including the Grounding Electrode Systems of the dairy farm. The step potential across the cow’s front and back legs was subjecting them to a continuous shock. I was up on a ladder installing new lighting with good solid Equipment Grounding on everything metallic when I saw a cow trying to sneak up on her water cup. These were brass and had a sort of nose treadle in them so that when the cows nose went into the cup the water would flow. The cow kept almost taking a drink and then shying away. Even I could figure out she was thirsty but unwilling to touch that brass fixture. The elevated water piping was mastic coated galvanized steel. I got curious and used my hard hat to bring her some water from the yard hydrant. She drank my size 8 broad brimmed hard hat dry in a few long gulps. I found some large plastic feed buckets stacked up waiting for another trip to the feed store and just out of pity I set one up for each cow. I made it down one side of a 50 cow barn and the first was finishing a ten gallon bucket of water. Yes at 85 pounds apiece they were very hard to carry but after the first 2 I filled them at the animals stocks using a wash down hose. I realized that what I was seeing wasn’t normal and went and found the farmer.

I ended up opening the neutral connection from the utility at the yard pole. The farm had a 3 pole transfer switch for attaching the Power Take Off (PTO) generator to the farms wiring. I opened it from the utility position to the center off and all of the weird voltages disappeared. This happened at about 10 in the morning. When the evening milking was done using the PTO generator for power the milk production went up by more than half. I got more from the gift that the couple gave me than I got for the barn lighting I was doing for them. I was so stunned by their gift I installed a special timer in the chicken house which caused the chickens to think that sunrise was happening twice a day. The agricultural agent suggested that when I asked what I could do to say thank you. That agricultural agent got the public utilities commission involved and after I showed their engineer how much current was flowing on the neutral with the service disconnecting means open they made the utility provide a neutral isolating transformer for the farm.

Their gift was a couple of hundred dollars but it was a fortune to a young electrician first ten years in the trade. Keep in mind what $200 dollars was worth 45 years ago. I just checked googles look up and that would be a little over $1000 in today’s money.


Tom Horne

I have one for ya. What if the service panel is located at the detached garage and the distribution panels are in the house? Theoretically, wouldn’t that have to be wired if it were the other way around? Or would bonding a distribution panel in the house to the water line suffice?

The type of structure is irrelevant, if the service is at the garage then the conductors running to the house are feeder conductors and a 4 wire feeder is required between the garage and the house.

Where/how are you supposed to ground the distribution panels in the house?

Are you asking about a service panel or a distribution panel? Is it in the same building as the service or detached?

Good question, you would need to connect the EGC bus to a grounding electrode (water pipe, CEE, ground rods, etc.). The neutral would be isolated from the panel and all of the EGC’s and neutrals would be kept on separate buses.

A house I have coming up on Saturday has the service panel on the garage. Now I know what to look for in the house!

If the service at the garage feeds the house it is the same as the house feeding the garage.

What if it’s an older installation and they used the 3 wire feeder exception?

That would be acceptable as long as the there are no other metallic paths between the two structures. If a phone or CATV line were run between the two structures then the 3-wire feeder, even if installed prior to the 2008 NEC would not be permitted.

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I have a lot to look for come tomorrow. I’ll keep you updated!