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Question About Separate Building Sub-Panel Bonding


msteger
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I have a question about another subpanel install I saw today. The detached garage's subpanel is fed with 3 wire (no ground) service cable. The service cable is fed from the home's main panel and protected on both sides with 100 Amp/240 Volt overload protection. The service cable was a 6 AWG aluminum. Yes, a 6 AWG AL with 100 Amp overcurrent protection.

There is a separate ground rod wired into this panel. The grounding and neutral conductors are not isolated but rather are bonded in the subpanel's only terminal bar. There is no metal fencing, plumbing, etc. connecting the detached garage to the home.

Besides the undersized service cable, is this subpanel install OK since it was NOT fed with a 3 conductor plus ground feeder? I seem to recall the rules are different for subpanel bonding and a separate ground rod if there is a metal fence or something connecting the home and garage.

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I have a question about another subpanel install I saw today. The detached garage's subpanel is fed with 3 wire (no ground) service cable. The service cable is fed from the home's main panel and protected on both sides with 100 Amp/240 Volt overload protection. The service cable was a 6 AWG aluminum. Yes, a 6 AWG AL with 100 Amp overcurrent protection.

There is a separate ground rod wired into this panel. The grounding and neutral conductors are not isolated but rather are bonded in the subpanel's only terminal bar. There is no metal fencing, plumbing, etc. connecting the detached garage to the home.

Besides the undersized service cable, is this subpanel install OK since it was NOT fed with a 3 conductor plus ground feeder? I seem to recall the rules are different for subpanel bonding and a separate ground rod if there is a metal fence or something connecting the home and garage.

Yes, it was ok to bond the neutral & ground in a detached building if there were no conductive metal paths between the buildings. That changed with the '08 NEC.

- Jim Katen, Oregon

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Jim: Is this ok because it was fed with 3 conductor (no ground) cable?

Yes. That and there were no metallic pathways between the buildings.

If the feeder had 4 wires, the NEC rules would have required that he separate the neutrals & the grounds.

But with no metallic pathways, there was no requirement (until the 2008 NEC) to use 4 wires.

Either way, the detached garage would have needed it's own grounding electrode system, so the ground rods are ok no matter what the feeder is.

- Jim Katen, Oregon

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  • 1 year later...

I am glad I found this question, because this almost my exact setup to my detached garage.

My setup( from previous owner) is such:

60 amp 2 pole in main panel

3 wire 4 awg out to garage ( in plastic conduit)

service panel in garage is a 6 slot square D with NO main breaker, and one neutral/ground bar. 6awg copper from the single bar to an 8 ft copper ground rod.

no ground wire carried from the house, just 2 hots and a neutral , and the neutral is not bare (i.e. identical to the 2 hots, except yellow stripe on insulation.)

There is nothing metal between the 2 structures, and they are 30 ft apart.

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I am glad I found this question, because this almost my exact setup to my detached garage.

My setup( from previous owner) is such:

60 amp 2 pole in main panel

3 wire 4 awg out to garage ( in plastic conduit)

service panel in garage is a 6 slot square D with NO main breaker, and one neutral/ground bar. 6awg copper from the single bar to an 8 ft copper ground rod.

no ground wire carried from the house, just 2 hots and a neutral , and the neutral is not bare (i.e. identical to the 2 hots, except yellow stripe on insulation.)

There is nothing metal between the 2 structures, and they are 30 ft apart.

If the subpanel was placed before Michegan adopted the '08 NEC, then it sounds fine.

- Jim Katen, Oregon

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Michigan has not adopted 2008 for single family dwellings. We are still on 2006. This I am extremely happy about. I just rewired the house, and I REALLY REALLY did not want to buy a whole bunch of arc flash breakers.

As for the garage, I am going to put a separate ground bar in the box when I get to it. Is that legal to do if the sub panel did not come with one?

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Michigan has not adopted 2008 for single family dwellings. We are still on 2006. This I am extremely happy about. I just rewired the house, and I REALLY REALLY did not want to buy a whole bunch of arc flash breakers.

As for the garage, I am going to put a separate ground bar in the box when I get to it. Is that legal to do if the sub panel did not come with one?

No, that would be a very bad idea unless you also run a new 4-wire feeder back to the service panel.

If you separate the grounds from the neutral, you'll no longer have a low-impedance path through which to clear faults.

Just leave it as it is.

- Jim Katen, Oregon

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Michigan has not adopted 2008 for single family dwellings. We are still on 2006. This I am extremely happy about. I just rewired the house, and I REALLY REALLY did not want to buy a whole bunch of arc flash breakers.

As for the garage, I am going to put a separate ground bar in the box when I get to it. Is that legal to do if the sub panel did not come with one?

No, that would be a very bad idea unless you also run a new 4-wire feeder back to the service panel.

If you separate the grounds from the neutral, you'll no longer have a low-impedance path through which to clear faults.

Just leave it as it is.

- Jim Katen, Oregon

Ok, will do. Thanks!

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  • 8 months later...

The only problem with that Matt, is that there IS a metallic path between the two buildings. Water line and the other is the wires feeding the shed. I am getting some magnetizing from "dual neutrals" I guess, and now some stray voltage. Should I isolate at garage now? Since my meter is on a pole 200ft from my house shouldn't the main panel's neutral be isolated as well? I thought you only bonded at the disconnect when the meter in on the pole?

Please explain. Do you own the house that Matt was asking about two years ago or is this an different, unrelated house?

Are you saying that there's a metal water line between the house and the garage?

What's the shed, is that yet another building? How does it relate to the house and the garage?

Please explain what "magnetizing from dual neutrals" is.

Also, what are the symptoms of your stray voltage?

What is it that you're hoping to isolate at the garage?

Do you have any relationship to anything that's been discussed on this thread before, or are you presenting a new set of questions about a house we haven't talked about before?

- Jim Katen, Oregon

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Just joined, shoulda waited 'til I had permission and sent this as a email to him,, sorry Matt. BTW, this is where you say "thats sounds similar, but is not the same job" it won't be disputed by me. Actually he may be correct in his installation anyway, IDK. 400 amp service goes from pot to sub-pole aerial, comes down riser into meter then strait to disconnect box. It is Grounded and Bonded at this pole. Underground from there to the house at a distance of about 200 ft then runs under the new basement slab about 32 ft. to the panel. 4 Wires were run 2ht 1n 1gnd. Neutral and ground are again bonded here at both of the 200 amp panels.# 1) is that OK? From the right panel goes 100 amp service (existing 3 wire, underground) to my existing garage 200 ft the other way. Ground rod here, neutral and ground bonded again in panel. #2) is this OK? I was always told only to bond at the meter/disconnect, isolate gnd/neutral everywhere else. #3 why not isolate? what problems does that create? Frost free, 6ft metal pipe(rated for 4 ft bury) at garage. From the bottom of that it is pvc black pipe (guess this doesn't count as metallic lines) that goes directly into the basement and hooks directly to a pressure tank that sits on 3 small blocks. This leaves it up about 3in. from the concrete, which is cracked Only under this tank. Frost free pipe is not grounded neither is pressure tank. #4 possible to jump on at frost free and ride up to pressure tank? Ok also shed is between these 2 buildings. inside shed is-2 receptical's on the wall, another one on ceiling controlled by a switch that's for a light, and two seperate outside lights. One of these is powered from a breaker in the garage, this one can be switched from the garage and/or the house. The other light and the recept's are powered from the house service and switched from the shed or house. both of these feeds had to be spliced and are now direct buried. Water line is crossed by the following; main service, 100 amp service, both shed feeds, dog fence, well pump feed, propane line parrallels well line.

Sorry, I missed that. When the amphetamines wear off, please repost your questions in English. And if it's not too much trouble, try organizing your thoughts into paragraphs, if only as a courtesy to those who you'd like to read them.

- Jim Katen, Oregon

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