Replacing Garage Sub Panel

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Old 03-16-13, 08:33 PM
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Replacing Garage Sub Panel

Hi, Great forum, hope someone can help.

My 90 year old detached garage has a small rusty fuse box. I would like to replace it with a 50 amp 110/220 Main breaker panel that I already have.

The feed to garage is #6 copper triplex overhead 100' or a little less, (2 covered one bare stranded) coming out of a 50A double pole breaker with the bare wire going to the neutral/ground buss of the 200A main panel. Really don't want to replace this feed wire if I don't have to, at least not now. Can I get 220 out of this? What grounding scheme should I use? Can I use GFI's?

There is no metallic connection between the garage and barn where the main panel is located.

Anybody have any garage door questions, feel free to ask. That's my thing.


Thanks in advance for the help.
 
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  #2  
Old 03-16-13, 08:40 PM
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You can't get 220 volts because modern nominal residential voltage is 240 volts. You can't use a 3-wire feed for anything but 120 volts. For 120/240 volts by modern code you need four wires, two,hots, insulated neutral and a ground. It is no longer code compliant to use 3-wire if there is no metallic path other then the supply.
 

Last edited by ray2047; 03-16-13 at 09:35 PM.
  #3  
Old 03-16-13, 11:55 PM
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You CAN use the three-wire feeder only because this is replacement of an existing panel and not an upgrade. That is, if your LOCAL code does not have a requirement for upgrade in this particular instance. You WILL need to have an eight-foot long grounding electrode (ground rod) at the garage and connected with no less than #6 copper. Connect both ground and neutral to the bonded neutral bus. Yes, GFCI circuit breakers or receptacles may be used with no problem.

You need to have a main circuit breaker, or at least a disconnect, at the garage if the panel is capable of having more than six circuit breakers.
 
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Old 03-17-13, 01:21 AM
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I agree with Furd, sounds like that feed should be grandfathered in. Your local inspector would know for sure. I'd inspect all the parts of the run. Cracked or failing insulation, bad splices, rusted out conduit or straps would make me rethink keeping the old run.

Can someone explain to me why one ground rod is ok at outbuildings, but we need two at a service, unless we prove 25 ohms?
 
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Old 03-17-13, 04:44 AM
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Thanks....220-240 whatever it takes

Thanks for the replies. I was going to use two grounding rods just because one already exists and I am adding a second. Keeping in mind that the feed is two wire plus a bare stranded wire, if I leave the original feed, do I separate the ground and neutral at the bus or treat the sub panel like a main and leave them connected?

I am going to change out the feed sometime but will hire somebody as the feed runs back to the barn where the main comes in from the street and they are all jumbled up up there with five weather heads and a whole cluster of wires. I am going nowhere near live street power 20' off the ground in that mess. (picture attached) Right now just need to get the garage up and running.
 
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Old 03-17-13, 05:42 PM
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I was going to use two grounding rods just because one already exists and I am adding a second.
That's the best practice if you aren't prepared to prove that resistance to earth is less than 25 ohms, as Glensparky mentioned. Be sure to drive the new rod at least 6' away from the existing one and to connect both of them to your new panel with one piece of #6 AWG or larger copper.**

Keeping in mind that the feed is two wire plus a bare stranded wire, if I leave the original feed, do I separate the ground and neutral at the bus or treat the sub panel like a main and leave them connected?
I recognize that Furd advised you to bond the incoming bare conductor, the new GEC you made with the two ground wires, the branch neutrals and grounds and the panel enclosure in the garage. I wouldn't, even for the time until you get the overhead feeders squared away at the barn. I would treat the new panel in the garage as a subpanel, as you suggest, so that the neutrals are bonded together but isolated from all paths to earth, including the panel enclosure, and all grounds are separately bonded together and to the panel box. I would treat the bare conductor from the main panel as neutral and the #6 copper from the ground rods as ground.

Yes, doing this now will make the job easier when you get the new 4-wire feeder run to the garage, but the reason I would do it now is that you told us that
The feed to garage is #6 copper triplex overhead 100' or a little less, (2 covered one bare stranded) coming out of a 50A double pole breaker with the bare wire going to the neutral/ground buss of the 200A main panel.
Separating and isolating the neutrals from the grounds, and especially from the new GEC, will avoid creating a parallel path.

Best way to go? Ask you permitting authority/inspector. It's their codes and standards, and their interpretation of what will meet those codes and standards, that you have to satisfy.

------------------

**Tech note: Neither requirement is in the NEC, BTW. That model code just requires "a low-resistance path to ground." The 25 ohm rule has become an industry standard, adopted in the field, to meet that requirement. Then, because very few of us, including many permitting authorities, are prepared to test the actual resistance to the ground, the two-rods-6'-apart technique has become accepted as being so likely to satisfy the 25 ohm rule that many authorities simply accept it, so long as the installation is properly done.

The soil in your area may be less resistive - more conductive - than it is elsewhere, though. I would ask the AHJ about this too, unless you know that two rods are pretty much done universally where you are.
 
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Old 03-17-13, 11:47 PM
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I recognize that Furd advised you to bond the incoming bare conductor, the new GEC you made with the two ground wires, the branch neutrals and grounds and the panel enclosure in the garage. I wouldn't, even for the time until you get the overhead feeders squared away at the barn. I would treat the new panel in the garage as a subpanel, as you suggest, so that the neutrals are bonded together but isolated from all paths to earth, including the panel enclosure, and all grounds are separately bonded together and to the panel box. I would treat the bare conductor from the main panel as neutral and the #6 copper from the ground rods as ground.
I wrote it that way for a purpose. The equipment ground is supposed to provide a low impedance path back to the source to facilitate the tripping of the overcurrent protective device, specifically the circuit breaker on the feeder to this panel. By not tying the equipment ground back to the neutral in this particular arrangement you are relying on the earth to provide that fault current return path. It is extremely doubtful that you will get 50+ amperes through the earth.

While I agree that utilizing the neutral for an equipment ground is not the best practice it was done that way for decades as long as there were no other electrically conductive paths between the buildings. This really does need to be installed EXACTLY as it was originally for safety. AFTER the new four-conductor feeder is installed then yes, separate the neutrals and equipment grounds, but until then keep them together.
 
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Old 03-18-13, 01:46 AM
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Even though Nashkat1 was mistaken about keeping the grounds and neutrals seperate, I very much like his idea of wiring the panel as if it were a four wire feed.

That is, buy the ground bar kit for your panel. Install it and only land grounds on it. Then use the green bonding screw through the neutral bar. Or a jumper, a short piece of #8(?) or thicker wire between the neutral and ground bars.

Then, when you replace the feed with the four wire one, just remove the bond screw or jumper. Voila! The bars are seperated, proper isolated neutral bar.
 
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Old 03-18-13, 04:03 PM
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Even though Nashkat1 was mistaken about keeping the grounds and neutrals seperate, I very much like his idea of wiring the panel as if it were a four wire feed.
Thanks. I'll just add thet your disagreeing with me is not evidence of a mistake on my part. It's evidence of a disagreement on code interpretation and methods. Happens all the time.

Originally Posted by Furd
By not tying the equipment ground back to the neutral in this particular arrangement you are relying on the earth to provide that fault current return path. It is extremely doubtful that you will get 50+ amperes through the earth.
Yep, I can see that. OK, we'll go with bonding everything until the 4-wire feeder is in place.

Slbradley, you'll need to buy a separate ground bar. They don't come with panels. I would use a wire brush to remove the paint where that ground bar will sit, just as insurance toward getting a good bond to the can.
 
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