garage sub panel


Old 11-07-05, 07:39 AM
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garage sub panel

I currently have a badly wired garage sub panel...

There is a #6 line spliced off the stove, running though 2" underground PVC to the garage where there are 4 breakers running to a few plugs one of which is 220. There are a couple hot wires that hare just cut and hanging, and you have to use the breaker as the light switch... I looked into the sub panel, the cable running in from the house has two hots and a ground, no nutral. And all the ground and nutral wires are connected to that ground via the ground bus bar.

So here is my plan...

Put a new 40A breaker in the main panel in the house.

Pull the old #6 cable out and disconnect it from the stove.

Run a new 4 conductor #6 cable from my new 40A breaker to the sub panel in the garage through the existing 2" PVC.

Rip all the wireing out of the garage.

Install a 40A main breaker in the box.

Install a 15A 110V breaker for the plugs the lights will be pluged into along with a switch.

Install a 20A 110V breaker for most of the plugs.

Install a 20A 220V breaker for anything that may need 220(dont have anything yet but may at some point.)

Make all these circuits with #12 wire.

Connect nutrals to the nutral bus bar that is connected though the nutral wire back to the main box in the house.

Connect the grounds to the ground bus bar that is connected through the ground wire to the main box in the house.

Live happily ever after with a well wired garage.
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Old 11-07-05, 07:58 AM
Join Date: Sep 2003
Location: Central New York State
Posts: 13,973
What you have now sounds very scary. How did it end up this way? It apparently was never inspected. You would do well to turn it all off until you address the problem.

Your proposed fis sounds good for th most part. I do have several comments.

If you are using all 12 gage wire in the garage itself, use all 20 amp breakers. It makes no sense to use a 15 amp breaker for the lights if they are wired with 12 gage wire. Do put switches on the lights. Using a breaker is okay if the breaker is designed for that purpose, but switches are still better.

Don't install a 240 volt breakjer in the garage panel until you have a need for it. You may never need one, or you may find that the 20 amp breaker is too small.

You need a ground rod or two for thisi subpanel.

USe waterproof cable in the conduit, or for the whole length.

Make sure that the garage receptacles are GFCI protected.

Unless you have en extremely long distance, 6 gage wire is more than you need. But that's okay.
Old 11-07-05, 08:15 AM
Join Date: Jan 2004
Location: Oregon
Posts: 1,219
Looks to me like a pretty reasonable plan. There are a couple of errors in the details, and I have a couple of suggestions.

1) Since this is a detached structure, you need grounding electrodes (ground rods) even though you are running a 4 wire feed with a grounding conductor. The ground rods get attached to the ground bus.

2) Please double check the location of your subpanel; I am having a brain-fart on the location requirements for the disconnect in a detached structure.

3) Don't run a cable through the conduit; run individual conductors. The individual conductors will follow the bends of the conduit more easily. Of course, if you do this, then you will need to use conduit inside the house as well, or transition to a cable assembly. Pull all 4 conductors at the same time, and be sure to use lots of cable pulling lube. (Note: the individual conductors part is a suggestion; if the conduit is large enough you are allowed to run a cable assembly)

4) Use copper conductors. They last better when exposed to moisture. (Again a suggestion. #6Al is safe for 40A)

5) #6 copper is good for 55A with 60C terminations, and 65A with 75C terminations. In either case you can use a 60A breaker. If you have 60C terminations, then the 60A breaker is okay as long as the calculation shows less than 55A load.

6) If you are using #6 copper wire with a 40A breaker for reasons of voltage drop, then your ground conductor must be increased in size from #10 Cu to #8Cu (This point might preclude your use of a cable rather than individual wires.)

7) If you wire the lights using #12 wire, then just use a 20A breaker. There is no particularly good reason for using the 15A breaker in this case. (Again just a suggestion.)

Old 11-07-05, 08:25 AM
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Both of these are good sets of ideas, only question I have is with the grounding rods. If I attach a couple grounding rods to the ground bus in the sub, do I still need 4 conductor? or would I be ok with 3 conductor and then using single conductor to attach the ground to the grounding rods?

As for some of my oversizing, I know I can put more than 40A on a #6 cable but I figure its much easier to upgrade from 40 to 60 by replacing one breaker than by running new cable, but since I dont as of yet need that much current why put in that big a breaker? Same reason I was planing on using the 15 amp on the lighting circuit with 12 ga wire. Dont need more than 15A but if I do its just a breaker change. or is this all just me being stupid? Ive been working on the idea of using the biggest wire I think I will ever possibly need, but using breakers that are only big enough to support my current needs and then upgrading the breaker when necissary.
Old 11-07-05, 08:50 AM
Join Date: Jan 2004
Location: Oregon
Posts: 1,219
You must _always_ have grounding electrodes at a detached structure.

With a detached structure, if you have no continuous metallic path bonded to ground at both structures, and no ground fault detection on the feed, you _may_ run a feed that does not have a ground conductor (three wires in your case), and then _bond_ ground an neutral at the panel, just like a service entrance panel. But this is pretty strict; no water pipes, no signal wires with ground connections (some signal cables are not ground bonded, and are thus okay), no fence connected to the buildings, etc., no lighting circuit run between the buildings, etc. In general, deciding to do this is more effort than simply running the four wire feed. All other things being equal, I would put in the 4 wire feed.

It is not too unreasonable to run larger wire than you need, and then simply use a smaller breaker. But it doesn't offer much benefit in your case. A 40A breaker and a 60A breaker cost the same, and provide essentially the same protection from short circuits. The 40A breaker might provide slightly more protection from some sort of high impedance fault...but the 60A breaker will run cooler at high loading. Basically the change that the 40A breaker will provide better protection is _very_ _very_ small, IMHO not worth it if there is _any_ chance that you will ever upgrade to 60A. A similar argument applies for the 15A vs 20A breakers on your 12ga circuits.

I personally would put in the largest breaker suitable for the conductors and the panelboard, and use 20A breakers for all 12ga circuits...but I don't think that putting a smaller breaker (if you do not need the additional capacity) qualifies as stupid; slightly amusing, but not stupid

Remember, if you 'upsize' the wire for any reason, then you must upsize the ground conductors proportionally. If you use #6 conductors when #8 would do, then you must use a #8 EGC when a #10 would ordinarily do.

Old 11-07-05, 09:28 AM
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Sounds reasonable to me, Might as well go with the 60A breakers and 20A breakers, I was just figureing the extra protection would be benificial, but if it isnt really significant then I have to agree with why bother.

As for grounding...

Basicaly I stick with my original plan, nutral and ground to seperate busses, both get wired back to the main panel seperately, but the ground bus also gets wired to a couple grounding rods out at the garage with ground wire of one size below the cable I use for power, ie 8ga if im running power on 6ga.
Old 11-07-05, 04:05 PM
Join Date: Jan 2004
Location: Oregon
Posts: 1,219
Basicaly I stick with my original plan, nutral and ground to seperate busses, both get wired back to the main panel seperately, but the ground bus also gets wired to a couple grounding rods out at the garage with ground wire of one size below the cable I use for power, ie 8ga if im running power on 6ga
Correct except for the wire sizing.

For the equipment grounding conductor run back to your main panel, you use NEC table 250.122 to determine the correct size, adjusting the size larger if you increase the size of the circuit conductors. For 30,40, or 60A circuits, you are permitted to use a #10 EGC. So you are fine if you use #6 circuit conductors, a #10 equipment ground conductor, and 60A breakers.

The connection to the ground rods is set by a different part of the rules. Rather than going down all of the options, IMHO you should simply use #6 copper conductors to the ground rods, #4 if they are subject to physical damage.

The point that I made about 'upsizing' the conductors can lead to something of a paradox: If you use #6 conductors with a 60A breaker, then you get to use a #10 EGC, but if you use #6 conductors with a 40A breaker, you must use a #8 EGC! The paradox is resolved when you consider _why_ you might use a 40A breaker with #6 conductors: voltage drop. If the conductors are so long that you need to upsize the circuit conductors, then you must upsize the EGC as well. But if you increase the size of the circuit conductors because it feels good, then you bump into the same rule, and your EGC gets larger.

You have never mentioned the _distance_ to the garage, so we've not even touched on the voltage drop issue, and this would be worth double checking.

Old 11-08-05, 08:25 AM
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I havnt measured that distance but I would say 100' would be safe, probably a bit less. The distance between the house and garage is only maybe 25', but the house main is on the far side, so it needs to run clear across the house bacement.
Old 11-08-05, 08:31 AM
Join Date: Jan 2004
Location: Oregon
Posts: 1,219
At a distance of 100 feet, a 60A circuit on #6 copper is fine from a voltage drop perspective.


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