Adding 60 AMP breaker to existing box...

wicrulesDecember 27, 2007

I recently built a studio for my wife behind our house. I put a GE 8/16 125 AMP subpanel breaker box in the studio. I was going to install a 60 AMP breaker into the breaker box in the house and run 2-2-2-4 aluminum in PVC conduit underneath the house, through the block foundation and in a trench out to the studio. The problem I have run into is that I can't figure out where to tie the ground and common wire into the the house breaker box. I had a 70 AMP circuit run to an exterior sub-panel box for a greenhouse a couple of years ago. The electrician used the main lug on the ground bar for the ground on that circuit. I also noticed he did not run a common to the sub-panel.

I guess my question is, do I need to just run a ground and two hots to the panel in the studio? Is there some type of modification I need to do to the panel in the studio to tie the ground and common bars together? Also, is there some type of adapter to put on the ground bar in the main house panel to put the #4 ground wire into?

Thanks!

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brickeyee

A 3 wire service (hot-hot-neutral) vs. 4 wire service (hot-hot-neutral-ground) may limit what you can connect to the separate building.

If there are NO other metallic connections to the separate building (no phone lines, no water lines, etc.) you can use a 3 wire service.

If there is ANY metallic connection besides the power wiring to the detached building you MUST use a 4 wire service.
Current is very likely to try and return on the 'other' connections and create a hazard.
A run ground line provides a low impedance path and limits the current that will flow on other paths.

You never run "a ground and two hots" to anything except a 240 volt load.
Since you will have 120 V loads you need at least a neutral and one (or more) hots.

If you are not experienced with supplying panels in separate structures you should probably hire this on out.

    Bookmark   December 27, 2007 at 10:50AM
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wicrules

Thanks for the info. I typically have this kind of work done simply because I don't have the time, but the prices I have been quoted for the work are rediculous based on what I priced materials out for.

I guess the main question I have left now is how to tie in #2/#4 gauge wire to the main breaker box. I don't believe they will fit in the ground/neutral bar holes.

    Bookmark   December 27, 2007 at 11:11AM
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brickeyee

Most box vendors have adapters that use multiple small holes in the ground/neutral bar for a larger conductor.
Look around the web site for the box manufacturer.

    Bookmark   December 27, 2007 at 2:03PM
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wicrules

Thanks for the feedback. I will stop at Lowes on the way home today and look for the adapter.

    Bookmark   December 27, 2007 at 2:37PM
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brickeyee

You may have to go to an electrical supply house.
There is no standard spacing for the ground bar screws, so the fingers on the adapters are specific to each manufacturer.

    Bookmark   December 27, 2007 at 4:01PM
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cobraguy

Brick, where it's a separate structure from the house (and main panel) isn't a ground rod required?

    Bookmark   December 28, 2007 at 8:12AM
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wicrules

I talked to an electrical supply house yesterday looking for a lockdown kit for my box. I threw in the question about a ground rod and his response was since I was running a ground back to the main panel it wasn't necessary, but putting one would ensure it was really grounded.

So, I am planning on running the ground from the main box and sinking a grounding rod outside the studio and hooking it in as well.

    Bookmark   December 28, 2007 at 8:56AM
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brickeyee

"Brick, where it's a separate structure from the house (and main panel) isn't a ground rod required?"

Not just one, but two unless you measure the first one at 25 ohms or less.
Of course to perform the measurement you have to drive a test rod...might as well just drive two rods and be done with it.

The OP should read the FAQ linked below.
It covers almost all the basics.

Here is a link that might be useful: Sub-panel FAQ

    Bookmark   December 28, 2007 at 10:04AM
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wicrules

OK, I want to run this by you guys....

1) 60 AMP breaker in main panel.
2) 2-2-4 aluminum between main panel and sub-panel. (2 hots and neutral)
3) 2-2-4 in conduit beginning at the top of the crawlspace blocks under the house where it exits the wall, the entire length of the run under the house, in the trench (18" down)and up into the crawlspace of the studio.
4) 60 AMP breaker in sub-panel (> 6 circuits) (does the cable need to be in flexible conduit within the wall until it enters the sub-panel?)
5) 10' grounding rod connected to grounding bus in sub-panel via copper wire.

I have tried to locate a "lockdown" kit for the 60AMP breaker in the subpanel, but nobody seems to carry them. Even the supply houses don't.

I believe this will be sufficient. Comments/suggestions appreciated.

Thanks for all the help!

    Bookmark   December 28, 2007 at 10:46AM
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terribletom

If you decide on a three-wire service, then I believe you'll need to run 2-2-2, not 2-2-4. A true equipment ground--not a neutral--can be a size lower (#4), but I don't believe that's the case when neutral and ground are combined.

[Since you'll have more than six breakers, I'm guessing that a good number of the circuits will be 120V. As Brick noted, if you had only 240V loads, no neutral would be needed at all, but that doesn't seem to be the case here, and usually isn't with multiple-circuit outbuildings.]

I know for certain that the three-wire service you're proposing would not fly in my county, but that is because local restrictions here (not necessarily the NEC) won't allow it. (My jursidiction doesn't buy into the "no other connections" exception for 240/120V service.) Perhaps they figure that sooner or later someone's going to run a phone line or intercom or water pipe, and anything like that--any other connection!--instantly puts the wiring out of code.

I can't speak for your area, of course, but that begs the question of whether you plan to pull a permit and have this installation inspected. In any event, it might be a good idea to consult with your AHJ to verify that your plan meets local code.

For a variety of reasons--not least the flexibility of adding things in the future--you really might be better off with four-wire service (2-2-2-4).

As noted above, you'll probably need 2 ground rods driven at least 6' apart. While it is true that one will suffice if it can be demonstrated to have a resistance of 25 ohms or less, don't bank on that. I've put in three ground rods in the past six months in three different states (Virginia, Maine and California) and none of the three passed the 25 ohm test, but that depends upon local soil conditions, moisture etc. at any given time. Use #6 wire for the ground connection and avoid any splices! (That's one of the problems with adding a second rod "after the fact".)

I've also run into the problem of not being able to find subpanel "main" breakers under 100amps with the code-required lockdown feature. Keep in mind that you can use a larger breaker there (i.e., 100amps) because it is serving simply as a disconnect rather than primary overcurrent protection for the wiring and your box is rated at 125A. With #2 wire coming in, it'll probably fit easier into a 100A breaker as well. The one slight disadvantage is that if the "main" does trip, it'll be the one in the house not the outbuilding. But this is not something that should be happening often. (We hope!)

Just out of curiosity, how long will the service wiring run be? I ask that because you might want to price #4 (or possibly even #6) copper instead of #2/AL for the sake of comparison, if you haven't already. There's no question that #2/AL will do the job (unless it is an inordinately long run), but #4/CU (with #6/CU ground) would be a good bit easier to pull and work with. Still, I know that copper's gotten very expensive in recent years, so YMMV.

    Bookmark   December 28, 2007 at 1:19PM
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mike13

As mentioned above, 3-wire service is not code-compliant in many jurisdictions. Have you checked w/ your local AHJ?

I'd recommend going 4-wire. Also, you need to keep the neutral & ground isolated.

    Bookmark   December 28, 2007 at 1:49PM
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wicrules

terribletom,

The run is about 75' from the main breaker to the sub-panel. I called around looking for 4/3 w/grnd romex and nobody carries it. 6/3 is the largest they had and from my research, its only rated at 55 amps.

I have another electrician coming out Wednesday to look at doing the work. Its raining cats and dogs here so I won't be able to do anything until then anyway.

Thanks!

    Bookmark   December 28, 2007 at 1:56PM
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brickeyee

"I called around looking for 4/3 w/grnd romex and nobody carries it. 6/3 is the largest they had and from my research, its only rated at 55 amps."

You do know that you cannot pull NM into conduit (except for short lengths for physical protection) and that it cannot be used underground (a wet location)?

    Bookmark   December 28, 2007 at 2:13PM
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Ron Natalie

While they do make 4 gauge NM, it can be difficult to find. We usually end up with SE cable in the larger sizes. It by and large can be installed legally anywhere NM would be.

A lot of guys still call it "ROMEX" anyhow.

    Bookmark   December 28, 2007 at 2:35PM
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wicrules

brickeyee,

Yes, I do. The first electrician I talked to was planning on running romex in watertight conduit between the house and the studio. Otherwise, it would be exposed.

    Bookmark   December 28, 2007 at 2:54PM
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terribletom

wicrules,

When you mentioned a conduit run, I assumed you were talking about pulling individual THWN wires, *not* NM cable (a.k.a. "romex"). As I'm sure your electrican will confirm, THWN is the wire of choice in this case and you shouldn't have trouble finding that in #4/CU.

You basically have two practical choices here: Pull THWN in conduit or bury a "Type UF" (underground feeder) or "USE" cable without conduit. Either of these are suitable for underground use. You'd have to trench deeper if you use UF, though.

BTW, although #6/CU may be rated at 55A for many purposes (depending on which temperature column is applicable from the code), it can be protected by a 60A breaker due to the so-called "round-up rule" since 55A breakers are not a standard size. If you want to ensure a full 60-amp capability under continuous loads, though, #4/CU or #2/AL is what you want and either will also reduce voltage drop over the run. (If your run length of 75' includes parts of the run inside the buildings, VD shouldn't be much of a problem in any case.)

    Bookmark   December 28, 2007 at 3:18PM
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solarpowered

Also, "romex in watertight conduit" is not acceptable. Underground conduit, whether watertight or not, will get wet, and is considered a "wet" area as far as the Code is concerned.

    Bookmark   December 28, 2007 at 3:52PM
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brickeyee

"brickeyee,

Yes, I do. The first electrician I talked to was planning on running romex in watertight conduit between the house and the studio. Otherwise, it would be exposed. "
Romex is actually a band name, and they make a number of types of cable.

ALL underground conduit is a WET location.

ANY electrician even considering running a cable in conduit, or NM cable should have their ticket pulled.

If he was planning on using UF, he still has to get past using it in conduit.

Cable is still basically not allowed in conduit systems.
In can be placed in conduit when required for mechanical protection.
A full run from house to a separate building would NOT be acceptable under the 'protection from damage' rules.
You need single wores in a wet rated insulation.
THWN is very common.

    Bookmark   December 28, 2007 at 7:41PM
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