60A subpanel installation in detached garage

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Old 05-08-17, 12:40 PM
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60A subpanel installation in detached garage

Hey folks. I'm in the planning stage of getting (reliable) electricity run to a detached garage. Currently there are two BX cables run through a buried steel conduit to the garage. Both are elderly, and of the cloth-jacketed wire variety. Obviously those need to go.

The conduit has a 1.5" internal diameter, and passes through the foundation on the house end. On the garage end, it opens into the bottom of a well that looks like it was made with a clay chimney pipe. Getting out of the conduit is a relatively sharp bend: the well is around 8" across. The actual route distance looks like it's going to be just about 75 feet.

The eventual goal is two 20A/110V circuits for power (woodworking tools, primarily), a 15A/110V circuit for lighting, and a 240V circuit, probably 30A, for future large tool purchases. These numbers are based on a 60A panel. Here's my current thinking:

1) Feed. Line the existing steel conduit with plastic. While it's in good shape at the ends, I have no idea what the middle looks like, and currently code seems to suggest plastic conduit anyway. I'm figuring 4-4-4-6 SEU cable from the main panel to a junction box over the end of the conduit, switch to four separate THWN cables (to ease the curve at each end), and run the four separate cables to the new subpanel.

2) Ground. It looks to me as if code says 6ga bare copper, two grounding rods 6' apart, so that's what I'll do. If that's not the code, please let me know.

3) In-garage. I'll probably do 12-2 for everything that's 110, so as to reduce the number of types of wire I need to buy. It looks like 10ga wire is right for the 220V circuit.

So here are my questions.

1) It looks like I can run up to 1" plastic conduit (possibly 1.25") in the existing steel conduit. Is that adequate for the wires I'll need? And is there really a good reason to line the steel conduit with plastic?

2) My understanding is that the limitation is 9 outlets on a 110V/20A circuit, and 1 on a 220V/30A circuit. Is that correct? If so, I'll probably run EMT or something for the 220V, and not actually wire it until later.

3) I'm reasonably certain that the main panel I have doesn't have sufficient capacity for a sub panel larger than 60A. If I'm wrong, should I just accept the cost upgrade and go to 100A? That would require upgrading the supply cable, which brings up more problems like "will that fit safely in the conduit I have available".

4) What else have I screwed up in this plan? I've done enough electrical work to be comfortable doing the work, but not enough to be certain of my planning.

Thanks in advance, all!
 
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Old 05-08-17, 01:42 PM
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Can you do some more recon on the steel conduit? Is it heavy wall threaded? If it is thinwall it is probably rotted away in the middle.

1. You can run a 70A feeder (3 #6 + 1 #10 copper THWN) through a 3/4" PVC conduit, so I think you'll be fine if you can get 3/4" or 1" PVC conduit.

2. There is no limit on the number of outlets per circuit unless you have a local code which adds that limitation.

3. When you calculate load on a service you only look at actual loads, not max rating of the panel. A "demand load calculation" is the proper approach to determine if you service is adequate. If you Google you can find some calculators to help with this.

4. You would want to use SER cable from the main panel to the junction box for the indoor portion of the run, not SEU.

When you install the new conduit it has to be fully assembled from junction box to junction box using proper fittings and sweep elbows. No more than 360 degrees of bends are allowed between accessible pull points. The THHN conductors cannot be hanging loose.

Inside the garage, you will need to provide protection for NM cable below ceiling height. The two most common approaches would be to either sleeve the NM in conduit where is runs down the walls, or use some kind of wall sheathing (drywall, plywood, OSB) to cover the stud bays where NM cable runs.

All 120V receptacles in the garage need to have GFCI protection. You'll also need at least one switched interior and exterior light fixture at each human door.

I wouldn't run the 30A circuit until you actually buy the tool(s) you want. At this point it's a guess and you'll probably just waste money on the wrong materials at this time.

Finally, this may be a good application for 6-3/g PVC-jacket MC cable, brand name Armorlite. It might be special order, but this cable can be used indoors or underground, it can be sleeved directly through the existing pipe, and can run through the garage. Might be easier to deal with than sleeving the old pipe and setting junction boxes.
 
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Old 05-08-17, 01:46 PM
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Currently there are two BX cables run through a buried steel conduit to the garage.
Was probably never code compliant for multiple reasons.
The eventual goal is two 20A/110V circuits
Nominal voltage is 120/240 not 110.
While it's in good shape at the ends, I have no idea what the middle looks like,
If it is EMT it is most likely rusted out.
I'm figuring 4-4-4-6 SEU
Type SEU and SER cables, NEC Section 338.12
Type SEU and SER Cables are not allowed to be installed underground.

I'd keep it simple and future proof. A 60 amp feed to a 100 amp main breaker panel. This gives you plenty of slots for future circuits and the main breaker panel kits are cheapest usually. (The 100 amp breaker is okay because it is only used as a disconnect.)

Wire wise either 6-3 UF direct buried or if the metal conduit is threaded metal conduit and intact main panel to subpanel with no gaps individual wires through the conduit.
 
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Old 05-08-17, 03:13 PM
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Sorry, I should have included everything about the existing conduit. It was almost certainly installed when the house was built, in 1920. The wall isn't quite the same all the way around, but it's about 1/4" thick. It slopes very slightly down into the house, and there's no evidence that water has ever gotten into it, which I figure is hopeful anyway.

1) Excellent. The plastic conduit at HD appears to have a maximum diameter of 1.3something inches for a 1" ID, so it should fit fine (barring major problems with the conduit).

2) I'm pretty sure the 9 outlets/circuit is a local weirdness: I live in a suburb of Boston, and there's some weird leftover bits of code in my town.

3) Yup. I pulled numbers from my power tools, and figured startup draw for the two heaviest plus a shopvac, then refigured with numbers for a low-end dust collector. Lights plus that number was below 30A, though I don't recall the final number. I don't use many power tools, so I have a little more leeway than most woodworkers. That seemed like a good margin for new tools.

4) You're right, of course. It's been a long few days, and my internal spellcheck isn't working so well.

I've just about finished studding out the walls, and wiring is the next step. The final product will be insulated and sheetrocked, so I'm fine there. Given the tendency for nails in the walls of a shop space, I'm trying to decide whether I'm paranoid enough to go with metallic conduit (or armored cable) all the way around. It's clearly not required, and it's quite a bit of extra work, but still...

I'm planning on GFCI circuit breakers. I've been in the situation of having the reset button behind a cabinet, and I'd rather pay extra for the breakers than have to deal with it again. Switched lighting isn't a problem, although I hadn't been thinking about outside... thanks for the reminder on that!

Armorlite looks like an excellent option. The extra cost (including tools) will probably be pretty thoroughly made up by the savings in time. You'd recommend 6-3 rather than 4-3? I was figuring on assuming I'd cross the 75' mark, which I thought was where you needed to move up to 4ga.

Thanks!
 
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Old 05-08-17, 03:24 PM
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It's always a little unclear what code was or wasn't... there's good reason to believe that the conduit has been there since the house was built in 1920, and it's quite possible the cable is original as well.

120/240: Sorry about that! I'll keep it in mind.

The conduit is not EMT: the wall isn't consistent all the way around, but is roughly 1/4" thick, neither end shows any sign of water penetration. That's not a guarantee, of course, but it does offer some hope that it may not be broken.

Thanks for the recommendations!
 
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Old 05-08-17, 03:25 PM
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Your good for at least 100 ft with #6 H-H-N and #10 G.
 
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Old 05-08-17, 03:35 PM
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Excellent. That'll make things simpler.

Thanks again!
 
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Old 05-08-17, 05:18 PM
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To clarify what you have is rigid conduit. It is still acceptable to run underground and as long as it's intact it doesn't have to be sleeved. I honestly think trying to sleeve it is a bad idea, it's going to cause you issues. You also can't actually connect the sections either, because couplings/bell ends aren't going to fit through. So if you get 50' into it and hit a snag, you're not going to be able to pull it all out.

My suggestion would be to run 2-2-2-4 aluminum RHH/RHW/USE-2 (also called MHF/Mobile Home Feeder)right through the pipe. You can also direct bury it if you wish instead of using the existing conduit (but it must be in conduit inside the structures). That is actually comparable in price to what you'd spend on #6 copper (it's about $3.00 a foot at home depot, I hear it's cheaper at Lowes) and will give you 90A capacity.
 
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Old 05-09-17, 08:14 AM
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Agreed, now that we have verified this is actually rigid conduit, you should use it as-is without an internal sleeve. Just pull the old conductors out and swab the pipe by pulling through a rag tied to a rope. You can get threadless couplings for the open ends so you don't have to thread it in place.

The MFH cable is a good option here too as long as you can get conduit fittings on each end of the pipe and connect them up to the panels.
 
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Old 05-10-17, 08:04 AM
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Fantastic, and thanks to both of you!

I have Lowe's, HD, and a few independent supply companies nearby, so I can shop around some to find the best prices.

Originally Posted by ibpooks
You can get threadless couplings for the open ends so you don't have to thread it in place.

The MFH cable is a good option here too as long as you can get conduit fittings on each end of the pipe and connect them up to the panels.
This sounds like I need to run the MHF through conduit inside the basement and garage. Is that correct? It's not a big deal if so, but it would be good to know for planning ahead....
 
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Old 05-10-17, 08:08 AM
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Yes, it needs to be run through continuous conduit.
 
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Old 05-10-17, 02:12 PM
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You know, I just went back and re-read what I had typed before and I made a mistake.. 2-2-2-4 is actually about $1.75 a foot. The price I quoted was for 4/0-4/0-4/0-2/0 (200 amp) MHF.
 
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Old 05-11-17, 11:06 AM
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Originally Posted by ray2047
Yes, it needs to be run through continuous conduit.
Thanks. That'll be inconvenient, but certainly not a major problem.
 
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Old 05-11-17, 11:15 AM
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Originally Posted by taz420
You know, I just went back and re-read what I had typed before and I made a mistake.. 2-2-2-4 is actually about $1.75 a foot. The price I quoted was for 4/0-4/0-4/0-2/0 (200 amp) MHF.
Even better! Thanks again, for both that and the rest of your advice.
 
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