Detached Garage, conduit question

Old 01-25-08, 10:57 PM
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Detached Garage, conduit question

Ive been reading up on wiring my garage, but I have a few questions for you all. I plan on wiring my garage for 240, 60 amps max. Rough plan is to run 6/3 with ground, about 50' from the main 200A panel to a new 100A panel in the detached garage. First question, should I, or rather can I use 60 amp breakers at both ends? Ive read others are using 60amp breakers at the main, then 100amp at the sub. Or do I have that backwards? Id prefer to use 60 on both ends since thats the size circuit Im running. It doesnt make sense to me to have a larger circuit breaker than the circuit itself.

Next, I have an existing 1" pvc conduit running from the house, above ground for about 40' along the side of the house, then it shoots under a 4" concrete slab about 10' long, then it elbows up to the outside of the garage. Question, what type of wire should I run? UF or THHN, NMB or what? I havent the foggiest idea what the differences are between them. Can I get away with running any of those wires in pvc above ground? Whats cheaper in the long run? I will be using copper and would prefer to use the pvc thats there, if i can.

I will have a small shop setup eventually, with a table saw at 15amps, a radial arm saw about the same, a lathe about 12 amps, a compressor and other such things around the 7-10 amp range. Wont have more than one of the larger amp suckers running at once. I will be rewiring my radial arm saw and maybe one other tool for 240 for sure and Ill have several runs of lights and receptacles. Question, should I add the double pole breakers for the 240 machines as I need them, or start with as many as Ill ever need right off the bat? What awg and type wire should I run for something like a table saw at 15amps 240v? The garage is unfinished, are there any code considerations I should take into account when running the wire through the framing? I understand local codes vary, but in a general sense, whats common practice?

And last question, for now , Im short on space in my existing main panel. I have one open slot, and one open space on a tandem breaker. I had hoped to move an existing 15amp circuit down to the open space on the tandem breaker, leaving 2 open slots for a double pole breaker for the sub. Does that sound reasonable? The circuit I would be moving is far below 15 amp draw so I cant imagine it being a problem, but Id like to know for sure it will pass code inspection, should I ever get one.

Thanks alot for the time, and sorry for being windy
Old 01-25-08, 11:58 PM
Join Date: Mar 2006
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First off, I am not an electrician but I have done some work similar to what you are proposing.

If you use a 60 ampere breaker in your main (service) panel then what size you use in the garage panel is irrelevant as it becomes only a panel disconnect. You could, of course, use a 50 ampere breaker in the garage panel and preclude the house breaker from ever opening on overload.

I think that you will find it easier to purchase a 100 or 125 ampere main breaker panel than having to buy a main lug panel and then a smaller breaker (with auxiliary hold-down clamp) to back-feed the main lug panel. Also, you want a panel with sufficient "spaces" to install all necessary breakers. Remember that unless you use tandem breakers you will need two spaces for every 240 volt breaker.

Local code will dictate if you can use the existing PVC conduit. I don't care for PVC in "exposed" areas where it may be subject to damage like someone slamming a car door open against the conduit. You also need to use PVC conduit that is "listed" for UV (ultra violet e.g. sunlight) exposure. Some localities will require schedule 80 PVC where it is exposed. Note that you may have no more than 360 degrees of bends (four 90 degree bends) between "pulling points". You would probably use at least one and maybe two or more "conduit bodies" that act as pull points.

For wire you want individual conductors rated THWN and these may have a dual rating of THHN. You must have the W in the rating for any conduit outside or underground. Stranded wire pulls easier than solid. You MUST have white, black and green wires; re-marking of wires smaller than #4 is not allowed under the National code although your local inspector MAY (or may not) allow remarking. The "second hot" wire may be red or it may also be black unless your local inspector requires it to be a different color from the first "hot" wire. You may NOT use type NM cable outside or underground, even when in conduit.

Although your local code may allow type NM cable in the garage / shop I prefer conduit for the added protection. Follow the general rules for ampacity, i.e. 15 amperes uses #14 wire, 20 amperes uses #12 wire and 30 amperes uses #10 wire. Remember that each 240 volt machine will require its own circuit.

I would not purchase the circuit breakers too far in advance of their necessity unless (a) you know what breakers you will need in the future, and (b) you can get a good deal on them now.

I would add no less than two 120 volt general purpose receptacle circuits. Remember that these do need ground fault circuit interruption. I would also have the general lighting an a circuit separate from the GP receptacles. Two lighting circuits may be nice if you will be moving large pieces of material that may have the possibility of smashing a light bulb.

You most likely will have the ability to move the circuits in your service (main, house) panel to allow the use of the double-pole breaker for the shop / garage.

Don't forget that you will need to have a separate equipment grounding bus bar in the new shop / garage panel and that the neutral in this panel must NOT be "bonded" to the panel. If your shop / garage is a detached building then (unless local code dictates otherwise) you must have a grounding electrode at the new panel.
Old 01-26-08, 05:52 AM
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Go ahead with a 100 amp panel in the garage. You can then use the 100 amp breaker it probably came from as the breaker for the panel. The 60 amp breaker in the main panel protects the wire itself.

Use THWN wire in conduit. Do not use NM outdoors for any reason. In theory UF cable could be used, but it harder to pull through conduit than individual wires. Use plenty of wire lube on the wires. Do not use anything but wire lube. You do not want to damage the wires with other products.

14 gage wire is fine for 15 amp circuits. However, I would probably not use 14 gage wire. Most of the time you can use 20 amp circuits for 15 amp devices, so I would use all 12 gage wire, as a minimum. This may provide more flexibility foir moving tools around.

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