Detached Garage Power

Old 06-16-14, 01:56 PM
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Detached Garage Power

I bought a house last year (built in '76) and it has a detached garage. Currently, there is a sub-panel in the garage that supplies 120v via what appears to be buried 12-2 romex. I'd like to have both 120v and 240v at the sub-panel.

I want to get a 240v MIG welder (20A) , and my wife has a 240v kiln (drawing about 15A) that she uses once in great while for pottery. They will NOT be used concurrently.

The run will be 70' from panel to panel, and I plan to bury it in 1-1/2" PVC conduit.

The 120v side has the normal garage stuff (overhead florescents, garage door, and power tools such as an air compressor or table saw)

My question is what wire size and type should I use. #8 or #6 ? Copper or aluminum? Since it'll stay single phase, do I need 4 conductor, or can I stick with 3 conductor?
Old 06-16-14, 02:31 PM
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via what appears to be buried 12-2 romex
Note if it really is NM-b it was a cde violation however since it will be a abandoned a moot point.

A 40 amp feed with #8 copper should be enough but you night as well future proof it with a 60 amp feed using #6.

Since it'll stay single phase, do I need 4 conductor, or can I stick with 3 conductor?
Current code requires 4 wires, two hot, one neutral, one EGC (ground). You can either use THWN individual conductors in PVC conduit or direct burial xx-3 UF-b. If you use conduit the minimal depth is 18" and the EGC can be #10. If you use UF-b minimum burial depth is 24". You would sleeve the UF-b for protection with conduit where it enters and leaves the ground.
I plan to bury it in 1-1/2" PVC conduit.
Best practice is to not run cable in conduit so use copper THWN. 1" conduit would be large enough for #6. (Will hold 7 #6)

I would suggest a 100 amp 12 space main breaker panel for a sub panel. The main breaker is not for protection, just used as the code required disconnect. If the wire you use as supply is smaller then minimum wire size for the main breaker you would pigtail on larger gauge wire in the panel.

Some would suggest using a main lug panel but then you are going to have to add a back fed breaker with hold down kit to act as a disconnect or provide a separate unfused disconnect ahead of the panel. You can often get a main breaker panel kit that includes some branch circuit breaker for less then what a main lug will cost when you add the cost of breakers you will need to buy for the main lug.

For either type panel you will need to buy and add a ground bar kit. The ground bar is bonded and the neutral bar is isolated. In addition you will need one or two 8' ground rods at the subpanel.

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