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I am trying to determine the best way possible to get a 240v outlet to my detached garage approximately 25' from the breaker panel inside the house. I currently have 2 separate feeds to the garage now run in conduit. Each 12/2 w/ground feed is connected to its own 20 amp breaker inside the house. I will still need 120v service for existing electrical devices in addition to the new 240v. Any way I can utilize the existing feeds to wire a sub panel perhaps???

Note: Existing 120v feeds each serve half the garage.

Thanks in advance!!!!

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  • So the 12/2 is installed in conduit? What size is the conduit? – JACK Apr 19 '20 at 17:39
  • Are the two existing runs cable in conduit or individual wires? Also, what are you planning on running that you need 240v for? That it's such a short run it wouldn't be too costly to re-run a 4 wire service with larger conductors. All depends upon size of conduit, and amperage needs. – George Anderson Apr 19 '20 at 17:50
  • Also, need to add that if you install a sub-panel in a detached building will require ground rods. – George Anderson Apr 19 '20 at 17:52
  • 240v is needed for a welder. The existing runs are still in sheath. Conduit is 1". Hope that helps. Also, ground rods are not an issue. Have about 6' left of rod. – alwdonovan Apr 19 '20 at 18:25
  • What type of conduit do you have installed already? PVC? Rigid (threaded metal)? EMT? – ThreePhaseEel Apr 19 '20 at 18:49
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As far as using the existing feeds to power a sub panel, no you can't. You can't parallel cables that small. The good news is, depending on the size of your conduit, you could pull some THHN in it, after removing the 2-12/2 cables. This could feed a sub panel and then you could reroute your two 120 Volt circuits into the sub panel and also add your 240 Volt outlet.

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  • Well my conduit is 1". I'd definitely have to remove the 12/2 to get it to fit anyway. 10AWG or 8AWG? As mentioned above the 240v is for a welder. I should also mention the conduit only runs from side of house to side of garage. Inside the house to the breaker box is sheathing. Same for once inside the garage. – alwdonovan Apr 19 '20 at 18:32
  • No problem running 4#10 or 4#8 in 1"conduit but you'd have to complete the conduit runs on each end. You could just run a new run of 10/3 or 8/3 UF cable from your panel to your garage – JACK Apr 19 '20 at 18:54
  • 8/3 UF cable won't fit inside 1" conduit, not even in the hillbilly electrical code. 10/3 UF wil just fit in 1" trade size, but not inside Schedule 80 PVC 1". – Harper - Reinstate Monica Apr 19 '20 at 19:39
  • The 4 wire sub makes more sense to me move to a double pole breaker for the feeder go big you could pull up to 4 awg wire thhn/thwn providing a nice sized sub with plenty of future adds.+ – Ed Beal Apr 19 '20 at 19:40
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  • Dual 12/2 is not legal in a 1" conduit. (to be more precise, 12/2NM is not allowed outdoors, and two 12/2UF are too big for a 1" conduit - that's why they bind.)
  • You can't run 2 circuits to an outbuilding of the same voltage and usage. It would be OK if one of them was on a switch at the house, or had some other characteristic distinguishing it.

So the existing arrangement has been illegal for some time.

At least some of your conduit should be Schedule 80, so you also cannot run any /3 UF cable larger than #12. It won't fit, and will be a nightmare pull anyway. You probably don't know enough swear words :)

What you can do is install LARGE junction boxes at each end of the conduit run. Then run common indoor NM cable of your choice of size from panel to conduit, transition to THHN wires inside the box, run THHN wires up to #4 copper in the 1" conduit (with a #8 bare ground). Then at the other box, transition back to NM cable for the run to your future subpanel.

That should set you up for 80 amps @ 240V.

If you go #6 copper THHN + #4 cable outside the conduit, that'll let you breaker at 70A. #8 THHN + #6 cable will let you breaker at 50A. 6/6 will give you 60A. 8/8 will give you 40A.

I realize THHN individual wires is probably "new to you" and so you are not comfortable with it. Take the time to get comfortable with it!! Once you do, you'll laugh at how stupid-easy it is pulling fat wire like #4, when pulling thin cable like #12 was so darned hard!

A couple details.

Subpanel size: Go big. Go really big. Spaces are cheap, regrets are expensive. A lot of people just count on their fingers how many breakers they'll need, and look for a panel with X/2 spaces X circuits. Don't do that. Double, even triple the number of circuits you think you need today. Get 2*X spaces 4*X circuits. Disregard the number of circuits claimed; that number is a lie. If you shop carefully, and watch out for "value packs" that include breakers also, you'll find the difference for a much larger panel is like a pizza. And that will pay back dividends everytime you add a tool or device. Don't be the poor schlub who spends $200 too much buying needless wire size, but saves $30 by getting too small a panel.

Wire color markings. Use bare copper for ground wires. #10 for up to 60A, #8 for up to 100A. Neutral wires must be actual white wire, unless the wire is #4 or fatter, in which case black is fine. Mark it with white or gray tape to indicate neutral.

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  • I currently have junction boxes on each end already with UF 12/2 in the schedule 80 conduit. It was a tight squeeze. I would be comfortable running THHN inside the conduit I just didn't have conduit all the way to the breaker box so I ran the sheathed. What size traditional NM would I use between the breaker box and the junction box? – alwdonovan Apr 19 '20 at 21:47
  • @alwdonovan It's perfectly OK to use UF between breaker box and junction box. NM or UF is fine indoors. Size it for the breaker you want to run (using the 60C column of table 310.15(B)(16). The THHN run inside the conduit can use the 75C column of table 310.15(B)(16) for wires #8 or larger. I suggested some combinations in my answer. – Harper - Reinstate Monica Apr 19 '20 at 21:54
  • Right Now you have a code violation in that dual #12 UF cable, well, two codevios. So if you don't plan to replace it soon, replace it with THHN just inside the conduit. I'll tell you what, once you work with #12 THHN stranded wire, you'll be like "Wow, this stuff is so easy to work with, I can't believe it!" – Harper - Reinstate Monica Apr 19 '20 at 21:57

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