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I've got an existing garage about 30 feet from the house which I need to reconnect (wires were cut during house construction project). I have three 2-conductor cables running to the garage, as well as what looks like a low-voltage 2-conductor cable.

I'm just going to ignore the low voltage cable, which means I have 6 wires going to the garage: 3x 2-conductor cables.

There's no ground wire bundled in the cable. So what's the proper way to hook this up? Can I use two of the cables to run two 20A circuits out to the garage, using the third cable as ground? Or just use one cable for power and another wire for ground, ignoring the third cable? I have steel conduit in the house and in the garage, but not in between the two.

I was thinking of putting a panel in the garage with two breakers there, but now I'm thinking maybe just a shutoff there will be fine.

I'm not particularly asking about grounding or GFCI breakers/outlets (yet), as I think I have all that covered by other sources. I'm just not entirely sure how to hook things up given the existing wires.

And I will be rewiring the garage (house is all new wiring) as it's all a cluster-fun in there right now. IOW, there is nothing proper in the garage now that I could use for reference.

Thanks, John

  • What outlets/devices are you trying to provide for in the garage? I take it this is a detached garage, by the way? – ThreePhaseEel Jul 11 '16 at 22:30
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    Bottom line is what you have is already a code violation. You can only have one circuit or feeder to a detached structure. – Speedy Petey Jul 12 '16 at 15:27
  • @SpeedyPetey is exactly right. Plus you don't even know the cable size, so you don't even know if it is rated to be ran as far as it is being run. If you just want a couple of outlets and lights you might get by with 12G - depends on run length - but might need 10G or even bigger. Also if you do this incorrectly I would find it very hard to hide from a local inspector or anyone doing an inspection before buying house - which could lead to your ripping out all your electrical in garage and starting over. Ignoring inspector is one thing, skipping code is another. – DMoore Jul 12 '16 at 22:51
  • I have been assuming the wire gage is enough to at least run lights and a garage door opener, as well as the occasional table saw or compressor, which is all I really need. I recently bought the house, and the home inspector made no comment on the garage wiring other than needing to cover a few open boxes. And the city inspector had no comments on the garage either. So I guess I'll check the wire gage and worst case run a new wire if need be. Thx. – John E. Jablonski Jul 14 '16 at 18:52
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You can reuse the wires one of several ways.

Run a separate ground wire

Since it is a retrofit, you can add ground wire. (NEC 240.130). The 3 circuits can share one ground wire if they all originate in the same panel (as of NEC 2014‘s new 240.130D). The ground wire must be installed properly, and an appropriate size for each circuit it's grounding. If your fattest conductor is 10 AWG, a 10 AWG ground wire can protect all 3 circuits under NEC 2014. If I recall, 8 AWG bare copper wire is readily available.

Hots and neutrals from the same circuit must be routed together. A 240.130 retrofit ground may follow a different route.

If an existing wire is already bare and appropriate size, you already have your ground wire!

Wire larger than 6 AWG can be redesignated as ground by wrapping it in green tape at each accessible point.

Ask your inspector to re-designate

Generally you cannot re-designate a conductor to be ground, although an entirely bare copper wire can only be a ground.

Ask your local inspector if there's a way to allow this anyway. Local jurisdictions can override Code.

Forget ground and use GFCI

GFCI's may be required anyway for your new circuits.

You can use the GFCI rules, which allow ungrounded 3-prong outlets when fed by a GFCI breaker or outlet. All the outlets must be labeled "No Equipment Ground".

They must also be labeled "GFCI protected" if they are a plain outlet fed by a GFCI upstream.

In this case, take care not to connect ground to anything. Nothing is safe except a ground wire run all the way back to the main service panel.

Make the barn a main panel, isolated by a transformer

This is a complicated one. Wire the barn as a main panel. It must have its own ground rods and neutral must be bonded to ground in the panel. However this neutral must not bond to the neutral back at the house.
It is isolated from the house by a transformer. This is a "mini" version of the service the power company provides to you.

The key is a transformer of appropriate size. It must be single-phase and with a VA rating exceeding your planned load. I often see them on Craigslist at sane prices, e.g. 5K VA for $100. Typically, you can jumper the secondary to either supply 240/120 split phase like your house, or 120-only at twice the amperage.

For amp capacity of a transformer that you've jumped for 240V, easy peasy - divide VA by 240. For instance, A 5K VA transformer is 5000/240 or 20.83 amps. It can be supplied by a 20A breaker and 12 AWG wire, and can output 20A to each side of the panel. If jumpered for 120V-only, it can provide twice that or 40A.

Main breakers that size will be tough to come by, don't bother, just backfeed a regular breaker.

Multiple circuits? Somewhat.

Generally NEC allows only one circuit per voltage. So you can have 240V on one pair of wires, and 120V on a second pair.

You can have more circuits for specialized uses which require a separate circuit, for instance the third pair of wires can be used for lighting controlled from the house. I would use that loophole to power the barn interior lighting on a separate circuit from the outlets - that way if the table saw kicks and trips the breaker, you aren't plunged into darkness with your hands 3 inches from a spinning blade.

Re-designate wire? Mostly no.

Generally, wire function is decided by color, and they cannot be re-designated unless they are larger than 6 AWG (at the request of distributors, who do not want to stock multiple wire colors in large sizes). However, neutral (white or gray) wire can be re-designated hot (e.g. to allow 10/2 to feed 240V loads). Also, an entirely bare wire can only be used as ground.

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No, you can not use the existing wiring. All of the wires you use in a circuit must be in the same jacket or same conduit.

  • But if everything is in the same conduit between the house and garage? – John E. Jablonski Jul 11 '16 at 19:11
  • What gauge are the wires? Are they copper or aluminum? How long are they (approximately)? – longneck Jul 11 '16 at 19:20
  • But you also said they are not in the same conduit. What has changed? – longneck Jul 11 '16 at 19:21
  • Gage is 10 or 12 (need to double check, but fairly large wire). Copper. 30-ish feet between house and garage. CLARIFICATION: No steel conduit between house and garage. So no continuous ground connection between them. Sorry for the confusion there. – John E. Jablonski Jul 11 '16 at 19:34
  • If it's really continuous conduit, you could pull whatever you want... But often, underground wires are only in conduit where they enter the earth. Code allows that, and in fact, requires it for direct burial cable. But if it is full conduit, just pull a ground wire, done. I would use green THWN stranded wire because of that nice slippery nylon outer later. – Harper Jul 12 '16 at 22:42

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