Detached garage sub panel

3hotwiresDecember 8, 2012

There is an existing underground cable to the garage from a 200 amp main panel. The cable has 2 hot wires and a bare ground. How do I connect these wires to the new panel? There is a grounding rod outside the garage.

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Ron Natalie

You can not connect this wiring to a subpanel.
Even with the old codes, you're going to need a neutral and you can NOT use the bare wire for this.

You can feed a single 240V circuit with this wiring.

    Bookmark   December 9, 2012 at 10:03AM
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I don't know the CEC. In the US, if the wire is #10 or larger, you could pigtail one black wire and land it on both hot lugs in the panel. This would give you only 120 volts at that panel. The white wire goes to the floating neutral bus. The bare wire goes to the bonded ground bus. And you would need two ground rods.

    Bookmark   December 9, 2012 at 11:48AM
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It has nothing to do with qwire size.

you meed two hots, a ground, and a neutral.

You cannot share a ground wire to a separate panel as a neutral.

That was reserved for dryers and ranges and has not been allowed on new work for many years.

    Bookmark   December 9, 2012 at 12:08PM
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Brickeyee, there's nothing in the code that requires you to bring 240 volts to a panel. Black, white, bare is fine for 120 volts. There is a code section that requires a panel to be fed with no less than a 30 amp breaker. That's why I specified a wire size. Try reading the entire post next time.

This post was edited by glennsparky on Sun, Dec 9, 12 at 14:33

    Bookmark   December 9, 2012 at 2:29PM
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Ron Natalie

He's not even close to the dryer/range exemption. That allows the NEUTRAL to be used as a ground. He doesn't have a grounded conductor here, just an equipment ground. The more appropriate missing obsolete exemption is the one that allowed structures that have no other metallic connections to the feeding structure to not need to be fed with a ground. That exemption is gone in the later NEC, and again he still doesn't have a legal grounded conductor.

Sparky is right, if he only has 120V loads, there's no specific requirement that he run two hot legs. He can use one (appropriately remarked) as the grounded conductor.

    Bookmark   December 10, 2012 at 9:12AM
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I believe this three wire system was to code a few years ago for detached structures.The cable had been connected to an older panel,but was removed when it was decided to convert the garage into a workshop.A newer sub panel was mounted on the wall with a 70 amp double pole breaker and the same size was installed at the main panel.The previous installation seemed to function quite well,but it was hard to find breakers when I wanted to ad to the panel.The cable looks to have 2 #4 insulated aluminum wires with a similar bare ground.The 2 hot wires were connected to the breaker,and I think,but not sure that the ground wire was attached to the common bus bar with the bonding strap removed.All the equipment white wires were attached to the common bus and all the equipment grounds went to the ground bus which was attached to the panel box and to the grounding rod outside the building.Hoping this helps and thanks for any feedback you have.

    Bookmark   December 11, 2012 at 3:42PM
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You can only have a single circuit to a detached structure.

That is ONE 120 V circuit or a single multi-wire (but that requires 3 wires plus ground.

    Bookmark   December 11, 2012 at 3:49PM
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"Seemed to function quite well" is a lot different than "safe and to code".
It doesn't sound like the previous installation was safe and to code.
You don't have a neutral wire. The bare ground cannot safely act as a neutral wire.
As was already stated, one of the hot wires could be re-marked and re-used as a neutral wire, but then you'd only have 120V available at the subpanel.

The other option is to run a new cable.

    Bookmark   December 11, 2012 at 4:00PM
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Ron Natalie

You can put in a single (perhaps multiwire) branch circuit and avoid a grounding system for the building. As pointed out there's only enough conductors here to do a 240 (without neutral) or 120V circuit.

You can use the wire for a feeder to a subpanel (if you put in a grounding system), but as pointed out, there's not enough conductors here to put in a 120/240 panel.

    Bookmark   December 11, 2012 at 4:36PM
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