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I want to put in two pedestal lamps at the beginning of my driveway, as well as a weatherproof outlet box. The total run from the outside panel is 30 ft. I want at least 30 amperes at the box. The wires will be in conduit.

Presently I have enough AL #6 wire for my hot & neutral (I already had the wire), but not enough for the ground. I do have some CU #10. Can I use it?

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You can use the 10AWG copper for your EGC, provided the conductor is bare or insulated with a green or green + yellow stripes finish -- 250.118(A) doesn't say anything about not being able to use a grounding conductor made from a different metal than your circuit conductors:

(1) A copper, aluminum, or copper-clad aluminum conductor. This conductor shall be solid or stranded; insulated, covered, or bare; and in the form of a wire or a busbar of any shape.

and Table 250.122 allows the use of 10AWG copper for the EGC of all circuits up to 60A in ampacity. Since Table 310.15(B)(16) limits the ampacity of 6AWG aluminum THWN to 50A when in conduit, you'll be safe on that front.

HOWEVER, you must use receptacles of appropriate ampacity -- 15A receptacles are NOT allowed on 30 or 50A circuits! This restriction comes from 210.21(B)(3) and the associated Table:

Receptacle Ratings. Where connected to a branch circuit supplying two or more receptacles or outlets, receptacle ratings shall conform to the values listed in Table 210.21(B)(3), or, where rated higher than 50 amperes, the receptacle rating shall not be less than the branch-circuit rating.

Furthermore, you cannot run lighting in a dwelling unit occupancy off a 30A circuit -- this is prohibited by 210.23(B):

30-Ampere Branch Circuits. A 30-ampere branch circuit shall be permitted to supply fixed lighting units with heavy-duty lampholders in other than a dwelling unit(s) or utilization equipment in any occupancy. A rating of any one cord-and-plug-connected utilization equipment shall not exceed 80 percent of the branch-circuit ampere rating.

You'd need to install a weatherproof subpanel by your driveway with the 6AWG Al as a feeder to do this, following 312.2 for the enclosure:

Damp and Wet Locations. In damp or wet locations, surface-type enclosures within the scope of this article shall be placed or equipped so as to prevent moisture or water from entering and accumulating within the cabinet or cutout box, and shall be mounted so there is at least 6-mm (1/4-in.) airspace between the enclosure and the wall or other supporting surface. Enclosures installed in wet locations shall be weatherproof. For enclosures in wet locations, raceways or cables entering above the level of uninsulated live parts shall use fittings listed for wet locations.

Once that subpanel is in place, you could then use 14/2 w/ ground UF or USE cable rated for direct burial to run standard 15A branch circuits to the outlet box and light fixtures.

Either that, or just pull a bunch of THWN copper through the conduit (ensuring it's appropriately sized, of course), and run multiple branch circuits. You'll need at least 2 in this approach -- 1 15A for the lighting, and another branch circuit of the appropriate size to feed your outlets.

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There is nothing wrong with mixing wires when using them as separate conductors as long as the terminations are designed for the wire type you are using. That is mixing aluminium and copper is not inherently a problem. Just make sure you are taking the precautions needed for the terminations on the ends as required for each wire type (or in this case, both wire types).

Aluminum is the type the requires what are generally considered special precautions because it is the one used less often in lower ampacity circuits.

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    While you aren't wrong here (hence the lack of a downvote), you miss the other Code defects in his proposal -- wanting at least 30A at the box doesn't mix with having lights on the circuit! – ThreePhaseEel Feb 11 '15 at 1:24

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