Underground feeder in conduit -- allowing slack?

number6November 13, 2010

I am about to install a new subpanel in a detached garage, about 100 feet from the main panel. There is 2 inch pvc conduit in place. It is about 3 feet underground, with risers at the house and garage. (FYI, it contains 2 45-degree bends, plus the 90's for the risers before entering the buildings via conduit body fittings.) I will be installing an aluminum feeder of 3 1 Ga cables plus ground.

Now, my question is, do I allow slack for thermal expansion and contraction of the aluminum wire through the different seasons? (This is Wisconsin, the conduit is not below the frost line, plus there's the outside portion of risers at each end.) If so, how much slack is required, and how is this accomplished, given that I'll be pulling through a conduit, not burying the cable directly in the ground? Some slack can be left in the main and subpanels, of course, but is that enough?

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brickeyee

Any slack would be in the boxes at each end of the conduit.

You do not really need much though.

    Bookmark   November 15, 2010 at 10:00AM
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Ron Natalie

As long as the cable isn't pulled unreasonably tight, there should plenty of room for thermal expansion.

    Bookmark   November 15, 2010 at 10:19AM
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lbpod

It's not the thermal expansion/contraction that you have to
worry about. It's the settling of the trench, (I've seen
meter boxes ripped right off of houses for this reason).
But if your trench has been there for a while, it will
probably be alright.

    Bookmark   November 16, 2010 at 10:10AM
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maryland_irisman

I'd like to throw up a small flag since you are using aluminum wire...I'm sure you are aware but I thought I'd mention to make certain you use the proper method for terminating aluminum wire or your temperature swings will certainly have an impact.

    Bookmark   November 17, 2010 at 7:54AM
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brickeyee

"I'd like to throw up a small flag since you are using aluminum wire...I'm sure you are aware but I thought I'd mention to make certain you use the proper method for terminating aluminum wire or your temperature swings will certainly have an impact."

Yes, aluminum woire needs to be correctlt terminated.

There has not been any issue with correctly terminated Al stranded wire in larger sizes.

The issues have been with single solid conductor aluminum wire, and there is NO WAY to correctly terminate these.

The Al is not tempered hard enough to allow for installation and handling, but this then allows it to cold flow under screw heads and other pressure type connectors and become loose after repeated thermal cycles.

    Bookmark   November 17, 2010 at 9:34AM
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