I'm finally getting around to updating the service in our detached garage. I've decided on 6-3 NMWU, running inside 1-1/2" Schedule 40 Conduit for the underground run.

I remember seeing utility linesmen being careful to give themselves a few loops of slack when running new wires, to account for expansion/contraction, mistakes, etc. and leaving that nice neat loop on the telephone pole.

Do I need to add slack for household electrical? If so, how much? Do I trim it after to a precise fit, or leave a loop in the wall? 6-3 seems awfully thick to try and make loops!

The conduit run itself I've estimated at 55ft end-to-end. From the main panel <--> sub-panel is about a 60ft total run.

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    It is very expensive to come up 2" short, and a few dollars to be over a foot. There are good electricians on here to give you best practice but when I have a "younger" electrician working for me I make sure anything 8 gauge or bigger has slack if we are running before panel/appliances are up.
    – DMoore
    Jul 10 '20 at 16:54
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    First electricians law: any wir cut to length will be too short. Second electricians law: whichever end you start to test, the fault will be at the other end.
    – Solar Mike
    Jul 10 '20 at 17:36
  • More from a network cable POV, but if you can pull from a box, then simply don't pre-cut it. Doesn't help if you have to buy the length you need though.
    – Criggie
    Jul 11 '20 at 5:16

Wire expansion/contraction in conduit is quite minimal. I’ve never left more than the few inches you normally get when you don’t straightline through a junction box.

Part of the reason 6/3 is so stiff is it’s not intended to be run in conduit. It’s intended for direct underground burial. And it’s stiff because it’s four wires bundled together. It’s not illegal to run in conduit, just a lot of work.

Here, get 4 chopsticks and bind them in a spiral with tape, the whole length. Notice how stiff they are as a package? Notice how bending them makes the tape wrinkle, as they try to slide past each other? Much stiffer than 1 chopstick, or even 4 chopsticks able to freely slide. That’s what’s happening.

When you see linemen do that, they are working with individual wires.

The preferable way to work in conduit is to run the conduit all the way from panel to panel, and then, use individual wires. They bend easier because they are able to slide past each other. And that makes them easier to pull.

With individual wires, it’s easy to give yourself a couple feet of slack simply by having the wires “take the scenic route” from conduit entrance to the lugs they’re meant for. This doesn’t even need to make the box messy. That’s harder (but not impossible) to do with cable, because the sheath must extend into the service panel, so if you pull some out later, it’d need to still be sheathed.

Also, given your length, and the large size of your conduit, if you go individual wires you might want to think about switching up to #4 aluminum (with a #6 insulated aluminum ground, or a #8 copper bare or insulated). When used as feeder, aluminum wire’s safety is fully proven. And you might as well - the subpanel’s lugs are probably aluminum. (Aluminum lugs play well with both Al and Cu as far as thermal expansion goes). It’s less money, and it’s a bit less stiff (and lighter).

Lastly, I hope you’ve seen our strong recommendations to wildly oversize any subpanel you install. Spaces are dirt-cheap when you are buying a panel, but running out of spaces is an expensive problem. And the old cheat of using cheater/tandem/double-stuff breakers is fading fast, as AFCI/GFCI is being required on more and more circuits. Don’t hesitate to take a panel you already bought back in favor of one with 2-3x as many spaces. It’s OK for the busing (or even main breaker) in a panel to be larger than the feed breaker. In fact, it’s a good thing.

  • The house structure limits running the conduit all the way to the main box (insulation, structure, etc.) My plan was to terminate the conduit outside at both ends with an LB. Given that, does it make more sense then to run THWN stranded? Added benefit there is its rated for 90c too. Can I run THWN stranded for the ground as well?
    – Ray
    Jul 10 '20 at 17:47
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    One thing about running conduit that never seems to get mentioned is mitigating thee effect of water intrusion. This is especially important if there is an elevation change in the conduit run. The last thing you want is water coming into panel. When I wired my new house I had to deal with that. I left about 3 feet of wire between the elbow where it enters the house and the end of the conduit run directly buried to break the "pipe" and prevent water intrusion. Of course the wire was rated for direct bury. Maybe we should add this to advice on conduit. Thanks for listening. Jul 10 '20 at 17:48
  • You have good info in this answer - really good info. But I think you need to give this guy a solid example of how much slack you would have in his specific case. This is something that could help a lot of people out if maybe there was a diagram or picture.
    – DMoore
    Jul 10 '20 at 17:58
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    @Ray you could terminate in a proper junction box instead of an LB, and then splice to cable that is rated for direct use indoors. You cannot run THWN or XHHW wires outside of conduit indoors, not even if they’re bundled into MH feeder. Jul 10 '20 at 18:08
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    @Ray nah, you can still have the LB to make the turn, then into a junction box with a short pipe (nipple). All the better if the j.box is indoors. Get a nice big one, a proper electrical supply house can give you an idea of needed size for a splice. They’ll also have the Polaris connectors for splicing. Jul 10 '20 at 20:25

There is really no change in underground conduit. Above ground with pvc the conduit itself can change quite a bit steel only ~20% of pvc if I remember correctly.

I always have a small service loop it may be an additional 6” to a foot at each end depending on the size box but it is a good idea for several reasons.

If you think you need 60 get 65 unless you have pulled a mule tape or other string in the same pipe and measured it then still get a couple feet extra. You can burn 2-3 ‘in each end inside the panels.

Pulling wire many times damages a foot or more of the end that is usually cut off. After the install even if done with a torque driver I find loose and burned ends , extra cable allows repairs.

To prevent or reduce the chance of damaged with larger conductors: Torque, relax or back off torque (do this at least 3 times) next wiggle the Wires back and forth and retorque , you will notice the driver turning further each time if it no longer goes further all the extra space has been removed and it probably won’t burn the ends off.

It is better to have a small amount extra than be an inch short. Even on 500 and 750 MCM wire I have extra and that is big wire.

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