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I plan to install a NEMA 6-50 outlet in my garage, from a 50A breaker in the main panel. I plan to use 6/2 cable, unless there is a better recommendation. The run is about 50 feet, planned as follows:

a. about 20' along joist in basement

b. drill through block basement wall, exiting just above the floor of the garage

c. up wall of garage, through drywall ceiling into garage attic

d. across ceiling joists in attic

e. straight down through ceiling to outlet.

My questions pertain to steps b, d, and c/e.

b: When passing the wire through the block wall, is conduit required? I don't believe so, but please advise.

d: I under stand that the NEC permits the wire to run across the ceiling joists unsecured, provided that the attic is inaccessible (no permanent stairs, which is the case here--I have to use a ladder and climb up) and the wire is run at least 6' from the opening, which will also be the case here.

c and e: I assume conduit IS required for runs up and down the garage wall, otherwise the cable would be exposed to damage. So, I'm planning to use 3/4 PVC conduit, based on these factors:

NEC requires that cable take up no more than 53% of the space inside the conduit; Outside diameter 6/2 cable = .3664; Inside diameter 3/4 PVC conduit = .824 (Link to home depot 3/4 Conduit for specs); Thus, cable will occupy only 44% of the conduit.

Please let me know your thoughts on these plans.

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    Note that you're comparing the radius (or diameter) of the conduit to the radius of the cable, when you need to be comparing cross-section area (A = pi r^2). With the diameters you listed, the cable has an cross-section area of ~0.10 in^2 and the conduit has an area of ~0.53 in^2, leading to the cable occupying only ~20% of the conduit's space. Still within specs, but this means you can use a smaller pipe as conduit, or continue to use the 3/4" PVC but have room to run more wiring in the future (which is not a bad idea, given the small difference in price) – mmathis Nov 7 '16 at 19:54
  • In the future, please ask just one specific question per post. SE isn't a discussion forum, and it's not as beneficial to the community to have your entire project outlined in a list format. – isherwood Nov 8 '16 at 1:28
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In turn here:

  • For your case b), conduit is not required -- NM is routinely run through bored holes in structural members without conduit.
  • For d), again, conduit is not required -- you can get away without guards if you run the cable along the side of the joist and at least 1.25" away from its edges. (320.23(B) and 300.4(D))
  • As to your protective runs in c) and e) -- 3/4" Schedule 80 PVC conduit has 138mm^2 of usable room in it for a single cable or conductor, and a 6/2 W/G NM-B cable, according to your specs, takes up only 70mm^2, so you have ample space. This might be a tight pull, though, so you may wish to go with 1" instead.
  • There are two other things you need to address with your conduit runs:
    1. The conduit you linked is Schedule 40, which isn't sturdy enough to protect wires from physical damage -- the code requires the thicker, sturdier Schedule 80 instead.
    2. Make sure to put expansion fittings in your PVC runs lest they do the snake on you mid-summer, or pull apart mid-winter! Easy rule of thumb for conduit that is operating within oh, 24degC of room temp -- put them in every 4m/12'. (You might be able to get away without it for isolated runs like yours provided the supports are set up so they don't provide excessive longitudinal restraint to the conduit.)
  • NM cable tends to be oval so you really have to watch yourself in conduit. space. I wouldn't even think about 1/2" conduit, and I'd be wary of 3/4" because the cable itself could be wider than that in the long dimension. too close clearance will make pulls rather difficult. I'd go 1" or larger. – Harper - Reinstate Monica Nov 8 '16 at 8:05
  • @Harper -- yeah, the assumption in Code treats the cable as round, with a diameter equal to the major axis diameter if the cable actually is oval. It's conservative, at least... – ThreePhaseEel Nov 8 '16 at 12:42
  • Thank you all for the helpful comments. I'll use 1" conduit--schedule 80, as recommended. My conduit runs will each be about 8' or less, so I don't think expansion fitting will be necessary. I may staple the Romex to the ceiling joists anyway even if not required. – An Ex Parrot Nov 8 '16 at 15:47

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