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My main panel is outdoors on one end of my house. Planning to add a sub panel for very occasional welder use, electric car charging, and future expansion in the garage on the other side of the house.

By far the easiest path would be outdoors from the main panel along the wall (that side of the house is obscured from view, luckily) up to the attic. Into the house at the attic, across the attic and down the garage wall.

Is it sane to run individual conductors through liquid tight non metallic conduit for this? Would the conduit need to be continuous through the attic? Total run will be about 75 feet. Is there a better solution? I assume 3-3-3-5 SER is not viable since it can't be installed in conduit and I assume this will have to be in the outdoor exposed section.

As a side note, not sure if I can use the 75 degree chart or 60. The wire plus the breaker on the main panel and the connections sub panel determine this? If 75 degree, I could run 4 awg for an 85 degree breaker? That seems adequate. What size ground and conduit? If I have to use 60 degree, I'll go with 3 awg, and I believe 1" conduit and 5 awg ground?

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  • I can definitely plan to not run the welder when charging the car and vice versa, the welder is typically in my workshop, this is only for the off chance I need to fix something that needs to drive in instead of be carried into the shop. – Evan May 28 '19 at 16:58
  • Car charging can be up to 48 amp, but is completely viable at 32 amp as well. – Evan May 28 '19 at 16:58
  • It seems like UF-B might be an option for running in conduit on the exposed portions and bare in the attic? But I don't see 3-3-3-5 UF-B available. Not sure about 4-4-4-x (where X is the unknown to me ground wire size if I got with 4 awg). – Evan May 28 '19 at 17:13
  • I assume you have "appearance issues" with just running it along the exterior (at sill or eve level it's not that obnoxious if painted to match, but everybody has an aesthetic they can and can't live with.) Likewise some issue with running through the nice cool basement rather than the hot, hot attic (perhaps you have no basement?) – Ecnerwal May 28 '19 at 17:34
  • The main panel is in the center of the side of the house, and the sub panel will be the same. It would be a much longer run, and seems like more work to go around the attic rather than through it. Basement is finished, no way to easily get wire through it. – Evan May 28 '19 at 18:01
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You must use the 60 degree column if your feeder is < 100A. The 75C column is allowable if the feeder >= 100A, and all the other conditions for 75C are met, which they usually are.

Using 310.15B7 (#4Cu) is out of the question. That table is for service not feeders. Yours does not supply the entire facility, so no go. If you want to use #4Cu you must breaker 70A, since you don't get the 100A bump.

Keep in mind attic temperatures require a temperature compensation if they're high. It's also good to think about voltage drop if your distance >=120 feet, but don't think too much about it - base it on actual daily load, not breaker trip, and never put 3% into a voltage drop calc.

It's legal to run cable in conduit, it's just really masochistic. The stuff is so stiff that you will have a bear of a time manhandling it, and realistically will probably have to call an electrician to do it.

Also masochistic is spending on copper for anything this large. Copper is a semi-precious metal. Aluminum AA-8000 is a fantastic choice but you will need #1Al. Aluminum requires slightly larger conduit but the savings in wire easily pays for it. I only use copper on very short runs. If you heard something about aluminum being bad, what is true does not apply here.

There are several ways to do it.

  • conduit all the way, and individual THWN or XHHW wires in conduit.
  • conduit up the wall only, wet-location cable continuously the whole way including through the conduit.
  • conduit up the wall only to a junction box, individual wires in the conduit, then a splice to dry-location cable. The splice will be fairly expensive due to the very large junction box required and Polaris connectors, may cost more than just running conduit the whole way.

I see no reason for liquidtight conduit. You will still need to use outdoor rated wire or cable. The one place it might help is keeping water out of the breaker panel if you top exit. I strongly prefer to bottom exit for obvious reasons, but that will be quite a large radius turn.

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  • I'm dubious that liquid-tight would actually help much with water entry. PVC, properly glued, is just as water-tight, but either will be (as defined by code, and that's based on reality) wet inside anyway - from condensation, if nothing else. – Ecnerwal May 28 '19 at 18:57
  • Thanks for the detailed answer. So as a side note, is it true that 3-3-3-5 would be acceptable for 100 amp because you can use the 75 degree column, but dropping down to 4-4-4 would move to the 60 degree column and therefore only allow 70 amp? That's a big difference in allowable ampacity between that one step in wire size. – Evan May 28 '19 at 19:08
  • This will be a left exit from the main panel, top is taken by the integrated meter, bottom is full due to feeder for the shed, an air conditioner, and one other existing conduit that I'm unfamiliar with. The appeal of liquidtight flexible is making it from the attic to the main panel left side, but I suppose one turn in a schedule 40 wouldn't be too hard for a novice to pull off? – Evan May 28 '19 at 19:11
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    @Evan absolutely correct. And yes, it is. Also, take what I said about aluminum seriously. Copper isn't deluxe. In fact the lugs will be aluminum, not least because they expect you to use aluminum... – Harper - Reinstate Monica May 28 '19 at 19:12
  • You can use the 75&deg;C column for <100A as long as the equipment is rated/labeled for 75&deg;C terminations (breaker and loadcenter lugs are, receptacles...aren't.) – ThreePhaseEel May 28 '19 at 23:00
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Personally (yup, this is an opinion), I think using liquid-tight is nuts. It's expensive and also a relative pain to pull in (as compared to EMT or rigid PVC.) Should be a pretty straightforward run as described, with little need for fancy bends.

Typically you'll need, or at least want, some pull boxes (a junction box for access, but no actual junction in it) or LBs (possibly LRs or LLs) for access when pulling, rather than trying to make the whole shot in one go. Up the wall, LB, across the attic, LB, down the wall to subpanel, at the simplest level.

Edit to add:

Left exit from the box, either an LR (vertically) an LL (horizontally) or an LB (sideways) will get you done easily, so long as the conduit size is adequate for the wire.

LR conduit body

LL conduit body

LB conduit body

I believe that strictly speaking PVC conduit less than 8 feet from the ground should be schedule 80 for "damage resistance." You may need a real electrical supply rather than a box store to get that. You may find the real electrical supply to actually be cheaper, too. And I throughly agree about using aluminum wire.

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  • Thanks for the quick answer. So you are suggesting individual THHN/THWN conductors in some form of conduit the entire length of the run? And continuing that rigid conduit through the attic? – Evan May 28 '19 at 18:02
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    Yes. I've seen too many rodent-gnawed cables in attics to not want conduit in an attic; I also prefer to avoid avoidable junctions, and will choose wires over cable to pull in conduit all day long (pulling cable in a conduit is miserable.) Don't veer too close to the maximum fill percentage - more room makes for an easier pull, and does not cost a lot more, typically. – Ecnerwal May 28 '19 at 18:18
  • Upvoting as I really appreciate the response. – Evan May 28 '19 at 19:15
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    And I even recommend metal conduit where rodents are concerned. They could nibble through PVC, but they Do Not Like zinc... – Harper - Reinstate Monica May 28 '19 at 23:32

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