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I have a house in Seattle WA with a hot tub that was wired when I bought the house. Some of the wiring is in grey rigid conduit (PVC?) attached to the garage ceiling. I want to reroute these through the ceiling joists to accommodate a new garage door and for a tidy appearance. The drywall ceiling has been removed.

In the panel, there are two 50 Amp circuit breakers. There are 4 wires total: 2 hot, 1 neutral, and 1 ground, all the same size, I think #6. These exit the panel via a short run of flex conduit which goes into a junction box. Exiting the junction box, the wires are routed along the ceiling in PVC conduit. This conduit runs down a concrete block wall and into a large switch box. Exiting the switch box is a third type of conduit, flexible liquid tight PVC.

I want to replace the PVC conduit run which is attached to the ceiling, and instead run wire through the ceiling joists. I will still need some conduit since the walls of the garage are concrete block. For aesthetic reasons, I would prefer to replace the PVC conduit on the garage wall with EMT. The outdoor portions are a small trip hazard but that is a problem for another day.

In the ceiling joists, I believe I want 6/3 NM-B. What I'm unsure about is how to make the transitions: from the panel into the ceiling, and out of the ceiling to the switch box. I haven't worked with 6/3 NM-B, but I suspect it will be too large to fit through the existing conduit runs. The wires might fit it I remove the sheathing from the 6/3 NM-B. But both cases, I'm unclear on the code. The conduit runs on either end are short, perhaps 3' from panel to ceiling and 4' from ceiling to the switch box.

I could use junction boxes to make the transitions, but it feels cleaner & simpler to have continuous wire runs. The 6/3 NM-B wire comes in a roll so I will have plenty to spare.

So my questions are:

1) Is 6/3 NM-B the right wire for running thought the ceiling joists?

2) Can this wire be run through conduit as-is? 2a) If so what size flex and what size EMT?

3) Can this wire be run through conduit after removing the sheathing? 3a) If so what size flex and what size EMT?

4) Most generally, can I do a continuous run with the new wire or do I need junction boxes?

Any clues appreciated !

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6/3 NM-B is not wire, it is cable. Cables contain multiple wires. NM is fine for running through ceiling joists in dry areas where the cable will be protected. If water is involved in any way, you need UF cable.

Cable can be run through conduit, but only if you are a masochist. It will be a rather difficult pull. You will need much larger conduit, because the conduit fill is calculated by looking at the widest dimension of the flat cable, and imagining a round wire that diameter.

There's an exception that allows this imaginary wire to fill most of the conduit if it is the only cable in the conduit. But that only makes it an even more difficult pull, and paints you into a corner as far as any future use of that conduit.

You cannot unsheath cable and use the individual wires. That is because the individual wires do not have individual wire markings to disclose their type and grade of insulation, and generally their insulation is not tough enough to survive any type of use outside a junction box. They are allowed as short pigtails entirely inside junction boxes.

If you find cable whose interior wires are marked as some sort of wire that is legal for use in that application, then have a field day. This is rare but not unheard of, talk to your local electrical supply.

You cannot do a continuous run with new wire because you cannot use loose wires in through-joist wiring. You need to either fit conduit in the joists, or use some sort of cable rated for such use.

If you run all metal conduit, that is one less wire you need to carry, as the metal conduit serves as the ground wire.

Before you decide on the romance of continuous wire runs, count your bends. More than 360 degrees of bend between access points is illegal. More than one makes pulling exponentially harder.

It is very easy to build a conduit route that is so difficult to pull that you end up needing to call a professional to do it. Professionals don't like doing a small part of the work, and will want a big chunk of the total job. Access points are the key to avoiding this, but of course, access points are the enemy of "a finished look" for some reason. I don't know why, the Victorians had no trouble fitting adorable cabinets where utility access was needed. It is a 20 minute job to change my tub faucet. Really.

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You cannot remove the cover from the NMB , the only advantage to NMB inside is it doesent require conduit but it cannot go outside (conduit outside is listed as a damp/ wet location) when using NMB it needs to be protected to a height of 8', since you already or it sounds like you have thhn/thwn wire I would continue using that and add a junction box to splice as it sounds like you will need a few more feet. This or use the existing wire outside since it should be listed for damp wet location then run the NMB in the attic area and splice at a box as it goes outside. PVC conduit is simple to work with and easy to reuse, if you use EMT outside make sure to use outdoor rated couplings and box ends (the ones that hold in with screws on the side are not for outside) the damp location have a compression fitting they only cost a little bit more.

  • Cool, thanks. Basically, it sounds like I should use junction boxes to transition between NMB in joist bays and thhn/thwn wire in conduit. Makes sense. FYI The EMT run is inside the garage, I think I will leave the outdoor segments alone. – Todd Hivnor Oct 21 '18 at 20:03
  • Yes a junction box is the correct way to make the change, sounds like your plan is a code compliant way, you should have a GFCI breaker feeding the tub. In your question it was not clear if there was one, in the main panel the double 50 or at the tub disconnect one of these locations should be GFCI. Home stores Cary them and plumbing and electric speciality stores cary small sub panels that have a 50 GFCI and many times slots for ~ 4 additional breakers. – Ed Beal Oct 22 '18 at 13:06

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