This question is similar to others, but some are older...

I am in the planning stages of running a 240V 50A circuit to a spa box in my back yard.

The spa manufacturer calls for 6AWG throughout and a 50A two pole breaker, plus the 50A GFCI two pole in the spa box. So..... here's the plan:

I'll be running straight up from the distribution box into the attic (I scoped the attic and the builder has left be a nice size hole to feed my 6/3NM-B into the distribution box from above).

Then break right and head through the attic (straight shot) to the back wall. Secure the cable every two feet (that's my OCD).

My question is how to transition outside. Do I mount a box on the last joist and transition inside the attic to the THWN+conduit, since NM-B can't be in "wet" areas?

That way I can have a clear mount on the joist and head down into a 3/4 PVC into the spa box?

After the spa box (mounted at 5 foot height), I am still trying to see whether this old body can take digging an 18 inch trench (PVC) or 8 inch trench (rigid metal) to make it to the spa (any thoughts on pro/con on 8 vs 18 inch trench are highly appreciated. especially when they decrease the amount of digging).

The numbers make sense. The voltage drop is way below 3% on the distance, load, voltage. unlike the cost... those numbers don't make sense anymore....

Thanks in advance on your input and for this awesome community.

  • 1
    How far from the spa box to the spa?
    – JACK
    Jun 21 at 12:30
  • 12 feet. straight line.
    – PeteS
    Jun 21 at 15:14

2 Answers 2


You can likely save a lot of money by using aluminum (larger size, but lower cost) instead of copper.

Normally the breakers & panels are rated for Cu/Al, even though equipment is often not, so you should be able to go with Al from your main panel (either all wire in conduit, or cable and transition to wire inside the attic as you described) to an outside small panel (spa box) on the wall.

From the small panel you go copper the rest of the way to the equipment. For that last part, rigid at 8" instead of PVC at 18" definitely makes sense.

A few other things to consider:

  • You only need 3 x 6 AWG. The 4th wire (green or bare for ground) can be 10 AWG.
  • If the cables in the attic are subject to damage then they need to be protected from damage - e.g., a piece of wood in front of or on both sides of the cable. If the attic has permanent access (built-in stairs or ladder) then this is generally a requirement. If it does not have permanent access then generally not a requirement.
  • If you use rigid metal conduit after the spa box then you might as well use it from the attic to the spa box as well.
  • You don't need a separate ground if you have properly installed rigid metal conduit. You connect the ground from the cable to the junction box and the rest is automatic. But if you feel better with a ground wire, that's fine too.
  • Looking at the options of wire in conduit (~$4/ft for single wires plus conduit cost) and 6/3NM-B (~4.40/ft). That number is the reason I opted to go NM. However, weaving the single cables through the hole to the distro box might be easier. I could run 3/4 PVC in the attic and that would eliminate the need to a protective 2x4's, which brings that cost lower, I guess. Plus if I did 6AWGx3 and 8AWG GND would lower a bit. Transition to outside would be easier, too. No need for a box anymore, just an elbow now and straight to the spa box, correct?
    – PeteS
    Jun 21 at 15:21
  • Correct about using conduit the whole way with respect to protection, no transition junction box, etc. As far as wire pricing: (a) only 10 AWG for the ground, saves a bit; (b) if you use rigid metal instead of PVC you can skip the ground. Jun 21 at 15:28
  • Thank you. What are your thoughts on coming up from the distro box, through the 2x6? Bring up the 4 cables, how do I transition into the PVC? a box?
    – PeteS
    Jun 21 at 15:36
  • Wires (as opposed to cables) can never be by themselves. Wires always need to be in conduit (PVC or metal) or junction boxes. You can run 3/4" conduit (which I just checked and is just over an 1" typical outer diameter) through a 2x6. Transitions in junction boxes. Jun 21 at 15:41
  • OK. so i'll run about 4 feet of the NM to the JB, transition to conduit and i'm done. What connectors are recommended?..... Thank you!
    – PeteS
    Jun 21 at 16:03

If at all possible I would do the whole shebang in conduit. That eliminates the costly splice. I myself use EMT metal conduit almost exclusively, though I'll use PVC or RMC underground since EMT is not allowed there. RMC is allowed buried and only requires 6" of cover burial depth. EMT is about the same cost as PVC, easy to work with (i.e. VERY novice-tolerant), and everything is reusable since it goes together like an Erector Set/Lego.

Conduit will allow a continuous run of THHN wire without any splices. Splices are a failure point, so it's really best to avoid splices. However, intermediate access (pulling) points would be highly helpful in a conduit installation, since all conduit must be built empty and the wires pulled in only after the conduit is complete.

There is nothing wrong with aluminum at these large sizes. Some people fear aluminum because of "some stuff they heard" but a thorough examination of that "stuff" will reveal two very particular mistakes were made (not using terminals properly rated for aluminum, and not setting screw torques with a torque wrench). Don't repeat them and you're fine. Terminals on disconnects are always rated for aluminum and 75C thermal. Note that aluminum wire is not allowed too close to hot tubs due to the chemicals, but it can usually make it to the disconnect and can definitely make it to your planned splice point.

Using copper over aluminum does nothing for safety. All the terminals on the hot tub disconnect and service panel are aluminum rated, and failing to torque properly is equally likely to cause a failure regardless of metal type. In fact the torque issue was uncovered because of copper failures.

Because of the 75 C thermal rating, you may use #8 copper or #6 aluminum for 50A, so long as they are the only circuit in the conduit. (A second circuit would require a derate, forcing one to #6 copper).

Splicing #6. #6 is best joined with the ILSCO "Mac Block Connector" which plays well with both aluminum and copper. In fact you could run #6AL SER cable across the house and copper #8 THHN for the homestretch, food for thought.

I am not a fan of "wire nuts" at this large size due to the difficulty of it getting tight enough. I'm talking to you Bruce Banner, you should really let "the other guy" nut the #6! Torque is a very big deal in wire failures. Each #6 wire requires 5 cubic inches in the splice box, so a large 4-11/16" box will be needed (for the 35 cubic inches you will need). Anyway #6 cable will probably need a 1" trade size knockout, requiring a larger box.

I would be much more frightened of a #6 wire nut splice than aluminum wire properly installed and torqued to spec.

  • Thank you. That was very informative. You mentioned a 1" knockout, that's the 1.050 for the 3/4 pipe, correct? those ILSCO connectors are pretty hard to come by from the looks of it....
    – PeteS
    Jun 21 at 20:54
  • @user2358893 -- no, a 1" trade size KO is really more like 1.3" or so. your best bet for the MAC-Blocks is a local electrical supply house that stocks ILSCO's product line, BTW Jun 22 at 0:08
  • 1
    @user2358893 -- you can upsize if you wish, but 3/4" Schedule 80 conduit will fit 3 6AWG THHN wires + a 10AWG bare ground, if only barely. (Note that you can run 50A over 8AWG THHN just fine, which makes 3/4" conduit much more practical for this.) Jun 22 at 2:30
  • 1
    @PeteS no, that's not allowed (unless the individual wires are marked, which I super duper doubt). Further the marking would need to specify an outdoor rated insulation type such as THWN or XHHW. Romex/NM is not legal outdoors and actually won't work, the paper wick sucks up water and rots the insulation. If you own the wrong cable, don't get stuck in "sunk cost fallacy", trade it on Craigslist/FB Marketplace for the right stuff. Jun 23 at 21:04
  • 1
    @PeteS lots of people want to believe outdoor conduit is indoors. But no, it's outdoors. Also Romex requires quite a large conduit - the conduit ID must be 138% of the widest part of the cable, which really sucks for oval cables like UF (outdoor rated Romex). #6UF requires 2" conduit. Jun 23 at 21:26

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