I recently installed a 100a exterior subpanel adjacent to my main panel to allow for the installation of 240V and 120V service to a shed on the opposite side of my house, along with a 240V dryer of which the panel spots were used for wiring the subpanel. My plan is to run conduit from the panel to the attic then bury the wire via conduit on the opposite side to enter the shed.

Not realizing how unruly 8/3 UF-B wire is to work with I purchased a long enough section for the entire run knowing this wire meets code for the use in exterior conduit. I quickly realized it is all but impossible to pull this through 1 1/4" Sch 40 conduit.

In this process, I have also noted what I believe are exisiting code deficiencies I am afraid they will be caught when my new electrical install is inspected.

Any input in regards to the following questions would be greatly appreciated!

1) My main service panel has several 14/2 romex wires exiting the panel into exterior conduit into the attic. This was done before I purchased the house, as I understand this is not up to code as romex is not rated for any type of outdoor use. Should I expect my electrical inspector to also check my main panel in order to certify my subpanel, and will I be required to mediate these deficiencies?

2a) Would best practice be to use the UF-B to cross the attic and use junction boxes on either end to transition to THWN prior to entering the conduit?
2b) Could I make multiple connections (8/3, 12/3, 10/2 to corresponding THWN) within a single junction box, what is the best way to calculate the necessary volume? Would this be an appropriate option Link

2c) Can I use a single common ground through the conduit to the subpanel

3) In the attic, which is not considered to be a liveable space can I run the wire over the insulation or does it need to be affixed to the rafters?

EDIT: I have now adjusted my plan to only use the UF-B cable to traverse the attic. Attached wiring diagram for transitions from UF/NM to THWN wire within conduit.

Wiring Schematic

Box Fill Calculation

  • 1
    Is taking that 8/3 UF back an option? Also, can you provide us more information about the other circuits that are involved with that junction box? Commented Oct 27, 2019 at 5:34
  • Not an option unfortunately as it was cut to order. In the junction box I’d have the following the 8/3 240V shed, a 10/3 240v for the dryer, and a 12/2 for shed lights and outlets. Commented Oct 27, 2019 at 6:16
  • Have you tried using a wire pulling compound? This might be your best bet to reduce friction and avoid unnecessary splicing. Commented Oct 27, 2019 at 14:00
  • I’ve tried with gel pulling compound and have been unable to get any degree of movement past the lb conduit body. I am thinking I may be able to configure the side from the subpanel with creative assembly of the pieces. On the opposite side I think I will need a splice given 3 90 degree direction changes in a relatively short segment. Commented Oct 27, 2019 at 14:22
  • 1
    .You do understand that you don't pull through an LB, right? You must take the cover off the LB and pull to there. Then pull from the LB to the next access point, etc. Also it is specifically a codevio to assemble conduit around wire. Commented Oct 27, 2019 at 14:35

2 Answers 2


I'd scrap this whole plan and start over (save for the existing buried conduit)

You started off on the right foot by burying a conduit from the house to the shed, and you are also correct that NM is nogoodnik in a wet environment, so we'll be replacing that as you indicate. However, you were only familiar with the cables (NM and UF) used for building wiring, so you figured that was what was used inside conduits as well, and then discovered to your dismay why that isn't so: they are stiff, chunky, and generally utterly miserable to pull down conduit.

As it turns out, what you need to be pulling down these conduits are individual THHN/THWN wires, not the cables you're used to. These take up far less fill than a fat, stiff, miserable cable, and are also far easier to pull down conduit, with their slick nylon outer coating and stranded cores giving them superior flexibility and slipperiness.

Going cable-free across your attic

One other faulty assumption you made was that you must transition from conduit to cables when you head inside. In fact, that's not the case; there's even a handy form of conduit that is basically purpose-made for light-duty indoor work. The Code calls it Electrical Nonmetallic Tubing, or ENT for short; however, it's much more commonly known as "smurf tube" for its bright blue color. It is made from thin PVC plastic, corrugated for strength and flexibility, and is legal for mains wiring within houses, although it's most commonly used for communications/low-voltage work.

With this, we can make the run (save for the dryer) entirely in conduit, allowing us to take a junction box entirely out of the picture, and giving us much more flexibility in locating the remaining box for where the dryer circuit exits the conduit.

As far what goes inside said conduit...

Your other issue with your plan for powering up the shed is the idea of running two separate branch circuits there, instead of simply fitting a subpanel at the shed and running a feeder to it. Given that your 1.25" Schedule 40 PVC can handle 374mm2 of wire fill, we can fit 3 1/0 Al XHHW-2 or THHN/THWN wires alongside an 8AWG bare copper ground down it with room to spare for 3 10AWG THHNs and a 10AWG bare copper ground for the dryer. However, that's overkill for the initial portion of the homerun, and 1.25" ENT is rather scarce on the ground for that matter.

This means we're better off running 3 2AWG Al XHHW-2 wires alongside that bare 8 AWG copper ground, giving us the ability to provide 90A (or 100A with some pigtailing trickery at each end) to the shed, while still being able to use 1" ENT for the run across the attic. The 4 10AWG THHNs for the dryer run out from the panel wil easily fit into the 1.25" PVC alongside the proposed feeder, and you can simply use a T-body, a reducer, and some 1/2" ENT for the rest of the dryer run. (If you wish to transition the dryer circuit to NM at a box in-line with the 1" ENT run, you could do that as well; I simply suggested using the ENT for the dryer run because it means you don't have splices or boxes to worry about.)

Once you get to the shed...

Once our wire run reaches the shed, we then land it on the line lugs of a 100A, 24-space, main breaker subpanel. This provides us with a convenient main disconnect for the shed, and also plenty of space for future expansion. In the subpanel, then, we can fit the breakers for the sauna heater and lights/outlets there as well. Note that you'll have to pull the bonding screw/strap (or make sure it's not fitted) and fit separate ground bars if the panel does not come with ground bars factory fitted. You'll also need to torque all the breaker and panel lugs/screws to spec using an inch-pound torque screwdriver or torque wrench, by the way; this is the new 110.14(D) requirement in the 2017 NEC, and is also a good idea anyway, lest your panel get a case of the loose lugnuts.

As to replacing that NM...

What I would do to replace the NM is fit a junction box at where the conduit terminates, then run individual 14AWG THHN hots and neutrals + a 14AWG bare ground down the existing conduit, and then continue the runs from the inside junction box with NM. Note that you may have to replace some of the NM inside due to water wicking up the paper separator in the cable.


You broke the first rule of buying wire: buy it after you talk to StackExchange. Unfortunately you may have to sell this cable on Craigslist.

This went off the rails quite awhile back

I mean this never had a chance, and I have no idea where you got your advice (Home Depot) but you were fantastically misled. Here. I actually plugged your cables into a voltage drop calc.

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Not how we are near 40% fill limits in a 2-1/2" conduit. A 2" conduit would be overfull. needless to say we'd be at 124% full in a 1-1/4" conduit, so the cable cannot even physically fit. In fact, the 8/3UF alone exceeds the fill limits for 1-1/4" conduit, which shows how horrible the advice has been. This is the problem with cable, particularly flat cable, in conduit. This is why you can't get around any curve.

If you received this advice at a big box store, and you told them your plan or the size of the conduit or otherwise relied on advice from a clerk at that store, then I would haul all the cut wire back and raise a huge, full-on Jersey Shore royal stink until they refund you. Just so you know, this is where all those cut sections of cable that you see come from - other customers they did refund.

No splicing inside conduit bodies

I hear you mention splicing, like you expect to run the cable between the LB conduit bodies etc. and splice there. No. When you splice inside a conduit body, it becomes a junction box and you must obey the cubic-inch conduit fill rules for junction boxes. Normal LB's are way, way, way too small for that to work if there is a sensible match between wire and conduit size. The fact that you can cram a wire nut in there does not make it OK.

No pulling through conduit bodies, either. You must remove the lid and pull the wire out and use it as an intermediate pulling point. Always maintain a loop of wire outside the box. Only when the pull is finished do you push the excess into the conduit body.

So I see three options for you.

  • Get rid of this wrong cable and use THWN wires the entire length of the conduit.
  • Lay 2-1/2" conduit instead of 1-1/4".
  • Switch to direct attach / direct burial of the UF cable. This will require a deeper trench.

Or some combination of the above, making splices inside junction boxes that remain accessible.

  • Thank you for taking the time for the detailed response. A few clarifications. I am not intending to splice within the conduit body. My plan is to use individual THWN wires from the service panel to the riser into the attic. Once inside the attic I will use a large junction box to make the necessary connections to romex/UF cable to traverse the attic followed by an additional junction box prior to the transition back to THWN and conduit leading to the shed. Commented Oct 27, 2019 at 15:39
  • This cable can be used the conduit is not needed with the small exception of exiting the house and going down , then coming up into the shop. Jump the conduit size for those short runs and it will work and no money will be wasted.
    – Ed Beal
    Commented Oct 27, 2019 at 19:11

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