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Junction box photo

The conduit coming in on the left carries a 15 A MWBC. In this junction box, it splits to the brown-to-black wire for the furnace, and the orange wires to some basement lights (top) and an upstairs bedroom (right) where there is an infrared sauna.

Problem is, the orange side of this MWBC also runs some outlets in the kitchen, and the outlet in our data closet. So when the furnace, sauna, and toaster all happen to be on at the same time, the circuit trips. Generally this only happens when someone is in a very important zoom call.

I would like to move the bedroom to its own separate circuit. Ideally I would take the connection out of this junction box into a new one. Problem is, there are no more accessible junction boxes on that branch -- to the right of this photo, the conduit goes into a wall and gets buried.

Is there any issue with bringing a completely separate circuit into this same junction box as a new run of NM from the panel? This approach has been used elsewhere in the basement when adding new branches, but not for new circuits, as far as I can tell. Would I need to cut the conduit back and add a new junction? If so... how do I cut the conduit off without cutting through the wires inside?

Some additional info...

  • The house is 100 years old, in Madison, WI.
  • Electrical is a mix of conduit (which is often full), BX, and NM. I haven't seen any knob and tube in use yet, but there's evidence of its existence in the past.
  • Conduit is only used on the basement ceiling. Basement is unfinished and too shallow to ever be finished.
  • The conduit leading to the junction box shown here follows a circuitous path of very full conduit and two other junction boxes, also full.
  • The MWBC is handle tied, but in such a way that one side can trip at a time. However, I do have to turn both legs off to reset the breaker.

EDIT: I think I figured out why this is so messed up...

There is a dedicated 20 A MWBC for most of the kitchen outlets. But at some point (probably during kitchen renovation) a junction box was buried in the wall -- for one outlet, the cable in the box is 12 ga, but where the other end comes out in the basement, it's 14 ga. So that outlet has to be on a 15 A circuit.

To solve my problem, I'm going to move the furnace to a dedicated circuit, and put the bedroom/sauna to this leg of the 15 A MWBC, so it will be on the opposite leg from the kitchen outlet. The sauna is rated for a 15 A circuit.

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  • Is pulling new wire for the kitchen outlets + a neutral for the furnace not an option? – ThreePhaseEel Feb 13 at 5:02
  • @ThreePhaseEel... Not without challenges. I added some details. – LShaver Feb 13 at 14:03
  • Since wiring code has changed many times the wiring that was installed to code is still legal today. The jurisdiction you are in also may allow modification of existing circuits (where in the world are you?). You can bring multiple branch circuits into a junction box, the circuits can even be spliced in the box. How many wires in your conduit that is full? Box fill and conduit fill are the limiting factors. Use caution as there are a lot of “internet only” electricians that only quote current code and not what may be code compliant. – Ed Beal Feb 13 at 16:06
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    Challenges can be a good thing. You may want to consult your sauna owners manual too they often specify a dedicated circuit. Grandfathering only allows circuits that were installed legally to begin with, at least by the time of install of this box and modern wire the conduit not going to the kitchen should have been separated. Also I've seen handle ties like that, but at least it still prevents you from manually turning off one circuit then accidently getting into a live neutral. – NoSparksPlease Feb 13 at 16:09
  • Your jurisdiction may not allow NM wire type. Very few homes have conduit except where required. Chicago is one example. So we need the location in the world to know what to advise. – Ed Beal Feb 13 at 16:16
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Yes - but you must carefully mark the "Sets of wires" to associate them - specifically to tell the next installer which hot(s) go with which neutral.

I'm a little gobsmacked that you know all about MWBCs but you seem stymied by not knowing how many amps a sauna draws vs how many amps a toaster draws. Not rocket science.

You have some "dedicated circuit" problems here.

First, the furnace requires a dedicated circuit, as per NEC 422.12. It can be one half of an MWBC, but the downside is, if something trips the other half of the MWBC, the furnace is out for the duration. MWBCs must land on handle-tied or 2-pole breakers.

Getting the furnace off the MWBC might be a worthy addition. Make that a simple 15A circuit.

Kitchen receptacles also must be on dedicated circuits, and they must be 20A. But they can still be one leg of a MWBC. You may ask "Can I run a MWBC with a 15A and a 20A leg?" The neutral would need to be #12 wire, but - aw heck, who cares? Since your house is conduit, you can simply upgrade both legs of the circuit. Certainly the (12A?) sauna will be better off on a 20A circuit than a 15A.

The wire typically used is THHN wire, and it's readily available by-the-foot many places. Note you cannot remark a white wire to be hot, so you'll need appropriate colors. Gray is an acceptable "alt" neutral color, but not all places sell that by-the-foot.

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  • I added some details. I'm thinking I should move the furnace off to a new circuit, but the kitchen is mixed up in a few places. – LShaver Feb 13 at 14:06
  • Dedicated circuits in a 100 year old house? 422.12 for the furnace. – Ed Beal Feb 13 at 15:52
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    Thhn wire is not required! It is the most common building wire type used with conduit. Gray building wire is sold by the foot. – Ed Beal Feb 13 at 16:08
  • @EdBeal good point. Fixed. Thx for the ref. With a 100 year old house, you have no idea when the wiring was renovated. – Harper - Reinstate Monica Feb 13 at 18:34
  • Understanding how much they draw is not the issue. It's unwrapping this single MWBC that seems to go to at least one fixture/outlet in every corner of the house, including a single outlet in the kitchen. This outlet can't be fixed without serious demo, because who ever renovated the kitchen buried a junction box in the wall that I'd need to get to in order to pull 12 ga wire to upgrade. By moving the furnace to a dedicated circuit, I can move the sauna to the other side of that 15 A MWBC and solve the problem. – LShaver Feb 13 at 22:30
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Likely no problem. But 3 considerations. Conduit fill, derating based on fill, capacity of the junction box. Both conduit fill and derating are dependent on wire size and insulation type.

Mixing wire types and sizes makes calculations for conduit fill confusing, but generally it looks like THHN/THWN or THW, not bigger than #12. You could fit 9 THHN or 6 THW #12 wires in 1/2" conduit. Adding a hot and neutral wouldn't be a fill problem with either of those wire types.

Adding one more circuit (or one more MWBC) adds two current carrying conductors count to 4, which requires derating to 80%, which if THHN/THWN or THW the capacity is still above the limits already established by 240.4(D), to 15A/#14 and 20A/#12. So normal breaker size is good.

The J-box fill requires 2.25 in³ for each #12, and 2.0 in³ for #14, with up to 4 grounds counting as 1 of the largest size ground, then 25% value above 4 grounds. I'm going to leave you with the math on that. If your wire count exceeds that using an extension ring (Raco 202) solves that.

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  • Actually the same as 9 current carrying conductors. And neutrals don't count in MWBCs. So you are guaranteed to hit fill limits before you hit ampacity derate limits on 1/2" conduit and #12. – Harper - Reinstate Monica Feb 13 at 5:36
  • Will edit to narrow answer to original question. – NoSparksPlease Feb 13 at 6:09
  • I added some more details to the question. – LShaver Feb 13 at 14:06
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    It's legal to add a hot NM cable, but would be a last resort for most electricians. Would mark the cable outside the box, the cover, and probably inside the box. There is no legal means to cut the conduit after wire installed, prohibited by NEC 300.18. DON'T USE A TUBING CUTTER, they leave a sharp edge that cannot merely be deburred. – NoSparksPlease Feb 13 at 15:55
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    @nosparks I agree with no tubing cutter but on occasions I have had to tap runs on inspected jobs infact the inspector mentioned threadless split couplings instead of pulling the pipe and shorten, rethread and then 3 pc couplings but we need threads in high vibration areas. The inspector mentioned the code was to prevent wire being laid then sliding the pipe over it not modifications of a system. – Ed Beal Feb 13 at 17:34

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