Can I have an MWBC plus another circuit in the same junction box?

Question

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.

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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.

Answer 1

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.

Answer 2

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.