Adding wires to EMT conduit to add circuits

Question

unfortunately, the kitchen wiring in our new condo is severely outdated. Just two circuits; one 15amp for lights and one 20amp for fridge, microwave, gas stove and oven, garbage disposal, dishwasher and countertop outlets. Surprisingly, the 20 amp only tripped once. But it's time to think about an upgrade.

We have a EMT conduit from the basement to the third floor currently with 3 circuits (2x 15amp, 1x 20amp) and one additional pair of wires for future use. I would like to add additional wires to the conduit. I researched this pretty thoroughly but it would be great to get confirmation and address some questions)

CONDUIT details

Wheatland EMT Tube, about 30' long. I think it is the 3/4'' size (outside 0.922''/ inside 0.824''). See picture with junction box below.

Current wires: 4x THWN 12 AWG, 4x THWN 14 AWG, ground wire (for 3 circuits and one unused wire pair)

Proposed additional wires for 4 additional circuits: 6x THWN 12 AWG, 2x THWN 14 AWG

Total wires: 10x THWN 12 AWG, 6x THWN 14 AWG, ground wire

Based on this calculator, the additional wires should fit in the conduit and be code compliant.

Questions

• Is this calculation correct? Can I add 8 additional wires to the conduit?

• Do I have to worry about the ground? I am not sure what gauge the ground wire is. But is it an issue with all the additional circuits or does the grounding work through the conduit anyways?

• I will also have to use larger junction boxes in the third floor and basement. Based on my own calculation, it should be 130 cubic inches. Any suggestions for junction boxes in that size with cut-outs in the back to put the wires in the wall?

Picture of EMT conduit with junction box:

Electrical panel:

Answer 1

You don't need a wired ground in EMT conduit between metal boxes.

Your calc is fine, but you forgot the other calc.

So many wires, so close, will make too much heat.

And that's settled here.

You have 16 conductors so a 50% derate. You derate off the thermal max for the wire you are using (for THHN, that's 90C column in table 310.15B16). Blah blah, the ultimate derate for 16 THHN wires in conduit is #14=unusable, #12=15A , and #10=20A.

That doesn't work. The ground is unnecessary in EMT, so get rid of that and bump your #14s to #12, but 15A is not legal for kitchen countertop circuits. At first blush, it appears you will have to cut circuits to make room for #10 wire on the 20A-mandatory circuits.

However, we're not going to do that!

Multi-wire branch circuits

These things are almost obsolete, because they play badly with AFCI and GFCI breakers, but they can be life savers in moments like these. This will require very judicuous breaker placement, which requires elbow room in your panel.

An MWBC has two hots which share a neutral. The hots are placed on opposite poles (240V opposed) so the neutral only carries differential current.

Interesting fact, if the neutral is carrying current, that means one of the hots is not - so if the neutral is seeing 10A, one hot may be 20A and the other is 10A. Heat is the square of current, so 20..10..10 actually runs cooler than 20..20. So neutrals in MWBCs are exempted when calculating thermal derate. Read that again. Get where this is going?

You now have two #14 hot wires, and six #12 hot wires. And four neutrals, but they don't count for derate. This puts us at 8 wires, which take a 30% derate.

30% derate on THHN is #14=17.5A... #12=21A... #10=28A. So we made it.

In MWBCs, neutrals must be kept fastidiously separate from one circuit to another. Further, groupings must be marked - I like colored tape for that. For what it's worth, 8 hot colors exist: black brown red orange yellow pink blue and purple. And I just bought some blue-red-stripe! But you must also mark which neutral is with which hots.

Also in MWBC, neutrals must be pigtailed at every splice. That way any device can be removed without severing the neutral for the other half of the circuit.

All breakers on an MWBC must be handle-tied for common maintenance shutoff. A 2-pole breaker is one way to get that, however it also provides common trip guaranteed, which is not a thing we need.

If GFCI is desired at the breaker, then it must be a 2-pole breaker. AFCI implementations differ by manufacturer; some don't allow shared-neutral AFCI.

Answer 2

Feeling the heat

While your conduit can physically fit all those wires, sure, there is a problem with your plan still, and that's the amount of heat stuffing 16 current-carrying conductors into a conduit throws off. In fact, due to the inability of the conduit to dissipate that much heat, the NEC limits your 10 12AWG wires to carrying a mere 15A per wire and your 6 14AWG wires to a puny 12.5A each, based on the derating factors from NEC 310.15(B)(3)(a) and its associated table.

One solution would be to upsize the wires to 10AWG for the 20A circuits and 12AWG for the 15A circuits. However, that would limit you to 7 10AWG (the existing 20A circuit, the existing spare circuit, and a single 20A MWBC) wires and 4 12AWG wires for the two existing 15A circuits, as well as a 10AWG bare ground wire (which must be bare, or else you'll overflow fill).

Consolidation is a better plan, here

However, this sort of situation (a shared homerun conduit) is where Multi-Wire Branch Circuits shine, as we can split each MWBC into two independent circuits before we reach the kitchen GFCIs, yet run 2 circuits' worth of power over 3 wires, of which only 2 count as current-carrying for derating purposes, as the neutral in a MWBC only carries the difference in current flow between the two (opposite leg) hot wires.

With this, we can get 6 20A branch circuits run using just 9 wires (vs 5 branch circuits run using 10 wires) and still have room for the 2 15A branch circuits (using 3 wires instead of 4). However, it requires the use of two-pole breakers in your panel (or handle ties if you don't have GFCI at the panel, but two-pole breakers tend to be easier to find), in order to provide a common maintenance shutoff for the MWBCs. You'll also have to pigtail neutral at any receptacles that are on the MWBC portion of the circuit, but that does not appear to be a concern here, and you'll need to distinguish the various neutrals as well.

Fortunately, since you're working in conduit, striped THHN is a readily available solution to this issue. I'd have the plain white neutral be the 14AWG wire, and then use yellow, red, and blue striped wires for each of the 12AWG neutrals, with black hots for the 15A MWBC and matching colored hots for each of the 20A MWBCs.

Overstuffed

One other thing you will have to deal with is the box fill at each end. The existing junction boxes that terminate this conduit run are 4 11/16" by 2 1/8" square boxes, with 42 in3 of fill. While more than adequate for your existing run, which only takes up 36.25 in3 of fill at each end (8 12AWGs, 8 14AWGs, and a 12AWG grounding allowance), this isn't enough space for any of the proposals you are floating, never mind filling your conduit to the brim with MWBCs.

The simplest option for fixing this would be to add an extension ring to both boxes -- a 4 11/16" square by 2 1/8" deep ring adds another 42 in3 of fill to the picture, giving you ample space for even the most extreme conduit cram job possible here. If replacing the boxes outright is an option, I would go to a large/deep metal two-gang or three-gang box as it provides better access to the wiring space than using an extension ring would.

Furthermore, keeping all the splicing straight will likely require doing it in an organized fashion instead of simply wirenutting seemingly-matching wires together and hoping for the best. Fortunately, Wago offers DIN rail carriers for their 221 series lever-type splicing connectors -- look for Wago 221-500s, although you may need to order them online as they are not commonly stocked here in the US. This, along with some Wago 221 lever-nuts and a few inches of DIN rail screwed into the back of the box, lets you do all the wire splicing in a neatly organized manner, much akin to having a set of terminal blocks you can use.

Answer 3

You could also consider running 3 #6 or possibly 3 #4 THHN conductors to a new sub panel in the kitchen. You wouldn't have to derate anything and I believe that would get you 75 to 95 amps to distribute throughout your kitchen.