EMT fill capacity and derates mixing 14ga & 12ga

Question

I have an existing 1/2" EMT conduit that has three switch runners, a hot for an unswitched outlet, a neutral and ground, for a total of six wires in 14ga THHN, all for the same 15 amp circuit.

I want to add two dedicated 20 amp circuits in the same area and I'm trying to determine if I can run four additional 12ga THHN wires, two hots and two neutrals, down that same conduit?

If I look just at fill capacity, and if my math is right, I'm okay. I'm using just under 72mm2 of an allowed 78mm2. (It's a ten foot run with one twenty degree bend, so the pull should not be too challenging.)

But since I have more that three current carrying conductors, I think I need to derate my ampacity. At which point I'm out of my league.

Answer 1

Deal with grounds.

Properly installed EMT provides its own grounding and does not need a ground wire. So ground wires in those conduit segments should be deleted altogether. Where ground is required, the presence of 20A circuits will oblige a #12 wire but it can be bare.

Note also that any ground wires must go to the metal box first - they are not run "to device first" like in plastic boxes. So where grounds do exist they should go (or be pigtailed) to the optional #10-32 screw on the metal box. Every box has at least one hole tapped 10-32 for a ground screw. When I do run ground wires, which isn't very often, I treat each EMT segment as a separate deal and run a ground wire box to box and that's it. I do that when I fear the EMT conduit will get hit or damaged.

Devices auto-pickup grounds off the metal box one of several ways.

• Switches just do.
• Cheap receptacles do it via hard flush clean metal-metal contact with bottomed-out screws.
• "Self grounding" receptacles do it via wipers on the mounting screws.

Back to your wires.

I count three switched-hots, one always-hot and one neutral.

You want to add four wires for hot-ground pairs.

For reasons, 15-20A circuits simplify to "no more than four circuits per conduit". You can grind through 310.15(B)(16), 240.4(D) and 310.15(B)(3)(a) if you really want to. Any circuit served from 120/240V panelboard counts as 2 wires.

When 2 or more wires "split" current any of them is rated to handle, the entire group of wires counts as 1 wire for thermal derate purposes. Why? Because wire heat is the square of current. To create an arbitrary "U"nit, count any 1 wire's heat as its amps squared. So a wire carrying 20A of heat makes 400U of heat. But split it on 4 wires carrying 16, 2, 1 and 1 amps - heat is 16² + 2² + 1² + 1². That's only 260U, see how quick that falls off? That means your 3 switched-hots are a nothingburger.

And the same principle applies to 120/240V circuits such as MWBCs. Worst case 20A on both hots, right? 400U + 400U = 800U both wires. Well, what if the current is 20/16A with 4A on the neutral? 400U + 256U + 16U. That's less, so really - any circuit counts as 2 wires.

Now if you get into 120/208V 3-phase, it's complicated.

So back to simplicity, four or fewer circuits = you're fine.

As far as conduit fill, you have five insulated #14 + four insulated #12 + an optional BARE #12 ground. Punch that into a conduit fill calc but based on my experience you're fine.

Answer 2

You're actually fine. See NEC 110.14(C), you can use the 90°C wire rating as baseline for derating THHN.

So from Table 310.16 you find for #14 and #12 max at 25A and 30A.

Nine conductors 70%, 17.5A and 21A, you are still within 240.4(D) conventional "small conductor" limits of 15A and 20A.

Using a MWBC wouldn't change derate because you would have 5 existing plus 2 new to get 7 conductors which is still 70%, but it would make the pull a little easier. (Assuming single phase linear loads, non-linear would be 5 +3. So many details...)