My understanding is I can have up to 9 #12 CCC before derating down to 15A, is this correct?

That is the foundation of how we get to that, yes.

If so then that means maximum of four 20A circuits with 4 hots and 4 neutral total of 8 CCC.

That is correct and "4 circuits" is the answer. (pertaining to 15-20A circuits).

Are there exceptions whether those CCCs if some of them are impossible to be energized at the same time?

For example, in a 3 or 4 way wiring, if I have multiple travelers but only one can be engergized at one time, do all travelers count as CCCs?

Absolutely correct, those "extras" do not count. When the nature of the circuit means that 1 circuit's current (in that direction will be split across n wires, they count as one (1) wire.

The reason for this is thermal. The heat from a wire is voltage drop x current. But voltage drop itself is resistance x current. Thus, heat is resistance x current squared. Got it? So take a 20A wire, and say at 1 amp, the heat generated is 1 something's. At 2 amps, this heat is 4 something's (2 squared). At 10A, 100 something's (10 squared).

So the thermal rules are planning for 400 somethings out of that 20A wire. What if that is split to 4 wires, and they are 3+3+4+10 amps? What is the "somethings" of heat on that wire? It is those squared, or 9+9+16+100. Well, that's WAY better! Gosh that is much less thermal load on the pipe.

So what's the worst case? The worst case is the 4 wires being 0+0+0+20 amps, giving 0+0+0+400 somethings. And that's no worse than 1 wire.

So the extra wires do not count.

However, if I use MWBC then I can squeeze in six circuits with 6 hots and 3 neutrals?

Now we come back to this. Is your power Split-Phase (120/240V)? Then what is the thermal worst case for a MWBC? It is 20A on L1, 0A on neutral, 20A on other phase. Well we know that is 400+0+400 somethings. Totalling 800. Same as 2 wires obviously.

But when you start splitting current, the same thing happens that we already saw. If the current is 20+4+16 amps, then the heat is 400+256+16 somethings. That's less!

So the worst case for a MWBC, again in 120/240V split-phase country, is 2 wires' worth of heat. Thus the neutral doesn't count. Same thing applies to 120/240V loads like dryers and RV outlets.

The upshot is that in split-phase country, ALL circuits count as 2 wires and this can be simplified down to "no more than 4 circuits" in 15-20A circuits.





Now what about 3-phase?

With 3-phase if you have 3 hots and a neutral, the same thing happens - with all 3 phases maxed neutral doesn't count, but imbalances only improve the thermal picture. So 3-phase+neutral counts as 3 wires not 4. That's why Code stops at 9 wires instead of 8.

Because of Tesla weirdness, if 2 hot wires are at 20A and the 3rd hot is at 0A, the neutral is at 20A and you still count as 3 wires...... even if the 3rd hot wire is MISSING altogether. That's the gotcha -- MWBCs sourced from 120/208V panels e.g. in high-rise apartments must count their MWBCs as 3 wires not 2.

But, MWBCs in 3-phase are insane. You are getting 9 circuits worth at 20A, right? So that's 22KW right there at 120V. But those can also run at 277V in industrial, and that means 50,000 watts on twelve #12 wires in one 3/4" conduit. Light up a whole CostCo!

 

That is the foundation of how we get to that, yes.

That is correct and "4 circuits" is the answer. (pertaining to 15-20A circuits).

Absolutely correct, those "extras" do not count. When the nature of the circuit means that 1 circuit's current (in that direction will be split across n wires, they count as one (1) wire.

The reason for this is thermal. The heat from a wire is voltage drop x current. But voltage drop itself is resistance x current. Thus, heat is resistance x current squared. Got it? So take a 20A wire, and say at 1 amp, the heat generated is 1 something's. At 2 amps, this heat is 4 something's (2 squared). At 10A, 100 something's (10 squared).

So the thermal rules are planning for 400 somethings out of that 20A wire. What if that is split to 4 wires, and they are 3+3+4+10 amps? What is the "somethings" of heat on that wire? It is those squared, or 9+9+16+100. Well, that's WAY better! Gosh that is much less thermal load on the pipe.

So what's the worst case? The worst case is the 4 wires being 0+0+0+20 amps, giving 0+0+0+400 somethings. And that's no worse than 1 wire.

So the extra wires do not count.

Now we come back to this. Is your power Split-Phase (120/240V)? Then what is the thermal worst case for a MWBC? It is 20A on L1, 0A on neutral, 20A on other phase. Well we know that is 400+0+400 somethings. Totalling 800. Same as 2 wires obviously.

But when you start splitting current, the same thing happens that we already saw. If the current is 20+4+16 amps, then the heat is 400+256+16 somethings. That's less!

So the worst case for a MWBC, again in 120/240V split-phase country, is 2 wires' worth of heat. Thus the neutral doesn't count. Same thing applies to 120/240V loads like dryers and RV outlets.

The upshot is that in split-phase country, ALL circuits count as 2 wires and this can be simplified down to "no more than 4 circuits" in 15-20A circuits.

Now what about 3-phase?

With 3-phase if you have 3 hots and a neutral, the same thing happens - with all 3 phases maxed neutral doesn't count, but imbalances only improve the thermal picture. So 3-phase+neutral counts as 3 wires not 4. That's why Code stops at 9 wires instead of 8.

Because of Tesla weirdness, if 2 hot wires are at 20A and the 3rd hot is at 0A, the neutral is at 20A and you still count as 3 wires...... even if the 3rd hot wire is MISSING altogether. That's the gotcha -- MWBCs sourced from 120/208V panels e.g. in high-rise apartments must count their MWBCs as 3 wires not 2.

But, MWBCs in 3-phase are insane. You are getting 9 circuits worth at 20A, right? So that's 22KW right there at 120V. But those can also run at 277V in industrial, and that means 50,000 watts on twelve #12 wires in one 3/4" conduit. Light up a whole CostCo!

@seharper, this is extremely helpful. Thanks for taking the time to explain. I had to read it twice before it sank in.

Just to clarify for split phase MWBC, you said "It is 20A on L1, 0A on neutral, 20A on other phase. Well we know that is 400+0+400 somethings. Totalling 800. Same as 2 wires obviously." So if it's drawing 20A on one phase, say 15A on the opposite phase, then the neutral may carry up to 5A of the unbalanced current back, that's 400+25+225 which is still way less than 400+0+400. Hence even though the rule says "CCC" in a MWBC the neutral is not thermally generating more heat than the two hots drawing full 20A, it doesn't count.

Does that follow then theoretically, one can pull 8 circuits if they are all MWBCs, 8 hots and 4 neutrals that do not count? Even though I will not even attempt to pull 12 THHNs through a 3/4" EMT...I think I once pulled 6 or 7 and it was quite tight.

 

I think the fill table is bullshit. As in good luck trying to get 16 12awg wires in a 3/4 conduit. screw all that noise.

neutral from what i read is always a ccc. However might not count for derating purposes.

sounds to me like you need to run another conduit. way above my paygrade

 

I think the fill table is bullshit. As in good luck trying to get 16 12awg wires in a 3/4 conduit. screw all that noise.

It is for novices. To achieve the highest allowed fills, you need "the electrician's truck full of pulling tools".

That's why I advice novices/DIYers stay significantly under the max fill. If you push it, you end up needing the truck full of pulling tools. You can go to the electrical supply and buy them, but then you're sinking money for a tool you'll use once.

Just to clarify for split phase MWBC, you said "It is 20A on L1, 0A on neutral, 20A on other phase. Well we know that is 400+0+400 somethings. Totalling 800. Same as 2 wires obviously." So if it's drawing 20A on one phase, say 15A on the opposite phase, then the neutral may carry up to 5A of the unbalanced current back, that's 400+25+225 which is still way less than 400+0+400. Hence even though the rule says "CCC" in a MWBC the neutral is not thermally generating more heat than the two hots drawing full 20A, it doesn't count.

Right.

Does that follow then theoretically, one can pull 8 circuits if they are all MWBCs, 8 hots and 4 neutrals that do not count? Even though I will not even attempt to pull 12 THHNs through a 3/4" EMT...I think I once pulled 6 or 7 and it was quite tight.

That thing which is "half an MWBC", which you have 8 of in that case, I do not call that a circuit. NEC considers it to be one circuit with 2 hot wires. And so do I. I call that 4 MWBCs with 8 half-circuits.

 

when it's in a nipple. just a guess.

wow look at that jumble of ****.

 

An example-- 12 is rated 30 amps @ 90C so if you have 9ccc in a pipe then you have to derate to 70%. 30 amps x 70% is 21 amps so you are good to go. If you had 3 wires in the conduit you could not use the 25 amp rating in 75C column or 30 amps in the 90C column.

There are time, with motors and a few other situations where #12 can be used at the higher ampacity but we don't need to go into that here.

 

Cross the T dot the I divided by 13 plus 20 but only on a sunny day when it's over 50 degrees.

or


My solution:

Run another damn conduit

 

Cross the T dot the I divided by 13 plus 20 but only on a sunny day when it's over 50 degrees.

or


My solution:

Run another damn conduit

Well, I think the question is, how would you know you need to run another conduit in the first place? If you just stick with no more then 3 CCCs in one conduit and don't derate, then you can only do one circuit with two conductors or two MWBCs. If you have 4 circuits between points A and B, at what point do you decide to go with another conduit? Is it just by a hunch?

So now with derating I know I can safely do (and this safely implies code, which implies insurance etc...) 9 CCCs in a conduit. In my case above, I have 14 CCCs inside a 2" conduit. According to the conduit fill chart I can jam 101 #12 THHNs into a 2" EMT, doesn't mean I would do it or even anywhere close to it, just that it helps me to understand fill capacity and ambient temperature correction are two different components and they must both be satisfied. In my case the conduit was only 16", but if it were 25" long, then yes I would have to run two conduits, but I also could have up the THHN sizes to #10 which would not have been a bad option depending on the situation.

To me it's all about knowing the rules, and in some cases understanding some logic behind the rules. Not necessarily to push against it, but it helps me sleep better at night.

 

Remember: Conduit fill has nothing to do with whether or not a wire is current carrying.

Wire fill is wire fill.

Every wire counts towards the fill.

 

Yes, I am counting them separately. With what I have planned I will have 6 CCC and 9 total THHN conductors.

Now, with what Jim Port just posted, I can skip pulling the extra neutral conductor for the switch until later if needed (I doubt it will unless I do some smart switching), that will reduce to 5 CCC and 8 total conductors. 5 CCC is 80% derating so I am fine on the temperature adjustment and 8 total is less than the 16 maximum so I am good there too.

 

Neutrals for switches do not need to be installed in conduit since they can be added later if needed.

 

That's good to know. I thought the neutral has to be made available whether used or not, connected or not. Thanks!