Derating of general conductors in conduit

[howardrichman]

If I install 12-12 THHN conductors in 3/4 pvc conduit. According to table 310.15(b)(2)(A), I should be able to install up to 9 conductors- 70% of 30 amps(90c column) to maintain 20 amp rating on those conductors and conduit fill. If I'm interpreting this correctly, Wouldn't the nuetrals be excluded in those calculations and of those 12 conductors. Are the nuetrals and ground current carrying conductors, and counted in the # of conductors? if so would I have to either have to run # 10 conductors for derate, or install another conduit?

please reply;
HR...

 

[ADub]

Is this 6 hots and 6 neutrals or is this 9 hots and 3 neutrals.

 

[GoldDigger]

The EGCs are always not counted. The neutrals may or may not be counted. Unless you have MWBCs in the mix, neutrals are counted.
BTW, it could also be 8 hots and four neutrals.

 

[howardrichman]

Well; yea, figure 7 hots and 4 neutrals, and 1 ground. I don't think I ever counted the neutrals as current carrying conductors. To be safe, for derate; I pulled three #10's for two feeders(common neutral) to one location for a definite 20 amp requirement, and have will to check conduit fill, but cant switch all out unless I install either larger conduit or add additional pipe.

Thanks for reply
HR...

 

[GoldDigger]

For one three MWBC you count two CCCs. For the other four circuits with individual neutrals you and the inspector will count 8 more CCCs for a total of 10.
For each pair of circuits that you convert to an MWBC you save TWO neutrals from the CCC count.

 

[Smart $]

Well; yea, figure 7 hots and 4 neutrals, and 1 ground. I don't think I ever counted the neutrals as current carrying conductors. To be safe, for derate; I pulled three #10's for two feeders(common neutral) to one location for a definite 20 amp requirement, and have will to check conduit fill, but cant switch all out unless I install either larger conduit or add additional pipe.

Thanks for reply
HR...

You have to break it down to circuit level in order to determine. For example, your 7 hots and 4 neutrals can be from a) 1? 3W system, b) 3? 4W delta system, or c) 3? 4W wye system. I was going to list all the possible combinations for these systems, but its too much of a headache this early in the morning... when you could just divvy up the circuits in your description.

For N to not count*:
1? 3W system, L1, L2, N circuit in the conduit.
3? 4W delta system, A, C, N circuit in the conduit.
3? 4W wye system, A, B, C, N circuit in the conduit.

*Where the load is not more than 50% non-linear.

 

[howardrichman]

That's what I'm working with. I figure single phase multiple 2 live lines 120V and one neutral. Would one line w/one neutral also not be counted, or just where neutral is shared?

HR...

 

[howardrichman]

For one three MWBC you count two CCCs. For the other four circuits with individual neutrals you and the inspector will count 8 more CCCs for a total of 10.
For each pair of circuits that you convert to an MWBC you save TWO neutrals from the CCC count.

please define mwbc's and ccc's

 

[Dennis Alwon]

please define mwbc's and ccc's

multiwire branch circuit
current carrying conductor

 

[howardrichman]

For one three MWBC you count two CCCs. For the other four circuits with individual neutrals you and the inspector will count 8 more CCCs for a total of 10.
For each pair of circuits that you convert to an MWBC you save TWO neutrals from the CCC count.

OK , I understand it now. If conduit fill allows, I may switch to one multi wire circuit to #10 conductors for a particular heavy load.
Thanks...HR...

 

[infinity]

What is the system that you're using? 1?, 120/240 or 3?, 208Y/120 or something else?

 

[howardrichman]

1 phase 120/240. For example there will be 6 feeders and 3 neutrals, w/1 gnd. What I did was use #10's for 2 feeders w/# 10 neutral to a heavier load and still meet conduit fill. I should only have to count 6 conductors, unless there's an odd # of neutrals which would have to be factored in .

HR...

 

[Dennis Alwon]

1 phase 120/240. For example there will be 6 feeders and 3 neutrals, w/1 gnd. What I did was use #10's for 2 feeders w/# 10 neutral to a heavier load and still meet conduit fill. I should only have to count 6 conductors, unless there's an odd # of neutrals which would have to be factored in .

HR...

For conduit fill you have to use all the conductors. For derating then you only need the current carrying conductor. So if you have a single phase system with 6 ungrounded conductors and 3 grounded conductors then the de-rating would only need to be based on 6 conductors not 9

 

[infinity]

For derating then you only need the current carrying conductor. So if you have a single phase system with 6 ungrounded conductors and 3 grounded conductors then the de-rating would only need to be based on 6 conductors not 9

Exactly that's why knowing the system that you're dealing with is important when doing the calculation. For the same 9 conductors with a 3?, WYE system you would have 9 CCC's.