Metal Wireway & Derating

[Joe_Medarac]

I have two questions regarding metal wireways and conduit. First off some of the information for project.

1. Metal wireway is 4? X 4? duct
2. Conduit is 2-1/2? EMT
3. All wire is 12 AWG
4. All branch circuits are 20 amps

Question 1. In reading article 376-22 I know I can run up to 30 current-carrying conductors in the wireway (raceway) without derating. If I go over this at what percentage do I start derating conductors at? If I look at table 310.15(2)(B)(2)(a) do I start at 4-6 conductors (80%) or do I start at 31-40 conductors (40%)?

Question 2. The sub-contractor wants to install 2-1/2? conduit from the distribution panel to the 4? X 4? wireway. This is to gain access to the wireway (raceway) for all the branch circuits. Now I know that I can have up to 30 current-carrying conductors in the wireway but what about the conduit? Do I follow the guidance from Table 310.16 and derate in accordance with table 310.15(B)(2)(a)? Which would mean if I have 30 current-carrying conductors I would have to derate to 45% of the ampacity of 12 AWG

 

[Smart $]

Q1: You start at 31-40 @ 40%.

Q2: Yes, you'd have to derate to 45% [unless the conduit is 24" or less?310.15(B)(2)(a, Excpt.#2)]

 

[batch]

For clarification on my part.

Does current carrying conductors include grounded conductors?

I would assume so. But, I rarely deal with more then a couple of circuits at a time.

Would any thing change it you used rigid non-metalic conduit?

Just curious, as I have seen this a couple of times now...

 

[infinity]

For clarification on my part.

1-Does current carrying conductors include grounded conductors?

2-Would any thing change it you used rigid non-metalic conduit?

1) Maybe. You'll need to look at 310.15(B)(4).

2) No, but you'll need to add an EGC in the conduit.

 

[Joe_Medarac]

Wireway & Derating

Wireway & Derating

Batch, yes it includes grounded (neutral) conductors.

Infinity, I thought that 310.15(B)(4)(a) would apply at first but then these are branch circuits and believe that you would have to count the grounded (neutral) conductors in this case as current-carrying.

On the conduit from the distribution panel to the 4"X4" this would be at least 3' but less than 6' in length. My recommendation to the sub was to install 4"X4" from the distribution panel for the full run then there would be no question in how many current-carrying conductors that can be placed within the duct without derating. He disagrees.

There will be a grounding conductor with each branch circuit.

 

[Smart $]

...

Infinity, I thought that 310.15(B)(4)(a) would apply at first but then these are branch circuits and believe that you would have to count the grounded (neutral) conductors in this case as current-carrying.

Simply being branch circuits is not the qualifying criteria for counting the grounded conductors as current carrying, or not. Your comment, as it is, leads me to believe you do not understand which count and which do not. Do you mean each circuit has a dedicated grounded conductor? If so, your conclusion would be correct. If not, then circuits share a grounded conductor, for which there are a possibilities both ways and would require more detail on the power configuration (such as 120/240 1? 3W, 208Y/120 3? 4W, etc.), the wiring method(s) utilized, and the loads being powered.

On the conduit from the distribution panel to the 4"X4" this would be at least 3' but less than 6' in length. My recommendation to the sub was to install 4"X4" from the distribution panel for the full run then there would be no question in how many current-carrying conductors that can be placed within the duct without derating. He disagrees.

Actually, there should be no question either way :grin: , but I vote for the method you suggested as easier way to be compliant.

There will be a grounding conductor with each branch circuit.

Is there a point to bringing this into the discussion? Not that doing so isn't compliant, but unless there is a specific and justified purpose for doing so, I believe it to be a waste of wire... and it also adds to wire fill at the same time...

If we are looking 30 current-carrying conductors, where each branch circuit is 20A wired with three 12 AWG THHN using 1 ungrounded, 1 grounded, and 1 grounding conductor per circuit, you'd have 45 wires. If these wires all enter at the same point and proceed through the wireway in the same direction, you'd have a wire fill of 45 wires ? ~0.130 in?/wire = 5.85 in?. You are only permitted to 20% of the cross-sectional area for fill, which is 4" ? 4" ? 0.20 = 3.20 in?...!!!

 

[augie47]

derating

derating

a definite case of the "devil is in the details"
I'm manged to become thoroughly confused (not a difficult thing to cause). To me, before we can make a determination.. How many phase conductors and how many grounded conductors, what tyope of load (if the grounded are shared) and how many equipment ground conductors ? If that was clear in the question, I misread.

 

[Joe_Medarac]

Simply being branch circuits is not the qualifying criteria for counting the grounded conductors as current carrying, or not. Your comment, as it is, leads me to believe you do not understand which count and which do not. Do you mean each circuit has a dedicated grounded conductor? If so, your conclusion would be correct. If not, then circuits share a grounded conductor, for which there are a possibilities both ways and would require more detail on the power configuration (such as 120/240 1? 3W, 208Y/120 3? 4W, etc.), the wiring method(s) utilized, and the loads being powered.


Actually, there should be no question either way :grin: , but I vote for the method you suggested as easier way to be compliant.


Is there a point to bringing this into the discussion? Not that doing so isn't compliant, but unless there is a specific and justified purpose for doing so, I believe it to be a waste of wire... and it also adds to wire fill at the same time...

If we are looking 30 current-carrying conductors, where each branch circuit is 20A wired with three 12 AWG THHN using 1 ungrounded, 1 grounded, and 1 grounding conductor per circuit, you'd have 45 wires. If these wires all enter at the same point and proceed through the wireway in the same direction, you'd have a wire fill of 45 wires ? ~0.130 in?/wire = 5.85 in?. You are only permitted to 20% of the cross-sectional area for fill, which is 4" ? 4" ? 0.20 = 3.20 in?...!!!

1. You are correct in this but since this work is being done a military base and is being installed for communications equipment we do not run multi-wire branch circuits. All branch circuits have 1 ungrounded (Phase) 1 grounded (neutral) and 1 grounding (fault protective) conductor. Just the way it is done in military communication facilities.


2. I agree it is a waste of wire and adds to the % of fill in the raceway but as I stated above this is just the way it is done in military communication facilities.


3. I think your math is correct but using the wrong numbers. My calculations are as follows.

Number of conductors = 45
20% fill of 4? X 4? duct = 4 X 4 X .2 = 3.20 in2
Square Area of one 12 AWG THHN conductor = .0133 in2

45 X .0133 in2 = .5985 in2

.5985 in2 < 3.20 in2

 

[Smart $]

1. You are correct in this but...Just the way it is done in military communication facilities.


2. I agree it is a waste of wire and adds to the % of fill in the raceway but...just the way it is done in military communication facilities.

To both 1 & 2, I had a feeling you would say that... but it's also done in many commercial and institutional applications, too. I wish those that do specify such would consider using MWBC's w/an oversized grounded conductor instead.

3. I think your math is correct but using the wrong numbers. My calculations are as follows.

Number of conductors = 45
20% fill of 4? X 4? duct = 4 X 4 X .2 = 3.20 in2
Square Area of one 12 AWG THHN conductor = .0133 in2

45 X .0133 in2 = .5985 in2

.5985 in2 < 3.20 in2

Ahhh ...(expletive withheld)...you are right. In my haste to do the math I pulled the diameter out of the table instead of the area. My bad :-( ...and I apologize.