Current carrying conductors?

[Michael gurries]

Where in the code does it actually specify the number of current carrying conductors per raceway? The general rule of thumb I’ve always been taught is 9, but where can I find the literature?

 

[LarryFine]

There's no limit per se, but the 9 is a rule-of-thumb limit before you have to up-size.

 

[oldsparky52]

310.15(B)(3) requires you adjust the ampacity of conductors when there are more than 3 CCCs. Table 310.15(B)(3)(a) shows how much you need to adjust the ampacity based on the number of CCCs.

You may use the 90 degree column to start your adjustments and it works out that for most of the conductors we use, a 70% adjustment to the 90 degree column gives an allowed ampacity that fits into the 75 degree column (use Table 310.15(B)(16) ).

Example, a #12 THHN @ 90 has an ampacity of 30 amps. You adjust to 70% (because of 9 CCCs) and you get 21 amps. So you are allowed to use it for a 20-amp circuit. If you had more than 9 CCCs, then you would adjust using 50% and the allowed ampacity of that #12 would be 15-amps and you could no longer use it on a 20-amp circuit, you would have to drop down to a 15-amp circuit. So, the "rule" became no more than 9 CCCs in a conduit.

 

[infinity]

Generally this "rule" only works with smaller conductors.

 

[anthonysolino]

310.15(B)(3) requires you adjust the ampacity of conductors when there are more than 3 CCCs. Table 310.15(B)(3)(a) shows how much you need to adjust the ampacity based on the number of CCCs.

You may use the 90 degree column to start your adjustments and it works out that for most of the conductors we use, a 70% adjustment to the 90 degree column gives an allowed ampacity that fits into the 75 degree column (use Table 310.15(B)(16) ).

Example, a #12 THHN @ 90 has an ampacity of 30 amps. You adjust to 70% (because of 9 CCCs) and you get 21 amps. So you are allowed to use it for a 20-amp circuit. If you had more than 9 CCCs, then you would adjust using 50% and the allowed ampacity of that #12 would be 15-amps and you could no longer use it on a 20-amp circuit, you would have to drop down to a 15-amp circuit. So, the "rule" became no more than 9 CCCs in a conduit.

double like

 

[Michael gurries]

310.15(B)(3) requires you adjust the ampacity of conductors when there are more than 3 CCCs. Table 310.15(B)(3)(a) shows how much you need to adjust the ampacity based on the number of CCCs.

You may use the 90 degree column to start your adjustments and it works out that for most of the conductors we use, a 70% adjustment to the 90 degree column gives an allowed ampacity that fits into the 75 degree column (use Table 310.15(B)(16) ).

Example, a #12 THHN @ 90 has an ampacity of 30 amps. You adjust to 70% (because of 9 CCCs) and you get 21 amps. So you are allowed to use it for a 20-amp circuit. If you had more than 9 CCCs, then you would adjust using 50% and the allowed ampacity of that #12 would be 15-amps and you could no longer use it on a 20-amp circuit, you would have to drop down to a 15-amp circuit. So, the "rule" became no more than 9 CCCs in a conduit.

Wow, this was extremely helpful I never fully grasped this till now, thank you!

 

[SSDriver]

I've always found it weird that the code only goes by number of CCC and not number of CCC and fill percentage. I know its unlikely but if I run 10 #12 CCC's in an existing 1.5" (6.95% fill) conduit I would have to make the same corrections(50%) as if I ran those 10 #12 CCC's in 3/4" (26.54% Fill) conduit. I'm sure its just due to complexity and lack of it being common. But it would be great to have some type of exception. IE: if your total conduit fill is under 15% or 10% you dont have to add the correction factor for number of CCC's.

 

[LarryFine]

CCC fill is based on heat generation. Conduit fill is based on cross-section area, especially as it relates to wire-pulling force.

 

[ramsy]

310.15(B)(3) requires you adjust the ampacity of conductors when there are more than 3 CCCs. Table 310.15(B)(3)(a) shows how much you need to adjust the ampacity based on the number of CCCs.

You may use the 90 degree column to start your adjustments and it works out that for most of the conductors we use, a 70% adjustment to the 90 degree column gives an allowed ampacity that fits into the 75 degree column (use Table 310.15(B)(16) ).

Example, a #12 THHN @ 90 has an ampacity of 30 amps. You adjust to 70% (because of 9 CCCs) and you get 21 amps. So you are allowed to use it for a 20-amp circuit. If you had more than 9 CCCs, then you would adjust using 50% and the allowed ampacity of that #12 would be 15-amps and you could no longer use it on a 20-amp circuit, you would have to drop down to a 15-amp circuit. So, the "rule" became no more than 9 CCCs in a conduit.

Never heard such an elegant antidote to NFPA's convoluted idiot-table adjustments.

It could also be argued NFPA tables overheat equipment, and perpetuate the cortege industry of thermal imaging.

NEC 310.1 mentions 1 of many equipment types that break these tables, when Chapter-3 derating or adjustments are relied upon exclusively for ampacity limits.

The issues that break overheat equipment are not found anywhere in NEC chapter 3. Finding these items requires a wild-goose chase thru fragmented puzzles of unrelated sections, hidden far away from Tables, derating, or adjustments.

1) NEC 220.18 points out a few sections that break these tables, referring to inductive loads, LED drivers, ballasts, Motors, and HVACR.

2) NEC 334.80 is another section that breaks all idiot-table temperature columns, except 60ºC. Most old work NM cables, or new cable bundling prohibit operating at 75ºC.

3) NEC 110.14(C)

A similar rule of thumb for old-work equipment limited to 60° would be

 

[roger]

I've always found it weird that the code only goes by number of CCC and not number of CCC and fill percentage. I know its unlikely but if I run 10 #12 CCC's in an existing 1.5" (6.95% fill) conduit I would have to make the same corrections(50%) as if I ran those 10 #12 CCC's in 3/4" (26.54% Fill) conduit. I'm sure its just due to complexity and lack of it being common. But it would be great to have some type of exception. IE: if your total conduit fill is under 15% or 10% you dont have to add the correction factor for number of CCC's.

I believe augie47 explained a few years back that a big oven will heat to the same temperature as a small oven using the same element, it just takes longer.

Roger

 

[infinity]

I agree that this entire concept is all over the place and poorly implemented by the NEC. I find it hard to believe that 9 CCC's in 1/2" run of EMT will be subject to the same amount of heat as those same 9 CCC's in a run of 4" EMT. And when look at wireways you can have a wireway with less cross-sectional area than a circular raceway and have still have up to 30 CCC's without any derating. Doesn't really make much sense.

 

[kwired]

I've always found it weird that the code only goes by number of CCC and not number of CCC and fill percentage. I know its unlikely but if I run 10 #12 CCC's in an existing 1.5" (6.95% fill) conduit I would have to make the same corrections(50%) as if I ran those 10 #12 CCC's in 3/4" (26.54% Fill) conduit. I'm sure its just due to complexity and lack of it being common. But it would be great to have some type of exception. IE: if your total conduit fill is under 15% or 10% you dont have to add the correction factor for number of CCC's.

Same rules apply to cables that are bundled together but otherwise in free air other than the occasional method of support.

The rules we have probably are not perfect, but guessing is about the simplest way they could find to make as much one size fits all as possible, and are on the conservative side in case the worst happens.

 

[anthonysolino]

I agree that this entire concept is all over the place and poorly implemented by the NEC. I find it hard to believe that 9 CCC's in 1/2" run of EMT will be subject to the same amount of heat as those same 9 CCC's in a run of 4" EMT. And when look at wireways you can have a wireway with less cross-sectional area than a circular raceway and have still have up to 30 CCC's without any derating. Doesn't really make much sense.

when those conductors are pulled in they are all intertwined and laying on top of each other, you have one conductor laying inside two conductors and it has no way for the heat to escape, its not the pipe being able to breath its the wire being able to, if your laying in bed and wrapped up in 3-4 quilts you get hot don't you? so you shed one, you get less hot, shed another, so on and so fourth. thats how I kinda of look at it, I had a teacher explain this to me once before and drew out on the board and how the wires lay in the tube it does make a lot of sense.

 

[kwired]

when those conductors are pulled in they are all intertwined and laying on top of each other, you have one conductor laying inside two conductors and it has no way for the heat to escape, its not the pipe being able to breath its the wire being able to, if your laying in bed and wrapped up in 3-4 quilts you get hot don't you? so you shed one, you get less hot, shed another, so on and so fourth. thats how I kinda of look at it, I had a teacher explain this to me once before and drew out on the board and how the wires lay in the tube it does make a lot of sense.

That logic applies, but if you place 10 #12's in a 4 inch conduit it is much easier to have them all laying right next to one another in the bottom of the pipe compared to having no choice but to have some surrounded by other conductors if in a 3/4 conduit.

 

[anthonysolino]

That logic applies, but if you place 10 #12's in a 4 inch conduit it is much easier to have them all laying right next to one another in the bottom of the pipe compared to having no choice but to have some surrounded by other conductors if in a 3/4 conduit.

hey I agree but at the same token, how do we know once those wires are pulled how they will land ?? I see it from your stand point as well, logically it makes sense to think of it in this manner. how ever I cant help but think if you have a continuous load on 3 or 4 of those circuits you won't have any heat syncing occurring on the other conductors they will all just be generating heat on top of each other, I think thats why they decided to force us to make these adjustments simply the risk is too high to just go on a hunch that "eh they might not land on top of eachother" what at the same token what is the practicality that any of us are going to run a 4 inch conduit for 8 CCC's in the field? slim to none I don't think. 9 times out of 10 its going to be a 3/4 or 1"

 

[kwired]

hey I agree but at the same token, how do we know once those wires are pulled how they will land ?? I see it from your stand point as well, logically it makes sense to think of it in this manner. how ever I cant help but think if you have a continuous load on 3 or 4 of those circuits you won't have any heat syncing occurring on the other conductors they will all just be generating heat on top of each other, I think thats why they decided to force us to make these adjustments simply the risk is too high to just go on a hunch that "eh they might not land on top of eachother" what at the same token what is the practicality that any of us are going to run a 4 inch conduit for 8 CCC's in the field? slim to none I don't think. 9 times out of 10 its going to be a 3/4 or 1"

4 inch conduit with parallel conductors inside does happen, but yes is fairly impractical to have 4 inch for a small number of 12 AWG conductors, but should you have that situation same adjustment rules still apply.

 

[ramsy]

There's no limit per se, but the 9 is a rule-of-thumb limit before you have to up-size.

Denis Alwon points out 240.4(D) prohibits the 75°C operating column for these small conductors.

I believe its a dangerous rule of thumb for common #12 conductors. After Chap. 3 derating & adjustments, my temperature spreadsheets based on Chap. 9, Table 8, Note 2 equation shows 9 ccc's in raceway begin to exceed 60°C at 15 Amps each, rather than 21 Amps, overheating in violation of 240.4(D).

 

[kwired]

Denis Alwon points out 240.4(D) prohibits the 75°C operating column for these small conductors.

My temperature calc spreadsheet based Chap. 9, Table 8, Note 2 equation shows 9 ccc's in raceway begin to exceed 60°C at 15 Amps each, and overheat in violation of 240.4(D).

Maybe so, but most of what is used today uses 90C conductors so you typically have a higher starting point to adjust from.

ADD: has been mostly 90C conductors for about 35 maybe even 40 years now for new conductors.

 

[ramsy]

If thermal Thermal Imaging reports prove operating temps near 75°C in violation of 240.4(D), inspectors can fail it, and clients can demand rework, regardless of 90°C insulation ratings.

My point is, if everybody has screwed this up for years, which apparently they have during installation or expansion of new loads to existing installs, then Chap. 3 is to blame, with no warning, or reference to 240.4(D), 110.14(C), 220.18, 334.80, common equipment with inductive loads, or listing limits to 60ºC.

 

[kwired]

If thermal Thermal Imaging reports prove operating temps near 75°C in violation of 240.4(D), inspectors can fail it, and clients can demand rework, regardless of 90°C insulation ratings.

My point is, if everybody has screwed this up for years, which apparently they have during installation or expansion of new loads to existing installs, then Chap. 3 is to blame, with no warning, or reference to 240.4(D), 110.14(C), 220.18, 334.80, common equipment with inductive loads, or listing limits to 60ºC.

I see no mentioning of temperature at all in 240.4(D).

If you read 75C at a 60C terminal, that is understandable reason for some concern and also 99% of the time is likely a failed termination more so than too much current. If you read 75C in middle of a raceway and the conductor is rated 90C there is nothing wrong, without more supporting information.