switching between current carrying conductors

[malachi constant]

We have a bunch of single phase motors being installed that require an extra phase wire. The extra phase wire is connected & controlled such that only one of the two phase wires operates at the same time. (If wire 1 is live the motor runs forward, if wire 2 is live the motor runs backward.) Do BOTH these need to be considered current carrying conductors (CCC) for the purposes of 310.15(3)(a) adjustment factors?

I ask because adding the extra wire is a change to the contract. I assume the contractor planned to run home runs to each group of three motors, which would be three CCC per 3/4" home run. If these additional wires are required to be considered CCCs, that would put this at (6) CCCs per home run. The motors are 3/4HP 120V, so FLA of 13.8A each. I don't get into adjustment factors very often, but my understanding of 310.15(B)(2)(b) is "10 or less / 75C" would apply, so derate these by a factor of 1.36. 13.8A x 1.36 = 18.8A. I'm not sure I'm doing that right, but if so maybe the "switching" question isn't even an issue?

Thanks!

 

[kwired]

No additional current carrying conductor count needed, same would even apply to multi-speed motors that have a common and individual input leads for each speed. On such motors you can never have current on more then two conductors at any time - it just physically doesn't work right if you had current on a third conductor.

 

[charlie b]

I agree. The thing I look for is an interlock or other control mechanism that physically prevents both wires from carrying current at the same time. If that is present, I don't count both wires. Keep in mind that all wires count towards conduit fill calculations. But that was not your question.

 

[infinity]

It's strange because if these interlocked conductors were just spares they would count towards the derating even though they carried no current.

 

[kwired]

Actual "interlock" isn't exactly necessary, If you try to run motor forward and reverse at same time - something is going to trip or burn out long before ampacity adjustments and temperature in the raceway becomes an issue.

 

[SceneryDriver]

It's strange because if these interlocked conductors were just spares they would count towards the derating even though they carried no current.

I believe the rationale is: spares count for derating because they are not prevented from being used at the same time. Interlocked conductors cannot be used at the same time.


SceneryDriver

 

[infinity]

I believe the rationale is: spares count for derating because they are not prevented from being used at the same time. Interlocked conductors cannot be used at the same time.


SceneryDriver

That probably true but IMO it's still pretty stupid. The burden should be on the next installer to do the calculation if they're going to use them, if it works great if not then go to plan B.

 

[kwired]

That probably true but IMO it's still pretty stupid. The burden should be on the next installer to do the calculation if they're going to use them, if it works great if not then go to plan B.

Spares for known future use, I think you should be able to count per the intended use - and if the intended use means not all will carry current at same time then you don't have to count all of them as current carrying.

Spares for unknown future use - I kind of agree with you. They do need to count for raceway fill but it is hard to assign them to the unknown otherwise.

 

[infinity]

Spares for known future use, I think you should be able to count per the intended use - and if the intended use means not all will carry current at same time then you don't have to count all of them as current carrying.

Spares for unknown future use - I kind of agree with you. They do need to count for raceway fill but it is hard to assign them to the unknown otherwise.

Exactly my point, the NEC gives you no guidance on how you are to figure how they could possibly be used in the future if at all. This type of lunacy only diminishes the usability of the code. If they're used in the future it is at that time the they should be evaluated.

If I use an over-sized conduit should I assume that someone may use it in the future to pull in more conductors and possibly altering the derating?

This one kind of infuriates me with it's stupidity. :rant:

 

[JFletcher]

We have a bunch of single phase motors being installed that require an extra phase wire. The extra phase wire is connected & controlled such that only one of the two phase wires operates at the same time. (If wire 1 is live the motor runs forward, if wire 2 is live the motor runs backward.) Do BOTH these need to be considered current carrying conductors (CCC) for the purposes of 310.15(3)(a) adjustment factors?

I ask because adding the extra wire is a change to the contract. I assume the contractor planned to run home runs to each group of three motors, which would be three CCC per 3/4" home run. If these additional wires are required to be considered CCCs, that would put this at (6) CCCs per home run. The motors are 3/4HP 120V, so FLA of 13.8A each. I don't get into adjustment factors very often, but my understanding of 310.15(B)(2)(b) is "10 or less / 75C" would apply, so derate these by a factor of 1.36. 13.8A x 1.36 = 18.8A. I'm not sure I'm doing that right, but if so maybe the "switching" question isn't even an issue?

Thanks!

I dont think the extras count as CCC since only one is supposed to be energized at a time, like a 3 way traveler. Sure, it's possible thru some gross failure to energize both at once but by design it cant happen. ofc I'm not the inspector on that job.

I'm not getting how a group of 3 single phase motors would have 3CCC - line to neutral or line to line, each motor would have 2CCC, not 1, giving 6 total. Even with another CCC per group of 3, that puts you at 9 CCC, still good with #12 wire.

 

[Dennis Alwon]

For those who don't know where this info is coming from..

Note after Table 310.15(B)(3)(a)

Number of conductors is the total number of conductors in the
raceway or cable, including spare conductors. The count shall be
adjusted in accordance with 310.15(B)(5) and (6). The count shall not
include conductors that are connected to electrical components that
cannot be simultaneously energized.

 

[JFletcher]

Exactly my point, the NEC gives you no guidance on how you are to figure how they could possibly be used in the future if at all. This type of lunacy only diminishes the usability of the code. If they're used in the future it is at that time the they should be evaluated.

If I use an over-sized conduit should I assume that someone may use it in the future to pull in more conductors and possibly altering the derating?

This one kind of infuriates me with its stupidity. :rant:

FIFY. and I agree with you. spares that are run for future use arent CCC, tho otoh it would be pretty dumb imo to pull extra wire in now that would have to be removed or modified in the future if the extra wires were CCC.

Does anyone actually pull in spare wiring that often? I could see it in a real limited fashion in commercial, say a machine shop, where 6 mo down the road they are getting another piece of eqpt... especially if running the conduit was a bear or required shutdown/nightwork. istm running a spare conduit would be much less a headache and give more flexibility...

 

[charlie b]

Actual "interlock" isn't exactly necessary. . . .

No argument from me. But please note that I said "interlock or other control mechanism."

 

[charlie b]

It's strange because if these interlocked conductors were just spares they would count towards the derating even though they carried no current.

I don't see that as a code requirement. But it would be my design choice to do that anyway. If a future engineer or EC connects the spare wires, and if doing so alters the ampacity of the wires that I had installed, then I would have done an injustice to my client. So I would select my wires (the ones being energized for my project) under the assumption that the spare wires would be carrying current in the future. I will repeat, however, that this is no more than a design choice.

 

[infinity]

I don't see that as a code requirement. But it would be my design choice to do that anyway. If a future engineer or EC connects the spare wires, and if doing so alters the ampacity of the wires that I had installed, then I would have done an injustice to my client. So I would select my wires (the ones being energized for my project) under the assumption that the spare wires would be carrying current in the future. I will repeat, however, that this is no more than a design choice.

It is a code requirement to count the spares. From post #11:

Number of conductors is the total number of conductors in the
raceway or cable, including spare conductors. The count shall be
adjusted in accordance with 310.15(B)(5) and (6). The count shall not
include conductors that are connected to electrical components that
cannot be simultaneously energized.

 

[kwired]

FIFY. and I agree with you. spares that are run for future use arent CCC, tho otoh it would be pretty dumb imo to pull extra wire in now that would have to be removed or modified in the future if the extra wires were CCC.

Does anyone actually pull in spare wiring that often? I could see it in a real limited fashion in commercial, say a machine shop, where 6 mo down the road they are getting another piece of eqpt... especially if running the conduit was a bear or required shutdown/nightwork. istm running a spare conduit would be much less a headache and give more flexibility...

I have pulled in spare conductors at times in past.

Maybe from a panel or motor controllers remote from where the loads are to a location closer to where the loads are. If you have a process that utilizes a lot of smaller motors say under 5 HP 480 volt three phase - you can pull a 1.25 or even 2 inch chocked full of 12 AWG conductors from a control room to the production area and have spares for future equipment and not need to worry about ampacity adjustments when you add those loads. If you should end up adding say a 20 HP motor sometime in the future - you didn't lose out too much because you would have had to run new raceway for it anyway.

Done this on dwellings at times even. New panel install - need to relocate as old panel just won't work where it was - maybe in a closet, bath, etc. have to locate outdoors because that is quickest and least cost. Run maybe three 3/4 inch raceways to a junction box in a crawlspace where you branch out to the loads, pull spare conductors to make it easier to add a circuit down the road, but again if they should add a 50 amp hot tub down the road you are running new raceway regardless so you don't try to accommodate that before hand unless they know early on they want to add that.

 

[kwired]

We have a bunch of single phase motors being installed that require an extra phase wire. The extra phase wire is connected & controlled such that only one of the two phase wires operates at the same time. (If wire 1 is live the motor runs forward, if wire 2 is live the motor runs backward.) Do BOTH these need to be considered current carrying conductors (CCC) for the purposes of 310.15(3)(a) adjustment factors?

I ask because adding the extra wire is a change to the contract. I assume the contractor planned to run home runs to each group of three motors, which would be three CCC per 3/4" home run. If these additional wires are required to be considered CCCs, that would put this at (6) CCCs per home run. The motors are 3/4HP 120V, so FLA of 13.8A each. I don't get into adjustment factors very often, but my understanding of 310.15(B)(2)(b) is "10 or less / 75C" would apply, so derate these by a factor of 1.36. 13.8A x 1.36 = 18.8A. I'm not sure I'm doing that right, but if so maybe the "switching" question isn't even an issue?

Thanks!

I never paid attention to the numbers the first time through OP, mostly focused on the fact you shouldn't need to count the extra multispeed conductors as additional current carrying conductors.

430 full load motor tables does give you 13.8 A FLA for this motor. Unless it is an intermittent duty motor your minimum conductor ampacity does need to be 125% of the FLA so you start out needing at least 17.25 amp conductor.

When you do adjust for number of conductors in the raceway - if you have 6 current carrying conductors the adjustment factor is 80% You can either select a conductor and multiply it's ampacity by .80 and see if that is more then the needed 17.25 or you can just multiply 17.25 by the reciprocal or .8 which is 1.25 and that gives you 21.6 amps - and select whatever conductor has an ampacity of 21.6 or more. @ 75C 12 AWG copper is rated 25 amps and would be acceptable. If for some reason you have to use 60 deg conductors you would need to go with 10 AWG, but that would generally only apply to old equipment, older conductors, (like more then 30-35 years old) or NM cable.

 

[JFletcher]

I have pulled in spare conductors at times in past.

Maybe from a panel or motor controllers remote from where the loads are to a location closer to where the loads are. If you have a process that utilizes a lot of smaller motors say under 5 HP 480 volt three phase - you can pull a 1.25 or even 2 inch chocked full of 12 AWG conductors from a control room to the production area and have spares for future equipment and not need to worry about ampacity adjustments when you add those loads. If you should end up adding say a 20 HP motor sometime in the future - you didn't lose out too much because you would have had to run new raceway for it anyway.

Done this on dwellings at times even. New panel install - need to relocate as old panel just won't work where it was - maybe in a closet, bath, etc. have to locate outdoors because that is quickest and least cost. Run maybe three 3/4 inch raceways to a junction box in a crawlspace where you branch out to the loads, pull spare conductors to make it easier to add a circuit down the road, but again if they should add a 50 amp hot tub down the road you are running new raceway regardless so you don't try to accommodate that before hand unless they know early on they want to add that.

Yeah I've pulled a spare 12/3 and 14/3 to j-boxes in the attic and basement of a house, maybe run an extra piece of ENT to the basement if the HO is thinking about a spa in the near future, to save me from a difficult/impossible wall-fish to the panel. I guess in commercial/industrial it's more a case-by-case basis, especially if, as you say, you have a bunch of tiny 480V loads, and several 'soon to be added' loads.