Another wire derating question

[tony_psuee]

I have always used the derating factors in table 310.15(B)(2)(a) when determining wire size for multiple motors, or two speed motors run in the same conduit. Now, I have another engineer telling me that for motors that are mechanically interlocked from running simultaneously and two speed motors the conductors for each motor or motor winding do not have to be derated because they will not be carrying current at the same time. I can follow the logic but am uncomfortable with it. Looking at the Article and the table it makes no mention of if the conductors are carrying current at the same time or not. Only if you have X number of wires that are intended to carry current (current carrying conductors) they are limited to Y% of full ampacity of that wire size (percent derate). I appreciate any feedback.

Thanks,

Tony

 

[derf48]

Re: Another wire derating question

Tony,

There are two ways to select ampacities of conductors, per 310.15 (A). The first is the way you are doing it, as per the tables provided 310.15 (B), and the second is under engineering supervision as provided in 310.15(C). If you do not qualify as a engineer, then you must select ampacities as you are presently doing.

Fred Bender

 

[wirenut1980]

Re: Another wire derating question

As Fred has pointed out, you will need a licensed PE to sign and seal the load calculations basically saying that the loads are coincident and will not be running at the same time, ever. This is what some refer to as load diversity and is not addressed in the NEC.

 

[charlie b]

Re: Another wire derating question

There are some ?magic tricks? that can only be justified with a PE seal, but I don?t think this is one of them. If a design has six conductors in a conduit, and if an interlock prevents more than three from being energized at a time, then I think (1) You don?t have to derate for six conductors, and (2) You don?t need a PE to seal the design. I?m not trying to take business away from my brother and sister PEs, but this is not a calculation or design process that requires our participation. That being said, however, we may note that Tony is a EE. And since he is involved in the design, we can presume that a PE will eventually seal and issue the design.

What you will have to do, however, is to get the AHJ to accept the interlock as the means of preventing the simultaneous operation of all components powered through that single conduit. There is nothing in the NEC that would compel the AHJ to accept this (i.e., no statement along the lines of, ?it shall be permissible?), so it becomes a judgment call.

 

[physis]

Re: Another wire derating question

There are some ideas in the proposal area to generate a proposal on this very topic.

I don't think the intention is to require derating for non-simultaneously current carrying conductors but there is no exception or other mention of it.

 

[pierre]

Re: Another wire derating question

It is not just the number of current carrying conductors in the raceway, but their proximity to each other, with the fact that they will occupy space that may otherwise help in dissipating the heat generated by the other conductors.
Who is to say that the motors may not be in use seconds after one is turned off and there is still heat in the other conductors. We always have to think worst case scenario.
Just a thought.

 

[physis]

Re: Another wire derating question

Just to make it interesting.

Which would conduct heat out of a coduit faster?

Air or insulated conductors.

 

[pierre]

Re: Another wire derating question

Sam
Think of the conductors that are in the center of the mass of other conductors.

 

[physis]

Re: Another wire derating question

And?

Edit: Considering the conductors in the center, if they're all carring current, it's hard for heat to escape.

If they're not all carring current, different deal. I would suggest that non-current carrying conductors would coduct the heat out of the center faster than air would.

[ May 12, 2005, 08:37 PM: Message edited by: physis ]

 

[W6SJK]

Re: Another wire derating question

If non current carrying conductors were an issue with conduction of heat (colling) then the code would make you count neutrals and ground conductors in every case.

 

[physis]

Re: Another wire derating question

I think this whole concept is being taken backwards.

Non-current carrying conductors isn't why we derate.

 

[kiloamp7]

Re: Another wire derating question

We are getting a little far-fetched here.

NEC is not a training manual that imparts a knowledge of the entire spectrum of electrical technology/engineering. IMO, I do not want NEC to even think about trying to do that. That is one reason that I have a job. I have acquired this tidbit of knowledge from other sources than just the NEC.

Thus, it comes as no surprise that NEC does not necessarily explain why, in a 3-Phase 2-speed motor, only (3) of the (6) conductors going to the motor are current carrying when motor is running.

Let us not forget common sense.

Yes physis, - the whole concept is being looked at backwards by some of us.

 

[mc5w]

Re: Another wire derating question

If the 3-phase motor is a 4 pole by 6 pole machine with 2 separate windings then only 3 wires will be conducting at a time. If the motor is say 4 pole by 8 pole using a 6 wire delta winding then on 1 of the speed all 6 wires are carrying current.

 

[tony_psuee]

Re: Another wire derating question

mc5w's post is one of the situations we have as is pierre's point about residual heat in the conductors when not carrying current. My application would have the motors or motor windings being energized seconds apart. One goes off, the other goes on, the time delay in between is minimal, long enough to allow the fan to slow down for the low speed. I have asked the engineer to provide his documentation that this is acceptable per NEC and have yet to receive anything.
The more time I spend on earth the more I realize that what is common sense to me is not universal. If so, the guy I almost t-boned on my way to work this morning would have looked to his left first and not his right before making a turn across oncoming traffic. Thanks all for the feedback provided.

Tony

 

[physis]

Re: Another wire derating question

pierre's point about residual heat in the conductors when not carrying current

My application would have the motors or motor windings being energized seconds apart

You cannot get more heat from two conductors, that never carry current at the same time, than you can from one conductor. (Given the same current anyway).

[ May 13, 2005, 11:26 AM: Message edited by: physis ]

 

[wirenut1980]

Re: Another wire derating question

You cannot get more heat from two conductors, that never carry current at the same time, than you can from one conductor. (Given the same current anyway).

I disagree with that statement, because I believe the conductors that were just carrying current and the load switches to the other conductors, the conductors feeding the load that just shuts off will raise the temperature of the ambient around them for some amount of time (the residual heat). Now whether or not that increase in ambient temperature is significant or not for the conductors feeding the load that just turned on is something I do not know.

 

[physis]

Re: Another wire derating question

We've been looking for free energy. I don't think it's here.

Remember the wire that just started conducting is still cool.

If anything there is going to be less total heat because that cool wire has lower resistance for a while where that wouldn't happen with one wire.