Hot Fan

[Donny C]

Hi, I am new to this forum. I am a Broadcast Engineer with a Journeyman's License. Most of my experience is industrial and for the last few years I have only had to deal with Coax and Cat 6 and the occasional 480 Volts that powers our transmitter. Any how, My boss came to me with switch that controls a two speed motor that in turn runs an attic fan. The fan would only run in the low setting, he had the switch wired wrong, he went on his way. He comes in the next day and says his fan caught fire. Come to find out he ran two runs of 12/2 to the fan, he used the black out of one for high and the white for neutral. In the other wire he used white for low speed instead of the black which he left unused. Well he wired it back up wrong and ended up using the wrong white and you know what happened next. Well after some head shaking, I told him please use the proper colored wire and or permanently mark the wire he is using. I told him he should not run a hot conductor in a cable without the return neutral which he was doing with the second wire, I told him this could cause the metal appliance or box to heat up. He said that the motor had been heating up and that the thermistor would open after about ten minutes. The question I have is, would there be any harm with him using both the neutrals in his two wires as the neutral for his fan? Then each wire would have a hot and a neutral. Or he could just run the proper cable, which he should have done from the get go.
Thanks

 

[GoldDigger]

It would violate the rule against paralleling small wires and it could still lead to magnetic heating from unblanced currents.

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[Donny C]

It would violate the rule against paralleling small wires and it could still lead to magnetic heating from unblanced currents.

Sent from my XT1585 using Tapatalk

Understood, and thanks for the reply.

 

[kwired]

300.3(B)(3) would allow you to run conductors of this circuit in two different cables if non ferrous method such as NM cable.

It tells you it must also comply with 300.20(B) which basically tells you they must be routed in close proximity to each other.

 

[Donny C]

300.3(B)(3) would allow you to run conductors of this circuit in two different cables if non ferrous method such as NM cable.

It tells you it must also comply with 300.20(B) which basically tells you they must be routed in close proximity to each other.

kwired,
This is what had me confused, he is using NM cable and I don't know how close he has wired this. So based on the incremental information I was getting and the fact that his fan motor appeared to be over heating, I told him it would be safer to run the proper cable. Having not run into this issue before I was curious about the combing the two neutrals from the separate cables.

Thanks for the help.
Donny

 

[infinity]

kwired,
This is what had me confused, he is using NM cable and I don't know how close he has wired this. So based on the incremental information I was getting and the fact that his fan motor appeared to be over heating, I told him it would be safer to run the proper cable. Having not run into this issue before I was curious about the combing the two neutrals from the separate cables.

Thanks for the help.
Donny

A single properly sized cable should be used. The restrictions on running two NM cables make that the wrong solution to the problem.

 

[kwired]

I am failing to see what running conductors in two separate cables had to do with burning up the motor.

It can cause inductive heating effects if each cable entered separate holes into a ferrous enclosure, but this likely a small fan and effects would not be enough to overheat the motor.

You sure something wasn't miswired and trying to power both high and low speeds simultaneously or maybe common lead of the motor wasn't hooked to common lead of the supply?

 

[grich]

...for the last few years I have only had to deal with Coax and Cat 6 and the occasional 480 Volts that powers our transmitter...

TV transmitter? I've worked on one or two of those.

...You sure something wasn't miswired and trying to power both high and low speeds simultaneously or maybe common lead of the motor wasn't hooked to common lead of the supply?

I would start at the panel and re-engineer this whole thing.

 

[JFletcher]

Welcome to the forum. There are so many unknowns as to why the fan burnt up that I cant hazard even a wag. You can use multiple (2) NM cables to power fans provided they meet 300.3 and 300.20(B). As other have mentioned, paralleling the neutrals is a code violation, though not a safety violation here imho.

If this is a long run of wire, you may need to upsize to prevent excessive voltage drop. Your ambient temps are already high enough to bring derating tables into play. I'd also consider the fan a continuous load.

Even if both 12/2s didnt go thru the exact same KO, I doubt inductive heating would have caused the fan to catch fire. There's something else going on there.

How many amps is the fan supposed to pull on the high setting?

eta: even if the fan and motor are a single mfd'd unit, I'd look at mechanical reasons for failure too, like a misaligned or over tightened belt, bearing lubrication failure, excessive vibration, fan oversized for the motor/motor undersized for the fan, etc.