I am a second-year apprentice, working on an NEC problem where I have an electric heater protected by 60-amp fuses with a control circuit with a thermostat, also protected by the same 60-amp fuses. I'm trying to find out what size the control circuit conductor should be. I'm using THWN Class 1 control circuit wire protected only by the 60-amp heater fuses. I come up with #4 wire, as do many of my classmates, but my instructor says that is incorrect and that it should be smaller, but he couldn't recall the code reference that allows us to de-rate this conductor. Do you know of any code reference that could help me with this problem?
Thermostat Conductor Size
- Thread starter fairfax
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russ
Senior Member
- Location
- Burbank IL
Re: Thermostat Conductor Size
240.21(B)(1)
240.21(B)(1)
- Location
- Bremerton, Washington
- Occupation
- Master Electrician
Re: Thermostat Conductor Size
Fixed electric space heating equipment is art 424,
424.2 refers to other articles shall apply.
430.72 covers overcurrent protection
See table 430.72(B). the size of the control circuit conductor depends if the control circuit leaves the heater.
I've not run inot this application before, but I didn't see anything in 242 covering control circuits we then look at 424.2, and then art 430.
Fixed electric space heating equipment is art 424,
424.2 refers to other articles shall apply.
430.72 covers overcurrent protection
See table 430.72(B). the size of the control circuit conductor depends if the control circuit leaves the heater.
I've not run inot this application before, but I didn't see anything in 242 covering control circuits we then look at 424.2, and then art 430.
russ
Senior Member
- Location
- Burbank IL
Re: Thermostat Conductor Size
Bill, I believe your right,( thank you) and I was wrong.
430.71 and 725.23 and 24, I'll let fairfax read it himself and ask questions
Bill, I believe your right,( thank you) and I was wrong.
430.71 and 725.23 and 24, I'll let fairfax read it himself and ask questions
Re: Thermostat Conductor Size
Thank you both for your answers. I'm still confused. I probably didn't explain the problem clearly enough, so here it is word-for-word, if you still want to address this issue!
An electric heater (resistance type) circuit is protected with 60-amp fuses, and the heater is controlled with a contactor. A line voltage thermostat operates the coil of the contactor. All wires are in conduit. All conductor terminations are 75 degrees C rated, except the terminations at the thermostat are only 60 degrees C rated. Determine the minimum size of the copper type THWN Class 1 control circuit wire permitted if it is only protected with the 60-amp heater circuit fuses.
The instructor says that my answer of #4 is incorrect. We are not given line voltage and we are not given the size of the heater resistance in KVA's. (Perhaps the question is incomplete or the instructor left out some vital information.)
I assume from the way this question is worded and drawn that there is no motor involved so I don't see how 430.72 applies. I've looked at 725.23 & 24 and I can't figure how they will let me do anything but size the wire to 60 amps at 60 degrees C. That gives me #4!
Thanks much, fairfax
Thank you both for your answers. I'm still confused. I probably didn't explain the problem clearly enough, so here it is word-for-word, if you still want to address this issue!
An electric heater (resistance type) circuit is protected with 60-amp fuses, and the heater is controlled with a contactor. A line voltage thermostat operates the coil of the contactor. All wires are in conduit. All conductor terminations are 75 degrees C rated, except the terminations at the thermostat are only 60 degrees C rated. Determine the minimum size of the copper type THWN Class 1 control circuit wire permitted if it is only protected with the 60-amp heater circuit fuses.
The instructor says that my answer of #4 is incorrect. We are not given line voltage and we are not given the size of the heater resistance in KVA's. (Perhaps the question is incomplete or the instructor left out some vital information.)
I assume from the way this question is worded and drawn that there is no motor involved so I don't see how 430.72 applies. I've looked at 725.23 & 24 and I can't figure how they will let me do anything but size the wire to 60 amps at 60 degrees C. That gives me #4!
Thanks much, fairfax
websparky
Senior Member
- Location
- Cleveland, Ohio
Re: Thermostat Conductor Size
Interesting.........
In my opinion, another poorly worded test question!
Here is the dumb part:
It says the heater is controled by a contactor which means that the 60A rating has nothing to do with the question. A contactor has a coil which is always supplied by another circuit to operate the contactor. The wire size would be the code minimum, 14AWG.
If your instructor is trying to say other wise, I doubt that he has ever installed a contactor. The coil is never supplied by the same conductors as the load. If it were, what would be the use of having a contactor in the first place?
Even if you were to follow his logic, the smallest size is 6AWG. I'd like to see a contactor with lugs large enough for 6AWG on the coil terminations!
Interesting.........
In my opinion, another poorly worded test question!
Here is the dumb part:
It says the heater is controled by a contactor which means that the 60A rating has nothing to do with the question. A contactor has a coil which is always supplied by another circuit to operate the contactor. The wire size would be the code minimum, 14AWG.
If your instructor is trying to say other wise, I doubt that he has ever installed a contactor. The coil is never supplied by the same conductors as the load. If it were, what would be the use of having a contactor in the first place?
Even if you were to follow his logic, the smallest size is 6AWG. I'd like to see a contactor with lugs large enough for 6AWG on the coil terminations!
- Location
- Illinois
- Occupation
- retired electrician
Re: Thermostat Conductor Size
Dave,
Don
Dave,
The control circut is often supplied by the same power circuit that feeds the equipment. In most cases a fuse is used for protection of the control circuit.
Don
pierre
Senior Member
- Location
- Westchester County, New York
Re: Thermostat Conductor Size
I cannot see how 430.72 pertains to this question. The overcurrent protection of motors and motor feeder and branch circuit conductors is treated differently, because of the overload protection provided for the motors.
The contactor controlling the heater is how we find our answer. Because we are left with inadequate info it is hard if not impossible to answer this question.
My answer would be similar to Dave's, in that the smallest building wire permitted is 14 AWG. Since they do mention that the Tstat is line voltage, that to me rules out low voltage/limited energy type wiring.
240.21(B) is feeder taps.
JMO
Pierre
[ November 06, 2003, 02:23 PM: Message edited by: pierre ]
I cannot see how 430.72 pertains to this question. The overcurrent protection of motors and motor feeder and branch circuit conductors is treated differently, because of the overload protection provided for the motors.
The contactor controlling the heater is how we find our answer. Because we are left with inadequate info it is hard if not impossible to answer this question.
My answer would be similar to Dave's, in that the smallest building wire permitted is 14 AWG. Since they do mention that the Tstat is line voltage, that to me rules out low voltage/limited energy type wiring.
240.21(B) is feeder taps.
JMO
Pierre
[ November 06, 2003, 02:23 PM: Message edited by: pierre ]
russ
Senior Member
- Location
- Burbank IL
Re: Thermostat Conductor Size
Mistake 430.72 not 430.71 referred me to 725.23, I kept reading thru 725.24. and took 725.24(C) as a good answer. ( which wood have allowed a 20 amp conductor). Now I don't know. Back to square one.
Mistake 430.72 not 430.71 referred me to 725.23, I kept reading thru 725.24. and took 725.24(C) as a good answer. ( which wood have allowed a 20 amp conductor). Now I don't know. Back to square one.
Re: Thermostat Conductor Size
Thanks for your answers and info. I, too, think it's a poorly worded question. I shared it with a journeyman at work (Ford plant), and he suggested 725.21(A)(2). We assumed line voltage to be 120 volts. Then, per 725.21(A)(2), we divided by 60 amps and multiplied the result by 167% and came up with .835 amps. According to 310.16, #18 wire would work, but the journeyman wasn't sure, so he said, "If you can't dazzle them with brilliance, baffle them with b--- s---." I will show all of your replies to this journeyman, and we may revise our answer. Thanks again. Fairfax
Thanks for your answers and info. I, too, think it's a poorly worded question. I shared it with a journeyman at work (Ford plant), and he suggested 725.21(A)(2). We assumed line voltage to be 120 volts. Then, per 725.21(A)(2), we divided by 60 amps and multiplied the result by 167% and came up with .835 amps. According to 310.16, #18 wire would work, but the journeyman wasn't sure, so he said, "If you can't dazzle them with brilliance, baffle them with b--- s---." I will show all of your replies to this journeyman, and we may revise our answer. Thanks again. Fairfax
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