Protection of #10 Conductors

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I think you need to size the supply conductors in accordance with 670.4 (A). There is no need to split hairs. #10 either makes the grade or it does not.
 
vpower1989 said:
Yeah I guess this probably falls under 310.15B(5)(a). I guess I'm not a 100% certain on the harmonics, or whether there are other non-linear loads.
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Harmonics typically aren't a problem unless you have a majority of the load being non linear - like a data center. Your PLC creates some harmonic content but isn't even close to being a problem here.
 
vpower1989 said:
I am running into a situation where a piece of equipment came in with an FLA listed on the nameplate that is higher than we were anticipating. We were told to provide a 30A circuit with a L21-30R. #10 conductors have been installed. The equipment shows up and we have an FLA of 25.14A listed on the nameplate. The electricians are asking if they need to pull a new 40A circuit. So I have been going down the rabbit hole of splitting hairs to see if we can use the #10s.

  • So nameplate FLA is 25.14A, we treat it as a continuous load, 25.14 * 1.25 = 31.4A required rating for the conductors
  • We likely have 75C terminations, #10 is rated for up to 35A.
  • No temperature adjustments required.
  • Conduit Derating – 90C cable (THHN) – 40A * derating factor ( .8) = 32A.

So in my mind the ampacity of conductors are fine for the load. I may have to ditch the RCPT and go to a disconnect. One question I have is on the "next size up" rule. Could I protect the conductors described above with a 40A breaker?
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Though barking up the wrong tree to begin with, next size up from 32 would be 35 not 40.
 
petersonra said:
I think you need to size the supply conductors in accordance with 670.4 (A). There is no need to split hairs. #10 either makes the grade or it does not.
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670.3 tells us what should be included on the nameplate which is great. When a company provides an "FLA" on their nameplate, and I know the machine will be running continuously, then I simply do FLA * 125 right to size my feeder. Correct?

Also, if I know the main OCPD in the machine is rated for a certain ampacity - conventional wisdom would say that I can simply provide a circuit rated at that same ampacity. I do not know if that aligns with the NEC though.
 
vpower1989 said:
670.3 tells us what should be included on the nameplate which is great. When a company provides an "FLA" on their nameplate, and I know the machine will be running continuously, then I simply do FLA * 125 right to size my feeder. Correct?

Also, if I know the main OCPD in the machine is rated for a certain ampacity - conventional wisdom would say that I can simply provide a circuit rated at that same ampacity. I do not know if that aligns with the NEC though.
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No. 670.4 tells you how to calculate the minimum ampacity.

670.4 Supply Conductors and Overcurrent Protection.
(A) Size. The size of the supply conductor shall be such as to
have an ampacity not less than 125 percent of the full-load
current rating of all resistance heating loads plus 125 percent
of the full-load current rating of the highest rated motor plus
the sum of the full-load current ratings of all other connected
motors and apparatus, based on their duty cycle, that may be in
operation at the same time.
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I wonder what "based on their duty cycle" means.
 
petersonra said:
No. 670.4 tells you how to calculate the minimum ampacity.



I wonder what "based on their duty cycle" means.
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My problem with 670.4 is that it is possible that you would not have all the required information to calculate as it describes. A lot of the time all I have is info on a nameplate. I don't know what devices within the machine operate at the same time. I don't know any duty cycles. Most of the time I know the voltage, FLA, and SCCR. That is it. The process since I've been with my new company is simply take FLA*125 and call it a day. That may work out fine but I am unsure if that is what the code actually calls for.

Is there any significance in the fact that the code says "full load current" and the commentary at the end of 670.4 in the handbook uses the phrase "full load amperes"? That commentary says that the full load amperes is all you need.

With duty cycle, they are talking about motors that may be turning on and off a lot ( not running continuous) within the machine. I recall in NEC there is some ampacity corrections based on duty cycle. I've only ever looked at it once on an elevator project.
 
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Here’s one fundamental question I have: If 670.4 tells us how to calculate conductor ampacity and OCPD size by looking at individual loads that comprise the industrial machine, what is the FLA on the nameplate actually used for?
 
vpower1989 said:
So, I could protect those as described above #10 conductors with a 35A breaker, right?
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Maybe. If motor loads are involved (or other loads mentioned in 240.4(G)), more likely. Otherwise the general rule in 240.4(D) still requires 10 AWG to be protected at maximum of 30 amps.
 
vpower1989 said:
Here’s one fundamental question I have: If 670.4 tells us how to calculate conductor ampacity and OCPD size by looking at individual loads that comprise the industrial machine, what is the FLA on the nameplate actually used for?
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The FLA on the nameplate should be the FLA of the packaged unit. For instance, if an AC unit has multiple compressors, the nameplate of the unit may have the FLA, MCA & MOCP. Sometimes there is a breakdown of each loads FLA so you can calculate the loads individually. In my opinion, if the nameplate is on the unit housing and not an individual device, I would take the FLA to be the packaged units FLA.
 
Rock86 said:
The FLA on the nameplate should be the FLA of the packaged unit. For instance, if an AC unit has multiple compressors, the nameplate of the unit may have the FLA, MCA & MOCP. Sometimes there is a breakdown of each loads FLA so you can calculate the loads individually. In my opinion, if the nameplate is on the unit housing and not an individual device, I would take the FLA to be the packaged units FLA.
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I believe he said this unit also contained 30 amp fuses, seems the manufacturer didn't intend it to draw more then that. Only problem would possibly be a breaker with lower magnetic trip than the fuses can handle upon motor starting or something of that nature, though would probably be somewhat rare to have a problem with this.
 
kwired said:
I believe he said this unit also contained 30 amp fuses, seems the manufacturer didn't intend it to draw more then that. Only problem would possibly be a breaker with lower magnetic trip than the fuses can handle upon motor starting or something of that nature, though would probably be somewhat rare to have a problem with this.
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I had seen that in earlies posts. From there I figured it was cut and dry to run #10s (based on his distance) and call it done, but he seems to be worried about the nameplate rating.
 
Rock86 said:
I had seen that in earlies posts. From there I figured it was cut and dry to run #10s (based on his distance) and call it done, but he seems to be worried about the nameplate rating.
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Yeah I am inclined to leave the #10s. It will just be different than the way Tool installs have been done prior to my time here. For years everyone has simply taken nameplate FLA multiplied it by 1.25 and that was the conductor size (minus any adjustments). In many cases I suppose this ended up being a conservative approach as the code only requires the 25% buffer on heating loads and largest motor.
 
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