# Thread: Derating for low voltage

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Originally Posted by gar
180112-1036 EST

Figure out what the word "derate" means, what it applies to, and why you have no problem.

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Derate - to reduce the power rating of (a component or device).

And yes that's exactly what I'm talking about...!

See 310.15(B)(3)(a)

2. Originally Posted by Jody Boehs
Derate - to reduce the power rating of (a component or device). See 310.15(B)(3)(a)
That's the right article, but the definition you are quoting doesn't work in this context. It is not a "power rating" that is being reduced. It is the amount of current that we are allowed to pass through the wire under the conditions of use.

3. gar
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180112-1150 EST

Jody Boehs:

There is something you don't get.

Your panel picture has nothing to do with your original post question.

Presumably your use of the word "derate" relates to current, which really means heat, or temperature rise.

Suppose you have wire with some insulation around it. This wire is rated for 10 A in free space (open to air around it) in a horizontal position at ambient 100 F. What does rating mean? Is it 1000 hours life, or 10^12 hours life, or something else. Somewhere someplace someone has this information and some criteria to define life. Further this is empirically determined.

Raise your ambient to 200 F. Is the rating the same?

Why would you derate that cable when used in a bundle inside a conduit as compared to its datasheet rating? The answer is heat and temperature rise and how the insulation behaves with temperature.

If you were to fully fill that conduit with your insulated wires and and simultaneously pass 1 A thru each wire would the center most wire in the bundle exceed its tempetrature rating. If not, then the class of wiring is of no concern. If the maximum wire current is 1 A you are nowhere near any derated value from some table so you just do not care.

Focus on fundamentals which are basically a heat problem (how things work), and from information you have provided there is no indication of a heat problem.

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4. Originally Posted by gar
180112-0925 EST

Jody Boehs:

Why did you start this thread? How or to what is the use of the word "derating" supposed apply?

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I sort of got it from the beginning that you were concerned whether you needed to apply "ampacity adjustments" to your conductors, though it maybe how you worded it made it unclear to some.

You said you had 16 AWG TFFN conductors. Those conductors are not in any of the 310.15 tables.

within 310.15(B)(3) it does say: Where conductors of different systems, as provided in 300.3, are installed in a common raceway or cable, the adjustment factors shown in Table 310.15(B)(3)(a) shall apply only to the number of power and lighting conductors (Articles 210, 215, 220, and 230).

Your conductors are all art 725 conductors, I don't think 310.15 or (B)(3) applies to them.

This from 2014 NEC:
725.43 Class 1 Circuit Overcurrent Protection.

Overcurrent protection for conductors 14 AWG and larger shall be provided in accordance with the conductor ampacity, without applying the ampacity adjustment and correction factors of 310.15 to the ampacity calculation. Overcurrent protection shall not exceed 7 amperes for 18 AWG conductors and 10 amperes for 16 AWG.
That said even if you tried to apply (B)(3) adjustment factors to them - you still likely have an adjusted ampacity well above the load each actually carries.

If you have class 2 or 3 control circuits they are power limited to begin with - so likely lower overall potential energy involved in those circuits then class 1.

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Originally Posted by Jody Boehs
"Since these are not power conductors but rather control circuits, this will depend on the class of the circuit. If they are class 1 see 725.51. If they are class 2 or 3 then no derating would apply."

Ok so i may be good on the derating and raceway fill aspects of this discussion, but help me understand the difference in Class 1, 2, and 3 circuits. Class 1 circuits 725.41 and 725.41(A) look to me like that's what I have, but then 725.121(A) talks about information technology and thermocouples (which I am using on this project). So which is it? Here is a pic of the panel I am terminating all those wires...I don't know if this helps or not...
That's why its much easier to just assume they are power conductors, look at the worst case derating, and realize PLC inputs and outputs are probably much lower than the ampacity of the derated conductors.

Class 2 or Class 3 power supplies have voltage and current limits listed in the NEC Table 11 in the back of the book. Class 2 and 3 power supplies should also be listed. So if you look at the power supply and it says Listed Class 2 Power supply, or Listed Class 3 power supply, then you know what you have.

Class 1 is a little harder. 725.41 gives you the current and voltage limitations. They are also divided into power source limited, and remote control and signaling. I think you probably have class 1 remote control and signaling wires.

If you look at 725.51(A), with Remote control and signaling wires, you still don't have to worry about derating unless the current is more than 10% of the wire ampacity.

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Originally Posted by kwired
I sort of got it from the beginning that you were concerned whether you needed to apply "ampacity adjustments" to your conductors, though it maybe how you worded it made it unclear to some.

You said you had 16 AWG TFFN conductors. Those conductors are not in any of the 310.15 tables.

within 310.15(B)(3) it does say: Where conductors of different systems, as provided in 300.3, are installed in a common raceway or cable, the adjustment factors shown in Table 310.15(B)(3)(a) shall apply only to the number of power and lighting conductors (Articles 210, 215, 220, and 230).

Your conductors are all art 725 conductors, I don't think 310.15 or (B)(3) applies to them.

This from 2014 NEC:

That said even if you tried to apply (B)(3) adjustment factors to them - you still likely have an adjusted ampacity well above the load each actually carries.

If you have class 2 or 3 control circuits they are power limited to begin with - so likely lower overall potential energy involved in those circuits then class 1.

Thank You!! Yes i may not have worded it right....

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Originally Posted by steve66
That's why its much easier to just assume they are power conductors, look at the worst case derating, and realize PLC inputs and outputs are probably much lower than the ampacity of the derated conductors.

Class 2 or Class 3 power supplies have voltage and current limits listed in the NEC Table 11 in the back of the book. Class 2 and 3 power supplies should also be listed. So if you look at the power supply and it says Listed Class 2 Power supply, or Listed Class 3 power supply, then you know what you have.

Class 1 is a little harder. 725.41 gives you the current and voltage limitations. They are also divided into power source limited, and remote control and signaling. I think you probably have class 1 remote control and signaling wires.

If you look at 725.51(A), with Remote control and signaling wires, you still don't have to worry about derating unless the current is more than 10% of the wire ampacity.
Thank You!! That makes better sense.....

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