Derating for low voltage

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Jody Boehs

Jody Boehs

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Location
Fairview, Oklahoma, USA
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Controls and Electrical Manager
Does derating apply to low voltage (24VDC)? I have a 1" EMT pipe that will have 36 - 16awg TFFN wires in it if I use the plan given to me. All those wires will only be either +24VDC or -24VDC. They are used to carry I/O signals to and from a PLC. Is it against code to put that many wires in 1 raceway? I know on higher voltages and currents it is. Just was curious about the lower end. thanks in advance!
 
180111-1613 EST
Jody Boehs:

I don't know.

You have not specified current.

An insulated #16 is just over 0.1", but I will assume 0.1 With 36 wires of 0.1 dia in one configuration they occupy about 0.6 * 0.6 square inches, 0.36 sq-in. A 1" round pipe has an approx ID of 1, or about 3.2 * 0.5 * 0.5 = about 3/4 sq-in. Thus, wires will fit.

#16 copper is about 4 ohms/1000 ft, or 0.04 ohms/10 ft. Suppose all wires each had 1 A of current on continuously. That is 0.04 W per wire or less than 0.04 * 40 = 1.6 W total per 10ft. You won't feel the temperature rise.

Change the assumption to 10 A per wire and you get 160 W for 10 ft. 16 W / ft you should feel.

The above numbers are for easy calculation.

.
 
gar said:
180111-1613 EST
Jody Boehs:

I don't know.

You have not specified current.

An insulated #16 is just over 0.1", but I will assume 0.1 With 36 wires of 0.1 dia in one configuration they occupy about 0.6 * 0.6 square inches, 0.36 sq-in. A 1" round pipe has an approx ID of 1, or about 3.2 * 0.5 * 0.5 = about 3/4 sq-in. Thus, wires will fit.

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Yes I know the wires will fit no problem...I think it was only like 30% fill or so. I'm only allowed 40%, although, I would never fill a conduit that full.

As for the current, I said all these wires are used as PLC inputs and outputs. There's really no current to speak of on the inputs...the outputs, however, will be used for air solenoids, contactors, and relays. The current on these devices isn't a lot to speak of either.
 
180111-1713 EST

Yes it was clear what you said, but the really critical information was not and still is not provided.

If all your currents are under 1 A, and they might be, then you do not have a problem.

.
 
Wire fill is about annular space in the raceway, a ratio of the inside area of the raceway and the aggregate outside diameter of the conductors, INCLUDING the insulation. It has no direct relationship to voltage. But if all circuits are 24V, the insulation of the wire can be thinner than say, 600V rated wire, so the diameter of each wire could be less, hence more actual wires could fit in a raceway up to the fill ratio. But if you are using say #14 THHN wire, the fill ratio is the same regardless of the voltage on the wire.
 
Jody Boehs said:
Does derating apply to low voltage (24VDC)? I have a 1" EMT pipe that will have 36 - 16awg TFFN wires in it if I use the plan given to me. All those wires will only be either +24VDC or -24VDC. They are used to carry I/O signals to and from a PLC. Is it against code to put that many wires in 1 raceway? I know on higher voltages and currents it is. Just was curious about the lower end. thanks in advance!
Click to expand...

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.
 
texie said:
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.
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I agree, but even if they are class 1 and you need to derate the wire to 40% that still gives you 7 amps per wire. I'm sure a PLC I/O is much less than that. Probably a fraction of an amp.

If you think about the PLC power supply rating, how many 24V, 7 amp outputs could it drive before it blew the fuse? Not very many.

IMO, you don't need to worry about it.
 
steve66 said:
I agree, but even if they are class 1 and you need to derate the wire to 40% that still gives you 7 amps per wire. I'm sure a PLC I/O is much less than that. Probably a fraction of an amp.

If you think about the PLC power supply rating, how many 24V, 7 amp outputs could it drive before it blew the fuse? Not very many.

IMO, you don't need to worry about it.
Click to expand...

An earlier reply/post said this:
"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...

Liquid Rack Control Panel.jpg
 
Jody Boehs said:
Derate - to reduce the power rating of (a component or device). See 310.15(B)(3)(a)
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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.

 
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.

.
 
gar said:
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.
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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.
 
Jody Boehs said:
An earlier reply/post said this:
"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...
Click to expand...

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.
 
kwired said:
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.
Click to expand...


Thank You!! Yes i may not have worded it right....
 
steve66 said:
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.
Click to expand...

Thank You!! That makes better sense.....
 
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