derating help

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  #1  
Old 04-14-15, 07:49 PM
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derating help

so, I'm kinda getting the idea from reading various stuff on the web that "derating" only concerns wires running in a conduit. is that true? if that's not true and there are other times you need to "derate",then what are those occasions? getting back to conduit,what nasty things might happen if you don't derate? I get the idea that heat inside a conduit has something to do with "derating". is it that regular wiring (Romex) inside a structure has lots of air around it to dissipate any heat? I guess my main question is: "can somebody explain "derating" in simple terms?

tnx,
 
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  #2  
Old 04-15-15, 12:33 AM
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Derating can also apply to bundled cables or cables run through insulating materials. If you have several type NM cables run together with minimal space between cables AND a high load (amperage on each cable near its limit) then overheating CAN occur. This is one reason why it is bad practice to use tape or wire ties to try to make a multi-cable run look neat. Generally lengths of less than 24 inches do not require derating.

Yes, it is a matter of heat dissipation. All wire has some resistance to the flow of electricity and that resistance ends up as heat. The more current (amperage) flowing through any given size of wire the higher the resistance and the greater the heating. Eventually you can reach a temperature at which the insulation will break down and allow for a short circuit. Different insulation types have different temperature ratings and there are also temperature ratings for the terminals on the various types of utilization equipment.

Simply put, the derating factors take into account the insulation type, the enclosure (conduit, cable or free air) the size of the conductor, the number of current-carrying conductors (more on this if you ask) and the size of the fuse or circuit breaker protecting the wiring.
 
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Old 04-16-15, 07:11 PM
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I'm wondering: in the "big picture",just how important is "derating" anyway? what I mean is, with a conductor like THHN,isn't there a certain amount of "slop" where the rated operating temp. is concerned?

tnx,
 
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Old 04-16-15, 07:19 PM
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Derating in a home environment is rarely an issue. You need to visit a commercial site's electric room and feel how hot the conduits get.

Insulation is rated for a certain amount of heat. Push that rating and the insulation breaks down.

Do you have a particular application in mind when referring to derating ?
 
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Old 04-16-15, 07:26 PM
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If it wasn't an issue the code would not be concerned with it. The code addresses derating for several issues like bundling and ambient temperature.
 
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Old 04-17-15, 06:38 AM
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Wire insulations are expected to hold up to continuous use for a very long time -- 50 years or more. Overheating them on a regular basis shortens the lifespan and eventually creates a fire hazard. For a practical example, open up the junction box over a light fixture in which too large a bulb has been installed for a couple years. The wire insulation will crack off and turn to dust when you touch it.

Yes there is some "slop" which is why the derating calculations allow us to operate on groups of wires, use ranges and general wire classifications instead of detailed engineering calculations for each individual wire. The wire standards also permit variations in manufacturing tolerances. In the end, a derated conductor has been given a reasonable chance of maintaining the insulation integrity for the functional life of the building.
 
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Old 04-17-15, 06:38 PM
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well,I guess then, the "next size up" rule should apply to most every "derating" situation,right? what I mean is say for a circuit protected by a 20A breaker where a 12 AWG conductor would be called for,use #10 wire if going through a connduit. isn't this a good "rule of thumb"? (eliminates messing with "tables" in the NEC when you'd probably end up using the next size up anyway)

thx,
 
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Old 04-17-15, 07:14 PM
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It all depends on job at hand. By up sizing the wires you now take up more space in the conduit. You can get 9 #12 in 1/2" EMT (THHN) but only 5 #10. So now you have to increase the size of your conduit as well

But all this might be for nothing because if you only have to derate for conductor fill, if you keep your count to 9 or less it really doesn't matter. Here's why:

#12 THHN is rated at 30 amps. (310.15(b)(16) 9 conductors you derate 70%. 70% of 30 = 21 amps. 240.4(D) says max amps on #12 is 20 amps. Everything is fine.

The only hitch on this is you may have to derate for ambient temp if it is higher then 86 degrees F.
 
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Old 04-17-15, 07:35 PM
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I'm starting to get this (I think :-) So,if I'm running 4 #10 conductors in a PVC conduit on the outside of my house for my dryer,I don't have to "derate" since the fill count of 1/2 in. PVC for #10 wire is around 5 (I think) and I believe only 2 of those conductors will carry an appreciable current. I hope I got that right since it's already done! :-)
as far as that "ambient temperature" thing,I'm totally in the dark,sice the outside temp. around here can swing from minus 10 or 15 to over 100. are you supposed to take the average?

tnx,
 
  #10  
Old 04-17-15, 08:24 PM
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Not quite. You only have to derate when there are more than three current-carrying conductors in the conduit. The neutral, when it only carries the unbalanced current from the two "hot" conductors doesn't count and an equipment grounding conductor never counts for derating purposes. For four through six current-carrying conductors you derate to 80% of the column three Ampacities and for seven through nine current-carrying conductors you derate to 70% of the column three Ampacities. There are additional derating factors for more than nine current-carrying conductors but I don't offhand remember them.
 
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Old 04-18-15, 04:22 AM
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The derating factor for over 9 wires is 50% reduction.
 
  #12  
Old 04-18-15, 09:00 PM
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ok,4 conductors in my conduit. 3 of them carry current,NO derating,right? (the 4 th is ground which never counts,right?)

tnx,
 
  #13  
Old 04-18-15, 11:45 PM
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The neutral in this particular instance is not counted because it is only carrying the unbalanced current in a three-wire dual voltage circuit. That means in this particular circuit you have only two current-carrying conductors for the purpose of calculating any derating figures. So, no, you do not need to make any adjustments for derating.
 
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Old 04-19-15, 06:28 AM
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As I mentioned before, even with 4 wires the derating factored will not have any effect on the allowable ampacity of your wires.

4 - #10 THHN/THWN = 40 amps
40 x 80% = 32 amps
 
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