tripple derations

[jason sleeth]

I was wondering when you derate for number of conductors and ambient temperature you have a double derate. You have to derate the wires ampacity for the number of wires then take that derated number and derate it for the ambient temperature. I am being told that if the load is continous you have to derate from your double derated number. I disagree. The way that I understand the code is that you have to take your double derate. You also have to calculate the available ampacity of the conductors if the load is continuos. (@ 80%). Then you compare the two numbers and have to use the more stringent of the two. You do not triple derate your wires. I was wondering what others think on this topic?

 

[iwire]

Re: tripple derations

1)De-rate for temp

2)De-rate for number of conductors

Now you know the rating of a particular conductor.

If you have a continuous load the code requires the conductor to have a rating 125% of the load.

So essentially your friend is correct.

If after temp derating and conductor count derating you have an adjusted conductor ampacity of 20 amps and your continuous load is 20 amps you will have to choose a larger conductor.

[ September 07, 2005, 03:41 PM: Message edited by: iwire ]

 

[charlie b]

Re: tripple derations

I agree. You are derating for three different reasons, each one of which impacts the ability of the conductor to survive the environment into which it is placed.

Keep in mind, for what it is worth, that if your conductor's insulation system is rated for 90C, such as a THHN conductor, then you start the derating process at the 90C ampacity. After you "triple derate," you compare with the 75C rating, and use the smaller of the two numbers.

 

[charlie b]

Re: tripple derations

Here's an example:

Let's put 6 current-carrying #12 THHN in a conduit, give them a continuous load of 19 amps, and subject them to an environment of 32C. That gives us derating factors of 80% for the multiple conductors, and another 96% for the ambient temperature.

Start with the 90C rating of #12 THHN: 30 amps. Multiply by 0.8 and again by 0.96, and you get 23 amps. This is below the 75C rating of 25 amps, so we must assign an ampacity of 23 amps, and not the 25.

But you have two other constraints. First, the #12 conductors must have overcurrent protection no higher than 20 amps. Secondly, their continuous load can be no higher than 80% of the overcurrent protection, or 16 amps. Therefore, even though you meet the ampacity requirements (20 amps of ampacity as compared to 19 amps of load), we do not meet the "continuous load" requirement. So a #12 will not be acceptable.

So let's do the same with #10. Start with the 90C rating of #10 THHN: 40 amps. Multiply by 0.8 and again by 0.96, and you get 30.7 amps. This is below the 75C rating of 35 amps, so we must assign an ampacity of 30.7 amps, and not the 35.

With a #10, you can use overcurrent protection of 30 amps. 80% of that is 24 amps. Your continuous load is less than 24 amps (it is 19). Therefore, a #10 THHN is acceptable.

{Edit: Corrected the 75C numbers.}

[ September 07, 2005, 04:24 PM: Message edited by: charlie b ]

 

[jason sleeth]

Re: tripple derations

If we had 6 #12 THHN conductors with a continous load of 16 amps we would be fine with a 20 amp breaker?

 

[charlie b]

Re: tripple derations

Given the other condition of my example (i.e., 32C ambient), yes.

 

[lew3611]

Re: tripple derations

charlie b,
wouldn't a commercial lighting circuit be considered a continuous load?

 

[charlie b]

Re: tripple derations

I would call commercial lighting "continuous," as it will be on for more than 3 hours at a time. As to whether lights in residential applications are "continuous," that is a subject worthy of some debate. But it is also outside the scope of this thread, so I won't offer an invitation to debate it here.

 

[benaround]

Re: tripple derations

Gents,

For what it's worth, while attending code review classes,tought by two highly respected names in the electrical field ,'Triple Derating' is never required.

 

[charlie b]

Re: tripple derations

May I suggest that calling it "triple derating" is not accurate? You are "derating" for temperature, and you are "derating" for multiple conductors in a raceway. In both contexts, you obtain a new value for the conductor's "ampacity," by multiplying the value in the Table by a derating factor. But that is not the process you use, when you look into issues of "continuous loads."

Suppose I get done with the two "derating factors," and I conclude that a conductor's ampacity is "40 amps." Now let be consider the load as being "continuous." I can only supply 32 amps worth of continuous load with these conductors. But their ampacity is still 40. I have not "derated" the conductors; I have not multiplied their ampacity by a "derating factor" of 80%.

This may be mere semantics, but I think we should be precise in our terms here. I am limited to loading a conductor, for which the ampacity of 40 amps, to a continuous load of 32 amps. That is not a "triple derating." It is two "derating factors," followed by a loading constraint.

 

[charlie]

Re: tripple derations

For what it's worth, while attending code review classes,taught by two highly respected names in the electrical field ,'Triple Derating' is never required.

Frank, for what it is worth, there are some highly respected names in the electrical field on this forum. Charlie is absolutely correct, you can take that to the bank. If you presented this issue to those guys, I am sure they would answer the same because this is not considered to be triple derating.

 

[bphgravity]

Re: tripple derations

The ampacity of a conductor is based on a set condition that if changed requires adjustment or correction. So instead of thinking that you are "derating" the ampacity, what you are really doing is "correcting" Table 310.16 for the actual ambient temperature, and you are "adjusting" the ampacity for more than three current-carrying conductors in the raceway or cable.

 

[George Stolz]

Re: tripple derations

Another way to think about it: The ambient temperature is perceived by the NEC to be a constant. The number of wires is a constant.

You wouldn't logically be permitted to ignore a constant requiring derating, just because another condition has come along, while the other conditions are still in effect.

If you could prove that suddenly there would be fewer wires present, or that the ambient temperature would magically go down for the duration of the continuous load, maybe something could be done with special permission.

 

[gregsparky1]

Re: tripple derations

To complicate matters worse, the derating of the conductors must be based around the conductor with the lowest insulation value. Correct?
What if there were 3(three) #12 THHN and 1 (one) 12/3 NM (romex)in a 3/4 inch conduit? Could I add 2(two) more #12 THHN wires and put them on a 20 amp breaker? Also, now what have I done to the rating of the short circuit protection of the other circuits? They will need to be adjusted also.

 

[charlie b]

Re: tripple derations

Originally posted by gregsparky1:. . . the derating of the conductors must be based around the conductor with the lowest insulation value. Correct?

Sorry. Not correct. The derating of any single conductor will be based on the rating of its own insulation system. The impact of other nearby conductors is limited to causing a derating for multiple conductors in the same raceway.

What if there were 3(three) #12 THHN and 1 (one) 12/3 NM (romex)in a 3/4 inch conduit? Could I add 2(two) more #12 THHN wires and put them on a 20 amp breaker?

That would give you a total of 7 current-carrying conductors, and a derating factor of 70%. The 90C rating of #12 THHN is 30 amps. 70% of 30 is 21 amps, and that is the ampacity of the #12 THHN. You can protect that with a 20 amp breaker, but not with a breaker of any higher rating, because of 240.4(D).

Also, now what have I done to the rating of the short circuit protection of the other circuits? They will need to be adjusted also.

The number and nature of conductors in a raceway has no impact on short circuit currents. So there will be no need for adjustments.

[ September 19, 2005, 10:39 AM: Message edited by: charlie b ]

 

[derf48]

Re: tripple derations

Please explain where I am wrong in this calculation.

9 THHN #12 CU conductors in a raceway, all current carrying. Ampacity before adjustment - 30 amps.
30 x .7 = 21 amps. 310.15(B)(2)(a)
ambient temperature of 51 degree C
21 amps x .76 = 15.96 correction factor 310.16
New ampacity of conductor = 16.0 amps
OCD size = 20 amp
Maximum continous load = 16 amps. 210.19 and 210.20

 

[bphgravity]

Re: tripple derations

The way I see it, you are doing the calculation backwards, or least, your logic is backwards.

210.19(A)(1) tells us the conductor size shall an allowable amapcity not less than 125% of the continuous load before the application of adjustment or correction factors.

So based on that, the #12 thhn conductors must have an ampacity of at least 20A 16A x 125%). Youmeet that, next you start your adjustments and corrections. Based on your numbers, the conductors are only good for 16A which will be exceeded with the use of a 20A OCD.

However, 240.4(B) permits the next standar size OCD which would indeed be a 20A breaker. So you are all set.

[ September 19, 2005, 05:06 PM: Message edited by: bphgravity ]

 

[gregsparky1]

Re: tripple derations

charlie b-

OK. It was early and my coffee hadn't kicked in yet.
The short circuit protection should have been the over current protection of the existing conductors. If I derate for the new conductors being installed doesn't the same rule apply for the existing conductors? Does that make more sense?

 

[charlie b]

Re: tripple derations

If you have conductors in a raceway, and if you add more conductors, and if that brings the total number of conductors to a point that you have to derate, then you have to derate every conductor in that raceway (including the ones that were there first). After you derate every conductor, you need to make sure that each conductor is properly protected by its breaker. You may even have to replace a breaker with one of a lower rating. That may or may not impact its ability to serve its load.

Basically, you need to re-design, or at least verify the design, of all circuits within the raceway.

Does that answer your question?

 

[gregsparky1]

Re: tripple derations

Thank you charlie b!