de-rating conductors

[wyboy]

I am really confused. I have always used the 90 degree table for de-rating conductors for temperature or over three conductors in a conduit to determine conductor ampacity. I just read an article that says, according to NEC 110.14, I must de-rate from the 60 degree column for circuits under 100 amps. For instance I figured I could put (9)#10 THHN in a conduit and have an ampacity of 28 amps. I could attach the THHN to a 30 amp breaker rated 60 degrees (assuming the load is 28 amps or less). According to the article I would have to de-rate from the 60 degree table and the conductor would be good only to 21 amps and I could only put a 21 amp load on the conductors. Have I been doing this wrong?

 

[augie47]

You can derate the conductors based on the conductor ampacity, such as 90? for THHN, but your actual ampacity can not exceed the termination temperatures shown in 110.14
Note that most devices allow for 75? terminations, so even your 100 amp or less may be terminated based on 110.14(c)(1)(a)(3).

Your (9) #10s would derate to 28 amps as you stated. 110.14 would prescribe a 30 amp breaker, based on the 60? column, but with 75? devices you could go to the 75? column, or 35 amps keeping in mimng 240.4(D)(7) restyricts the OCP to 30 amps {except as allowed by 240.4(E) & (G) }

 

[charlie b]

What matters is the temperature rating of the conductor's insulation system. Under each of the three columns of Table 310.26 (i.e., the 60, 75, and 90C columns), there is a list of conductor insulation systems that have that value as their temperature rating. THHN is rated for 90C. Therefore, when you derate for ambient temperature or for more than three CCCs in a raceway, you can start with the value in the 90C column. The final ampacity, as Gus pointed out, cannot be higher than the ampacity in the 75C column, if you have terminations rated for 75C (which most of them are). If your terminations are only rated for 60C, then you still start the derating process from the 90C column, but you can't use a final ampacity value that is higher than the one in the 60C column.

 

[ptrombley]

Charlie- I've been trying to understand 110.14(C)(1)(a) myself. Given the following, am I calculating the ampacity of my conductors correctly?

- 60-amp disconnect with 75 degree C lugs
- 2 8AWG copper current carrying conductors (XHHW-2) insulation in 3/4" EMT conduit
- ambient temperature for both the conduit and disconnect location = 110F

From the 90 degree column of Table 310.16... 8AWG = 55Amps
55A x .87 (temp adjustment) = 47.85A

Compare to 75 degree column of 310.16-- 8AWG = 50A
47.85A is less than 50A, so 8AWG connected to this equipment can carry 47.85Amps.

OR- Because the disconnect is rated 100 amps or less, does 110.14(C)(1) say that I must adjust the conductor ampacity (for temperature) starting from the 75 degree column? (50A x .82 = 41A)

Thanks, Paul

 

[eprice]

Charlie- I've been trying to understand 110.14(C)(1)(a) myself. Given the following, am I calculating the ampacity of my conductors correctly?

- 60-amp disconnect with 75 degree C lugs
- 2 8AWG copper current carrying conductors (XHHW-2) insulation in 3/4" EMT conduit
- ambient temperature for both the conduit and disconnect location = 110F

From the 90 degree column of Table 310.16... 8AWG = 55Amps
55A x .87 (temp adjustment) = 47.85A

Compare to 75 degree column of 310.16-- 8AWG = 50A
47.85A is less than 50A, so 8AWG connected to this equipment can carry 47.85Amps.

OR- Because the disconnect is rated 100 amps or less, does 110.14(C)(1) say that I must adjust the conductor ampacity (for temperature) starting from the 75 degree column? (50A x .82 = 41A)

Thanks, Paul

I would say you are correct the way you described it at first. The easiest way for me to keep this sorted out is to remember that there are two things that we are trying to protect.

When we derate conductors for ambient temperature, or for number of conductors in a raceway, we are protecting the conductor's insulation from the heat generated by the conductor combined with heat from other closely associated conductors, and from the environment. This part of it doesn't depend on what is connected to the end of the conductor. Therefore, we start with the ampacity of the conductor in the column of Table 310.16 appropriate for the conductor's insulation.

Also, we need to protect the equipment on the end of the conductor from the heat generated by the conductor. It doesn't matter to the equipment what insulation is on the conductor, so for this purpose we use the appropriate column in table 310.16 according to the temperature rating of the equipment.

These two considerations give us two answers. The final ampacity allowed is the lesser of the two answers.

 

[ptrombley]

Thank you for your response eprice. Your explanation is helpful.
Can you explain the reason for the distinction between equipment rated above or below 100-amps in 110.14?

 

[eprice]

Can you explain the reason for the distinction between equipment rated above or below 100-amps in 110.14?

I look at 110.14(C)(1) as providing a "default" temperature rating to use for equipment when it is not otherwise listed and identified. 110.14(C)(1)(a)(3), In the case of 100 amp or less, and 110.14(C)(1)(b)(2), in the case of over 100 amp, each make allowance for using a higher temperature rating than the default if the equipment is so listed and identified. There apparently is no equipment over 100 amps that is not suitable use at 75 degrees, whereas there is some 100 amp or less equipment that is only suitable for use at 60 degrees. I'm guessing it has something to do with the UL standard used to list the equipment.

 

[ptrombley]

My actual situation that brought this question up is this-- The 60A disco is installed outdoors in Arizona. The ambient temperature is HOT for sure. It is listed and labeld as 75C equipment (actual disco is SQD DU222RB). We are using THWN-2 conductors (I think I said XHHW-2 before, but it shouldn't matter for this discussion). Some here say that 110.14 forces us to calculate the ampacity of the 90 degree conductor from the 75C column of 310.16. Their thought is that if we adjust the conductor ampacity from the 90 degree column (for ambient temperature) that the conductor could still reach 90C at the lug... thereby over-heating the 75C lug. Does that make sense to you?

 

[kevin]

Paul, you pose an interesting question, but the statement that occurs below CORRECTION FACTORS in Table 310.16 clearly requires that adjustment factors be applied to the (otherwise) allowable ampacities for each conductor, not to each terminal. In your particular case .87 x 55 = 47.85 A, the amperage at which within the raceway the temperature of both the copper conductor and the THWN-2 insulation will reach 90 degC. In the few inches from the raceway to the terminal things have cooled down to a maximum of 75 degC, according the Code. I haven't got the slighest idea how the Correction Factors to Table 310.16 were derived, but apparently they have stood the test of time.

I hope that helps.