Question on practice test about ampacity derating

[Clayton79]

Im reviewing some practice questions for my license exam, the question is the following.
2014 nec cycle
A 120v/208v wye service has a computed load of 350 amps. the neutral is considered a current carrying conductor as 52 percent of the load is nonlinear. it will be installed in RMC. what is the smallest THHN copper conductor that may be used.

choices are 400, 500, 600, 750 Kcmil
soo, the answer states that its 4 conductors. that i understand.
so we adjust by adjust by 80%
they show 600 as the correct answer
then it states if over 100 amps that we use 75deg column per 110.14 (C)1(b)1 and shows how 600mcm is used based on 90deg table to get 380amps after adjustment.
but doesnt 110.14 mean we have to use 75deg column? so 750mcm is actually correct??

would really like some clarification if possible thank you.:?:?

 

[Carultch]

Im reviewing some practice questions for my license exam, the question is the following.
2014 nec cycle
A 120v/208v wye service has a computed load of 350 amps. the neutral is considered a current carrying conductor as 52 percent of the load is nonlinear. it will be installed in RMC. what is the smallest THHN copper conductor that may be used.

choices are 400, 500, 600, 750 Kcmil
soo, the answer states that its 4 conductors. that i understand.
so we adjust by adjust by 80%
they show 600 as the correct answer
then it states if over 100 amps that we use 75deg column per 110.14 (C)1(b)1 and shows how 600mcm is used based on 90deg table to get 380amps after adjustment.
but doesnt 110.14 mean we have to use 75deg column? so 750mcm is actually correct??

would really like some clarification if possible thank you.:?:?

I'll give you an analogy to think about this problem.

Consider a rope connected between two hooks. Suppose in a dry environment, the rope is rated for 100 pounds, and the hooks (which are unaffected by wetness) are rated for 90 pounds. The whole assembly is rated for 90 pounds, because the hooks are the weakest link. Now suppose this rope is wetted, and no longer can carry its full 100 pounds, but can only carry 80 pounds safely. The wet assembly is rated for 80 pounds. The fact that it the rope is wet doesn't affect the hook strength, it only affects the rope strength, and the rope becomes the weekest link. The assembly strength is the minimum of each hook strength, and the conditions of use rope strength.

Similar thinking should be applied to ampacity calculations. The wire in the two terminations need enough ampactiy, and the wire in the rest of the circuit at conditions of use needs enough ampacity. Both conditions need to be satisfied. Conditions of use derate factors (temperature correction and bundling adjustments) do not apply to terminal ratings, only to wire ratings.

600 kcmil wire has a rating at 90C that is used with your 0.8 derate factor, and any temperature corrections that may apply. None are specified, so we assume a 30C ambient with none that apply. The fact that the conductors are bundled with 4ccc's in a conduit doesn't affect terminal performance. So this is the most common reason why you would 90C wire, even if you only have 75C terminals. It gives you flexibility for these calculations. Read the rest of 110.14(C), and it confirms this.

 

[JFletcher]

Im reviewing some practice questions for my license exam, the question is the following.
2014 nec cycle
A 120v/208v wye service has a computed load of 350 amps. the neutral is considered a current carrying conductor as 52 percent of the load is nonlinear. it will be installed in RMC. what is the smallest THHN copper conductor that may be used.

choices are 400, 500, 600, 750 Kcmil
soo, the answer states that its 4 conductors. that i understand.
so we adjust by adjust by 80%
they show 600 as the correct answer
then it states if over 100 amps that we use 75deg column per 110.14 (C)1(b)1 and shows how 600mcm is used based on 90deg table to get 380amps after adjustment.
but doesnt 110.14 mean we have to use 75deg column? so 750mcm is actually correct??

would really like some clarification if possible thank you.:?:?

If you have 90*C wire (and most of it is), then you can use the 90* column for a derating starting point. However, the terminals are only designed for 75*C, so the current the wire carries cannot be above the 75*C column.

A 600MCM Cu THHN wire is 475A @ 90*C. For 4 CCC, you derate to 80% by 310.15(B)(2)(a). 475A x .8 = 380A, which is less than the 75*C column max amperage (good), and enough to carry a calculated 350A load (good)

500MCM Cu with 4 CCC is 430 x .8 = 344, which is less than the 75*C column (good), but less than the calculated load of 350A (not good), so it cannot be used.

NM and UF can only be used at the 60*C table, and conductors 10ga or smaller are subject to 240.4 as well

 

[Clayton79]

Thanks to both of you Gentlemen for your time and explanations. I understand now. but to be sure let me reiterate that which i think i know.

because the resultant 380amps is less than the 420 of the 75deg column and greater than that of the calculated 350amps its ok to use.

My confusion came from where they stated about the 75deg column, i was reading it to mean i had to use the 75deg column to start with and then derate which was really confusing me.

thanks again for the timely replies.!!

 

[ggunn]

Thanks to both of you Gentlemen for your time and explanations. I understand now. but to be sure let me reiterate that which i think i know.

because the resultant 380amps is less than the 420 of the 75deg column and greater than that of the calculated 350amps its ok to use.

My confusion came from where they stated about the 75deg column, i was reading it to mean i had to use the 75deg column to start with and then derate which was really confusing me.

thanks again for the timely replies.!!

For a given conductor with 90 degree insulation supplying continuous loads through 75 degree terminals, you derate the ampacity in the 90 degree column for conditions of use, which protects the insulation, and you derate the ampacity in the 75 degree column for continuous use, which protects the terminals. Whichever derated ampacity is lower is the one you use.

 

[Carultch]

For a given conductor with 90 degree insulation supplying continuous loads, you derate the ampacity in the 90 degree column for conditions of use, which protects the insulation, and you derate the ampacity in the 75 degree column for continuous use, which protects the terminals. Whichever derated ampacity is lower is the one you use.

You might want to be more specific on the second half of your statement. The "derating" that you do for terminals, is multiplying the continuous load current by 1.25, to account for the fact that it is a continuous load. This doesn't ever depend on ambient temperature, elevated air temperature due to sunlight, or conductor bundling. It is only due to whether or not the load is continuous, or the 100%+125% mixture of both.

 

[ggunn]

You might want to be more specific on the second half of your statement. The "derating" that you do for terminals, is multiplying the continuous load current by 1.25, to account for the fact that it is a continuous load. This doesn't ever depend on ambient temperature, elevated air temperature due to sunlight, or conductor bundling. It is only due to whether or not the load is continuous, or the 100%+125% mixture of both.

Mulitplying the current by 1.25 is the same as multiplying the ampacity by 0.8. If your loads are 100% continuous, it's the same thing. Conditions of use derating is ambient temperature, sunlight heating, and CCC's in a conduit. Continuous use... isn't. To me that's a simpler way of looking at it when the loads are 100% continuous. I work in solar, so it's all continuous to me.

 

[Carultch]

Mulitplying the current by 1.25 is the same as multiplying the ampacity by 0.8. If your loads are 100% continuous, it's the same thing. Conditions of use derating is ambient temperature, sunlight heating, and CCC's in a conduit. Continuous use... isn't. To me that's a simpler way of looking at it when the loads are 100% continuous. I work in solar, so it's all continuous to me.

True. My point is, not to confuse the 0.8 that is due to 1/125% for continuous loads, with the 0.8 that applies for 4-6 CCC's in the raceway.

 

[Clayton79]

Thanks, for the replies I appreciate the feedback. took the ICC national standard Masters today. Passed it.