310.15(B)(2)(a) question

[dre120]

A 120/208 volt, 3?, 4-wire feeder is installed in a raceway in free air to supply a 25,000 VA, non continuous, electric discharge lighting load. This THW feeder will pass through an ambient temperature of 120 degrees F. What is the minimum allowable ampacity of this feeder?

25000= 69.4 or 69
360

The answer key says 116 amperes

What am I doing wrong?

 

[wirenut1980]

There are two ampacity adjustment factors that need to be factored in.

The first is that the load is non-linear and the neutral needs to be counted as a current carrying conductor. - 80% reduced ampacity

The second is the ambient temp - 75% reduced ampacity

Take your original ampacity requirement of 69.4 Amps / 0.8 /0.75 = 116 Amps required ampacity.

 

[don_resqcapt19]

If the question is worded as stated in the post the answer key is wrong....the minimum required ampacity of the feeder is 69.4 amps. The adjustment and correction factors have nothing to do with the required ampacity of the feeder....they only change the ampacity of the conductors used for the feeder.

 

[dre120]

If the question is worded as stated in the post the answer key is wrong....the minimum required ampacity of the feeder is 69.4 amps. The adjustment and correction factors have nothing to do with the required ampacity of the feeder....they only change the ampacity of the conductors used for the feeder.

wirenut is right because the question is asking about non continous electric discharge lighting so the wiring would be a b c n= 4 conductors
80% under 310.15(B)(2)(a) and the .75 from table 310.16. Where I get confused is when you have to multiply and when you have to divide.

 

[infinity]

It would *80% of the full required conductor ampacity or 125% of the 69 amps. Same thing is true for the .75. That would be *.75 of the full required conductor ampacity or 133% of the 69 amps.

So 69.5 * 1.25 * 1.33 = 115.54

 

[bob]

wirenut is right because the question is asking about non continous electric discharge lighting so the wiring would be a b c n= 4 conductors
80% under 310.15(B)(2)(a) and the .75 from table 310.16. Where I get confused is when you have to multiply and when you have to divide.

DRE
You are missing the point that Don is making. The required ampacity of the feeder is 69 amps. You must choose a conductor ampacity that is larger than 69 amps so that when you multiply the ampacity by 0.75 and 0.80 you end up with an ampacity of 69 amps. In this case you need to use #1 cu with an ampacity of 130 amps. 130 x 0.80 x 0.75 = 78 amps. If you choose #2 cu with an ampacity of 115 amps you would get 115 x 0.80 x 0.75 = 69 amps also. If you use 69.4 amps as the minimum, then #2 cu would not meet the requirements. So much for rounding down.

 

[don_resqcapt19]

wirenut is right because the question is asking about non continous electric discharge lighting so the wiring would be a b c n= 4 conductors
80% under 310.15(B)(2)(a) and the .75 from table 310.16. Where I get confused is when you have to multiply and when you have to divide.

None of that changes the required ampacity of the feeders. The required ampacity for this circuit is 69.4 amps. The things that you are talking about are adjustment and correction factors...they change the ampacity of the conductor. You need a conductor that has an ampacity of 69.4 amps after you have applied the adjustment and correction factors.

 

[LarryFine]

The adjustment and correction factors have nothing to do with the required ampacity of the feeder....they only change the ampacity of the conductors used for the feeder.

Or, to say it another way, the adjustments change the size of the conductor it takes to end up with an adequate ampacity.

 

[wirenut1980]

If the question is worded as stated in the post the answer key is wrong....the minimum required ampacity of the feeder is 69.4 amps. The adjustment and correction factors have nothing to do with the required ampacity of the feeder....they only change the ampacity of the conductors used for the feeder.

I agree that the adjustment factor do not change the ampacity of the feeder, but rather they affect the allowable ampacity of the conductors. I had to think differently when answering this question, because it is worded incorrectly. But one can tell that the spirit of the question is adjustment factors. It is an important distinction to make in understanding this section of the code.

 

[don_resqcapt19]

I agree that the adjustment factor do not change the ampacity of the feeder, but rather they affect the allowable ampacity of the conductors. I had to think differently when answering this question, because it is worded incorrectly. But one can tell that the spirit of the question is adjustment factors. It is an important distinction to make in understanding this section of the code.

I have no idea what the question is trying to ask...it may be asking exactly what it says. The correct answer as written is 69.4. I If you want to ask the question to include the correction and adjustment factors, then it should ask for the required wire size, not the ampacity of the feeder.