ampacities of parallel conductors

[powersystems]

I have a couple of issues but they pertain to the same problem.
I have a feeder coming from a 400 amp main switch we ran (2) 2" w/ 3/0 thhn my two questions are

1)For derating for the # of conductors do I need to use 4 (80%)or do I need to count all of the conductors as if they were in 1 conduit and use 8 conductors (70%).

2) If I use 80%(which I think is correct) of 3/0 in the 90 degree column (225*2*.80) I get 360 amps and go to the next standard size CB I should be able to install these wires to a 400 amp CB does this sound accurate or am I missing something??

Thank you for any input

Kevin

 

[pierre]

Re: ampacities of parallel conductors

If you have installed 3 phase conductors and 1 grounded (neutral) conductor in each conduit, you will not need to do any ampacity adjustment for the number of conductors per conduit.

 

[powersystems]

Re: ampacities of parallel conductors

I think because they are serving a lot of non linear loads it puts me into the 4-6 current carrying conductors according to 310-15(B)(2)(c)but the inspector says I need to use 7-9 because I have to count all parallel conductors. My argument is that this would only be true if all conductors were in the same conduit.

Thanks

Kevin

 

[pierre]

Re: ampacities of parallel conductors

Kevin
If in fact you are supplying Nonlinear loads, 310.15(B)(4)(c) is the reference and yes you will need to count the neutral conductor as a current carrying conductor.
The reference this inspector is following is confusing him, or I am confused as to your installation.

He is reading the last sentence of 310.15(B)(2)(a) - "each current-carrying conductor of a paralled set of conductors shall be counted as a current-carrying conductor.

You are doing that, but from what I am reading here, you have 2 sets of conduits carrying 4 conductors each. You will apply ampacity adjustment as per Table 310.15(B)(2)(a) - of which you have 4 conductors at 80%.

 

[charlie b]

Re: ampacities of parallel conductors

Originally posted by powersystems: My argument is that this would only be true if all conductors were in the same conduit.

You are correct. The derating is described in Table 310.15(B)(2)(a). In the title of that Table and in the text of the related paragraph, you will see the key word "a." More than three current-carrying conductors in a raceway. If you count the neutral because of non-linear loads, you have no more than four conductors in a raceway, and four more in another raceway. You use the derating factor of 80%.

Originally posted by powersystems:. . . does this sound accurate or am I missing something??

You might be missing something. What is the calculated load on the panel? If the calculated load is more than 360 amps, then you cannot use a conductor with an ampacity of 360 amps. You can protect a pair of 3/0 with a 400 amp breaker, but you cannot design the system to push a known (i.e., calculated) load of 370 amps through a pair of 3/0.

If by chance you do not have a calculated load for the panel, then I would say you need to supply conductors that can handle every bit of the 400 amps for which the panel is rated. You would have to go with 4/0, and you might want to think about a 2.5 inch conduit.

 

[powersystems]

Re: ampacities of parallel conductors

My calculated load is well under 300 amps
Thank you very much for the input

Now I need to tactfully tell the inspector that I think he is mistaken

Thanks again

Kevin

 

[pierre]

Re: ampacities of parallel conductors

Charlie
Why can't he have a calculated load of larger than 360 amp with paralled 3/0?
Isn't 3/0 rated at 200 amps for 75 degree C?

 

[charlie b]

Re: ampacities of parallel conductors

Originally posted by pierre: He is reading the last sentence of 310.15(B)(2)(a) - "each current-carrying conductor of a paralleled set of conductors shall be counted as a current-carrying conductor.

You might be right about the Inspector's misreading of that sentence. I believe that sentence is saying that if you put (for example) six conductors in the same conduit (that is big enough to hold them all), and if they consist of 2 parallel Phase A, 2 parallel Phase B, and 2 parallel Phase C, you don't get to call this three conductors, and skip the derating. It is counted as six conductors, and you must apply the 80% derating factor.

 

[charlie b]

Re: ampacities of parallel conductors

Originally posted by pierre: Charlie
Why can't he have a calculated load of larger than 360 amp with paralled 3/0?

Only because he is counting the neutral as a current-carrying conductor, and must use an 80% derating factor.

 

[powersystems]

Re: ampacities of parallel conductors

Pierre,
I think because I have to derate to 80% of the 90 degree column and use the lower of that and the 75 degree column which are 360 amps (80% of 450) and 400 amps respectively.

Kevin

 

[powersystems]

Re: ampacities of parallel conductors

Originally posted by charlie b:

Originally posted by pierre: He is reading the last sentence of 310.15(B)(2)(a) - "each current-carrying conductor of a paralleled set of conductors shall be counted as a current-carrying conductor.

You might be right about the Inspector's misreading of that sentence. I believe that sentence is saying that if you put (for example) six conductors in the same conduit (that is big enough to hold them all), and if they consist of 2 parallel Phase A, 2 parallel Phase B, and 2 parallel Phase C, you don't get to call this three conductors, and skip the derating. It is counted as six conductors, and you must apply the 80% derating factor.

I talked to another inspector and he said the same thing , that is exactly what that sentence refers to

Thanks again

Kevin

 

[pierre]

Re: ampacities of parallel conductors

Okay, I see that - thank you.

Now, do you take ampacity adjustment of the 3/0 (200 amps) per raceway or do you take ampacity adjustment of paralled 3/0s at 450 amps? That would be 2 different numbers.

 

[charlie b]

Re: ampacities of parallel conductors

You may take ampacity adjustment of the 3/0 (225 amps) per raceway or you make take ampacity adjustment of paralled 3/0s at 450 amps. Those would be the same numbers.