2014 NEC 310.15

[moonlite]

I would like confirmation on my interpretation of NEC 310.15(A)(2) and which ampacity table should be used for the base value. Below is the text in the 2014 NEC that I have found to support my interpretation followed by an example. The example is used to help show why different ampacity tables would be used for determining the allowable ampacity for the whole length of the circuit.

1) 310.15(A)(2) Selection of Ampacity ?Where more than one ampacity applies for a given circuit length, the lowest value shall be used.

2) 310.15(B)(3)(a) ?Where the number of current-carrying conductors in a raceway or cable exceeds three, or where single conductors or multiconductor cables are installed without maintaining spacing for a continuous length longer than 24 in. and are not installed in a raceway, the allowable ampacity of each conductor shall be reduced as shown in Table 310.15(B)(3)(a).?

3) The note in Table 310.15(B)(3)(a), ?Percent of Values in Table 310.15(B)(16) through Table 310.15(B0(19)??

The above language appears to allow the typical adjustment factors to be applied to Table 310.15(B)(17) and Table 310.15(B)(16) for a circuit made of multiple single conductors cables that are installed in an open cable tray and in conduit for different sections of the circuit. For example, a group of twelve #10 AWG copper single PV Wire cables are routed together in a 2? wide, solid bottom cable tray under the ground mounted PV array. Then the same group of cables transition into underground conduit. I?ll work through specific calculation for the two sections to determine the lowest value per 310.15(A)(2).

Cable Tray Section;
The ambient temperature is elevated near the PV module so I used 14?C adder from Table 310.15(B)(3)(c) for guidance. Therefore, the ambient temperature is ASHRAE 2% plus 14?C, 34? + 14?C = 48?C. Using Table 310.15(B)(17) with the adjustment factors we get the following ampacity; 55A x 0.82 x 0.5 = 22.6A

Underground Conduit Section:
The ambient temperature is 34? for the conduit section. Using Table 310.15(B)(16) with the adjustment factors we get the following ampacity; 40 x 0.96 x 0.5 = 19.2A

Therefore the ampacity of the #10 AWG copper single PV Wire cable in this example is 19.2A for the whole length of the circuit, including the cable tray section. Please confirm my method of selecting the appropriate ampacity and adjustment factor tables.

Thank you.

 

[Smart $]

Looks good to me.

 

[moonlite]

Looks good to me.

Thanks for the feedback.

I'm looking forward to a review from a moderator.

 

[Smart $]

IF I were mistaken, you can be assured someone else would have corrected me by now... :happyyes:

 

[Dennis Alwon]

Thanks for the feedback.

I'm looking forward to a review from a moderator.

Being a moderator does not makes us smarter than any one else. Smart is one the sharpest guys on this forum. I did not check it out but if he did I would bet he is right.

 

[Dennis Alwon]

BTW it looks good to me also unless you meet the exception in 310.15(A)(2)

 

[Smart $]

Being a moderator does not makes us smarter than any one else. Smart is one the sharpest guys on this forum. I did not check it out but if he did I would bet he is right.

Thanks for the compliment, intended or not...

 

[Dennis Alwon]

Thanks for the compliment, intended or not...

It was intended and I believe what I said. :thumbsup:

 

[Smart $]

BTW it looks good to me also unless you meet the exception in 310.15(A)(2)

:lol: (<--- related to another current thread for those that wonder)

However, just to clarify, it would still be good even if the exception applies, where the final ampacity would only be greater than as calculated.

But it is a good point. :happyyes:

 

[moonlite]

Smart, I hope you have not taken my request for a moderator to chime in as an insult. If you have, I do apologize, that was not my intention. I was trying to get a pole or consensus on how to calculate the ampacity of a #10 copper PV Wire in the conditions of use that are described in the original post (a bundle of single conductor cables in an open solid bottom cable tray near a PV array). If a couple of members and a moderator confirm my calculation method, then I will feel more confident in my interpretation of the code.

I have also reached out to NFPA. I'll post their response when I get it.

 

[Smart $]

Smart, I hope you have not taken my request for a moderator to chime in as an insult. If you have, I do apologize, that was not my intention. I was trying to get a pole or consensus on how to calculate the ampacity of a #10 copper PV Wire in the conditions of use that are described in the original post (a bundle of single conductor cables in an open solid bottom cable tray near a PV array). If a couple of members and a moderator confirm my calculation method, then I will feel more confident in my interpretation of the code.

I have also reached out to NFPA. I'll post their response when I get it.

I did not take your preference as an insult. I understood what you were asking, and aware you are new here. FWIW, you can somewhat gage the accuracy of a reply by comparing the number of replies to the number of views. This reply will make 10, and prior to entering the reply page the thread has had 181 views..

Is there a reason you are using solid bottom tray? In my experience not very common, but there is nothing which prohibits such. Not sure if you meant the cables are technically bundled, but you'll have to be cautious if so, because the AHJ may say spacing is not maintained for a distance greater than 24". Article 392 does not address true bundling.

If you get an NFPA reply anytime soon, it will be an unofficial interpretation.

 

[moonlite]

Is there a reason you are using solid bottom tray? In my experience not very common, but there is nothing which prohibits such. Not sure if you meant the cables are technically bundled, but you'll have to be cautious if so, because the AHJ may say spacing is not maintained for a distance greater than 24". Article 392 does not address true bundling.

This is the muddy part. The "cable tray" is made up of C-channel that are the structural members for the ground mounted PV array. Article 690.31(C)(2) in the NEC 2014 version allows all sizes of PV Wire to be installed in cable tray. Since the single conductor is a #10 AWG copper, 2kV PV Wire, it appears 392.22(3)(b) is used to determine the maximum number of conductors. This section point to Column 3 of Table 392.22(A) to determine the maximum fill area. For the inside width of 2", Column 3 allows a maximum fill area of 2 in^2 and therefore (37) #10 AWG copper, 2kV PV Wire conductors (with an outside diameter of 0.260") is the maximum. I used Table 310.15(B)(3)(a) for the bundling adjustment factor because 392.80 does not address the #10 conductor size. See attached sketch for a cross sectional view of the C-channel.