Ampacity Ratings of Cables

[Designer371141]

I have been hired to evaluate the main service for a manufacturing facility, there typical installation is an underground feeder from the utility to the main transformer primary side, the secondary side is routed through a concrete manufactured wireway with steel plates covering it. The feeder cables consist of XHHW-2 cables laid inside this wireway as individual feeders, the transformer produces a 4000a secondary, they have installed 8 sets of 4 #750 for the service. If I use the open air ampacity then they are correct, but these feeders have not been segregated into sets of 4 but just all laid in the tray and I do not feel they meet the open air requirement installation to allow the extra ampacity. Article 392 for cable tray list specific requirements, when researching the code I find that the cables are to configured in a triangular fashion and 'spaced' apart from each set a distance equal to the width of the bundled conductors. Am I correct, they have applied the open air ampacity where in fact they do not meet the criteria just on spacing let alone that its in a concrete covered trench?

What's your thoughts

 

[drktmplr12]

You say "concrete manufactured wireway with steel plates covering it" as in a Trenwa covered concrete trench with cables laying inside?

Those cables on the interior "wire piles" will get hotter than the others assuming all are loaded equally. As heat builds up, the interior conductors will not share the load with other paralleled conductors as intended. It sounds like derating is needed, although I haven't designed anything like this before. Whether to and how to derate are questions I do not know the answer to. Apologies

I think the intent of "cables in air" ratings assumes convection is possible. In this case, the air flow is restricted due to the cover and heat will build up inside the trench.

The cables really should be bundled in groups of 3 (or 4) with spacing equal to cable diameter between them. Then enclosed cable tray ratings would apply, in my opinion.

Further reading in related forum post. (c. 2009).

 

[Designer371141]

Thank you for the reply; with additional investigation I found that the cables are segregated into bundles of four, and spaced as if they were in open tray, the wires are XHHW-2 supported in Aluminum Cable tray, half way submurged under water all the time. The trough is covered on three sides solid concrete and the top is steel plate, solid, so no chance of 'air' passing through to keep the overheating down. You mentioned the de-rating and I agree with that, under Article 392.80(2), covered tray requires a de-rating for this many conductors to be 70% of Article 310.15 allowances using open air ampacity, but, that's not the only de-rating required; under the Code and parallel conductors you have to treat each bundled set as one current carrying conductor, now that there are 8 parallel runs, you are required to again de-rate the cables to 70% of the value after de-rating for the covered tray. Hence the 750mcm cables because of the way they are installed can only be used with an ampacity rating of 384 amps per set. Now I am faced with overcurrent protection (4000 amps) with wire value rated at maximum 3077 amps.

 

[petersonra]

Thank you for the reply; with additional investigation I found that the cables are segregated into bundles of four, and spaced as if they were in open tray, the wires are XHHW-2 supported in Aluminum Cable tray, half way submurged under water all the time. The trough is covered on three sides solid concrete and the top is steel plate, solid, so no chance of 'air' passing through to keep the overheating down. You mentioned the de-rating and I agree with that, under Article 392.80(2), covered tray requires a de-rating for this many conductors to be 70% of Article 310.15 allowances using open air ampacity, but, that's not the only de-rating required; under the Code and parallel conductors you have to treat each bundled set as one current carrying conductor, now that there are 8 parallel runs, you are required to again de-rate the cables to 70% of the value after de-rating for the covered tray. Hence the 750mcm cables because of the way they are installed can only be used with an ampacity rating of 384 amps per set. Now I am faced with overcurrent protection (4000 amps) with wire value rated at maximum 3077 amps.

Where did you get the idea that each bundled paralleled set is treated as one current carrying conductor?

 

[ramsy]

..Hence the 750mcm cables because of the way they are installed can only be used with an ampacity rating of 384 amps per set. Now I am faced with overcurrent protection (4000 amps) with wire value rated at maximum 3077 amps.

Using Table 310.17 de-rate only once, 70% for covered tray per 392.80(A)(2)(a)
De-rate further only if failing 392.22(B), ambient > 30°C in tray compartment, or + 125% motor loads per 220.18A / Sec.430

If 70% is the only derating, I get 500A per set.
With 45% motor loads @ Pf=0.85, I get 536A per set, and 74.9°C at termination terminals per 110.14(C)

 

[ramsy]

..these feeders have not been segregated into sets of 4 but just all laid in the tray and I do not feel they meet the open air requirement installation to allow the extra ampacity. ..

If prints don't specify installation per 392.22(B) w/ proper parallel bundles, then get change order approved, and re-submit new prints to installers for redo, so you can use Tbl. 310.17