Power cable calculation

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A friend of mine sent this calculation question to me from their electrical article. I did not get the same answer as the author, especially about the 75 degree rating equipment.

article said:
What is the ampacity of three parallel 4/0 Type W portable power cables with a
temperature rating of 90⁰C connected to equipment terminals having a
temperature rating of 75⁰C? - See correct answer on Page 2.

answer they gave is 1080 amperes ? 2008 NEC 400.5(A), 400.2.

The only way I can see how they even come close to that is they used column E2 in the 90 degree section. 361 X 3=1083, but even then that's still not exactly right. But how can they use E2 since its only for 2 conductor cables and other cables when only 2 are current carrying?

In addition to that, I thought when you had 75 degree equipment that you had to still derate for that even for cables. Let me know what I am missing here.
 
They are using Column D1, under 75degC section. 360*3=1080. This would presume that the conductors are not in physical contact.
 
They are using Column D1, under 75degC section. 360*3=1080. This would presume that the conductors are not in physical contact.

Thanks. I guess I should not 'assume' these cables would be in some type of raceway for protection especially for that level of amps and voltage. Even when I did temp power for trailers on construction sites, we still put some type of protection (most of the time raceway) for these cables.
 
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In addition to that, I thought when you had 75 degree equipment that you had to still derate for that even for cables. Let me know what I am missing here.
Not missing anything. If 600V or less, termination temperature limitations per 110.14(C) will limit the installation to max 690A (3*230A).
 
Not missing anything. If 600V or less, termination temperature limitations per 110.14(C) will limit the installation to max 690A (3*230A).

How is that? (I also don't see 230a on the 4/0 line.)

110.14(C)(1)(a) doesn't apply (<100a), and (1)(b) tells us to use 75 deg. That puts us back to the table 400.5(B) 75 deg columns, and D gives 360a per cable (360 * 3 = 1080). Given that these are 4/0, it's rational to assume single conductors, which keeps us in column D, and since the problem doesn't tell us about a raceway, we shouldn't assume there is one.
 
How is that? (I also don't see 230a on the 4/0 line.)

110.14(C)(1)(a) doesn't apply (<100a), and (1)(b) tells us to use 75 deg. That puts us back to the table 400.5(B) 75 deg columns, and D gives 360a per cable (360 * 3 = 1080). Given that these are 4/0, it's rational to assume single conductors, which keeps us in column D, and since the problem doesn't tell us about a raceway, we shouldn't assume there is one.
What table does 110.14(C)(1) general statement say to base its determination on?
 
What table does 110.14(C)(1) general statement say to base its determination on?

Are you thinking of "Unless the equipment is listed and marked otherwise, conductor ampacities used in determining equipment termination provisions shall be based on Table 310.16 as appropriately modified by 310.15(B)(6)."

According to the OP, the terminals are rated for 75 deg, so that above shouldn't enter the discussion. And we shouldn't assume an ambient temperature of more than 30 deg.

Table 310.16 is for Raceway, Cable, or Earth, and 310.15(B)(6) doesn't apply to this problem. Table 310.17 is for free air, and gives 360a per conductor for 75 deg.

Article 400.5 tells us to use table 400.5(A) and (B) for flexible cords and cable, and all of the recent type W I've handled specifically says to use "NEC table 400.5" for ampacity, sometimes embossed in the jacket.

AFAICT, it all points back to the 75 deg columns, not grouped, and 360a/conductor.
 
Are you thinking of "Unless the equipment is listed and marked otherwise, conductor ampacities used in determining equipment termination provisions shall be based on Table 310.16 as appropriately modified by 310.15(B)(6)."

According to the OP, the terminals are rated for 75 deg, so that above shouldn't enter the discussion. And we shouldn't assume an ambient temperature of more than 30 deg.

Table 310.16 is for Raceway, Cable, or Earth, and 310.15(B)(6) doesn't apply to this problem. Table 310.17 is for free air, and gives 360a per conductor for 75 deg.

Article 400.5 tells us to use table 400.5(A) and (B) for flexible cords and cable, and all of the recent type W I've handled specifically says to use "NEC table 400.5" for ampacity, sometimes embossed in the jacket.

AFAICT, it all points back to the 75 deg columns, not grouped, and 360a/conductor.
I know your assessment to be in error. 75?C terminal temperature rating is commonplace. The OP does not mention anything about the equipment being listed and marked otherwise, so you must use Table 310.16 to determine terminal temperature limitations (i.e. maximum permitted conductor ampacity).
 
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