diesel locomotive cable ampacity at 75 degrees

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dm9289

Industrial Maintenance Electrician
Location
Pennsylvania
Occupation
Industrial process repair/ maintenance Electrician
I was asked to look over a cable plan and was having some trouble determining code compliance.

We have a control panel feeding a disconnect the disconnect has 75C terminals 2 wires in conduit. 115 degrees ambient the load is continuos 310 amp

continuos 310 amp x 1.25 = 387amp

temperature is 115f so 387/.82=472.56amp

South wire dlc cable is rated like RHHW- 2 but has odd sizing for example if my terminations were 90C 646 cable would work at 493Amps
The contractor/ designer felt313 DLC was OK and rated it at 477 amp southwire says 326amps


However my terminations at the disconnect are 75C so if the designer/ contractor was married to using dlc cable (which they are) how would I calculate the necessary size??
Unless I am missing something his use of 313 DLC is pretty far off with 90c terminations. I dont know how to convert this to 75C terminations but my guess is it would be much worse yet?
 
When I use dual rated DLO, I use the ampacities from Table 310.15(B)(16) based on the next smaller standard size conductor. 300kcmil is 320 amps in the 90°C column. That is what I would normally use as most of the time the cable manufacturer gives you the 90°C free air ampacity. In this case it appears that the Southwire value is a 90°C, not more than 30 conductors in a raceway ampacity, so I would probably use the 326 amps as the rated value....much too small for your application.

The 646kcmil DLO would work fine. The 75°C ampacity for 600 kcmil is 420 amps, greater than your required conductor ampacity of 387 amps, so you would be good to go with equipment having 75°C terminals.
 
don_resqcapt19 said:
When I use dual rated DLO, I use the ampacities from Table 310.15(B)(16) based on the next smaller standard size conductor. 300kcmil is 320 amps in the 90°C column. That is what I would normally use as most of the time the cable manufacturer gives you the 90°C free air ampacity. In this case it appears that the Southwire value is a 90°C, not more than 30 conductors in a raceway ampacity, so I would probably use the 326 amps as the rated value....much too small for your application.

The 646kcmil DLO would work fine. The 75°C ampacity for 600 kcmil is 420 amps, greater than your required conductor ampacity of 387 amps, so you would be good to go with equipment having 75°C terminals.
Click to expand...

I agree that would work. Southwire's DLO cable is also rated RHH/RHW so it seems to me you are completely ok with looking up the 75 degree ampacity in 310.15(B)(16) that is required for your terminations
 
don_resqcapt19 said:
When I use dual rated DLO, I use the ampacities from Table 310.15(B)(16) based on the next smaller standard size conductor.
Click to expand...
What do you think about interpolating based on cross-sectional area? Ampacity as a function of cross-sectional area is a concave down function, so a linear interpolation is conservative.

Cheers,
Wayne
 
wwhitney said:
What do you think about interpolating based on cross-sectional area? Ampacity as a function of cross-sectional area is a concave down function, so a linear interpolation is conservative.

Cheers,
Wayne
Click to expand...
That should be ok. Usually I am just to lazy to to do the math and the next smaller size ampacity is even more conservative.:)
 
jumper said:
Oh good gravy! Is that even English?:D
Click to expand...
Let me try to translate that to mostly English:

If you take the ampacity values from an NEC table and the corresponding kcmil values, and you graph ampacity on the y-axis versus kcmils on the x-axis, the points don't lie on a straight line. If you connect them with a smooth curve, you'll find that the curve is steeper at low kcmil values and consistently "bends downard" to become less and less steep as the kcmil value increases. That means if you draw a straight line between two of the points it will lie below the curve on this graph. Therefore if you use the straight line to estimate an ampacity for a kcmil value not in the table, the result will lower than the "actual" (curve) value, and your estimate errs on the side of safety.

Cheers, Wayne
 
Here's the picture:

cvxg3.gif


Cheers, Wayne
 
wwhitney said:
What do you think about interpolating based on cross-sectional area? Ampacity as a function of cross-sectional area is a concave down function, so a linear interpolation is conservative.

Cheers,
Wayne
Click to expand...

I would not argue against that as science, but the code does not appear to allow for it.
 
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