Derating for Cables feeding 12 100HP motors in cable tray

[kenaslan]

Situation:
I have 12 each 100Hp 460V motors. Ambiient air temp is 114F. Cables to be 3/C 2/0 XHHW-2. Uncovered Ladder type cable tray.

NEC 392.80(A)(1)(c) states
"Where multiconductor cables are installed in a single layer in uncovered trays, with a maintained spacing of not less than one cable diameter between cables, the ampacity shall not exceed the allowable ambient temperature corrected ampacities of multiconductor cables, with not more than three insulated conductors rated 0 through 2000 volts in free air, in accordance with 310.15(C).

My question is, what is the derating for conductors CLOSER than one cable diameter between cables?

 

[david luchini]

You would use the ampacity table 310.15(B)(16) with the temperature correction factor from 310.15(B)(2)(a).

 

[kenaslan]

This is correct, but only for 100% spaced cable.

(A) Ampacity of Cables, Rated 2000 Volts or Less, in Cable Trays.

(1) Multiconductor Cables. The allowable ampacity of multiconductor cables, nominally rated 2000 volts or less, installed according to the requirements of 392.22(A) shall be as given in Table 310.15(B)(16) and Table 310.15(B)(18), subject to the provisions of (A)(1)(a), (b), (c), and 310.15(A)(2).

IMO the provisions of (A)(1)(c) allow temp derating as the only derating factor only if 100% spaced cable.

(A)(1)(c) Where multiconductor cables are installed in a single layer in uncovered trays, with a maintained spacing of not less than one cable diameter between cables, the ampacity shall not exceed the allowable ambient temperaturecorrected ampacities of multiconductor cables, with not more than three insulated conductors rated 0 through 2000 volts in free air, in accordance with 310.15(C).

 

[rcwilson]

I thought that if the cables are spaced with at least one diameter between them the higher ampacities for a conductor in free air apply, subject to temperature limitations. If the cables are stacked next to and on top of each other, then the "normal" ampacities in table 310.15 (B)(16) apply. Ambient temperature correction applies to both installation conditions.

 

[kenaslan]

Here is the complete text from the 2012 NEC handbook for 392.80
This is true only "subject to the provisions of" statement. Dont forget you may not stack cables 4/0 and larger. 392.22(A)(1)(a),(b),(c).


392.80 Ampacity of Conductors.

(A) Ampacity of Cables, Rated 2000 Volts or Less, in Cable Trays.

(1) Multiconductor Cables. The allowable ampacity of multiconductor cables, nominally rated 2000 volts or less, installed according to the requirements of 392.22(A) shall be as given in Table 310.15(B)(16) and Table 310.15(B)(18), subject to the provisions of (A)(1)(a), (b), (c), and 310.15(A)(2).

(a) The adjustment factors of 310.15(B)(3)(a) shall apply only to multiconductor cables with more than three current-carrying conductors. Adjustment factors shall be limited to the number of current-carrying conductors in the cable and not to the number of conductors in the cable tray.

(b) Where cable trays are continuously covered for more than 1.8 m (6 ft) with solid unventilated covers, not over 95 percent of the allowable ampacities of Table 310.15(B)(16) and Table 310.15(B)(18) shall be permitted for multiconductor cables.

(c) Where multiconductor cables are installed in a single layer in uncovered trays, with a maintained spacing of not less than one cable diameter between cables, the ampacity shall not exceed the allowable ambient temperaturecorrected ampacities of multiconductor cables, with not more than three insulated conductors rated 0 through 2000 volts in free air, in accordance with 310.15(C).

 

[rcwilson]

100 HP = 124FLA. Ampacity needed = 1.25 * 124= 155A. XHHW cable is 90 C rated. 114F derating factor is 0.82. 155A/0.82 = 189A. Per Table 310.15 (B)(16) #2/0 AWG Cu cable is good for 195A.

195A x 0.89 = 173 amps, which is larger than teh required 155A. Assume the motor starter is in 30 C ambient and the lugs are rated for 75C cable. #2/0 AWG, 75C XHHW is rated 175 A at 30C so the terminations are OK.

Since each cable is only 3/C + EGC there is no need to derate for more than 3 conductors, even though there may be 30 current carrying cables in the tray. See 392.80 (A) (1) (a).

3/C #2/0 w/ EGC is about 1.4" in diameter; 10 of them will take 14" in width and have about 15.4 sqin cross section area. 16" tray has an allowable cross section of 18.5 sqin.

Bottom Line - ten 3/c, #2/0 awg, XHHW cables in a 16 " tray feeding ten-100HP motors in a 114F environment will meet code requirements.

I guess I don't understand your point about derating not applying?

 

[Phil Corso]

Kenaslan... some out-ofthe box thoughts:

1) Presuming voltage-drop or short-circuit withstand capability are not problematic why not consider increasing separation to 2 diameters, then 1/0 cable can be used on single level ladder-rack.


2) If installed in two rows, and VD and SCW are still not problematic, then maybe #1 cable can be used. Of course, it will require two level ladder-rack spaced about a foot apart in height!


3) If you are stuck with reducing separation, how about "compact" conductor shape!


Regards, Phil Corso

 

[Phil Corso]

Sorry... should have said "Sector-Shaped!"

Phil

 

[david luchini]

This is correct, but only for 100% spaced cable.

(A) Ampacity of Cables, Rated 2000 Volts or Less, in Cable Trays.

(1) Multiconductor Cables. The allowable ampacity of multiconductor cables, nominally rated 2000 volts or less, installed according to the requirements of 392.22(A) shall be as given in Table 310.15(B)(16) and Table 310.15(B)(18), subject to the provisions of (A)(1)(a), (b), (c), and 310.15(A)(2).

IMO the provisions of (A)(1)(c) allow temp derating as the only derating factor only if 100% spaced cable.

(A)(1)(c) Where multiconductor cables are installed in a single layer in uncovered trays, with a maintained spacing of not less than one cable diameter between cables, the ampacity shall not exceed the allowable ambient temperaturecorrected ampacities of multiconductor cables, with not more than three insulated conductors rated 0 through 2000 volts in free air, in accordance with 310.15(C).

I think you are misinterpreting 392.80(A)(1). The "Subject to the provisions of (A)(1)(a), (b), (c)" section tells you how you adjust the ampacity of the conductors in the multiconductor cables in cable tray.

The starting point for the ampacity of multiconductor cables is Table 310.15(B)(16).

(A)(1)(a) tells you the adjustment factor for more than three conductors applies to each individual cable, not to the total number of conductors in the cable tray.

(A)(1)(b) tells you that you must derate to 95% for covered cable trays.

(A)(1)(c) tells you that when you have cables in a single layer in uncovered trays with a spacing of not less than one cable diameter, the allowable ampacity can be based on the "free air" ampacity rating of the conductors in accordance with 310.15(C), rather than the Table 310.15(B)(16) ampacity.

(A)(1)(c) has the effect of allowing higher ampacities when the one diameter spacing is maintained between cables.

As Bob pointed out, the conductors appear to have sufficient ampacity based on T310.15(B)(16) after the temperature adjustment is applied.

 

[kenaslan]

God I love this Forum,

Thank you guys, you are right I misunderstood (c). I brain spaced the free air part. After 2 years of doing Canadian cable tray I am still thinking of the CEC code where different spacing has different derating up to an including 50% for 100% random fill tray, filled 100%.