cable tray and ampacity

[dm9289]

We have a 3 phase 480 volt ungrounded delta feeding an SCR control panel 75 c lugs that feeds 3 different 480 to 240 volt transformers for heating.


480Volt side from switch gear to 480v SCR cabinet 540Amp draw and end use 240v heating is a continuous load so 3 transformers 480-240 600amp draw each

continuous load 540 x 1.25 =675Amp

Room temp very hot 115F so temp factor .82 675/.82= 824Amp

392.11(B)(2) 65% allowable ampacity of free air table due to tray use so 824Amp/.65= 1266Amp

My opinion 2-sets of 3 500kcmil thhn individual conductor free air rating 2 x 700amp= 1400amp

Please let me know if my logic seems sound

 

[Smart $]

I didn't check your numbers because it appears you are doing what most do and forget that you cannot use a wire size smaller than permitted by 110.14(C)... and that size is based on Table 310.15(B)(16) regardless of conditions of use.

To base your wire size otherwise, you will have to do a transition beyond the confines of the terminal enclosure (i.e. switchgear and SCR panel). Cable tray is the only wiring method which permits splicing in it without a enclosure. The splicing means must be rated for 90°C (as well as the terminal section of wire) to obtain any advantage in conductor ampacity rating.

 

[dm9289]

But The 90c cable when derated will be below the 75c rating

 

[dm9289]

i could not find 310.15(B) 16 are you referring to the table

 

[Smart $]

But The 90c cable when derated will be below the 75c rating

Let's backtrack here...

Are the heating load 1Ø or 3Ø? I'm asking because 3 times 600A at 240V don't jive with 540A draw at 480V on switchgear.

Assuming 1Ø, 600A per 240V heating circuit is 300A at 480V 1Ø line-to-line. Converting to 480V 3Ø, that's 520A per line. Continuous requires the wire sized at 125%, so 520A × 125% = 650A. 2 × 500kcmil copper @ 380A each per table 310.15(B)(16) is 760A, so you are good there.

Assuming you use a 600A OCPD, also good there as long as derated conductor ampacity remains at or above 520A.

Assuming all in cable tray and none in conduit, and 90°C-rated conductor:
2 × 700A × 65% × 0.82 = 746A

So you're good to go if all assumptions are correct.

 

[Smart $]

i could not find 310.15(B) 16 are you referring to the table

It's Table 310.15(B)(16) on page 70-161... one page before Table 310.15(B)(17) based on Free Air on page 70-162.

 

[dm9289]

Let me try to express this better, circuit is being moved and rebuilt
480v three phase each leg draws 540Amp

3 480 to 240 volt transformers
1st transformer single phase A-B CURRENTLY ON CONTACTOR 600AMP OUT PUT
2nd A-C CURRENTLY ON CONTACTOR 600AMP OUT PUT
3RD B-C CURRENTLY ON CONTACTOR 600AMP OUT PUT

New circuit will use 3 scr to be able vary output.

Maybe this time my explanation is a bit clearer

So switch gear to scr panel

540 X 1.25 = 675 I think you may have had 650

115F room condition so .82 ambient correction 675/.82= 823

392.11(B)(2) 65% of allowable ampacity in free air table so my thought was 2-500kcm free air rated at 700 x 2 =1400
then 1400x.65 from 392.11(B)(2)=910

since the 910was less than 2x620=1240from 75C column i figured term temp after correction would be ok

Please straighten me out I sometimes am a bit slow with this

 

[dm9289]

woops I mesured the 540A primary and you had calculated 520A maybe I have a little extra due to wire lenth transformer losses etc. Old set up is pretty long new set up will be much shorter length long story short. I think we are on the same page.

I greatly appreciate you walking me thru this

 

[Smart $]

woops I mesured the 540A primary and you had calculated 520A maybe I have a little extra due to wire lenth transformer losses etc. Old set up is pretty long new set up will be much shorter length long story short. I think we are on the same page.

I greatly appreciate you walking me thru this

3 × 600A × 240V ÷ 480V ÷ 3^0.5= 520A