Cable Tray Derating

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kenaslan

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Billings MT
Are the derating sched used together, or does the highest take presidence.

I have a cable tray consisting of 8 cables with a total of 24 curent caring conductors. The ambient temp is 40C Using #1 Teck90 cable good for 140A temp derating at 40C 126A. Now the question is do I derate the .7 from the 140A or the 126A? I believe I need to use the 126A for a total derating of 88.2A

BTW I am using the CEC, the project is in Canada

Thanks
Ken
 
kenaslan said:
Are the derating sched used together, or does the highest take presidence.

I have a cable tray consisting of 8 cables with a total of 24 curent caring conductors. The ambient temp is 40C Using #1 Teck90 cable good for 140A temp derating at 40C 126A. Now the question is do I derate the .7 from the 140A or the 126A? I believe I need to use the 126A for a total derating of 88.2A

BTW I am using the CEC, the project is in Canada

Thanks
Ken
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Under NEC purview you'd multiply .7 ? 126A value.

Under CEC...???
 
Under the NEC, you may not have to derate for multiple conductors, assuming that there are only 3 current carrying conductors in each cable and you do not exceed the fill permitted in 392.9. See 392.11(A).
 
The CEC does not exempt cable tray from the derating for multiple power conductors. Calbe tray is defined as a raceway, just like conduit.

In practice though, installations have passed the AHJ inspection without derating. The cables were spaced neatly in the trays with enough air space to meet the free air rating under NEC rules. Legally, it was a violation, but practically it was acceptable.
 
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