cable ampacity installed in a cable tray

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reko19

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I have an application with 1000A continuous load with the conductors installed on a cable tray. So to support 1000A continuous, I need a 1250A CB. Based on 392.80(A)(2)(a), the ampacity shall not exceed 75% of ampacities listed in 310.15(B)(17). Using 90C column (assuming I could utilize 90C if insulation is rated for 90C), 700kcmil is rated for 850A. (At this point I would apply temperature derating if I have to, and check against 1000A load I am trying to support). Two sets are good for 2x850x.75=1,275A. Since the cable will be terminated at the CB with 75C rating, I take 75C column rating and compare it to CB rating: 755x2=1510 which is greater then the CB rating so I should be OK.

My question is how 110.14 applies to this. Based on 110.14, I need to use 310.15(B)(16) 75C rating instead of 310.15(B)(17) 75C rating for the last step above which doesn't make any sense to me.

thanks in advance
 

Carultch

Senior Member
Location
Massachusetts
I have an application with 1000A continuous load with the conductors installed on a cable tray. So to support 1000A continuous, I need a 1250A CB. Based on 392.80(A)(2)(a), the ampacity shall not exceed 75% of ampacities listed in 310.15(B)(17). Using 90C column (assuming I could utilize 90C if insulation is rated for 90C), 700kcmil is rated for 850A. (At this point I would apply temperature derating if I have to, and check against 1000A load I am trying to support). Two sets are good for 2x850x.75=1,275A. Since the cable will be terminated at the CB with 75C rating, I take 75C column rating and compare it to CB rating: 755x2=1510 which is greater then the CB rating so I should be OK.

My question is how 110.14 applies to this. Based on 110.14, I need to use 310.15(B)(16) 75C rating instead of 310.15(B)(17) 75C rating for the last step above which doesn't make any sense to me.

thanks in advance

110.14(C) and the 75C rating applies to the termination point, without any temperature correction or adjustment factors.
The 90C rating, when you have 90C wire insulation (most common), you can use for temperature correction and adjustment factors considered simultaneously.

In any event with over 800A of overcurrent device, you need as much wire ampacity as you have overcurrent device. And just because your 1.25 saftey factor yields 1250A of OCPD doesn't necessarily mean that you can find exactly that part. You need to round up to the next overcurrent device you can find and install, whether it is from a standard list or a custom part. And then you size the wire to the OCPD.

1200A is a standard OCPD size
1600A is the next standard OCPD size
Then 200A.
 

Smart $

Esteemed Member
Location
Ohio
...My question is how 110.14 applies to this. Based on 110.14, I need to use 310.15(B)(16) 75C rating instead of 310.15(B)(17) 75C rating for the last step above which doesn't make any sense to me.
To help it make sense, look at it like this. Only the wire size is of any concern. Insulation type and temperature rating has none. The ampacity tables are for conditions of use beyond the termination enclosure. However, Table 310.15(B)(16) is used to establish a termination temperature limitation ampacity value within the enclosure. Table 310.15(B)(16) is used essentially to omit having a table with redundant values.
 

Smart $

Esteemed Member
Location
Ohio
...

1200A is a standard OCPD size
1600A is the next standard OCPD size
Then 2000A.
True... but when you get to this size frame, the breaker trip rating is usually adjustable or offers a few levels of lower-rated trip setting modules.

Refer to 240.6(B) and (C).
 

ron

Senior Member
My question is how 110.14 applies to this. Based on 110.14, I need to use 310.15(B)(16) 75C rating instead of 310.15(B)(17) 75C rating for the last step above which doesn't make any sense to me.

110.14(C) specifically directs you to table 310.15(B)(16) for ampacity at 75deg. That doesn't jive with your use of the free air rating use from Table 310.15(B)(17).
 

reko19

Member
110.14(C) specifically directs you to table 310.15(B)(16) for ampacity at 75deg. That doesn't jive with your use of the free air rating use from Table 310.15(B)(17).

this is exactly my question, why do I have to go to 75C column of 310.15(B)(16) instead of 75C column of 310.15(B)(17). I loose all advantages of running my cable on the cable tray if I have to go back to the "in conduit" ampacity table. Inside an enclosure I should be able to use a free air rating, I know for a fact that all equipment manufacturer's use free air for interconnects inside an enclosure.
 

Smart $

Esteemed Member
Location
Ohio
.... Inside an enclosure I should be able to use a free air rating, I know for a fact that all equipment manufacturer's use free air for interconnects inside an enclosure.
Inside an enclosure is not in free air.


I'd like to see documentation confirming the latter part of your statement. Got any?
 

reko19

Member
I'll see if I could find some shop drawings... This is a bummer since it would definitely require more conductors on the cable tray.
 

reko19

Member
thank you for the link, that was the first thing that came to mind, some sort of a bus detail to transition from the enclosure to the cable tray.
 

Smart $

Esteemed Member
Location
Ohio
thank you for the link, that was the first thing that came to mind, some sort of a bus detail to transition from the enclosure to the cable tray.
Here's another tidbit of information...

Cable tray wiring method is the only wiring method I know of that don't require an enclosure for cable splices [392.56] (other than direct burial splices, but this isn't really a wiring method).
 

reko19

Member
interesting, so if I have the same number of conductors, I could just do an in-line splice to a smaller conductor once I am on the cable tray. thanks for the info
 

reko19

Member
Inside an enclosure is not in free air.


I'd like to see documentation confirming the latter part of your statement. Got any?
so i checked shop drawings for the power distribution units provided by Emerson on one of our jobs. They used two 1/O conductors per phase to connect line side of 400A to the distribution bus. This is clearly based on the free air rating 75C column.
 

Smart $

Esteemed Member
Location
Ohio
so i checked shop drawings for the power distribution units provided by Emerson on one of our jobs. They used two 1/O conductors per phase to connect line side of 400A to the distribution bus. This is clearly based on the free air rating 75C column.
By chance are you talking about manufacturers wiring inside listed equipment? If so, that wiring does not fall under the purview of the NEC (see 90.7), and is quite likely sized per the UL standard under which it was listed. Additionally, it is probably sized smaller than Table 310.15(B)(16) and closer to the free air table. This is not unusual. It is similar to POCO installing service drop wiring that is smaller than the NEC permits, which is another case of wiring not under NEC purview.

That said, field wiring under NEC purview must be installed NEC compliant, so unless you can submit a proposal (Public Input) that gets accepted and implemented which changes the criteria we must adhere to in the future, you are stuck with the NEC requirements as they are, just like the rest of us.
 

reko19

Member
if the load side of the CB I am connecting to is provided with a small bus detail for the cable termination, would the argument still apply or could I use free air rating at that point? How about fuses?
 

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
PE (Retired) - Power Systems
if the load side of the CB I am connecting to is provided with a small bus detail for the cable termination, would the argument still apply or could I use free air rating at that point? How about fuses?
Does your application require NRTL Listed equipment?
Did you see where the Square D paper said: there is no UL Listed protective device rated with terminations greater than 75C?
UL Listed enclosed equipment is typically designed and built using temperature rise testing assuming 75C terminations.
 
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