Cablebus terminated between "busbar" and "busway cable tap box"

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elec_eng

Senior Member
I have a equipment (indoor) that needs to be powered up with 4000A feeder (480V/3ph) from 4000A switchgear (outdoor). The equipment comes with a termination box (an enclosure with 4000A bus bar, nothing else) that I can terminate the cables. I have over (20) of them in this set up now and I will have more as the facility grows so I would like to have somewhat economical yet scalable solution I can use for a long term.

I am planning to use a cablebus for this feeder and trying to come up with a way to reduce the feeder size by utilizing the free air ampacity of 310.15(B)(17) to size the cablebus per 370.80. However, I realized that I still had to use table 310.15(B)(16) for the termination rating per 110.14(C).

In order to avoid this termination rating, I am planning to use "4000A busway flange" at the switchgear (directly connecting to switchgear bus) then the other side of the flange, I will have a "busway cable tap box", where I can connect my cablebus. Then this cablebus will be run to the equipment termination box.

That said, I have two questions:

1. Can I size my cablebus with 90 deg C column now instead of 75 deg C? I don't have any CBs or switchgear that I need to worry about.

2. Can I use the 90 deg C column from the free air ampacity table 310.15(B)(17) or still have to use the 310.15(B)(16)? Am I still under temperature limitation per 110.14(C)?
 
The first question depends on the rating of the cablebus termination box. I think cablebus and their termination boxes are usually suitable for 90C, but you definitely should check those ratings. A manufacturer should be able to tell you. Just remember that anything connecting inside of the equipment (swbd or xfmr) needs to be limited to the 75C column of whatever table you're using.

For the second question, you can, at minimum, use the 75C columns of the charts indicated in 370.80. You can potentially use the 90C columns if the cablebus and termination boxes are suitable for 90C.

A side note; if using the 90C column is only saving you 1 or maybe 2 sets on a short run, I'd consider just adding the conductors rather than having extra boxes and terminations on either end.
 
HarrySlother said:
A manufacturer should be able to tell you. Just remember that anything connecting inside of the equipment (swbd or xfmr) needs to be limited to the 75C column of whatever table you're using.
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I guess the question is, are the "termination box" and "busway cable tap box" considered as equipment and subject to 110.14(C)? I don't think there is any heat rise testing requirement for these boxes like swbd or xfmr?

HarrySlother said:
For the second question, you can, at minimum, use the 75C columns of the charts indicated in 370.80. You can potentially use the 90C columns if the cablebus and termination boxes are suitable for 90C.
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I would like this to be the case but how did you conclude that I can use the ampacity table described in 370.80?
 
elec_eng said:
I guess the question is, are the "termination box" and "busway cable tap box" considered as equipment and subject to 110.14(C)? I don't think there is any heat rise testing requirement for these boxes like swbd or xfmr?
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Yes they are electrical equipment, but they are not standalone devices. Their temperature rating will be based on the equipment they are connected to.
This sounds like custom equipment, have you consulted a manufacturer?
 
elec_eng said:
I guess the question is, are the "termination box" and "busway cable tap box" considered as equipment and subject to 110.14(C)? I don't think there is any heat rise testing requirement for these boxes like swbd or xfmr?
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I don't think anything associated with the cablebus would be considered as equipment in 110.14, maybe someone else knows. I'm sure a manufacturer would know if they're rated for 90C. Or maybe you can find a technical doc for one. Look for something like the note below. I found this on a switchboard technical specification doc - switchboards are explicitly limited to the use of the 75C column by UL.

SWBD Rating.PNG

elec_eng said:
I would like this to be the case but how did you conclude that I can use the ampacity table described in 370.80?
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Because you're using cablebus, and that is the cablebus section. If you were using cable tray, you would use the ampacities/tables noted in 392 instead.
 
HarrySlother said:
Because you're using cablebus, and that is the cablebus section. If you were using cable tray, you would use the ampacities/tables noted in 392 instead
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yes but you are terminating to the equipment ( boxes in this case) so if this termination at the box subject to 110.14 then, the ampacity in the 370.80 is useless. Just like the termination at the switchboard you noted.
 
jim dungar said:
Their temperature rating will be based on the equipment they are connected to.
This sounds like custom equipment, have you consulted a manufacturer?
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Jim,

Didn't know termination box would need a temperature rating before. What kind of standard or testing they need to comply/perform to get this temperature rating?

I am trying to get that information from them yet but have to jump thru hoops to get that info. It seems they are not that familiar with this issue.
 
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ron said:
Since you are terminating inside equipment,
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Even if this is just a junction box, (I think this termination box is a kind of junction box, unless there is a different UL listing or testing required?), this temp limitation apples?

ron said:
I don't use the free air table. I end up with 310.16 75degC column.
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Even if I use the 90 deg C lug for the termination? Nothing else will be in the enclosure.
 
The short answer is no. You are limited to the 75°C temp *ampacity*.

It has to do with listings and manufacturers not testing for using free air ampacities and 90°C enclosed ampacities.

To my knowledge there isn't a single switchgear manufacturer that currently rates at 90°C.
 
elec_eng said:
Didn't know termination box would need a temperature rating before.
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Every enclosed set of conductors will have some temperature rating. I am surprised your equipment engineers can't help you. Are they sizing your lug landing pads and bus bars by current density?

Bus duct manufacturers used to make open air lug terminations for pole style transformers.
 
jim dungar said:
Bus duct manufacturers used to make open air lug terminations for pole style transformers.
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This is good to know. I will check with busduct manufacturer on this. Thanks.

I don't know how they size the land pads/bus bars , that's something that I need to check with them. Will that make any difference? What do I need to ask in order for this to work then?
 
elec_eng said:
yes but you are terminating to the equipment ( boxes in this case) so if this termination at the box subject to 110.14 then, the ampacity in the 370.80 is useless. Just like the termination at the switchboard you noted.
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That is a separate issue. You can still use the ampacity tables noted in 370.80. You may be limited to the 75C column in those tables though. (110.14 determines what column you need to use, not what table).
 
HarrySlother said:
That is a separate issue. You can still use the ampacity tables noted in 370.80. You may be limited to the 75C column in those tables though. (110.14 determines what column you need to use, not what table).
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110. 14(C) specifically says we have to use Table 310.15(B)(16) for termination.

1747320375798.png
 
elec_eng said:
110. 14(C) specifically says we have to use Table 310.15(B)(16) for termination.

View attachment 2577582
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Aw thanks, I breezed right past that in the spec I even snipped as well 😅

That seems strange though. I would have thought the temperature rating of the run is what would matter. Using the 75C column for free air ampacity in appropriately spaced tray, or the 75C column for "normal" ampacity in conduit should result in the same temperature of the conductor/terminations.
 
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