Suppose I have a meter bank in the basement and I want to feed all apartments with MC cable coming out of the meter banks. How would I support the MC cables with cable tray and maintain adequate spacing? If I bundle MC cables in groups of 3, how can I space them out in the cable tray?
Supporting MC cable with Cable Tray
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masterinbama
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Calculate the size of the cables, and size the tray accordingly
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That’s not my question, would I have to derate the cables if I put too many in the cable tray? What conditions do I have to look out for and how do I space the cables accordingly so that it doesn’t touch other cables?
masterinbama
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That was my answer.
Tell me how many runs you have, and what size they are. And I will calculate the tray needed.
Defeats the purpose, I suppose.
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Tell me how many runs you have, and what size they are. And I will calculate the tray needed.
Defeats the purpose, I suppose.
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Suppose I’m running 55 2/0 3 conductor + ground MC cables. And what cable tray do you recommend? How do I maintain spacing between MC cable? Don’t I have to derate cable if I have a bunch of them in a cable tray?
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winnie
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- Electric motor research
IMHO it isn't clear what spacing is required to avoid derating.
There are specific spacing requirements to permit the use of the 'free air' rating, but if you are operating using the normal cable rating you simply need to avoid bundling.
Jon
There are specific spacing requirements to permit the use of the 'free air' rating, but if you are operating using the normal cable rating you simply need to avoid bundling.
Jon
- Location
- Illinois
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- retired electrician
See 392.80(A)(1). MC cable in a tray will not require an ampacity adjustment as long as the tray is filled per 392.22.
If you have a solid cover on the tray, you will have to use a ampacity adjustment factor of .95.
If you install the cables in a single layer with a one cable spacing between cables you would be permitted to use the free air ampacities, however, the rules in 110.14 do not permit the use of the free air ampacities at the terminations.
If you have a solid cover on the tray, you will have to use a ampacity adjustment factor of .95.
If you install the cables in a single layer with a one cable spacing between cables you would be permitted to use the free air ampacities, however, the rules in 110.14 do not permit the use of the free air ampacities at the terminations.
Julius Right
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See also:
Vertical MC Cable Support in the 2014 NECThe 2014 NEC has added language that affects only metal-clad (MC) cables with phase conductors 250 kcmil or larger. You may think of MC cable as primarily a branch circuit wiring...
iaeimagazine.org
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Cable support fastenersEaton B-Line series cable support fasteners are used to support metal clad cables (MC), armored cables (AC), non-metallic cables and flexible conduit in new or retrofit commercial construction.
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You mean something like this? I don’t see how a single
layer could ever be interpreted as “bundling”, especially with the larger sizes
The cable tray rules allow for “random wiring” only for control, single layer spaced 1 cable diameter apart, and bundled in groups of three with 1.5 diameter air gap. This also includes multi conductor cables like MC. Tie wraps is how you maintain spacing.
One of the ambiguities is whether the “one diameter” rule means using the jacket OD or the conductor OD which depending on jacket like MC can be a LOT smaller. Since the Code is ambiguous use the smaller diameter. With MC this could mean no actual gap,
There are derating rules for multiple layers, too. Just like exceeding 6 current carrying conductors in conductor I suggest not going there.
How much do I have to derate if I have another set layered on top of the mc cable?
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- Illinois
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- retired electrician
I don't see anything in 392 that requires an ampacity adjustment for more than one layer of multi-conuctor cable.
Ok suppose I have like (55) 3 conductor + ground MC cables, what size cable tray would I need? And there will be no derating?
wwhitney
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- Retired
Not very familiar with Article 392, but my impression is that the requirements of 392 will end up requiring a width of cable tray sufficient for a single layer for this application
Looking at (2017) 392.22(A)(1), "Ladder or Ventilated Trough Cable Trays Containing Any Mixture of Cables," part (b) applies for all conductors smaller than 4/0 AWG and says to see Table 392.22(A) Column 1. The biggest entry there is for a 36" wide tray, with an allowable fill area of 42 in^2. (2) of those is not quite enough. You could use (5) 16" cable trays, each one can be filled to 18.5 in^2, which is enough for 11 of the cables. And 11 * 1.42" = 15.6", so presumably they'd lay in a single layer. [(4) 20" cable trays would also work.]
At least if I've read Article 392 properly. One thing I'm not clear on is any stacking rules for cable trays, could you stack those 5 on top of each other for an assembly that is only 16" wide?
Cheers, Wayne
I looked up one web page with this cable in Al, the overall diameter is 1.42". So each cable has an area of 1.584 in^2, with a total area of 87 in^2 for all 55 cables.
Looking at (2017) 392.22(A)(1), "Ladder or Ventilated Trough Cable Trays Containing Any Mixture of Cables," part (b) applies for all conductors smaller than 4/0 AWG and says to see Table 392.22(A) Column 1. The biggest entry there is for a 36" wide tray, with an allowable fill area of 42 in^2. (2) of those is not quite enough. You could use (5) 16" cable trays, each one can be filled to 18.5 in^2, which is enough for 11 of the cables. And 11 * 1.42" = 15.6", so presumably they'd lay in a single layer. [(4) 20" cable trays would also work.]
At least if I've read Article 392 properly. One thing I'm not clear on is any stacking rules for cable trays, could you stack those 5 on top of each other for an assembly that is only 16" wide?
Cheers, Wayne
I see thanks you. Can I use 2 cable trays 42” wide instead of 5 16”?
wwhitney
Senior Member
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- Berkeley, CA
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- Retired
There's no entry for 42" wide cable trays in Table 392.22(A). If there were and that size is available, it would work, as (2) 36" is close to large enough (each one can hold 26 cables 1.42" in diameter per the table). From the sizes in that table, you could use (3) 30", (4) 20", or (5) 16".
Cheers, Wayne
- Location
- Illinois
- Occupation
- retired electrician
The B-Line catalog does not show any widths greater than 36".
I found an excel sheet calculator at eaton website and they recommended 42” for what I was trying to go for
https://www.eaton.com/content/dam/e...ire-mesh-basket/cable-tray-sizing-program.xls
- Location
- Illinois
- Occupation
- retired electrician
Interesting....since it is not shown in the code book, you would need to check with your AHJ on the use of tray wider than 36".
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