MC Bundling Question

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Dean83169

Dean83169

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I have read through the posts on this topic but still I am unclear of the answer.
In commercial buildings where we have the concrete ribbed decking, usually you have clear paths of ribs which will run the entire length of the floor. In using these ribs for homeruns of MC cables, and of multiple sizes, (6-4, 6-3, 12-2, 12-3, 12-4, 8-2 MC's) I am not actually tying these together but using the beams and unistrut horizontal to the rib to keep the cables in the ribs between the 6' span of I beams. Can someone let me know if this falls into the bundling category and how should I calculate this. The power is 3 phase 208 volt. Any help on this would be great.
 
Are the cables tied together? Or is it a long parallel run of how many cables?
Keep in mind for derating you use the 90C rating of 30 amps for 12 AWG and derate down from there, if you go below 20 amps and you need 20 amps, then you go up a size.
 
The question is are these cables considered bundled or not. If they are bundled for more than 24" then they must be derated. If derating applies the adjusted ampacity can be quite complicated since #12 AWG cables bundled and derated according 310.15(B)(2)Ex5 can have an adjustment factor of 60% in some cases. Other sized cables will have an ampacity adjustment based on table 310.15(B)(2)(a).

The exception in 310.15(B)(2)(Ex.5) negates any derating for #12 AWG bundled cables that aren't feeding multioutlet receptacles. So single receptacle circuits or lighting circuits effectively can be bundled without any effect on their ampacity.

For example:

25-12/2 MC cables feeding single receptacles or lighting would contain 50 current carrying conductors but would only require a derating factor of 60%. {310.15(B)(2)Ex.5}

30 amps(60%)=18 amps. Round up to next standard size breaker =20 amps.

So for these types of loads #12 MC cable could still be bundled and derating would not affect the final allowable ampacity of the circuits.

If however the bundled cable were feeding multi-receptacle circuits, than the final circuit ampacity of 18 amps would have to go on a 15 amp circuit breaker since rounding up to a 20 amp CB wouldn't be permitted.
 
Dean83169 said:
I have read through the posts on this topic but still I am unclear of the answer.
In commercial buildings where we have the concrete ribbed decking, usually you have clear paths of ribs which will run the entire length of the floor. In using these ribs for homeruns of MC cables, and of multiple sizes, (6-4, 6-3, 12-2, 12-3, 12-4, 8-2 MC's) I am not actually tying these together but using the beams and unistrut horizontal to the rib to keep the cables in the ribs between the 6' span of I beams. Can someone let me know if this falls into the bundling category and how should I calculate this. The power is 3 phase 208 volt. Any help on this would be great.
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If the cables are run parallel to each other then I would not consider them bundled, however, if they are laying on top of one another then they are bundled. The idea is that there needs to be air space around the cables so that they can dissipate heat. Just make sure that have some breathing room.
 
Hi guys, I am new to the forum this is my first post. Correct me if I am wrong but aren't you allowed up to 20 current carrying conductors before the derating must be taken into account? In this case you would be allowed to pull up to 10 12-2 MC's or 6 12-3 MC's without derating anything. 310.15(B)2 exception No. 5
 
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Tommy
Welcome to the forum. 310.16 is based on three current carrying conductors. Refer to 310.15(B)(2)(a) for derating more than 3 current carrying conductors. For 20 conductors you must derate 50%! The question here is if this installation qualifies as the conductors being "bundled". Sometimes it comes down to a judgement call.
 
mc

mc

If you can keep the mc's spaced apart in your installation you would't have to derate. More than likely you fishing them down the cell, so that would be hard to prove one way or the other, its going to be the AHJ's call.:smile:
 
TommyC said:
Hi guys, I am new to the forum this is my first post. Correct me if I am wrong but aren't you allowed up to 20 current carrying conductors before the derating must be taken into account? In this case you would be allowed to pull up to 10 12-2 MC's or 6 12-3 MC's without derating anything. 310.15(B)2 exception No. 5
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The 20 CCC exception only applies to #12 AWG cables, so in your example you would derate after you hit 20 at a derating of 60%.

In the OP he mentioned various sized cables. These would have to comply with the derating factors in table 310.15(B)(2)(a).
 
The cables are laying in the rib not tied or tywrapped nor in bridle rings. The code only states these ways to be considered bundling. I may have 1-6/4, 10-12/3, 3-12/2, 2-12/4 mc's in the rib. I am curious as to the code and not what or how its interpretted by others, even though the AHJ has his own. I am looking to clarify this so I know what is right and what is wrong according to the NEC. Thanks everyone.
 
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