Romex -- Cabling Effect

Old 02-06-02, 07:54 AM
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Romex -- Cabling Effect

I am in the process of finishing my basement and am beginning the wiring phase. I saw something brief on this website that talked about cabling effect and the lowering of amperage.

Can anyone explain more about cabling effect or know of a website that describes it better?

My main questions are:

1) How many romex cables (12-2 and/or 14-2) can be put through a single hole in celing joists?

2) If I drop a ceiling will that allow more cables to be run through a single hole (assuming the cables then have room to breath and not get too hot)?

3) What actually causes the loss of amperage in cabling effect?

Thank you!
Old 02-06-02, 10:19 AM
Join Date: Sep 2000
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1. No limit. But don't cram them in. And several smaller holes will weaken the joist less than one larger hole. Make sure you drill the holes in the dead center (vertically) of the joist. If possible, don't put a hole in the middle third of the span -- as near to the ends as possible is better.

2. If you have a drop ceiling, you won't need the holes at all.

3. Nothing causes a loss of amperage. I assume you mean a loss of ampacity (a wire's ability to safely carry a certain number of amps). This is caused by heat buildup of cables in near proximity to each other, due to the fact that they cannot dissipate heat as readily. But no derating is necessary simply because the cables go through the same hole.
Old 02-06-02, 10:49 AM
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I believe what you are referring to as "cabling effect" is a statement about 310-15 (2) (a)
" More then three current carrying conductors in a Raceway or cable. Where the number of current carrying conductors in a raceway or cable exceeds three or where single conductors or multiconductor cables are stacked or bundled longer then 24 in. (610mm) without maintaining spacing and are not installed in raceways, the allowable ampacity of each conductor shall be reduced as shown in Table 310-15(b)(2)(a)"
If you place more then three romax in a bundle and not allow proper space for more then the 24 in then you create this effect and you have to derate.

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