Lots of romex through 1 set of holes...

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Old 01-29-07, 08:24 AM
jn
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Lots of romex through 1 set of holes...

Can someone tell me the various derating percentages found in table:
310.15 (B) (2) Adjustment Factors
for derating multiple CCC's?
If you want to know why I want to know- read on--

In my area, I have been seeing a trend where electricians are pulling most of their wiring through large sets of holes that run the length of the entire house. (Trusses are similar to Silent Floor Systems engineered floor joists)
I've seen multiple sets holes that are about 3" in diameter, about a foot apart, drilled from one end of the house to the other. I can't tell you exactly how many cables I've seen in one hole, but its a bunch. The hole wont be tightly filled, but say 2/3rds full (at the panel end) before they start using the next set. This is often resulting in a stack on wires pulled together for 50 feet or so. (Of course runs are branching off and running with the joist every so often. They may be pulling 10 sets of wire at a time during rough in.

From some past posts I saw:
310.15 (B) (2) Adjustment Factors.

The NEC does say that when more than 3 current carrying conductors are in the same raceway or cables (like romex) are bundled leaving a total number of current carrying conductors over 3 we need to de-rate the conductors.

...the table in the code says for 7 to 9 CCC we need to de-rate the conductors to 70 percent of the original value. (4-6 is 80 percent, 7-9 is 70 percent 10-20 is 50 percent) To calculate this we take the value in the 90 deg columb for the wire size we plan to use on the correct chart from the code, and multiply by the adjustment factor. If the value in our answer is lower than the breaker we had chosen, we can either use a smaller breaker or a larger wire and try the calculations again...

Now..what I am seeing is possibly 30 CCC's or more in a run over 24".
Can someone tell me what the NEC's:
Table 310.15 (B) (2) Adjustment Factors
says for more and more CCC's or does derating stop at 50%?
 
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Old 01-29-07, 09:51 AM
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4-6: 80%
7-9: 70%
10-20: 50%
21-30: 45%
31-40: 40%
41+: 35%

I think the crux of the issue is that the cables are not tightly confined when they run through engineered trusses. Therefore, the traditional derating factors which were designed with sealed conduits in mind probably should not apply so strictly to the loosely bundled cables. Clearly, the cables will have a convection cooling effect that similar conductors in a conduit would not have. Many inspectors follow this line of reasoning when they allow numerous cables installed loosely through trusses.

I personally think it's more unsafe to bore unnecessary holes through engineered members than to loosely bundle Romex through the large knockouts.
 
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