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Holes in joists for cables - multiple small or single big?

Holes in joists for cables - multiple small or single big?


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Old 10-02-13, 07:58 AM
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Holes in joists for cables - multiple small or single big?

This is part of project to get back about 2 inches of basement headroom by changing from a drop to flush ceiling. I have up to 10 MC cables to run through several 70 year old 10" floor joists. So my question: Structurally is it better to have four 1" holes or a single bigger 2" hole? Either way it is the same area removed from the joist.

This is aside from any electrical bundling/derating issues.
 
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Old 10-02-13, 09:46 AM
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This is what I found....

Size? The holes can not be larger in diameter than 1/3 the actual depth of the joist. For a 2x10, that would mean a hole no larger than 3-5/32" diameter.

As for location:
1- You are not allowed to drill in the middle third of the span at all;
2- No part of the hole can be within 2" of either edge of the joist; and,
3- Maybe most often overlooked - no part of the hole can be within 2" of another hole (or edge of a notched area).


Seems like 2" would be fine unless it would impact the cables somehow.
 
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Old 10-02-13, 12:06 PM
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Structurally is it better to have four 1" holes or a single bigger 2" hole? Either way it is the same area removed from the joist.

This is aside from any electrical bundling/derating issues.
Structurally it may not matter. Just follow the advice Gunguy45 posted.

In terms of electrical code requirements, you should not run more that 3 cables through the same set of holes. I drill 3/4" or 7/8" holes 1' apart for running cables.
 
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Old 10-02-13, 12:28 PM
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And notice the last line in the info I found....

3- Maybe most often overlooked - no part of the hole can be within 2" of another hole (or edge of a notched area)
I don't know where that is in the code, but I found the info on an electrical Forum. I've seen recommendations to drill smaller holes at least the width of the holes apart. Apparently that wouldn't pass. Maybe inspectors aren't that picky or don't know themselves. Nashkats would easily pass, but you prob don't need to go that far.

Also...I doubt the typical DIY'r has a Hole Hawg and a 2" bit, but I'll bet most have multiple 1" and 7/8" bits laying around.
 
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Old 10-02-13, 03:06 PM
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All the above information is correct. I've seen few electricians follow those rules. Most just drill so close to the edge its frightening. But just as an exercise, think about bending a board enough to break it. Where does all the stress and material displacement take place? Not in the middle but at the edges. And at the edges is where the wood would begin to splinter and break. So tasking away material at that area is not a good idea. I always drill in the middle of the member. Same thing with steel bridge members. Notice all the holes in the middle? Less weight but still strong.
 
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Old 10-02-13, 05:55 PM
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You are confused on "no drilling" in middle 1/3 of span....no notches is correct, though. Chapter 5 - Floors

Here is the other code requirement; Chapter 5 - Floors

From the lumber manufacturer, Fig. 1- holes in middle third; http://arch.umd.edu/Tech/Structural_..._Guide_A11.pdf

Gary
 
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Old 10-03-13, 06:22 AM
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So still nothing saying that multiple holes are different than fewer, bigger holes as long as the rules are followed. I've been doing small but after seeing what plumbers do was thinking might be easier in the long run with a single large hole per joist.

Yes, most don't have hole hawgs but a lot of the multi-piece kits people get for Christmas include a basic right-angle drill.
 
 

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