Multiple Drilled Holes in Floor Joists

Does anyone know if there is a maximum number of holes that can be drilled in a joist. I have the general rules on size and location but I can’t find anything on a maximum number. This seems a little excessive and there are two joists that are starting to split.

Thanks in advance.



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Hey Brian, I have that info, I’m trying to find backup for stating the joist is over-bored. It seems they are actually allowed to bore the entire outer 1/3 if the spacing is correct (I measured the joist and the holes). I don’t understand why someone would bore every location on a floor joist that was allowed to be bored. I also don’t see any possibility that this boring hasn’t weakened the joist.

I think that graphic is incorrect as it shows hole distance from the edge as 2" max and it should be 2" minimum.
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Here’s a correct one:

660155802-notching-guide-hero-tcm122-2183894

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That’s correct, according to the WWPA holes shouldn’t be closer than 2 inches from the edge. I read somewhere today (I’ve researched several articles) that holes have to be 2 inches apart. That is exactly what they have done on this joist, drilled it every 2 inches out to almost the 1/3 of the length mark.

not sure off the specs. either, but sure doesn’t look right to me…

Nice catch Matt, glad you got the correct information posted.

Little point in all them holes in a crawl space. Staples would have worked just fine for most of that. Somebody just got a hole saw!

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As far as I know, you can’t staple anything smaller than #8 to the bottom of floor joists. You can either run them through bored holes or install a “runner” on the bottom of the joist and staple to that.

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No one has mentioned the span of the joist. They look like 2x10

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If you mean numerically smaller that makes perfect sense. if you mean physically smaller you need to cite code or quote a source. “As far as I know.” doesn’t carry a lot of weight.

My bad, should have clarified that. Yes, physically smaller.

334.15(C) In Unfinished Basements and Crawl Spaces. Where cable is run at angles with joists in unfinished basements and crawl spaces, it shall be permissible to secure cables not smaller than two 6 AWG or three 8 AWG conductors directly to the lower edges of the joists.

NMC is not physically smaller than 2 - 6awg wires.

2x10, spanning 15 feet

That’s an interesting take and does make me wonder why the NEC mentions number of conductors, and not just the gauge size.

My first exposure to this requirement was many moons ago when the state electrical inspector made me move some 12-2 I had stapled to the bottom of the floor joists in my basement. Said they had to have holes bored.

Maybe @rmeier2 can help clarify the requirement if he can see this thread.

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Those are the minimum conductor sizes and the number of conductors within a cable that are permitted on the underside of the joists. So if the conductors are #6 AWG then a 6-2 cable is permitted, #8 AWG conductors then you would need a minimum of 8-3 cable.

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This seems to be another one of those, “at the discretion of your local AHJ.” I’ve seen rewires in older homes with 14-3 and 12-3 stapled to the underside of joists framing in crawlspaces and basements with signed off permits. It would be interesting to know why the code authors care.

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Thanks Robert.
Any insight as to why this requirement exists in the code? I have heard anecdotally that it is to prevent damage to wiring that could be caused by floor joists flexing. That would somewhat make sense on why it is ok with larger wiring and not smaller, although I can’t imagine a real problem even with smaller wiring. But why the specification on number of conductors in the cable?

From my understanding it has always had something to do with potential damage to the cables (I’ve never heard of the flexing argument). The original requirement for cable smaller than 6-3 or 8-3 to be installed in holes or on running boards was only for basements. The thought was that someone could hang something from the cable and that small NM cables would not be as resistant to damage as larger cables. So it was settled on 6-2 or 8-3 and larger due to the strength afforded to the cable by the conductor sizes.

This is from Mark Ode from UL who addresses why crawl spaces were added to the requirement. I agree with him that a crawl space isn’t the same as a basement and find the crawlspace requirement to be somewhat unnecessary.

Scanning older editions of the NEC, NEC handbooks and other Code-related documents where only the basement NM cabling was addressed, a common concern for an unfinished basement was expressed. The concerns seemed to focus around the storage of clothing on hangers in an unfinished basement with the possible damage that could occur to smaller sizes of NM cables directly attached to the bottom of beams, joists and other support structures. The discussion considered NM cables with two 6 AWG or three 8 AWG conductors (but not smaller cables) substantially large enough to withstand possible damage and subsequently to permit attachment of these larger NM cables directly to the bottom of joists. The explanations stated that hanging heavy clothing on smaller NM cables could cause undue strain at the support staples with possible damage to the NM cables’ internal conductors. This damage could result in a fire, so the smaller NM cables should be installed on running boards or through bored holes in the joists.

The addition of “crawl spaces” to 334.15(C) now requires NM cables smaller than two 6 AWG or three 8 AWG conductors within the cable to have running boards for the cables or have bored holes. The proposal requiring crawl spaces to comply with the same open cable rules as unfinished basements did not have any substantiation of a problem but stated “334.15(C) did not give direction as to the requirements for a crawl space and since the same dangers exist in unfinished basements and crawl spaces, this subsection should apply to both locations.” The two locations do not seem to be equal, since no one would hang their clothes in a crawl space. A crawl space would more equate to an attic area and, if protection of the NM cable were necessary, compliance with 334.23 would be more appropriate. Protection of NM cables within 6 feet of the entrance into the crawl space seems more suitable.

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No IRC code that states how many holes are allowed.

Those joists look beefy. Like 2x12 but actually 2 inches thick.

It probably wasn’t the smartest thing I’ve seen. I’d probably make a statement about the holes. However, I would not elevate it. You mentioned splitting. Was it associated or linked to the holes or something else? I would just mention the splitting itself. Hope this helps.