Is it OK to drill vertically through a joist?
Don't do it.

The reason that you can drill through the face of a beam in the center is because the bending stress is essentially zero there.



If you drill from the top to the bottom, you're removing the portions that carry the brunt of the loading (the top and bottom of the beam). If a joist that big is sistered, there's a good bet that it's a major load bearing beam section that you don't want to mess with.

Find a different route for your wire. Ask yourself this question: Is it worth risking major structural problems in order to hide one unsightly wire?

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