I'm doing some remodeling in our basement, and came across the following (see pictures). It looks like the contractor simply wrapped tape around the conduit to secure the box to the wall.

How are exterior boxes usually secured to an exterior wall? What is the "correct" way to prevent weather and/or critters from accessing the hole in the siding? I saw a similar post that mentioned caulking the box to the exterior wall, but is that how it's usually done? Are there any IBC codes that apply in this situation?

inside outside

4 Answers 4


The IRC or the NEC doesn't apply here, common sense does. What you need to do is apply a paintable silicone acrylic or silicone caulk to the back of the box and around the entry hole before you screw the box to the wall. There should be holes inside the box or external ears for mounting. The idea is to stop water from going behind the box and around the conduit into the wall. Don't be afraid to use it, you can wipe off the excess with a damp cloth. Good luck.

  • 1
    also, for the wire itself coming out of the conduit, they make plastic inserts to protect the wire.
    – longneck
    Jun 13, 2011 at 2:03
  • yes there are plastic bushings you can use. I see from the pic that it is old wire. I would give it a good wrapping of electrical tape where it exits the conduit as well. Looks a little frayed. Jun 13, 2011 at 4:25
  • The cable is old enough that it probably doesn't have a ground wire, so it should be replaced. Jun 25, 2011 at 3:32
  • A lot of exterior boxes seem to have built-in female adapters - does this mean I need to assemble the entire box+thread adapter+conduit at once before installing it through the penetration?
    – alexw
    Apr 21, 2020 at 18:39
  • Thinking that there should be a small gap in the caulk at the bottom, providing an escape path for any moisture that makes its way into the gap between the box and wall. Aug 26, 2021 at 13:29

You can go the caulking route, but I personally like a putty like material called duct seal for sealing many of my protrusions (click the picture for the product link):

duct seal

When you work it in, leave a lot of it around the conduit outside of the wall so that when you reattach the box to the house, it squeezes the material to create a weather tight seal. Should you decide to caulk around the perimeter of the box, I would leave an opening at the bottom of the box to allow moisture to escape. Because more importantly than keeping the spiders out, you want to avoid trapping moisture and causing wood rot.

  • 1
    Moisture always finds a way in. I recall a story about power car window motors given a waterproof coating: cars heated up in the sun, then were cooled abruptly by water in a car wash. This caused enough vacuum that water was sucked in between the strands of the motor wires. The moisture didn't escape the same way. The motors corroded so badly they could be poured out of the waterproof coating. Aug 26, 2021 at 13:37

There are lots of ways to run cable through the sheathing as seen in the picture, but there are some issues with what's shown.

It looks like they used short rigid nipple as protection around the cable. Nothing wrong with that, as long as you do it right. The nipple doesn't extend too close to the inside finish wall, which is good. You'd need a proper bushing where the cable exits to protect the cable from abrasion. A threaded plastic bushing that threads onto the end of the nipple, rather than a lump of tape, would be suitable. If the box was securely mounted to the wall from the outside, and the space between the nipple and the hole filled with silicone sealant or duct seal, then it would be OK.

Although it's done as shown all the time, technically the NM-B indoor cable is not rated for the outdoor location in the box. It should be a UF cable. If you want to be super picky, I bet that nipple is a plumbing pipe nipple rather than an electrical conduit nipple.

A more common way to do this would be to use a cable connector fitting on the back of the box, instead of the nipple:

UF connector

Again you'd need to seal between the connector and the hole in the wall with silicone or duct seal to ensure that water doesn't seep in, and fasten the box securely to the outside wall.


The best way is to clean the area behind the box on the outside wall.. some sand paper make it kinda' rough. Use water resistant silicone and apply around the pipe area and on the edges of the square. Use screws and faster the box to the (looks like wood so use some longer wood screws about 6) and fasten it tightly to the wall. Apply water repellent silicone to the outside permiter of the box after you fastened, use your finger to create a curved area- don't be shy , all around the box one with no breaks. On the inside you can just put more silicone in the pipe where the wire is running.. will stop any spiders and possibly some minor amount of dampness.

This answer is based on experience. Home regulations only mention that exterior boxes need to be well insulated from water and in some countries grounded to a earth leakage breaker is applicable.. especially if you get freezing winters. Otherwise this box will stay fastened to your wall forever and ever.

  • Completely sealing the conduit is fine, but I wouldn't seal the entire perimeter of the box to the wall. If you decide to seal any of the perimeter, at least leave some opening in the bottom to allow moisture to escape so you don't create a moisture trap.
    – BMitch
    Jun 13, 2011 at 12:30
  • I suppose it depends on the climate. In south Africa it will be ok in Britain. Sure. It's always rains.
    – Piotr Kula
    Jun 13, 2011 at 21:30
  • See story about the sealed car power window motors. No water could get in! Except it got sucked in between the strands of the wires, and then couldn't escape. So the motors 'melted' (corroded away). Aug 26, 2021 at 15:00

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.