Running Electrical Wire Through Cinder Blocks

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Every DIY-er out there has to face the inevitable challenge of running an electrical wire through a brick wall or concrete. This can either be a new construction home or an existing one. Depending on whether or not you can do this correctly will determine your level of skill.

One of the biggest problems that you will most likely face when working on your project is Code compliance. Of course, you can run a wire anywhere, but knowing whether code allows it or not is the question. A question that arises here is it okay to run electrical wire through cinder blocks? If so, how can you do it?

You should be okay when running a wire through a cinder block so long as it’s an underground feeder (UF) wire. You should, under all circumstances, avoid using a non-metallic (NM) wire such as a Romex. If you are wiring a new construction home, you won’t have to break the wall – just drill into it. However, if you are planning on running electrical wire through a cinder block that has been filled up, you are in for a tough time.

Running an Electrical Wire Through Cinder Blocks

Cinder block closeup

One of the primary reasons why you might want to run an electrical wire through cinder blocks is if all your house’s walls, including the internal walls, are made of the same. Let’s say you want to run a fridge wire through the walls into one of the rooms. Your best bet would be to either run the same through the attic or basement.

You also have another option: running the wire through your walls. You may need to drill holes or even start breaking it a bit to run the wire and plaster the wall again. However, another option you may have is that of running the wire through the cinder blocks. The best time to do that is when your contractor is laying these blocks.

The problem is that if you are passing BX 12- or 14-gauge wires through a cinder block is against Code, regardless of whether it’s NM or UF wire. Instead, Code recommends that you put a sleeve through the block first to act as a conduit. This sleeve can be either EMT or PVC. Avoid metal conduits since they tend to erode and therefore may introduce openings leading to your wires.

When running these conduits through cinder blocks, it is important to remember that under no circumstances should you splice a wire that you are about to run through it, nor should you attach a junction box in the middle, i.e., where you don’t have direct access to it.

Some cinder blocks are firewall blocks, in which case you will need to caulk openings with a fire-rated caulk if possible. Otherwise, you can use any type of caulk.

The best course of action in our opinion would be to mount a junction box only at the sides of the cinder block, i.e., the entry and exit point, and then run a straight length of wire from box to box. You can then run the wire anywhere else from there – even if you want to run NM wires.

Plastic bushings at both ends are also a great way to protect the wire, so long as they are placed on both ends. This will protect your wire from the conduit, should the corners (or the junction boxes) heat up too much.

How to Run Wires Through Cinder Block

The first and most important consideration you need to make when running electrical wire through cinder blocks is to check whether the blocks have been filled or not. Contractors or builders usually don’t bother filling these holes on their own, unless the building or wall needs to be extremely sturdy, such as a retaining wall.

However, DIY-ers often fill these walls up, effectively making it impossible for wires to run through.

You can either ask the builder about whether or not they filled it up or simply tap the cinder block to check it – however, the latter method requires a lot of experience. You can also choose to drill a hole in the wall to check whether the blocks have been filled.

You will need;

  • A drill
  • Plastic anchor
  • Junction boxes (2)
  • conduit
  • A feeder wire
  • A wire stripper
  • Some electrical tape
  • A chisel
  • A hammer

Depending on the complexity of your task, you may also need more tools. For example, if the opening (or exit) of your cinder block holes are covered with tiles, to make sure you expose those wholes neatly, you will need an angle grinder to cut those tiles.

To feed the wire,

  1. Start by drilling a hole where you know the cinder block opening is. Again, ask your contractor or builder for a more reliable estimate. In the US, it isn’t uncommon to cover the cinder block with a piece of wood or directly with drywall, thus making it a bit of a trial-and-error process. In the UK, there are bricks with holes in them with a simple layer of paint or concrete in most instances. You can measure where the hole is going to be and start drilling there.
  2. Drill until you reach the hollow part of the cinder block or brick. Mark how far the drill has gone in and then pull it out.
  3. Take your hammer and chisel and carefully start breaking away the excess material. On the other hand, you can also drill more holes or use a bigger bit to open the entryway enough to ultimately push your conduit in.
  4. Once the hole is wide enough, push a bit of the conduit in at a time. Keep track of how much you want to insert and how much is already in.
  5. Now, install the first junction box close to the entry point.
  6. Drill a hole similar to the first one at the end and then widen it just like you did the other side. Here, you can get away with widening it just enough so that you can catch and pull the wire.
  7. Install the second junction box close to the exit point.
  8. Insert the UF wire from one end, making sure it is in the conduit as you push it along. Push it until it comes out the other end or until it touches it.
  9. With a pair of pliers or even tweezers, pull the wire out of the other end. Splice and strip the wire to install it into the junction box.
  10. Connect the wires at the other end as well to the junction box and cut off the excess from both sides.
  11. Now attach the NM wire at both ends, and voila! You are done!

All that remains now is to seal both sides. Now that there is a wire inside it and since there is no joint in the feeder wire, feel free to seal it up with concrete if you wish. However, in our opinion, it is much safer to use caulk or even silicone sealant.

This is because the concrete may fall and damage the wire while in the drying process unless you are a professional and are sure about your ability. When running an electrical wire through cinder blocks, a lot can go wrong pretty quickly, so stay sharp and attentive at all times.  

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About Charlie D Paige

Charlie is a massive DIY fan, with dozens of DIY projects under his belt - ranging from tiling to electrics, and concrete pads to walls. Charlie loves tinkering, seeing how things works, the outdoors and playing with power tools... so is it any wonder that he's completed so many DIY jobs over the years?

Charlie loves spreading his hard-won DIY experience with the world via this blog.