Beatrice Lang
Running electrical conduit under concrete is a common task for electricians and DIY enthusiasts alike. It involves creating a path for electrical wires to travel, usually by installing a conduit before pouring a concrete slab. The type of conduit used is important, with PVC being a popular choice due to its weather-tight properties and resistance to corrosion from acidic concrete. Other types of conduit include Cor-line/smurf tube/ENT, EMT, and rigid pipe. It is also important to consider the depth of the conduit, with some sources recommending a minimum burial depth of 12 inches under walkways and slabs. Running electrical conduit under concrete requires careful planning and may involve techniques such as trenching, drilling, or using a vacuum to pull wires through.
| Characteristics | Values |
|---|---|
| Time of installation | The best time to run conduit is before pouring the concrete slab. |
| Type of conduit | PVC, ENT (smurf tube), EMT, Cor-line, rigid pipe, FMC "flex" |
| Depth | If the slab is 4" or thicker, the conduit can be placed right under it. If it is thinner, then a 12" burial is required. |
| Width of trench | 3-5 inches |
| Installation method | Conduit can be laid in a trench and then covered. Alternatively, a hole can be dug on each side of the slab, and a steel pipe pushed through from one side to the other. |
| Bend limitations | Limit yourself to a total of 270 degrees of twist (three 90-degree bends). Beyond this, pulling wires through becomes much more difficult. |
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What You'll Learn
Use PVC conduit as it's weather-tight and doesn't corrode
When running electrical conduit under concrete, it is advisable to use PVC conduit. PVC stands for polyvinyl chloride, a plastic material that is lightweight, cost-effective, and resistant to various environmental factors.
PVC conduit is highly resistant to water and chemicals, making it ideal for underground installations and areas with high moisture. Unlike metal conduits, PVC does not corrode or rust, ensuring the long-term protection of electrical wiring even in humid or submerged conditions. This makes it perfect for use under concrete, where it will be exposed to high moisture levels and potentially corrosive substances in the soil.
PVC conduit is also permitted in locations subject to severe corrosive influences and where it is subject to chemicals for which the materials are specifically approved. It is one of the most widely used materials for electrical installations, especially in wet and outdoor environments.
However, there are some limitations to using PVC conduit. Extreme cold may cause PVC to become brittle and more susceptible to damage. Additionally, while PVC conduit offers excellent water resistance, it may not be suitable for extreme weather conditions, impacts, or long-term exposure to UV rays.
When installing PVC conduit under concrete, it is important to follow the necessary safety precautions and local regulations. Ensure that the conduit is properly sealed and protected to prevent any water ingress and potential corrosion of the wiring.
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Install conduit before pouring concrete
The best time to install electrical conduit is before pouring a concrete slab. This allows you to place the conduit in a protected location under the slab. The process involves several steps and careful planning to ensure a successful installation.
Firstly, it is essential to determine the location of the conduit and mark it out with a chalk line. Make sure to wear protective gear, including gloves and eye protection, during this process to avoid any injuries. Along the marked line, dig a trench with a depth of around 3 to 5 inches, removing any gravel and large rocks, and ensuring the base of the trench is level. This trench will accommodate the conduit.
The next step is to choose the right type of conduit. PVC conduit is a popular choice for this application due to its weather-tight properties and resistance to corrosion from acidic concrete. It is wise to install a conduit that is one size larger than strictly necessary to accommodate future expansion of the electrical system. Place the PVC conduit into the trench, ensuring it runs smoothly from the start to the end of the marked line.
Once the conduit is in place, fill the trench back in with the material that was removed earlier, carefully packing it down to ensure stability. This process secures the conduit in place, protecting it from damage and ensuring it remains hidden beneath the slab.
It is important to note that the depth requirements for conduit burial vary depending on the situation. For walkways, a burial depth of at least 12 inches is typically required. However, if the slab is made of poured concrete with a thickness of 4 inches or more, the conduit can be placed directly under the slab without the need for deeper burial.
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Bury conduit under the slab to avoid cracks
Burying conduit under a concrete slab provides a protected path for electrical wires. It is recommended to install the conduit before pouring the concrete slab. This method is also known as under-slab conduit installation.
To begin the installation process, mark the desired path of the conduit by snapping a chalk line from the start to the end of the intended conduit run. Along this line, dig a trench with a depth of around 3 to 5 inches, ensuring that you remove any large rocks and maintain a level base. It is important to note that the required burial depth may vary depending on local codes and regulations. Some sources suggest a minimum burial depth of 12 inches for walkways and thinner slabs, while others recommend placing the conduit just under the dirt beneath the slab. Check with local authorities or refer to specific code requirements, such as the National Electrical Code (NEC), to determine the appropriate depth for your installation.
After preparing the trench, lay the conduit in place, ensuring it runs smoothly from the start to the end of the trench. It is advisable to use PVC conduit, which is commonly used for under-slab installations due to its weather-tight properties and resistance to corrosion from acidic concrete. Additionally, consider using a conduit one size larger than strictly necessary to accommodate future expansion of your electrical system.
Once the conduit is in place, carefully fill the trench with the material you removed earlier, ensuring that the conduit is securely covered. This method of burying the conduit under the slab helps to avoid potential cracks that could form if the conduit were embedded within the concrete. By placing the conduit beneath the slab, you provide a protected pathway for electrical wiring while minimising the risk of concrete cracking due to the presence of the conduit.
For existing concrete slabs, it is possible to run conduit underneath by employing trenchless methods or using a thrusting tool to create a small excavation at each end. Alternatively, you can dig a hole or trench on one side of the slab and tunnel underneath, pushing or driving a steel pipe through to the other side. These methods allow for conduit installation without disturbing the existing concrete, providing a neat and effective solution for running electrical wiring.
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Use a trenching service for long installations
If you have a long electrical installation to do under a concrete path, it is much easier to use a trenching service than to do it yourself. They will only need to make a small excavation at each end (about a spade's width) and the whole process will take about an hour. They use a thrusting tool to push the conduit through with ease. This method is much more efficient than other methods and is very impressive.
If you choose to do it yourself, you will first need to dig a trench. The depth of the trench depends on the thickness of the concrete slab. If the slab is 4 inches or thicker, the conduit can be placed just under the slab. If it is thinner, then the conduit must be buried at least 12 inches deep. You should also be mindful of any water/electrical separation requirements.
After digging the trench, you can lay the conduit in place and fill the trench back in. It is wise to install a conduit one size larger than you need to accommodate future expansion of the electrical system. PVC conduit is typically used for these applications because it is weather-tight and doesn't corrode due to exposure to the acidic concrete.
If you are running electrical wire through the conduit, you can use a vacuum to run a string through the conduit and pull the wire through. Alternatively, you can use fish tape to pull the wire through. You should limit yourself to a total of 270 degrees of twist (three 90-degree turns) as beyond this, the pull becomes much more difficult.
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Avoid FMC flex and use ENT (smurf) instead
When it comes to running electrical conduit under concrete, it is important to consider the type of conduit that can be used. FMC "flex" conduit is not suitable for this application. FMC stands for Flexible Metal Conduit, and while it is flexible, it is not meant to be embedded in concrete. Using FMC in concrete may not meet local electrical codes and could cause issues with inspectors.
Instead, it is recommended to use ENT, also known as "smurf" tube. ENT stands for Electrical Nonmetallic Tubing and is specifically designed to be used in concrete installations. It is important to follow local electrical codes when installing ENT. In some cases, a steel plate or sleeve may be required to protect the conduit, but this is not always necessary. ENT is flexible and easy to work with, making it a popular choice for electrical installations.
One of the benefits of using ENT (smurf) tubing is that it doesn't rust like some other types of conduit. This makes it ideal for use in concrete, where moisture could be an issue. ENT is also designed to replace EMT (Electrical Metal Tubing) and other flexible metal conduit systems. It is important to note that smurf tubes are typically listed for low voltage applications, so for line voltage wiring, additional considerations may be needed.
When installing ENT (smurf) tubing in concrete, it is important to keep the tubing as straight as possible to avoid making installation of wires difficult. This can be achieved by frequently tying the tubing to avoid many small bends. The same principle applies when installing ENT inside a stud wall. Additionally, it is important to limit the number of elbows or sweeps in each run to ensure a clean and neat job.
In summary, when running electrical conduit under concrete, it is best to avoid using FMC flex conduit and opt for ENT (smurf) tubing instead. ENT is designed for concrete installations, is flexible and easy to work with, and won't rust over time. By following local electrical codes and proper installation techniques, ENT can provide a safe and effective solution for running electrical conduit under concrete.
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Frequently asked questions
The best time to run electrical conduit is before you pour the concrete slab. You can use PVC conduit, which is weather-tight and resistant to corrosion from exposure to acidic concrete. It is recommended to install a conduit one size larger than needed to accommodate future expansion of the electrical system.
Cor-line/smurf tube/ENT and PVC are the most commonly used conduits in concrete. EMT was once popular, but it has fallen out of favour due to the ease of use and rust resistance of ENT. Rigid pipe is also an option, but it requires threading.
The depth of burial depends on the thickness of the slab. If the slab is 4" or thicker, the conduit can be placed right underneath. If the slab is thinner or made of a different material, a burial depth of 12" is required.
Yes, it is possible to run electrical conduit under existing concrete. You can either dig a trench on one side and tunnel under the path or hire a trenching services company to install the conduit with minimal excavation.
