Molly Ayala
Underground electrical conduits are tubes that house electrical wires and protect them from elements such as moisture, short circuits, rust, impact, electric shock, and fire. The type of conduit pipe used for underground electrical wiring depends on several factors, including soil type, moisture levels, and exposure to corrosive elements. Some common types of conduit pipes used for underground electrical wiring include PVC, HDPE, fiberglass, rigid metal conduit (RMC), electrical metallic tubing (EMT), and flexible metal conduit (FMC). PVC conduits are known for their resistance to moisture, while RMC provides excellent protection in harsh environments but is expensive and challenging to install. EMT is lightweight and bendable but is not suitable for underground use as it can easily become damaged.
| Characteristics | Values |
|---|---|
| Purpose | To protect electrical wires from moisture, impact, chemical vapors, and animals |
| Types | Rigid metal conduit (RMC), electrical metallic tubing (EMT), flexible metal conduit (FMC), HDPE, PVC, CIC, fiberglass, geothermal HDPE, split PVC, GFCI-protected underground feeder cable |
| Factors to consider | Soil type, moisture level, exposure to corrosive elements, voltage, current levels |
| Pipe size | Dependent on the number of wires and wire size |
| Minimum depth | 18 inches for anything other than rigid steel conduit; specific areas may require a minimum of 24 inches |
| Conduit assembly | Plastic conduit assembles with solvent cement; no PVC cleaner is necessary |
| Bends | No more than four 90-degree bends between permanently accessible "pulling points" in a complete conduit run |
| National Electrical Code rule | If the circuit is protected with a GFCI, the minimum burial depth is 12 inches |
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What You'll Learn
Plastic conduit
PVC conduit is available in two main forms: rigid PVC and split PVC. Rigid PVC conduit, sometimes sold under brand names like Bore-Gard, comes in straight sections rather than being wound on a roll. This type of conduit is strong and flexible, similar to HDPE (high-density polyethylene) pipe, and is easy to handle and ship due to its straight sections. On the other hand, split PVC conduit is an interlocking type of conduit that snaps together to encase fiber optics and cables. It is impact and ultraviolet resistant, making it ideal for repairing lines without cutting cables. However, most split PVC conduit is not recommended for electrical applications as it is not UL listed.
When installing plastic conduit for underground electrical wiring, it is important to follow certain guidelines. Firstly, all conduit work must be completed before pulling the wires through. Additionally, there should be no more than four 90-degree bends between permanently accessible "pulling points" in a complete conduit run. The minimum depth for plastic conduit is typically 18 inches, but specific areas may require a minimum of 24 inches, or even 6 inches if encased in concrete. It is also important to note that underground conduit is considered a wet environment, so certain types of cables, such as Type NM cable, should not be used.
To install plastic conduit, you can start by digging a trench at the required depth based on your local regulations. Cut the plastic conduit to size, using a propane-powered heat gun to bend it if necessary. Feed the electrical cable through the conduit and secure it with joist hangers or cable staples if needed. Finally, lay the cable in the trench and repeat the process until all conduit and cable are installed. Remember to follow local electrical codes and guidelines to ensure a safe and proper installation.
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Rigid metal conduit
RMC is suitable for locations where the conduit is subject to physical damage, and it can be buried in normal soil conditions with just a galvanized coating. However, it is essential to follow the National Electrical Code (NEC) guidelines, which specify a minimum depth of 18 inches for RMC, with some areas requiring a minimum of 24 inches.
When compared to RMC, PVC conduit is a more cost-effective and durable alternative. PVC conduit is flexible and strong, cut into shorter, straight sections, and is easier to handle and ship. It is also resistant to corrosion and suitable for protecting underground cables and lines.
While RMC offers robust protection, it is important to consider the specific location and environmental conditions before selecting it for underground electrical applications. The weight and cost of RMC installation may be deterring factors, and alternatives like PVC and fiberglass conduit offer competitive advantages in terms of ease of installation, durability, and cost-effectiveness.
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Electrical metallic tubing
While EMT is a good option for electrical wiring, it is not suitable for underground use as it will rust out in a few years. EMT is commonly used in standard applications such as commercial buildings, schools, hospitals, and data centres. It is available in various trade sizes, lengths, and colours.
For underground electrical wiring, intermediate (IMC) or rigid (RMC) steel or aluminium conduit is recommended. These materials are more durable and less susceptible to rusting. The most common type of underground conduit is PVC plastic, also known as non-metallic conduit. PVC conduit is flexible, strong, and easy to handle, making it suitable for protecting underground electrical cables and lines.
It is important to note that the pipe size for underground electrical wiring depends on the number of wires and the wire size. Additionally, the minimum depth for anything other than rigid steel conduit is 18 inches, with some areas requiring a minimum of 24 inches.
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Geothermal HDPE pipe
Geothermal HDPE (High-Density Polyethylene) pipes are used for ground source energy systems, also known as geothermal heat pump systems. These pipes are typically buried or submerged underground, so they need to be durable enough to withstand harsh conditions and meet strong pressure requirements.
HDPE pipes have been used for municipal and industrial water applications for over 50 years. They are well-suited for ground source geothermal applications due to their unique performance properties. HDPE is a tough and durable piping material that offers corrosion resistance, chemical resistance, flexibility, impact resistance, and resistance to slow crack growth. It also provides long-term hydrostatic strength and temperature resistance, making it ideal for geothermal systems.
The Geo-Stripe® Geothermal Pipe by IPEX USA LLC is an example of a HDPE pipe designed for geothermal heating and cooling systems. It is made from high-density polyethylene resin, which is resistant to decomposition, oxidation, and other damaging elements. The flexibility of the Geo-Stripe® pipe allows for bending during installation, preventing breakage due to expansion and contraction. It also absorbs shock water hammer or sudden impact without suffering damage, even in extremely low-temperature conditions.
When choosing piping for underground electrical applications, it is important to consider the specific requirements of the project. PVC (Polyvinyl Chloride) conduit is commonly used for underground electrical applications as it is flexible, strong, and easy to handle. It is impact and ultraviolet resistant, making it suitable for protecting underground cables and lines. However, in areas where the conduit is subject to physical damage, Schedule 80 PVC may be required to provide additional protection.
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Split PVC conduit
While split PVC conduit offers convenience and protection, it is important to note that most varieties are not UL-listed and are therefore not recommended for direct electrical applications. However, some split PVC conduit products, such as those from Conduit Repair Systems, Inc. (CRS), have been independently tested and certified for electrical use according to the National Electrical Code.
When choosing a pipe for underground electrical use, it is crucial to consider the specific requirements of your project. While PVC conduit is a popular choice due to its flexibility and strength, other options such as HDPE and CIC pipes are also available, each with its advantages and disadvantages. Additionally, factors such as pipe size, number of wires, and local electrical codes should be carefully considered to ensure a safe and effective installation.
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Frequently asked questions
You should use electrical conduit, not pipe, for underground wiring. The best type of conduit depends on your specific project. Some common types of conduit include: PVC, HDPE, fiberglass, RMC, EMT, FMC, and split PVC.
PVC conduit is rigid, strong, and flexible. It is also known by the brand name Bore-Gard. HDPE conduit is highly flexible, has high impact resistance, and can withstand a broad range of temperatures.
RMC (rigid metal conduit) is thick, heavy, and provides excellent protection in harsh environments. However, it is expensive and difficult to install. EMT (electrical metallic tubing) is lightweight and bendable but is not suitable for underground use as it can easily become damaged. FMC (flexible metal conduit) is very bendable and is great for applications where tight bends are needed.
First, dig a trench at the required depth for your project. If connecting to an interior source, bore a hole through the wall. Cut and bend a plastic conduit to cover the area from the trench to the hole. Feed the cable and attach a plastic underground electrical conduit outlet body to the end of the conduit. Push a length of UF electrical cable into the conduit and feed it through the hole in the wall. Screw the conduit outlet body to the wall and apply caulk to seal out water. Cover the cable with sand and backfill the trench.
You should consider the soil type, level of moisture in the ground, and exposure to corrosive elements. You should also determine the voltage and current levels that the conduit will carry, as higher-voltage applications may require a specialized conduit. Some types of conduit work better underground than others, so it is important to choose the right material for your project.
