There are many reasons to bury conduits under the ground. To connect an outbuilding to the house’s electric circuit or take an electrical outlet to part of the garden, running wire in an underground conduit is a good way to do it. But what type of wire can be used in a buried conduit?
Two types of wire can be used in buried conduits: single conductors can be used in galvanized metal conduit buried at 6 inches; THWN-2 conductors can be used in PVC conduit buried at 18 inches. A GFCI-protected cable can be buried at 12 inches without a conduit, but a protective conduit is best.
Specific code regulations dictate what types of wire can be used in buried conduits. Always consult your local authorities and building inspection office for the full set of regulations to follow in your area. Keep reading to get all the information you need about what wire to use in buried conduits before you head off to the building inspection office.
NOTE: If you’re doing a basement renovation, check out this full how-to guide!
What Wire Or Cable Should Be Used In Underground Lines?
The correct wire or cable to use in a buried conduit is usually determined by the depth of the underground line. Single conductors can be buried in the galvanized metal conduit at a depth of 6 inches, and THWN-2 conductors can be buried in the PVC conduit at 18 inches.
NOTE: The closer to the surface the underground line is, the better protected it will need to be from digging and accidental breakage by garden tools.
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Underground lines buried at 6 inches need to be protected with a galvanized metal conduit because they are more likely to be dug up or damaged accidentally during gardening or other maintenance. A wire buried at 24 inches is less likely to be dug up or damaged accidentally, so it doesn’t need a conduit.
The following four methods of burying wire are recommended at the indicated depths:
- Bury single conductors or other wires in galvanized metal conduit, sold on Amazon, when the underground line is only buried at 6 inches.
- Direct-bury GFCI-protected cable when the trench will be 12 inches deep.
- Bury THWN-2 cables in a continuous length of PVC electrical conduit, when burying the wire at 18 inches.
- Direct-bury underground feeder cable when burying the wire at 24 inches.
In all cases, ensure that conduit is used where the wire comes out of the ground to protect the cable at ground level, and where possible, use a service entrance ell on the wall to make accessing and feeding new cable easier.
How To Lay Cable In Buried Conduit
When burying conduit to lay cables, you can use two types of conduit: galvanized metal conduit or PVC conduit. The correct conduit to use will depend on three factors:
- The depth the conduits will be laid.
- The likelihood of the conduit accidentally being dug up.
- How much power will be supplied through the wires in the conduit?
The deeper the conduit is laid, and the likelihood of it being accidentally dug up will determine if the conduit will need to be galvanized metal or if PVC is sufficient. A shallowly buried conduit with a high likelihood of being dug up should be metal to protect the wires inside the buried conduit. A deeper buried conduit with a low risk of being accidentally dug up can be a PVC conduit.
There are also specific code regulations to keep in mind. Follow these steps to determine which conduit to use and how to go about it:
Step #1: Get Necessary Permits and Permissions from Local Authorities
Depending on your area or jurisdiction, you may need to write permission to run buried cabling and create an outdoor circuit connected to the main house circuit. Contact your local authorities for the correct information. In some instances, only a qualified professional will be allowed to do the installation. In other instances, you will be able to do it yourself.
Step #2: Assess the Soil or Ground Type the Conduit Will be Buried In
It’s best to bury the conduit at only 6 inches using a galvanized metal conduit if the soil is:
- difficult to dig
If the soil is not difficult to dig, consider burying deeper, to 12 inches or 18 inches, and using PVC conduit. Dig the trench.
Step #3: Ensure the Conduit is the Right Size for the Wires
The diameter of the conduit you bury will depend on the gauge and the number of wires being run through the buried conduit. Check that the wires you intend to run through the conduit will fit.
Step #4: Use the Correct Type of Wire in the Buried Conduit
Whether running galvanized metal conduit or PVC conduit, the wire being run should have a waterproof rating. This is usually indicated by a “W” labeling. THWN-2 wire, found on Amazon, has this rating and is safe to use. If you are running single conductors, check that they have an appropriate “W” labeling or rating. Feed the wire through the conduit and lay it in the trench.
Step #5: Extend the Conduit out of the Ground at Both Ends of the Trench
Extending the buried conduit from the ground at the wall will further protect the wires from damage and leave a tidy finish. For conduit that will begin and terminate at a wall, place a service entrance ell on the end of the conduit to make it easier to access the wires in the buried conduit after the job is complete.
Step 6: Get the Installation Passed by Local Authorities
Where necessary, contact local electrical code authorities and have the installation passed. If a qualified electrician does the installation, this should be part of the service.
Can Wires Be Buried Directly In The Ground?
An underground feeder cable can be run directly into the ground. It is sometimes referred to as direct burial but be aware that there are certain restrictions and safety measures in place to do this. In many cases, a buried conduit is still recommended even at this depth.
Always ensure the wire being laid is:
- designed for direct burial
- designated safe for direct burial
In most cases, a cable that is protected by a ground fault circuit interrupter is recommended when the cable is not placed in a buried conduit to protect the circuit from shorts.
A cable laid directly in the ground must be buried at a depth of at least 24 inches below the surface. Any shallower than this requires the use of conduit and the entry and exit points of the cable to be housed in a rigid conduit of either PVC conduit or galvanized metal conduit, with an elbow fitting gently bending the wires when they reach the trench floor.
Two types of wire are generally used in buried conduits: single conductors placed in galvanized metal conduit and THWN-2 conductors in PVC conduit. Assess the requirements of the circuit you are laying and choose the appropriate wire. Also, assess the most appropriate depth to bury the conduit and the correct conduit to use before starting the project.
Always consult your local authorities and building inspection office to ensure compliance with relevant codes and safety measures. Specific code regulations relate to the types of wire used in buried conduits.
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