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What Kind Of Wire To Use In An Outdoor Conduit: Full Guide

A large stack of metal outdoor conduits.

With per-meter wiring costing between around $0.30 per foot and $4.45 per foot, installing the correct wiring for a new, outside installation can be costly. Over a long distance, this can result in a very high overall price. Unfortunately, there are no shortcuts if you want a safe, compliant system.

The type of wire to use in an outdoor conduit will be determined by the consumption (amps) of the electrical consumers in the circuit, as well as whether the wire is being installed above or below ground. When installed below ground, the depth of the trench will determine the type of cable used.

The category of wire ultimately used in an installation will be selected based on a number of risk factors identified in the National Electrical Code (NEC) or NFPA 70.

The Type Of Wire To Use In An Outdoor Conduit

There are several wire types available, and each is designed for a specific purpose. In broad terms, this is defined as whether the cables are installed above or below ground.

These are the two factors that an electrician uses to determine which wire to use.

#1: The Depth Of The Wire

The depth below the ground will determine the category of wire chosen. The building code details the requirements based on the following minimum depths:

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  • above ground
  • six inches below ground
  • twelve inches below ground
  • eighteen inches below ground
  • twenty-four inches below ground

#2: The Gauge (Diameter ) Of The Wire

The gauge of wire is chosen based on the amperage of the electrical consumers.

The following excerpt from the standardized American Wiring Guide shows the gauge cable required for the most common amperages.

American Wire Gauge RatingDiameterAreaBased on Copper Wire
(in)(mm)(mm2)(M Ω/ft)(A)

Where Are Conduits Installed?

Conduits are rigid or flexible, metal or plastic, pipes through which electrical wires run.

The purpose of the conduit is to protect the wires in exposed locations, such as along the exterior surface of a wall and underground trenches less than eighteen inches deep.

It is also used in unfinished areas, like:

  • basements
  • crawlspaces
  • attics
  • surface-mounted installations outdoors

When used outside, the conduits are installed either above or below ground.

An above ground conduit running above dirt.
Above-ground wire in conduits varies based on amperage

Above Ground Conduits

The type of wire that can be used above ground will depend largely on the amperage required to conduct.

Section 522 of the National Electrical Code (NEC)  lists several external influences that the electrician must consider when selecting the type of wire used in an above-ground installation.

Regulation 522.6.1 requires that the wiring used in an installation be chosen because of its ability to resist and minimize the damage that may arise from any impact during the:

  • installation
  • use
  • maintenance

The wire used in an application must protect against impact caused by:

  1. Providing optimal protection, such as using a steel-wired armored cable or enclosing cables in a conduit that can be bought on Amazon.
  2. Routing the cables along the installation to reduce the risk of being damaged.

NOTE: In areas where there is no risk of contact with moisture, an NM cable will suffice. If there is contact with moisture, a THWN-2 (water-resistant) building wire will be used in the installation.

Wire for Underground Conduits

While the gauge cable required for the application is still dictated by the electrical consumption (amps) of the electrical consumers on the circuit, the type of cable and its protection will differ from the ground installations.

The depth of the cable determines the type of cable you use underground. The deeper a cable is buried, the less protection to the cable can be provided.

#1: Twenty-Four Inches Deep Burial Depth (UFB)

For trenches that are 24 inches or deeper, an underground feeder cable (UFB), sold on Amazon, can be used. 

UFB cable is made up of:

  • insulated positive wire
  • insulate neutral wire
  • bare copper ground wire

The sheathing on the UF cable is a solid plastic that surrounds each wire.

With the 24 inches of cover to protect the cable, less protection is required. As a result, a conduit is not required at this depth, apart from the vertical feeds from 18 inches and above.

#2: Eighteen Inches Burial Depths (THHN and THWN)

The category of wire which can be used at eighteen inches of depth is the THHN/THWN-2 building wire, found on Amazon.

The acronym THHN stands for:

  • Thermoplastic
  • Highly Heat-resistant
  • Nylon-coated, for added protection

The acronym THWN stands for:

  • Thermoplastic
  • Heat-resistant.
  • Wet locations rated
  • Nylon-coated, for added protection

THHN and THWN wires are single conductor cords, each with color-coded insulation.

The wire is primarily intended for general-purpose applications as defined by the National Electrical Code (NEC) and is a commonly used category of wire.

HOT TIP: Cable can also be used without a conduit for a wire buried at eighteen inches depth or deeper.

A blue conduit for wire running several inches deep in an underground trench.
Many underground wires require conduits for protection

#3: Twelve Inches Deep Burial Depth

The less the cover over a wire, the more the wire must be protected. At twelve inches deep, a direct burial feeder cable (DBFC), may be used as long as it is GFCI-protected.

A ground fault circuit interrupter (GFCI) is a device that can either be installed in the circuit or built directly into the power wire.

The GFCI device is designed to protect people from electrical shocks if they damage the wire and break the circuit. For example, if it is unintentionally cut when digging in the vicinity.

The typical GFCI device works by constantly monitoring the current flowing through a circuit. If the current that flows into the circuit changes by a very small increment, the GFCI interrupts power. It prevents a life-threatening dose of electrical current from causing injury.

#4: Six Inches or Less Burial Depth

The electrical load will determine the type of wire used at this depth.

A stronger conduit is required with just six inches covering the cable. These cables can all be used:

  • direct burial feeder cable
  • low-voltage wire
  • THHN
  • THWN

These conduits must be used for all underground cables which are six inches or less in depth:

  • rigid metal conduit (RMC)
  • galvanized metal electrical conduit (EMT)
  • intermediate metal (IMC)

An IMC conduit is difficult to penetrate with a tool like a spade and therefore requires protection from people digging in the vicinity.

Several sizes and colors of plastic conduits in the process of being buried in dirt.
There are many types and sizes of conduits and wires to use


When choosing to wire for outside uses, the variables that must be considered include the current they will carry and the depth they will be buried at.

The final cost is often substantially more than the initial budget and can lead to some difficult decisions.

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