Why Romex Wiring Can’t be Used Outdoors
Romex© Outdoor Conduit
Romex is a type of electrical wire used in various applications. While Romex is a useful electrical wire for many applications, it is not often used in outdoor settings, despite containing thermoplastic high heat-resistant nylon-coated (THHN) thermoplastic high water-resistant nylon-coated (THWN) wires. While THHN and THWN wires can be used in outdoor settings, they cannot be put directly underground.
Romex features a bare copper ground wire inside the cable that is not fully protected by the polyvinyl chloride (PVC) jacket which covers the conductors. Romex wires cannot be used outdoors because, if it gets wet with water, gas, oil or another solvent, these liquids could penetrate the jacket and ruin the connection by getting in contact with the bare copper ground wire.
What Kind of Wire Do You Use for Outdoor Conduit?
The National Electrical Code (NEC) has specific requirements and standards for installing outdoor equipment and circuits. When placing wiring in an outdoor application, one of the most important safety concerns is preventing wire damage, protecting from corrosion ad moisture and managing any potential complications related to undergoing wiring.
With many outdoor wiring projects, you will need to follow applicable code requirements about placing outdoor lighting fixtures and receptacles and running wiring below and above the ground. While Romex contains an outer vinyl jacket and premium waterproof insulation around the conduction wires, it is not recommended for outdoor applications. When you are using an outdoor cable and conduit, there are also many additional standards and regulations to consider.
A metal-clad (MC) cable assembles one or more insulated circuit conductors enclosed in a smooth or corrugated metal sheath or interlocking metal tape. An MC cable can be used in outdoor and indoor applications because the ground wire is insulated in THHN wire and THWN wire. One of the most common uses of MC cable is in indoor applications because it can be used without a conduit for easier installation. Even if the MC jacket does not fully protect the THHN and THWN wires, they already have the approvals to withstand outdoor elements. Copper and aluminum MC cable can also be used in conduits.
Does Exterior Wiring Need to Be in Conduit?
In most cases, local codes require exterior wiring to be placed in a conduit when the wiring is installed above ground. You should check local regulations to determine if you should use underground feeder (UF) wire or TW wire and a conduit. Local codes typically require conduit protection for exterior wiring if the wiring is installed above ground.
If you are burying the wiring, most codes allow for underground fed cables, but some will require the use of a conduit. Buried or exposed wiring will need to be listed for its application. Underground feeder wiring can be directly buried without a conduit with a minimum of 24 inches of cover. Any wiring placed inside intermediate metal or rigid metal conduits will require at least six inches of ground cover.
On the other hand, wiring protected in a PVC conduit will require a minimum of 18 inches of cover. Wiring lower than 30 volts must be buried at a depth of at least six inches. If you are running wiring underneath outdoor decks, you will need to use a conduit, as conduits are required by code. You should also attempt to provide reasonable protection for the conduit and place it high enough that pests will not be likely to nibble the conduit.
Some of the most common outdoor conduits include:
- Rigid metal conduit (RMC): RMC is typically used for outdoor applications and is a heavy-duty galvanized steel tube installed with threaded fittings. RMC can protect from damage while providing support for panels, electrical cables and more. Generally, RMC is sold in 10 or 20-foot-long increments with threads on either end.
- Intermediate metal conduit (IMC): IMC is often used in new outdoor construction projects. IMC is lighter-weight and thinner than RMC and is ideal for use in the same projects as RMC conduit. Because IMC is often simpler to work with, it is more commonly used in new construction projects.
- Liquid-tight flexible metal conduit (LFMC): LFMC is often used for outdoor equipment and is a special kind of flexible metal conduit with a plastic coating. LFMC can be used to seal fittings for a watertight finish. Many people use LFMC when installing air conditioning units.
- Rigid polyvinyl chloride (rigid PVC): Rigid PVC is ideal for use in corrosive environments. Rigid PVC shares many qualities with plastic plumbing piping and is installed with plastic fittings secured in place with glue. Rigid PVC can be shaped after being heated. Because these conduit fittings and tubes are glued together, the conduit assemblies are watertight, making rigid PVC ideal for direct placement in the ground for various applications.
Does Exposed Romex Need to Be in Conduit?
When using any non-metallic wiring in an outdoor application, you should run the wiring through a conduit. Using a conduit is ideal because it is always better to add more protection to exterior wiring than not enough protection. If you are running wiring underground, you should also use a conduit, which is a best practice for ensuring longevity.
The copper ground wire inside is regularly exposed to various elements compared to conductors. When running the wiring underground, it will be exposed to weather, water, pressure and roots. If you are using Romex, especially stripped Romex, the need for a conduit becomes even more important. When Romex comes into contact with solvents, the bare copper ground wire can negatively impact the connection and trip the breaker repeatedly.
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