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Why Romex Can’t be Used Outdoors

Romex© is a type or electrical wire used indoors only for a few reasons. We get this question a lot since it has THHN THWN wires inside it that are allowed to be used outdoors.

Yes, THHN THWN wire can be used outdoors but not directly underground. Romex©, however, has a bare copper ground wire inside the cable that is NOT fully protected by the PVC jacket covering all of the conductors. So when Romex© wire is used outdoors and gets wet with water, gas, oil, or any type of solvent it could penetrate the jacket and ruin the connection by getting in contact with the bare copper ground wire.

Buy Romex Wire here

What should you use instead?

MC cable can be used indoors and outdoors because the ground wire is an insulated THHN THWN wire as well. The common use for MC cable is indoors because it can be used without conduit for a much easier installation. However, even if the metal-clad jacket doesn’t fully protect the THHN THWN wires they already have the approvals alone to withstand outdoor environments. Copper and Aluminum MC cable can also be used in conduits.

Why not just run THHN THWN wires individually?

You can. Depending on the current copper prices we’ll sometimes push our customers in that direction because the cost of all 4 electrical wires are cheaper than the cable. However, sometimes the cable is cheaper as well. There’s also the added labor involved in pushing each individual wire through conduit or tying all four together to push them through as a group.

We have a highly trained staff here at WesBell Electronics and are here to help you. We’ll give you the cheaper option before you ask and help you find the correct cable by the installation it’s going in. If you have questions please ask and we’ll gladly help you through the process and explain what we’re saying to help you understand.