Copper Romex cable has a few different types of stranding options. Romex 14/2 through Romex 10/2 come in solid version only to make the cable stiff enough for easy installation. Even if you're wiring your home it's better to have a more solid, less flexible, wire or cable to run because a flexible electrical wire would fall down and bunch up easier.

Romex® and THHN wire are very similar because Romex® wire actually has THHN conductors along with a bare copper ground wire. Using Romex® instead of THHN is an advantage because the electrical contractor can run more than one wire at a time.

If your electrical wire runs outdoors at all you must upgrade to UFB cable. UFB is an underground cable used to run power to a garage or barn from the original electrical power source. UFB cable can run directly underground with or without conduit. The protective underground feeder jacket is no match for the water, gas and solvents.

Why Can THHN Be Used Outdoors but Not Romex?

While Romex® contains THHN wires, it also contains a bare copper ground wire. Since the Romex® outer jacket cannot resist water or moisture it could penetrate the jacket and hit the ground wire.

You could use THHN wire as the ground wire so that it has insulation and run all the wires individually through conduit. Otherwise you will need to upgrade to a UFB underground cable.

 

Romex® Wire	UFB Underground Cable