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Romex wire is very popular because it saves time. It is mostly run through conduit or along the walls of your home to connect power to lights and other appliances. Romex comes in all different sizes depending on how much power is needed for the application.

Usually in homes Romex 14/2 and Romex 12/2 are the most popular. The 10 awg and the three conductors cables aren’t used as much because they carry way too much power for a standard home. The cable is filled with two THHN wires and a ground wire. The ground wire doen’t have any insulation over it so it has minimal protection. That’s the reason Romex can not go outdoors because the moisture will eat through the jacket and hit the ground wire fairly quickly.
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Sometimes the wire needs to be run outdoors for a short length. Even if you put it in conduit I still don’t recommend it. Romex wasn’t built for that reason and has no added protection against moisture of any kind. If you need to go outdoors you have two options. 1) You can run single THHN / THWN wires instead of the cable with an insulated ground wire. THWN wires can go outdoors without conduit because of the nylon coating over the insulation. 2) Use UFB cable which is an underground cable that doesn’t even need conduit. This cable can go anywhere including directly in the earth without any problems.

Romex 6/3 and Romex 8/3 are very popular to connect larger power sources to smaller power sources within the home. Again, Romex can not go outdoors. UFB cable isn’t that much more money to avoid the hassle of replacing the wire at some point. The same options from above apply in this situation as well.

If you are trying to supply power to a barn or garage then I highly recommend UFB 6/3 or maybe even a smaller size. UFB cable is the correct cable to use for that application.

Make sure you talk to your electrician about the size of the cable you need. Then talk to your sales person about the type of cable and insulation you will need for protection.