Romex wire and UFB cable are very similar for a few different reasons, which allows them to be interchanged with each other if needed. Both types of electrical wire are made with THHN wires and a bare copper ground wire. The main difference then lies in the outer jacket.

Romex® is made with a thin outer jacket without any added protection. The idea behind manufacturing this cable is for electrical contractors to have a cheaper version of an outdoor or underground cable. If the Romex® jacket is exposed to outdoor weather the bare copper ground wire could be affected by moisture, oil or gas.

UFB cable is also known as the underground cable in the electrical wire industry. You can see the difference in protection from the pictures below. UFB cable has insulation covering each conductor and it's much thicker and heavier. Romex® has a very thin jacket made cost effective for electrical contractors to use it indoors.

Using all THHN wires as an alternative to UFB cable could be a cheaper way to run the electrical power as long as it's used in conduit and not directly underground. Most THHN wire is now dual rated for THWN wire but you should still make sure it is because the "W" stands for water resistant. If your THHN isn't dual rated for THWN you will not be able to use in outdoors.

Romex® Wire	UFB Underground Cable