There aren’t too many types of electrical wire that are manufactured in 3 AWG but the industry using THHN wire has deemed it worth the time and effort. The gauge size bigger is 2 AWG and the gauge size smaller is 4 AWG which used to have a big gap in between but not with the inclusion of 3 AWG copper. The gauge size of the copper wire determines how many amps, or power, can travel through it and there were instances where the number 2 was too big (too much money) and the number 4 wasn’t quite enough for the application.
THHN is dual rated with THWN which gives it a water resistant. THHN wires are used by themselves as single conductor electrical wires and also in cables such as NMB, MC and UFB. electrical cable gland These cables could be as simple as putting a thin jacket over it for indoor use or as complex as having each wire individually covered for underground use. The piece of insulation or jacket that is most exposed does the most work at resisting and complying with the specifications set forth by engineering.
Therefore, NMB cables, for instance, have a thin PVC jacket covering the THHN wires and the bare copper ground wire. This jacket doesn’t have much added protection other than bundling the wires together in order to run all of them at once rather than individually. It’s considered an indoor cable only and actually does not come with 3 AWG wires. In order to have 3 AWG wires you’ll have to use MC cable which is approved for use indoors (without conduit), outdoors and in conduit outdoors and underground.
Another electrical cable that has 3 AWG conductors is SER cable which stands for Service Entrance (Round). There is also a flat version called SEU which lays all of the conductors flat to the ground. Service entrance cable is commonly used to bring power from a telephone pole to the electrical box of your home. Once it connects to the electrical box you can distribute smaller amounts of power to each outlet, light switch and appliance throughout the home. THHN wires and NMB cables are commonly used to drive the small power in a home or building.
Speak to your electrician about the size of copper wire you need to distribute the right amount of power in accordance to the NEC code. Then discuss possible options for insulation and jacket depending on how and where you need to run your cable.