Electrical wire comes with all different types of insulation, but they all have the common denominator of copper or aluminum stranding. The insulation changes the expense of the wire or cable based on the environment you need to place it in.
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THHN wire is the most common type of single conductor electrical wire. It’s manufactured with a dual rating of THHN and THWN so that it’s all water-resistant. It can be used indoors and in conduit only.
Using your copper wire and cable indoors requires the least amount of protection on the insulation because there isn’t much that can hurt it. There aren’t any moisture, oil, or solvents affecting the life of the insulation so it can be made for less money. Romex cable is the most common type of indoor electrical wire which uses THHN wire conductors.
THHN is still the single conductor wire used outdoors and in conduit, because the THWN rating can resist water, oil, gases, and solvents. However, running 3-4 single wires can be difficult for the installer because they’ll have to run each one individually or tie all 3 together and try to run them as one.
Romex will not work outdoors OR in conduit so you’ll have to upgrade in quality and price to an MC Cable. It’s manufactured with THHN wires and a Metal Clad jacket for easy installation in conduit. The ground wire in MC cable is also a THHN wire rather than a bare copper ground wire so the cable remains moisture and solvent resistant.
THHN cannot be buried directly in the ground so you’ll have to upgrade on a single conductor to USE-2 wire. It’s used mostly on solar installations and has rubber insulation.
If your installation is directly in the Earth instead of in conduit then you’ll have to upgrade to a UFB cable which is an underground feeder cable. It’s color-coded and flat instead of round with a bare copper ground wire. The insulation covers each conductor individually to keep it water and moisture tight directly underground.
Tray cable is also an underground cable for larger sizes since UFB cable only goes as large as 6 AWG 3 conductor. Tray cable goes as big as 500 mcm 4 conductors with ground. It has all-black numbered THHN conductors and an overall PVC jacket. We sell it per foot on our website so that you can get the exact length you need and avoid any waste.
Those are the basics of indoor and outdoor electrical wire. There are also basics on electronic wire and cable-based on heat and voltage ranges for different applications.