HOW TO CHOOSE ELECTRICAL WIRE

• Choose wire in this order: application (what it feeds), environment (dry, wet, buried, conduit), insulation, then gauge.
• Dry interior means Romex (NM-B). Commercial or exposed means MC cable or conduit. Wet or underground means UF-B or THWN-2/XHHW-2.
• The environment eliminates most options. Only one or two products survive it — then you pick between those.
• Size the gauge from NEC Table 310.16, respect the 240.4(D) small-conductor caps, and upsize long runs for voltage drop.
• When in doubt, the answer is on the wire's own markings — the rating that matters is printed on the jacket.

Before the decision steps, here is the whole field on one table. This is the page electricians screenshot. It is part of your friendly guide to all things electrical wire, where each of these products gets its own deep-dive guide.

Every wire decision starts with the same question: what is this circuit feeding, and where does it live? A 15-amp bedroom receptacle circuit, a 20-amp commercial lighting run, and a 60-amp feeder to a detached garage are three different jobs, and starting with the application keeps you from shopping by gauge — the most common beginner mistake. Write down the load (what it powers), the voltage (120, 240, or 208/480), and the location (which room, which building, inside or outside). That short description already points at a product family.

Let the building type narrow it.

Residential work lives in the cable world: NM-B for dry interiors, UF-B the moment you go outside or underground. Commercial work lives in the armor-and-pipe world: MC cable for fast branch-circuit runs, conduit with THHN or XHHW-2 where the spec, the future, or the local code demands pipe. Industrial work adds the heavy cable types — tray cable, armored feeders, MCM sizes — which have their own guides in this hub. If you can name the building type, you have already eliminated half the table above.

The environment is the great eliminator. Dry, damp, wet, buried, exposed to damage, or inside conduit — each answer knocks products off the table until one or two remain. This is the step people skip, and skipping it is how Romex ends up buried in a backyard.

The four environments that matter.

Dry interior is the easy one: inside walls, ceilings, and floors that stay dry. That is NM-B territory in homes, and MC or conduit territory in commercial buildings. Wet and damp locations — outdoors, bathrooms' wet zones, anywhere water reaches — rule out NM-B and AC entirely; UF-B cable or wet-rated conductors (THWN-2, XHHW-2) in conduit take over. Underground means direct-burial-rated cable (UF-B) or conduit with wet-rated conductors, buried at the depths the code requires. And exposed to physical damage means armor or pipe, full stop — no bare cable where it can be hit, which is the entire reason MC and conduit exist.

Conduit is a wet location.

This surprises people: conduit run outdoors is legally a wet location, because water gets into conduit no matter how well it is put together. So the wire inside outdoor conduit must be wet-rated — THHN/THWN-2 or XHHW-2, never NM-B. The rating that matters is on the wire, not the pipe. When you read a wire's markings and see THWN-2 or XHHW-2, that wet rating is exactly what you are looking at.

Once the environment has narrowed the field, the insulation type finishes the choice. For cable products (NM-B, UF-B, MC), the insulation comes with the package — you chose it when you chose the cable. For conduit work, you choose the conductor, and that means understanding the alphabet soup.

THHN: the default conductor.

THHN stands for thermoplastic, high heat-resistant, nylon-coated — the single insulated conductor electricians pull through conduit more than any other. It is rated 90°C in dry locations, and in practice it is almost always dual-marked THHN/THWN-2, where the THWN-2 marking extends the 90°C rating to wet locations. If you are pulling wire in pipe and nobody specified anything exotic, THHN/THWN-2 is the answer.

XHHW-2: the tougher conductor.

XHHW-2 uses cross-linked polyethylene insulation instead of thermoplastic, and it is rated 90°C in both wet and dry locations. It handles heat and abrasion better than THHN, which is why it shows up on feeders, service work, and harsh environments. It costs more and is stiffer to pull, so it is specified where its toughness earns its keep — not as the default for every pipe run.

What the temperature rating actually buys you.

The 90°C rating is mostly about derating math — adjusting ampacity for hot attics or bundled conductors — because the final ampacity is still limited by the lowest-rated termination, usually 75°C or 60°C. Do not let the big number tempt you into smaller wire: the terminations govern. This is exactly the kind of detail the licensed pro handles, and it is why the gauge step comes with a pro attached.

Last step, not first: pick the gauge. The process is simple to describe and easy to get wrong, which is why it belongs to the licensed pro on a real job. Here is how the thinking works.

Ampacity basics.

Ampacity is just how much current a wire can safely carry, and NEC Table 310.16 is the official chart. At the 75°C column — the one that governs most terminations — copper runs 14 AWG at 20 amps, 12 at 25, 10 at 35, 8 at 50, 6 at 65, and up from there. But the small-conductor rule (NEC 240.4(D)) caps the overcurrent protection at 15 amps for 14 AWG, 20 for 12 AWG, and 30 for 10 AWG — so in practice, 14/12/10 AWG mean 15/20/30-amp circuits. Our wire gauge chart lays out every gauge from 14 AWG through 4/0 with diameters and ampacity, and our ampacity explainer teaches the table in five minutes.

Distance changes the answer.

Long runs need bigger wire than the ampacity table alone suggests, because voltage drops over distance — and motors, lights, and electronics all notice. A 20-amp circuit that is fine on 12 AWG at 50 feet may want 10 AWG at 150 feet. Voltage drop is not a code violation at any particular number, but it is the difference between equipment that runs right and equipment that runs hot and short-lived. When the run is long, the pro upsizes. That judgment call is the job.

Three mistakes account for most bad wire choices. All three are avoided by the four steps above.

Indoor cable outdoors.

Running Romex outside, underground, or anywhere wet is the single most common wire-selection mistake, and it is a straight code violation under NEC Article 334. If the run leaves the dry interior, the product changes — UF-B, or wet-rated conductors in conduit. No exceptions, no "it will be fine."

Shopping by gauge first.

Picking "12 AWG" before picking the product is backwards. Twelve-gauge Romex and twelve-gauge THHN in conduit are different answers to different questions. Product first (by environment), gauge second (by load). The order matters.

Forgetting the local amendments.

The NEC is the national floor, and cities build on it. Chicago requires conduit where the NEC allows cable. Some jurisdictions restrict NM-B beyond the NEC's own limits. The wire that is perfect in one town fails inspection in the next — which is why "check your local amendments" is not boilerplate on this site. It is the step.