Wire safely.

Two rules decide the wire, and only one of them is on the label

Wire sizing answers two separate questions, and skipping the second is how a legal circuit ends up with dim lights and a hot motor. First, ampacity: can the conductor carry the current without overheating? That is a table lookup, and for ordinary house wiring it is short enough to memorize. 14 AWG copper is 15 amps, 12 AWG is 20, 10 AWG is 30, 8 AWG is 40, 6 AWG is 55. Second, voltage drop: over the distance you are running, does enough voltage actually arrive? A 12 AWG circuit is fine at 30 feet and marginal at 80, even though the table never changes.

The calculator above answers both at once and returns the larger of the two answers, which is the one that governs. This page covers the full ampacity chart for copper and aluminum, the 60 degree column rule that catches nearly every DIY mistake, the wire sizes for common appliances and services, and how far each gauge really runs.

The wire size chart

The rows people search for, stated plainly. A 20 amp circuit takes 12 AWG copper, never 14, and this is the single most common wiring error in DIY work because 14 AWG is cheaper and easier to bend. For 50 amp wire size, use 6 AWG copper or 4 AWG aluminum. The 100 amp service wire size is 4 AWG copper or 2 AWG aluminum, and 125 amp service, common on houses upgraded in the 1980s, takes 2 AWG copper or 1/0 aluminum. The 200 amp service wire size is 2/0 copper or 4/0 aluminum; service entrance conductors for dwellings get a small allowance under NEC 310.12 that lets them run slightly smaller than a general feeder of the same rating, which is why the service table looks generous next to the subpanel numbers.

Why the cable says 90 C and you still use 60

Every roll of NM-B cable is printed with a 90 degree C rating, and the 90 degree column of Table 310.16 shows 12 AWG at 30 amps. Both are true, and neither lets you put 12 AWG on a 30 amp breaker. NEC 334.80 requires NM cable to be sized from the 60 degree column, and the reason is the hardware at each end: breakers, receptacles, and panel lugs are listed for 60 or 75 degree terminations, so the weakest link governs the whole circuit. The 90 degree rating exists for derating math, where you start from the higher number and subtract for bundling or ambient heat, but the final answer can never exceed the 60 degree value.

Two derating cases come up in real houses. Bundling: once more than three current-carrying conductors share a conduit or pass bundled through more than 24 inches of framing, ampacity drops, 80 percent for four to six conductors and 70 percent for seven to nine. Ambient heat: an attic in Phoenix can run above 100 degrees F, and conductors up there lose ampacity accordingly, which is why long attic runs sometimes need the next size up even though the length alone would not demand it.

Voltage drop and how far a circuit really runs

Voltage drop is not a code violation. NEC 210.19 puts the 3 percent branch circuit and 5 percent total targets in an informational note, which means recommended rather than required. It is still worth respecting, because the symptoms are real: motors draw more current at lower voltage and run hotter, incandescent and some LED fixtures dim visibly, and heating elements lose output roughly with the square of the voltage. A detached garage 120 feet away on 12 AWG at 120 volts is the classic complaint, and the fix at the design stage costs one wire size.

Two structural facts make long runs easier. Doubling voltage roughly doubles the distance you can run at the same drop, which is why a detached shop is fed at 240 volts with a subpanel rather than by extending 120 volt circuits. And drop is proportional to current, so a lightly loaded long run is fine while a fully loaded one is not. If the load is a continuous one, such as an EV charger or electric heat, the 125 percent continuous load rule in NEC 210.19 already pushes the conductor up before voltage drop enters the picture.

Low-voltage DC: solar, automotive, and speaker wire

Wire sizing for 12, 24, and 48 volt DC follows the same two rules, but the balance between them flips completely. At 12 volts, voltage drop governs almost every run, because a 3 percent budget on 12 volts is only 0.36 volts. A 20 amp accessory circuit that would take 12 AWG in a house needs 10 AWG at 10 feet and 6 AWG at 25 feet in a 12 volt system. This is why marine and RV wiring looks absurdly oversized to anyone used to household work: it is not overbuilt, it is correctly built for the voltage.

Three practical notes for DC work. Stranded wire, not solid, for anything that vibrates or flexes, which is every vehicle and most boats. Solar arrays are usually wired at higher DC voltage specifically to escape this problem, which is why a 48 volt battery bank moves the same power through a quarter of the copper a 12 volt bank needs. And speaker wire is the one case where the 3 percent rule is the wrong target: speaker runs are sized against the load impedance, and the common guidance is to keep total wire resistance under about 5 percent of the speaker impedance, which for an 8 ohm speaker means 16 AWG covers most residential runs and 14 AWG handles long ones. Set the voltage selector on the calculator above to 12, 24, or 48 volts and it applies the DC math directly.

Aluminum: fine on feeders, a problem in old branch circuits

Modern aluminum building wire, the AA-8000 series alloy, is a legitimate and common choice for service entrances and large feeders. It is meaningfully cheaper per amp, it is what most utilities run to the meter, and 4/0 aluminum feeding a 200 amp panel is completely ordinary. It needs the right treatment: connectors listed AL or CU-AL, antioxidant compound on the terminations, and torque set with a torque screwdriver rather than by feel, because aluminum creeps under pressure in a way copper does not.

The bad reputation comes from a different product. Houses wired between roughly 1965 and 1973 used solid aluminum in 15 and 20 amp branch circuits, and the CPSC found those connections far more likely to reach fire hazard conditions than copper. If you open a receptacle box and find solid aluminum branch wiring, that is an evaluation-by-an-electrician situation and not a DIY repair; the accepted repairs are specific listed connectors or a rewire, and simply replacing devices does not fix it.

Where wire sizing goes wrong

Using 14 AWG on a 20 amp circuit. The most common one, and the most consequential. The breaker protects the wire, not the device; a 20 amp breaker will happily let 18 amps flow through a conductor rated for 15 until the insulation fails somewhere inside a wall.

Upsizing the breaker instead of the wire. When a breaker trips repeatedly, the breaker is doing its job. Replacing a 15 amp breaker with a 20 to stop the nuisance leaves 14 AWG protected at 20 amps, which is the textbook cause of an electrical fire.

Ignoring the continuous load rule. Anything running three hours or more without a break, such as EV charging, electric heat, or shop lighting, is sized at 125 percent of its load. A 40 amp charger is a 50 amp circuit, not a 40 amp one.

Forgetting the neutral and ground. A 240 volt appliance circuit may need a neutral depending on the appliance, and the equipment ground sizing comes from its own table, NEC 250.122, not from the ampacity table. Buying 6/2 cable when the range needs 6/3 is a return trip.

Circuits are part of a larger plan

Wire sizing usually shows up inside a bigger job. A new EV or shop circuit often accompanies a solar calculator estimate on the same panel, portable tools raise the separate question the extension cord calculator answers, and sizing the cooling load that a new circuit will serve runs through the BTU calculator.

Run your amps, volts, and distance through the calculator above, take the larger of the two answers, and bring that number to the electrician. Being right about the wire is the cheapest part of any electrical project.