Ask what size wire a circuit needs and you will get a confident answer that is right about eighty percent of the time: twelve gauge for twenty amps, fourteen for fifteen. It is right because most household circuits are short. Once a run leaves the house, whether for a garage, a shed, a well pump, or a subpanel at the far end of a basement, the confident answer starts being wrong, and the failure is quiet. Nothing trips. The lights are a little dim and the motor runs a little hot for the next twenty years.
Two separate questions decide a conductor size, and they have nothing to do with each other.
Question one: will it get too hot
This is the ampacity question, and a table answers it. NEC Table 310.16 lists, for every conductor size and insulation temperature rating, the current it can carry continuously without cooking its own insulation. It has three columns, 60 °C, 75 °C and 90 °C, because the same copper is rated differently depending on what the insulation around it can survive.
| Copper | 60 °C | 75 °C | 90 °C |
|---|---|---|---|
| 14 AWG | 15 A | 20 A | 25 A |
| 12 AWG | 20 A | 25 A | 30 A |
| 10 AWG | 30 A | 35 A | 40 A |
| 8 AWG | 40 A | 50 A | 55 A |
| 6 AWG | 55 A | 65 A | 75 A |
| 4 AWG | 70 A | 85 A | 95 A |
| 2 AWG | 95 A | 115 A | 130 A |
| 1/0 | 125 A | 150 A | 170 A |
You don’t choose which column to read. NEC 110.14(C) says the circuit is limited by the lowest-rated termination in it: the breaker lug, the receptacle screw, the equipment terminal. For equipment rated 100 amps or less, that is the 60 °C column unless both ends are marked for 75 °C. Modern breakers and panels usually are marked, but the receptacle at the far end frequently is not.
The rule that overrides the table
Look at 12 AWG copper in the 90 °C column: thirty amps. It is a real number, and you may never use it to justify a thirty amp breaker on twelve gauge wire. NEC 240.4(D) caps overcurrent protection on small conductors flatly:
14 AWG copper: 15 A. 12 AWG copper: 20 A. 10 AWG copper: 30 A. 12 AWG aluminum: 15 A. 10 AWG aluminum: 25 A.
The 90 °C column exists for derating arithmetic. You start at the higher number, apply the ambient-temperature and conduit-fill factors, and see where you land. It is not a licence to put more breaker behind small wire. Misreading this is the most common way an internet answer goes wrong.
Question two: will enough of it arrive
Copper is a good conductor, not a perfect one. Every foot of it has resistance, and current through resistance loses voltage as heat. The formula for a single-phase run is:
Volts lost = 2 × K × amps × one-way feet ÷ circular mils
K is 12.9 for copper and 21.2 for aluminum. The 2 is there because the current goes out on one conductor and comes back on the other. Measure the distance one way and the formula counts the return leg for you. Circular mils is the conductor’s cross-sectional area: 6,530 for 12 AWG, 10,380 for 10 AWG, and so on up the table.
The NEC does not enforce a voltage-drop limit on branch circuits. It recommends one in informational notes: 3% on a branch circuit, 5% across feeder and branch combined. Everyone treats those as the standard of care, because equipment is designed around them and because a lighting circuit at 6% looks visibly wrong.
What that means in feet
A fully loaded 20 amp circuit on 12 AWG copper at 120 volts reaches about 45 feet before it passes 3%. That number surprises people, and then they notice the qualifier: fully loaded. A bedroom circuit drawing four amps of LED lighting and a phone charger can run two hundred feet and never notice. The drop scales with the current actually flowing, not with the breaker size.
So run length matters most where the load is real and steady: a freezer in a detached garage, a well pump, a workshop with a table saw, a subpanel feeding all of it. Calculate those runs and don’t guess. The voltage drop calculator gives you the percentage for a size you are considering. The wire size calculator works the other way, starting from the load and the distance and telling you which of the two constraints, the table or the run, decided the answer.
The 240 volt shortcut
If the load can take it, doubling the voltage beats doubling the copper. The same watts at 240 volts draw half the amps, and half the current loses half the volts, measured against a supply twice as large. That is a quarter of the percentage drop for the price of a two-pole breaker.
This is why a long run to a workshop is worth wiring as a small 240 volt subpanel and not as two 120 volt circuits, and why baseboard heaters over about 1,500 watts are almost always 240 volt units. The electric heat calculator shows both currents side by side for this comparison.
Aluminum, honestly
Aluminum conductors are cheaper per amp and lighter to pull, and they are still completely standard for service entrances and large feeders. They are also about 60% more resistive than copper for the same size, so they run a size or two larger and drop more voltage over the same distance.
The bad reputation comes from 1960s and 70s branch-circuit wiring: small aluminum conductors on devices never designed for them, which loosened, oxidised and overheated at the terminations. Modern practice for large aluminum is unremarkable: listed AL/CU connectors, antioxidant compound, proper torque. For a fifteen or twenty amp branch circuit, buy copper and stop thinking about it.
What the calculators cannot do
- Derating. More than three current-carrying conductors in a raceway, or an ambient temperature above 30 °C, reduces ampacity by published factors. The wire size calculator takes a derate multiplier if you know yours, but it will not work it out for you.
- What is already on the circuit. Nothing in a formula knows that the bedroom receptacles you are extending already share a circuit with the hallway.
- Your local code. Jurisdictions adopt NEC editions on their own schedule and amend them. The tables here are the widely adopted ones, not necessarily yours.
- Whether you should be doing this at all. Sizing a circuit on paper is planning. Installing it is permitted, inspected work, and in most places some of it is licensed work.
Do the arithmetic before you talk to an electrician anyway. A quote that arrives with a wire size you already understand is a quote you can ask questions about.