Wire size comes from two checks, and the larger answer wins. A 16 amp load on a 120 volt circuit needs 12 AWG copper to carry the current (Table 310.16 and the 20 amp breaker it goes on), but if the load is 100 feet away, 12 AWG drops 5.27% of the voltage and you need 8 AWG to stay under the 3% the NEC recommends. Short runs are sized by ampacity; long runs by voltage drop.
How the wire size is calculated
Ampacity. Take the load in amps, multiplied by 1.25 if it runs three hours or more (a continuous load, 210.19(A)(1), as EC&M explains). Find the smallest conductor in Table 310.16 whose allowable ampacity is at least that. The column you may use is the lowest temperature rating in the circuit: the wire's insulation, and the terminals on the breaker and the device. Southwire states that NM-B cable (Romex) is limited to the 60 °C column even though its conductors are 90 °C wire.
Corrections. Above three current-carrying wires in one conduit or cable, multiply the ampacity by the Table 310.15(C)(1) factor (80% for 4 to 6 wires, 70% for 7 to 9, 50% for 10 to 20). In hot spaces, multiply by the Table 310.15(B)(1)(1) ambient factor. Derating starts from the insulation column, and the result may never exceed the terminal column.
Small conductor rule. 240.4(D) caps the breaker on small wire: 15 A on 14 AWG copper, 20 A on 12 AWG, 30 A on 10 AWG, 15 A on 12 AWG aluminum and 25 A on 10 AWG aluminum, whatever the ampacity table says.
Voltage drop. The wire must also hold the drop to 3% on a branch circuit:
CM = 2 × K × I × L ÷ (volts × 3%), with K = 12.9 for copper, 21.2 for aluminum
The first size in Chapter 9 Table 8 with at least that many circular mils passes.
Worked example: 16 A, 120 V, 100 ft
With the defaults (copper NM-B cable, 60 °C terminals, 16 A noncontinuous):
- Ampacity: 14 AWG is 15 A at 60 °C, too small. 12 AWG is 20 A, and the 20 A breaker the load needs is allowed on 12 AWG, so 12 AWG by ampacity.
- Voltage drop: 2 × 12.9 × 16 A × 100 ft ÷ 3.6 V = 11,467 cmil. 10 AWG has 10,380, so 8 AWG (16,510 cmil), which drops 2.50 V, 2.08%.
- Answer: 8 AWG copper. At 50 feet, 12 AWG would pass both checks.
Tick "continuous load" and the same 16 A needs 20 A of ampacity: 12 AWG still carries it at 60 °C, with no margin.
Wire gauge and amps: Table 310.16
Allowable ampacity with up to three current-carrying conductors at 86 °F (NEC 2023 Table 310.16):
| Size | Copper 60 °C | Copper 75 °C | Copper 90 °C | Aluminum 60 °C | Aluminum 75 °C |
|---|---|---|---|---|---|
| 14 AWG | 15 A | 20 A | 25 A | none | none |
| 12 AWG | 20 A | 25 A | 30 A | 15 A | 20 A |
| 10 AWG | 30 A | 35 A | 40 A | 25 A | 30 A |
| 8 AWG | 40 A | 50 A | 55 A | 35 A | 40 A |
| 6 AWG | 55 A | 65 A | 75 A | 40 A | 50 A |
| 4 AWG | 70 A | 85 A | 95 A | 55 A | 65 A |
| 2 AWG | 95 A | 115 A | 130 A | 75 A | 90 A |
| 1/0 AWG | 125 A | 150 A | 170 A | 100 A | 120 A |
| 2/0 AWG | 145 A | 175 A | 195 A | 115 A | 135 A |
| 4/0 AWG | 195 A | 230 A | 260 A | 150 A | 180 A |
For 14 to 10 AWG the 240.4(D) breaker limits apply on top of the table. The 90 °C column is mainly for derating: six 12 AWG THHN wires in one conduit get 30 A × 80% = 24 A, still inside the 25 A limit of 75 °C terminals, and still on a 20 A breaker.
Wire size for 12 V and DC circuits
Pick the DC tab for batteries, RVs, boats, solar and low-voltage lighting. Ampacity rarely decides at 12 V; voltage drop does. A 10 A load 20 feet from a 12 V battery could run on 16 AWG by current alone (240.4(D) allows 10 A on 16 AWG), but it would lose 2.00 V, 16.7%. Holding 3% takes 8 AWG. At 24 V, 10 AWG passes. The voltage drop calculator shows every size side by side.
Copper or aluminum
Aluminum carries less current per size and has a higher K (21.2 against 12.9), so it runs one or two sizes larger. It is common and economical for feeders and service entrance cable, less so for branch circuits. The calculator starts aluminum at 12 AWG and copper at 14 AWG, the smallest sizes in the ampacity table.
What the calculator leaves to you
The equipment grounding wire has its own sizing table. Motors, air conditioners and welders have nameplate rules (minimum circuit ampacity, maximum breaker) that override the general method: use the nameplate. The inspector enforces the NEC edition your state adopted, which may be older than 2023.
Next steps
With the size known, check the breaker size for the circuit, work out the conduit fill for those wires, or turn watts into amps with the amp calculator. Planning a new service or a big addition? Start with the electrical load calculator. All tools are on the electrical calculators page.
Sizing wire for a 12V circuit in an RV, trailer or solar setup? Use this wire size calculator with 12 volts and check the run with the voltage drop calculator, because low voltage loses proportionally more over distance.