All load and distance combinations · Wire size calculator · 20 A breaker
20 A · 25 ft · 120 V single phase
Sized for both ampacity and a 3% voltage drop limit, this run needs 12 AWG copper. Ampacity governs here; voltage drop is comfortably inside the limit.
Answer
NEC 2023This answer is transcribed from the 2023 NEC (NFPA 70, 2023). The code is republished on a three-year cycle and adoption varies by jurisdiction - confirm against your locally adopted edition and your authority having jurisdiction (AHJ) before you build.
| Material | By ampacity | By voltage drop | Use | Drop | Breaker |
|---|---|---|---|---|---|
| Copper | 12 AWG | 14 AWG | 12 AWG | 1.65% | 20 A |
| Aluminum | 10 AWG | 12 AWG | 10 AWG | 1.7% | 20 A |
Ampacity governs here. 12 AWG copper is the smallest conductor that carries 20 A at a 75°C termination rating, and over 25 ft its voltage drop of 1.65% is comfortably inside the 3% target.
That comfort disappears as the run gets longer. The same 20 A load at triple this distance would likely need a size or two more purely for voltage drop.
With a 20 A overcurrent device ahead of it, the equipment grounding conductor for this circuit is 12 AWG copper.
This page fixes several assumptions that may not match your job. It assumes a 75°C termination rating, which is typical but not universal - check what your breaker and equipment are actually listed for. It assumes a 30°C ambient, which an attic, a rooftop, or a boiler room will exceed. It assumes three current-carrying conductors in the raceway; more than that triggers an adjustment that reduces ampacity further.
It also assumes a non-continuous load. A load that runs three hours or more is sized at 125% of its rating, which frequently pushes the conductor up a size. And it does not model motor circuits, air conditioning equipment, or the other load types that have their own sizing articles.
Run your real numbers through the wire size calculator, then have the result confirmed by a licensed electrician before anything gets pulled.
FAQ
12 AWG copper. Under the 2023 NEC tables, that is the smallest copper conductor that both carries 20 A at a 75°C termination rating and stays inside a 3% voltage drop over a 25 ft one-way run at 120 V single phase, assuming a 30°C ambient and no more than three current-carrying conductors.
No, ampacity governs this run. 12 AWG copper is the smallest conductor that carries 20 A at a 75°C termination rating under the 2023 NEC table, and over 25 ft at 120 V its estimated drop of 1.65% stays inside the 3% target used on this page.
Yes, at 10 AWG. Aluminum conducts less per unit area than copper, so under the same 2023 NEC assumptions this circuit takes 10 AWG aluminum against 12 AWG copper, with an estimated drop of 1.7%. Terminations and lugs must be listed for aluminum conductors.
A 20 A breaker, the smallest standard overcurrent rating at or above the 20 A load under the 2023 NEC standard ratings. With a 20 A device ahead of it, the equipment grounding conductor for this circuit is 12 AWG copper. A continuous load, one running three hours or more, is sized at 125% and may need the next rating up.
Source: NFPA 70, National Electrical Code, Table 310.16; NFPA 70, National Electrical Code, 210.19(A) and 215.2(A) informational notes. Published by NFPA.NEC 2023Transcribed from the 2023 NEC (NFPA 70, 2023). Confirm against your locally adopted edition and your authority having jurisdiction (AHJ).
Related