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Wire Size Calculator
20 amps at 120 volts over a 50 ft run needs 10 AWG copper wire to keep voltage drop under 3% (2.07%, 2.48 V). Ampacity alone would allow 12 AWG. Enter your own amps or watts, voltage, length and wire type below.
Planning estimate based on NEC tables. Local codes, conditions of use and terminations can require a larger wire. Have a licensed electrician design and install circuits.
YOUR CIRCUIT
RECOMMENDED WIRE SIZE
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Planning estimate based on NEC tables. Local codes, conditions of use and terminations can require a larger wire. Have a licensed electrician design and install circuits.
How we calculate wire size
The calculator finds two sizes and recommends the larger one. First, the smallest conductor whose ampacity in NEC Table 310.16 (2023 edition, 30°C ambient, no more than three current-carrying conductors) covers your current, at the insulation temperature column you pick. Continuous loads are multiplied by 1.25 for this check. Second, the smallest conductor whose voltage drop stays within your limit.
Voltage drop is current × the DC resistance per foot × the length × 2 for single-phase and DC circuits (current goes out and back), or × 1.732 for three-phase. Resistances are the 75°C values for uncoated stranded copper and aluminum from NEC Chapter 9, Table 8. Aluminum starts at 12 AWG in Table 310.16, and the largest size here is 500 kcmil: beyond that, parallel conductors or an engineered design are needed.
The small-conductor limits of NEC 240.4(D) are applied: 14 AWG copper is limited to a 15 amp breaker, 12 AWG to 20 amps and 10 AWG to 30 amps (aluminum 12 AWG 15 amps, 10 AWG 25 amps), so a 25 amp load needs 10 AWG even though the table lists 12 AWG at 75°C as 25 amps. Watts are converted to amps with a power factor of 1 (for three-phase, watts ÷ (1.732 × volts)).
Not included: derating for ambient temperatures above 86°F (30°C) and for more than three current-carrying conductors in a raceway or cable, motor and other special rules, and local amendments. Hot attics, rooftops and bundled cables can require a larger wire.
Sources
- NFPA 70 (NEC) 2023, Table 310.16 – allowable ampacities, 30°C ambient, not more than three current-carrying conductors; values cross-checked against Cerrowire's ampacity chart and Portlandia Electric Supply's NEC 310.16 chart. Checked October 2026.
- NFPA 70 (NEC) 2023, Chapter 9 Table 8 – DC resistance in ohms per 1,000 ft at 75°C for uncoated stranded copper and aluminum; spot-checked against the ECM magazine and Mike Holt Table 8 references. Checked October 2026.
Frequently asked questions
What size wire for a 20 amp circuit?
12 AWG copper carries 20 amps (NEC 240.4(D) limits 12 AWG copper to a 20 amp breaker). On a 120 V run of 25 ft the drop is 1.65%. On a 100 ft run the voltage-drop size grows to 8 AWG copper (2.59% drop).
What size wire for a 30 amp circuit?
10 AWG copper is the usual answer for 30 amps (a 30 amp breaker is the NEC limit for 10 AWG copper). At 240 V over 25 ft the drop is 0.78%.
What size wire for a 50 amp circuit?
At 75°C ratings 8 AWG copper carries 50 amps, and on a 240 V run of 25 ft the drop is 0.81%. If the terminations are rated only 60°C, the table needs the next size up. A 150 ft run at 240 V needs 4 AWG copper for 3% (1.93% drop).
What size wire for a 100 ft run?
Length matters because voltage drop grows with it. A 20 amp, 120 V load 100 ft away needs 8 AWG copper for 3% drop, versus 12 AWG at 25 ft. Enter your own length in the calculator.
What wire size for 12 V DC solar, RV or boat circuits?
Low voltage makes voltage drop the main limit. 30 amps at 12 V DC over 20 ft one way needs 3 AWG copper for 3% (2.45%, 0.29 V), while ampacity alone would allow 10 AWG. Choose 12, 24 or 48 V DC in the calculator.
How do I calculate voltage drop?
Voltage drop = current × resistance per foot × wire length × 2 for single-phase and DC (out and back), or × 1.732 for three-phase. The calculator uses the DC resistance of each size from NEC Chapter 9 Table 8 at 75°C and compares it to your allowed percent of the system voltage.
What voltage drop is acceptable?
The NEC recommends (in informational notes, not as a requirement) up to 3% on a branch circuit or feeder and 5% total for feeder plus branch circuit. This calculator defaults to 3% and also offers 2% and 5%.
Is aluminum wire the same size as copper?
No. Aluminum carries less current and has higher resistance, so it needs a larger size. 60 amps at 240 V over 100 ft needs 4 AWG aluminum (2.54% drop). Aluminum is listed from 12 AWG up in NEC Table 310.16.
What do the 60°C, 75°C and 90°C ratings mean?
They are the insulation temperature columns in NEC Table 310.16. Terminals on breakers and equipment are usually rated 60°C or 75°C, so even 90°C wire such as THHN is normally sized from the 60 or 75°C column. Equipment for 100 amp circuits or less is treated as 60°C unless it is marked 75°C. The calculator defaults to 75°C; pick 60°C if your terminals are not marked 75°C.
What does continuous load mean?
A load that runs for 3 hours or more. The NEC sizes the conductor for 125% of a continuous load, so tick the box and the calculator multiplies your amps by 1.25 for the ampacity check (voltage drop still uses the real current).
Does this account for hot locations or many wires in one conduit?
No. Ambient above 86°F (30°C) and more than 3 current-carrying conductors in a raceway or cable need derating that is not applied here. Hot attics, rooftops and bundles can require a larger wire.
Can I use this to install wiring myself?
It is a planning estimate based on NEC tables. Local codes, conditions of use and terminations can require a larger wire. Have a licensed electrician design and install circuits.