How to use it
- Pick single- or three-phase and enter the voltage and load in amps.
- Mark the load as continuous if it runs 3 hours or more.
- Enter the one-way run length in feet and pick copper or aluminum.
- Choose the 60 °C, 75 °C or 90 °C ampacity column and the maximum voltage drop.
- Read the recommended AWG, breaker size and voltage drop.
What this wire gauge calculator checks
A wire has to pass two tests: it must carry the current without overheating (ampacity), and it must not lose too much voltage over the run. This wire size calculator checks both for every AWG from 14 to 4/0 and picks the smallest one that passes.
It follows the structure of the National Electrical Code (NFPA 70): ampacities from Table 310.16 (not more than three current-carrying conductors, 86 °F / 30 °C ambient) in the 60 °C, 75 °C and 90 °C columns, the small-conductor limits of 240.4(D), and the 125% rule for continuous loads. The results table shows ampacity, maximum breaker, voltage drop in volts and percent for each size, so you can see which test drives the answer. Metric (mm²) is one click away.
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Safety note: electrical work should be done by a licensed electrician and must meet your local code and inspection. This calculator is informational and does not replace a design or an inspection.
The rules and formulas
1. Design current. For a continuous load (3 hours or more), size for 125%: design amps = load × 1.25. The breaker is the next standard size at or above that.
2. Ampacity. The conductor's Table 310.16 ampacity in the chosen column must be at least the design amps. Use the 60 °C column unless every termination is rated for 75 °C; the 90 °C column is mainly a starting point for derating.
3. Small conductors, 240.4(D). Whatever the table says, copper 14 AWG is limited to a 15 A breaker, 12 AWG to 20 A and 10 AWG to 30 A. For aluminum, 12 AWG is limited to 15 A and 10 AWG to 25 A.
4. Voltage drop.
VD = k × L × I × R / 1000
- k = 2 for single-phase, 1.732 for three-phase
- L = one-way length in feet, I = load current in amps
- R = resistance in ohms per 1,000 ft from NEC Chapter 9, Table 8 (75 °C): copper 14 AWG 3.07, 12 AWG 1.93, 10 AWG 1.21, 8 AWG 0.764
Informational notes in the NEC suggest about 3% drop on a branch circuit and 5% total; the calculator lets you set your own limit.
Worked example: 16 A continuous load at 120 V
Inputs: 16 A continuous, 120 V single-phase, 50 ft one way, copper, 60 °C column, 3% maximum drop.
- Design current: 16 × 1.25 = 20 A, so a 20 A breaker.
- 14 AWG: 15 A at 60 °C, too small. 12 AWG: 20 A, and 240.4(D) allows a 20 A breaker. Passes.
- Voltage drop on 12 AWG: 2 × 50 × 16 × 1.93 / 1000 = 3.09 V, or 2.57%. Under 3%.
- Result: 12 AWG copper.
At 100 ft, 12 AWG drops 5.15% and 10 AWG drops 3.23%, so the calculator moves up to 8 AWG (2.04%).
What size wire for 30 amp? For a 30 A noncontinuous load, 10 AWG copper is the smallest size: 30 A in the 60 °C column, and 240.4(D) allows a 30 A breaker on it. At 240 V and 50 ft it drops 1.51%. At 120 V over the same 50 ft it drops 3.02%, just over 3%, so the calculator suggests 8 AWG (1.91%).
Column choice matters: a 40 A continuous load needs 50 A. In the 60 °C column that is 6 AWG (55 A); if every termination is rated 75 °C, 8 AWG (50 A) qualifies.
Copper ampacity table (NEC 310.16)
Copper, not more than three current-carrying conductors, 86 °F (30 °C) ambient, as used by the calculator:
| AWG | 60 °C | 75 °C | 90 °C | Max breaker, 240.4(D) |
|---|---|---|---|---|
| 14 | 15 | 20 | 25 | 15 A |
| 12 | 20 | 25 | 30 | 20 A |
| 10 | 30 | 35 | 40 | 30 A |
| 8 | 40 | 50 | 55 | – |
| 6 | 55 | 65 | 75 | – |
| 4 | 70 | 85 | 95 | – |
| 2 | 95 | 115 | 130 | – |
| 1/0 | 125 | 150 | 170 | – |
Aluminum typically needs one to two sizes larger for the same ampacity and has no 14 AWG entry. Higher ambient temperatures and more than three current-carrying conductors in a raceway require adjustment factors, which this simplified calculator does not apply.
Tips and common mistakes
- Using the 90 °C column for the final size. Breakers and devices are usually rated 60 °C or 75 °C; the lowest-rated termination sets the column.
- Forgetting 125% for continuous loads. EV chargers, water heaters and baseboard heat usually count as continuous.
- Measuring round trip. Enter the one-way distance; the formula already doubles it.
- Ignoring voltage drop on long runs. Detached garages, well pumps and outbuildings often need a larger wire than ampacity alone suggests. Check a specific run with the voltage drop calculator.
- Bundling and hot attics. Derating can push you up a size; ask your electrician.
Have a licensed electrician design and install the circuit; this tool is informational.
Frequently asked questions
What size wire do I need for 30 amps?
10 AWG copper for a 30 A noncontinuous load on a short run. Over longer runs, voltage drop may require 8 AWG; at 120 V that happens at about 50 ft with a 3% limit.
What size wire for a 20 amp circuit?
12 AWG copper. Under NEC 240.4(D), 12 AWG copper may be protected by up to a 20 A breaker, and 14 AWG only up to 15 A.
Why does a continuous load need a bigger wire?
The NEC sizes conductors and breakers for 125% of a continuous load, one that runs 3 hours or more. A 16 A continuous load is treated as 20 A.
How far can I run 12 gauge wire?
It depends on current and voltage. At 16 A and 120 V, 12 AWG copper stays under 3% drop up to about 58 ft one way. The calculator shows the drop for your exact numbers.
Can I use the 75 °C column?
Only if every termination on the circuit, breaker and device included, is rated for 75 °C. Otherwise use the 60 °C column, which is the safer default for 100 A and smaller circuits.
Can I wire the circuit myself with this result?
Electrical work should be done by a licensed electrician and inspected where required. The calculator is informational and a starting point for that conversation.
Sources
Last updated: November 13, 2026