Wire Size Calculator
Find the right conductor size for a circuit run — checks both ampacity and voltage drop, side by side for copper and aluminum.
Load
Wire size
Your result will appear here
Please add: Load, Load unit, Voltage, Phase, One-way distance, Max allowable voltage drop, Ambient temperature.
Wire it right
Build With UsThe Wire Size Calculator finds the appropriate conductor size for a circuit from its load and installation conditions.
Choosing the right size matters for safety and performance. An undersized conductor overheats, drops too much voltage, and raises fire risk. An oversized one is safe but wastes money on copper you don't need. The calculator gives a quick, defensible starting size before you pull conductors.
How to Use the Wire Size Calculator
The recommendation rests on a few inputs.
- The circuit load, in amps or watts, is the biggest driver — it sets the minimum current the conductor must carry.
- The one-way distance from the source to the load. Longer runs mean more resistance and more voltage drop, which can force a larger conductor than ampacity alone would.
- The ambient temperature. Hot attics, direct sun, and heated equipment rooms reduce how much current a conductor can safely carry, so the NEC derates ampacity above 86°F.
- The maximum allowable voltage drop, as a percentage. Most branch circuits target 3% or less to keep equipment running efficiently.
The calculator checks conductor ampacity and voltage drop against these, then recommends the smallest AWG size that satisfies whichever is stricter — reported for both copper and aluminum. If you're sizing a whole panel, start with the Electrical Load Calculator to find the service demand first.
Wire Size Calculator Example
You're installing a circuit for a 240V electric-vehicle charger:
- Circuit load: 40 amps
- One-way distance from the panel: 120 feet
- Ambient temperature: 95°F
- Maximum allowable voltage drop: 3%
At 95°F, ampacity is derated to 94% of the table value, so a conductor has to carry 40 A after that derate and still stay under 3% drop over 120 feet. The calculator lands on 6 AWG copper: it carries the load with margin and drops about 1.96% over the run. The next size down, 8 AWG, would carry the current but slip past 3% on voltage drop — which is exactly the trap the tool is built to catch. In aluminum, the same run needs 4 AWG to hold the drop within 3%.
Shorten the run, cool the environment, or lower the load and a smaller conductor may do. Lengthen the run or raise the load and it grows.
Why Proper Wire Sizing Matters
Correct sizing improves both safety and performance.
Right-sized conductors run cooler, hold voltage drop in check, and deliver adequate operating voltage to motors, appliances, and HVAC equipment — which extends their life. Undersized wire does the opposite, and the failure mode is heat.
Common circuit sizes (copper, 75°C, 86°F ambient)
Typical breaker sizes and their common copper conductor pairing:
Ampacity alone picks these sizes — a long run or a tight voltage-drop target can still push you to a larger conductor. Always run the numbers for your actual distance.
Frequently Asked Questions
Why does the recommended size sometimes exceed the ampacity minimum?
On long runs, voltage drop rather than ampacity is often the limiting factor. This tool checks both and recommends the smallest size that satisfies whichever is stricter.
Why is aluminum always a bigger size than copper?
Aluminum has roughly 1.6 times the resistance of copper for the same size, so it needs a larger cross-section to carry the same current at an equivalent voltage drop.
What voltage drop should I target?
The NEC recommends no more than 3% on a branch circuit and 5% total (branch plus feeder). It's standard practice rather than a hard code requirement.
Can the Wire Size Calculator replace the NEC or local codes?
No. It gives a reliable estimate from the values entered. Final conductor sizing has to be verified against the NEC and local amendments by a licensed electrician.
This is a simplified planning estimate using NEC ampacity and DC resistance tables, not a substitute for a licensed electrician's conductor sizing. Verify against local code before pulling wire.
Build the tools your trade needs.
Dalton Mills gives people in the trades the opportunity to create custom workflows and software tools without any prior technical experience.
Apply for early access