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Ampacity reference: NEC Table 310.15(B)(16), 75°C column. Always verify against the current code edition and local amendments.
How to Use This Wire Size Calculator
Enter the system voltage, load current, one-way circuit length, wire material, maximum allowed voltage drop, and conduit type to estimate the minimum wire size for your circuit. The calculator checks both ampacity and voltage drop so you can size conductors for common residential and commercial installations.
Use it for branch circuits, subpanel feeders, garage or workshop runs, HVAC equipment, EV charger circuits, and other long runs where voltage drop can push you into a larger AWG size than ampacity alone would suggest.
Why Correct Wire Size Matters
Choosing the correct wire size helps limit overheating, keeps equipment operating properly, and reduces unnecessary voltage loss across the circuit. A conductor that meets ampacity requirements may still be too small for a long run if the voltage drop is excessive.
That is why wire sizing often involves more than just amperage. Circuit length, conductor material, system voltage, and installation method all affect the final AWG recommendation.
What Affects Wire Size?
- Load current: Higher amperage requires a conductor with greater ampacity.
- Circuit length: Longer runs increase resistance and voltage drop.
- Wire material: Copper and aluminum have different resistance and ampacity characteristics.
- System voltage: The same load and distance can produce different percentage drop at 120V vs 240V or 480V.
- Voltage drop target: Designing around 3% or 5% can change the recommended AWG size.
- Conduit type: Raceway and installation conditions influence practical conductor sizing decisions.
Common Wire Sizing Scenarios
This calculator is useful for sizing wire for 20 amp and 30 amp branch circuits, 60 amp and 100 amp subpanels, detached garages, sheds, workshops, HVAC equipment, EV chargers, and longer copper or aluminum feeder runs. You can also compare the result against our voltage drop calculator, AWG wire gauge chart, and conduit fill calculator when planning a full installation.
Frequently Asked Questions
#12 AWG copper (75°C rating) or #10 AWG aluminum per NEC Table 310.15. Always confirm ampacity, termination rating, and voltage drop for your specific run length.
A common starting point is #4 AWG copper or #2 AWG aluminum, but the correct size depends on conductor material, run length, installation conditions, and allowable voltage drop.
The NEC ampacity table lists allowable ampacity by AWG size, conductor material, and temperature rating. This calculator uses the 75°C column as a practical reference for many common terminations.
Wire size is mainly affected by load current, circuit length, system voltage, conductor material, allowable voltage drop, and installation conditions such as conduit type and temperature rating.
Yes. Longer runs increase resistance and voltage drop, so upsizing the conductor is often needed even when the ampacity requirement is already met.
Copper usually allows a smaller conductor size for the same load because it has lower resistance. Aluminum can still be cost-effective, but it often requires a larger AWG size to meet the same performance target.