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Disclaimer: This calculator provides preliminary estimates for educational purposes only. It is not a substitute for a licensed professional engineer, architect, or structural engineer. Results depend on material grade and species, site conditions, local climate and snow loads, and amendments to the applicable building code, all of which vary by location. Always verify results against your local building codes and confirm requirements with your Authority Having Jurisdiction before proceeding. Do not use these estimates for permitting, fabrication, or construction without stamped, engineer-sealed drawings from a licensed professional. Improperly sized structural members, foundations, or gas lines can cause structural collapse, fire, gas leaks, serious injury, or death.

Example Calculations

120A Kitchen Circuit (120V, 50 ft, 12 AWG Copper)

Inputs

Current20 amps

Voltage120V

Distance50 ft

Wire12 AWG Copper

Result

Voltage Drop2.65% (3.18V)

Ampacity (75°C)25A

Max Distance (3%)56 ft

12 AWG copper resistance = 1.588 ohm/1000ft. Drop = 20 × 1.588 × 100/1000 = 3.18V. Percent = 3.18/120 = 2.65%.

230A Dryer Circuit (240V, 40 ft, 10 AWG Copper)

Inputs

Current30 amps

Voltage240V

Distance40 ft

Wire10 AWG Copper

Result

Voltage Drop1.0% (2.4V)

Ampacity (75°C)35A

Max Distance (3%)120 ft

10 AWG copper resistance = 0.999 ohm/1000ft. Drop = 30 × 0.999 × 80/1000 = 2.4V. Percent = 2.4/240 = 1.0%.

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Frequently Asked Questions

What is the maximum acceptable voltage drop?

NEC recommends no more than 3% voltage drop for branch circuits and 5% total for feeder + branch combined. At 120V, 3% = 3.6V drop. Longer wire runs need thicker gauge to stay within limits.

Branch circuit limit: 3% voltage drop (NEC recommendation)
Total system limit: 5% (feeder + branch combined)
At 120V, 3% = 3.6V maximum drop
At 240V, 3% = 7.2V maximum drop
Voltage drop increases linearly with distance and current

Voltage	3% Drop	5% Drop	Impact
12V DC	0.36V	0.6V	LED dimming, motor slowdown
120V AC	3.6V	6V	Lights dim, motors run hot
240V AC	7.2V	12V	Reduced appliance efficiency

What wire gauge do I need for 20 amps?

12 AWG copper wire handles 20 amps at 75°C rating per NEC Table 310.16. For runs over 50 feet at 120V, consider upsizing to 10 AWG to keep voltage drop under 3%. Aluminum wire requires one gauge larger than copper.

14 AWG: 15 amps (typical 15A household circuits)
12 AWG: 20 amps (kitchen, bathroom, garage circuits)
10 AWG: 30 amps (dryers, water heaters)
8 AWG: 40–50 amps (ranges, large appliances)
6 AWG: 55–65 amps (sub-panels, large A/C units)

How far can you run 12 gauge wire?

At 20A and 120V, 12 AWG copper can run about 50 feet one-way before hitting 3% voltage drop. At 10A, it can go about 100 feet. Lower current or higher voltage extends the safe distance.

20A at 120V: ~50 feet one-way (3% limit)
15A at 120V: ~65 feet one-way
20A at 240V: ~100 feet one-way
Double distance by halving current
Double distance by doubling voltage

What is the difference between copper and aluminum wire?

Aluminum has 61% the conductivity of copper and requires one gauge size larger for the same ampacity. A 20A copper circuit uses 12 AWG while aluminum needs 10 AWG. Aluminum is cheaper but requires special connectors.

Copper: Better conductivity, smaller diameter, easier to work with
Aluminum: 30–40% cheaper, lighter weight, requires larger gauge
Aluminum needs anti-oxidant compound at connections
Copper resistance: 10.37 ohm/1000ft for 12 AWG
Aluminum resistance: 16.51 ohm/1000ft for 12 AWG (1.6× copper)

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Last Updated: Jul 24, 2026

This calculator is provided for informational and educational purposes only. Results are estimates and should not be considered professional financial, medical, legal, or other advice. Always consult a qualified professional before making important decisions. UseCalcPro is not responsible for any actions taken based on calculator results.

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