Voltage Drop Calculator

Calculate voltage drop in volts and percent for a given AWG conductor, amperage, circuit length, and system voltage — compare copper and aluminum runs before you pull wire or size a feeder.

Circuit Details

Amperage

Circuit Length

Voltage

Conductor Material

Wire Gauge

Live Results

Voltage Drop

3.19V

Voltage Drop

2.65%

Amperage

20A

Circuit Length

50ft

Voltage

120V

Wire Gauge

12 AWG

Conductor Material

Copper

Voltage drop calculations use standard NEC resistivity values. Always verify final wire sizing with local electrical code.

How to Use This Voltage Drop Calculator

Enter the circuit amperage. Input the load current in amperes — use the breaker rating, nameplate FLA, or calculated load for the circuit. Common branch-circuit values are 15 A for lighting, 20 A for general receptacles, and 30 A or higher for dedicated appliance circuits.
Measure one-way circuit length. Enter the one-way distance in feet from the panel to the load. The calculator applies the standard factor of 2 in the voltage drop formula to account for both outbound and return conductors in a single-phase circuit.
Select system voltage. Choose 120 V for standard single-pole branch circuits or 240 V for large appliances, HVAC equipment, and double-pole circuits. The same absolute voltage drop in volts produces a lower percentage drop on 240 V systems.
Choose conductor material. Select copper for most branch circuits and short runs, or aluminum for feeders and larger conductors where material cost matters. Aluminum has higher resistivity and produces more voltage drop for the same AWG size compared to copper.
Select wire gauge. Pick the AWG size of the conductor you plan to use or want to evaluate. Results update instantly — compare voltage drop across gauges to see whether upsizing wire or shortening the run is needed to stay within recommended limits.

Formulas & Example

Voltage drop uses the standard single-phase formula with a factor of 2 for the outbound and return conductors. Resistivity is expressed in ohm-cmil per foot for copper and aluminum conductors.

Voltage Drop (V) = (2 × Circuit Length (ft) × Resistivity × Amperage) ÷ Circular Mil Area
Voltage Drop (%) = (Voltage Drop ÷ System Voltage) × 100

Resistivity (ohm-cmil/ft):
  Copper:    10.4
  Aluminum:  17.0

Worked Example

A 20 A load on a 50 ft one-way run at 120 V with 12 AWG copper conductors:

CMIL (12 AWG) = 6,530
Resistivity (copper) = 10.4 ohm-cmil/ft

Voltage Drop = (2 × 50 × 10.4 × 20) ÷ 6,530 = 3.19 V
Voltage Drop % = (3.19 ÷ 120) × 100 = 2.66%

Result: 3.19 V drop (2.66%) — within the recommended 3% branch-circuit limit

The same circuit with 12 AWG aluminum would produce about 5.21 V (4.34%) because aluminum resistivity is higher. Upsizing to 10 AWG aluminum or using copper would bring drop back within recommended limits. Pair this tool with the Wire Gauge (AWG) Calculator to find the minimum gauge that satisfies both ampacity and drop requirements.

Frequently Asked Questions

What is voltage drop?▾

Voltage drop is the reduction in voltage between the source and the load caused by conductor resistance over the circuit length. As current flows through wire, a portion of the system voltage is lost as heat in the conductors. Excessive drop means equipment receives less than nominal voltage, which can cause dim lights, slow motors, and poor appliance performance.

What is acceptable voltage drop?▾

The NEC recommends limiting voltage drop to 3% on branch circuits and 5% total from the service entrance to the farthest outlet. These are informational recommendations rather than mandatory code rules in most jurisdictions, but they represent good engineering practice for efficient equipment operation. Sensitive electronics, long outdoor runs, and motor loads often benefit from sizing to 2% or less.

Does wire gauge affect voltage drop?▾

Yes — larger wire (smaller AWG number) has greater cross-sectional area and lower resistance, which reduces voltage drop for the same current and length. Moving from 14 AWG to 12 AWG can cut drop roughly in half because circular mil area increases significantly. When drop exceeds 3%, upsizing one or two AWG sizes is usually the first corrective step before reducing circuit length.

What are the copper vs aluminum voltage drop differences?▾

Aluminum has higher resistivity (17.0 ohm-cmil/ft) than copper (10.4 ohm-cmil/ft), so the same AWG aluminum conductor produces about 63% more voltage drop than copper at equal amperage and length. For the same drop limit, aluminum typically requires one or two AWG sizes larger than copper. Aluminum also requires listed connectors, anti-oxidant compound, and proper torque at terminations.

How does circuit length affect voltage drop?▾

Voltage drop is directly proportional to one-way circuit length because resistance accumulates over the run. Doubling the distance from panel to load doubles the voltage drop in volts. This is why a 20 A garage circuit 150 ft from the panel may need 10 AWG or 8 AWG copper while a 25 ft kitchen receptacle circuit can use 12 AWG — length is often the limiting factor rather than ampacity alone.

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