Voltage Drop Calculator
Voltage drop is one of the most important considerations in electrical circuit design. Every wire has resistance, and as current flows through it, some voltage is lost as heat. The longer the wire run and the higher the current, the more voltage is lost. If too much voltage is dropped, equipment at the end of the circuit will not operate correctly—motors overheat, lights dim, and sensitive electronics malfunction.
The National Electrical Code (NEC) recommends a maximum voltage drop of 3% for branch circuits and 5% total for the combined feeder and branch circuit (NEC 210.19(A) Informational Note No. 4 and 215.2(A) Informational Note No. 2). While these are recommendations rather than requirements, they represent good practice that every electrician should follow. Exceeding these limits leads to wasted energy, poor equipment performance, and potential safety issues.
This calculator uses the standard voltage drop formula with copper conductor resistance values from NEC Chapter 9, Table 8. Enter your circuit parameters and instantly see the voltage drop in volts, the percentage drop, and the voltage that arrives at the load end of the circuit. If the percent drop exceeds 3%, consider upsizing the wire gauge or shortening the run.
Voltage Drop Formula
VD = 2 × L × I × R / 1000
VD = voltage drop (V), L = one-way length (ft), I = current (A), R = wire resistance per 1000 ft (Ω)
% Drop = (VD / Source Voltage) × 100
Copper Wire Resistance (per 1,000 ft)
| AWG Gauge | Resistance (Ω/1000ft) | Typical Use |
|---|---|---|
| 14 AWG | 3.14 | 15A circuits, lighting |
| 12 AWG | 1.98 | 20A circuits, receptacles |
| 10 AWG | 1.24 | 30A circuits, dryers |
| 8 AWG | 0.778 | 40A circuits, ranges |
| 6 AWG | 0.491 | 55A circuits, sub-panels |
| 4 AWG | 0.308 | 70A circuits, sub-panels |
| 2 AWG | 0.194 | 95A circuits, feeders |
| 1 AWG | 0.154 | 110A circuits, feeders |
| 1/0 AWG | 0.122 | 125A circuits, service |
How to Use This Calculator
- Enter the current — the load current in amps that will flow through the circuit.
- Enter the wire length — the one-way distance in feet from the panel to the load. The calculator automatically accounts for the return path.
- Select the wire gauge — choose the AWG size of the copper conductor.
- Enter the source voltage — defaults to 120V. Change as needed for 240V or other systems.
- Read the results — check that the percent drop is within the NEC 3% recommendation.
Frequently Asked Questions
What is voltage drop and why does it matter?
Voltage drop is the reduction in voltage as electrical current flows through a conductor. All wires have some resistance, and this resistance causes a loss of voltage over distance. The NEC recommends no more than 3% voltage drop on branch circuits and no more than 5% total (feeder plus branch circuit). Excessive voltage drop causes equipment to run inefficiently, lights to dim, and motors to overheat.
How do you calculate voltage drop in a circuit?
Voltage drop is calculated using the formula: VD = 2 × Length × Current × Resistance per foot / 1000. The factor of 2 accounts for the round trip (hot and neutral conductors). Length is the one-way distance in feet, current is in amps, and resistance is the wire resistance per 1,000 feet based on the AWG gauge.
What wire size do I need to keep voltage drop under 3%?
The required wire size depends on the current, distance, and voltage. For longer runs, you need larger wire to keep voltage drop within the NEC 3% recommendation. Use this calculator to test different wire gauges until the percent drop is at or below 3%. As a general rule, for 120V circuits over 100 feet, you should consider upsizing from the minimum wire gauge required by the breaker.