Voltage Drop Calculator — Check Your Circuit's Voltage Loss
Calculate the exact voltage drop in volts and percentage for any circuit. Enter wire gauge, amperage, distance, and voltage to check NEC compliance.
How to Use This Calculator
- Select the wire gauge you have (or plan to use)
- Enter the circuit current in amps
- Enter the one-way distance from panel to the last load in feet
- Enter the source voltage (120V or 240V for most residential)
- Click "Calculate Voltage Drop" to see drop in volts, percentage, and NEC compliance
How the Calculation Works
// Resistance per 1000ft (from NEC Table 9):
// 14 AWG copper: 3.14 Ω | 12 AWG: 1.98 Ω | 10 AWG: 1.24 Ω
// 8 AWG: 0.778 Ω | 6 AWG: 0.491 Ω | 4 AWG: 0.308 Ω
// 2 AWG: 0.194 Ω | 1/0 AWG: 0.122 Ω | 2/0: 0.0967 Ω
VD_volts = (2 × distance × amps × resistance_per_1000ft) / 1000
VD_percent = (VD_volts / source_voltage) × 100
voltage_at_load = source_voltage - VD_volts Voltage drop is calculated using wire resistance values from NEC Table 9. The factor of 2 accounts for both the hot wire and the return neutral/ground wire. For 3-phase circuits, the factor is √3 (1.732) instead of 2. The result is compared against NEC recommendations: 3% for branch circuits, 5% total for combined feeder and branch.
Variables:
- resistance_per_1000ft: Wire resistance in ohms per 1000 feet from NEC Table 9
- distance: One-way wire run length in feet
- amps: Current flowing through the circuit
- VD_percent: Voltage drop as a percentage of source voltage
- voltage_at_load: Actual voltage delivered to the end of the circuit
Pro Tips
- Voltage drop over 3% on a branch circuit causes dimmer lights, slower motors, and hotter wire
- To reduce voltage drop: shorten the run, upsize the wire gauge, or increase the source voltage
- Long outdoor lighting runs (150ft+) often require 10 AWG or larger to maintain proper voltage
- Electric vehicle chargers (EVSE) are very sensitive to voltage drop — keep it under 2% for Level 2 charging
- Voltage drop is cumulative — add branch circuit drop to feeder drop for total system drop
Recommended Products
Frequently Asked Questions
What is an acceptable voltage drop?
NEC recommends 3% maximum on branch circuits and 2% on feeders, for a combined 5% total. Most electricians target 3% or less. Sensitive electronics and motors prefer 2% or less.
How does wire length affect voltage drop?
Voltage drop is directly proportional to wire length. Double the run = double the voltage drop. A 12 AWG circuit at 20A has a 3% drop at 50ft but a 6% drop at 100ft. Long runs require upsized wire.
Why does my 120V outlet show only 110–112V?
Utility supply variations (typically 114–126V), combined with wiring voltage drop, cause lower measured voltage. A 1–3% drop on a 120V circuit reduces voltage by 1.2–3.6V. This is normal and within acceptable range.
Does voltage drop depend on the wire gauge?
Yes — larger gauge wire (smaller AWG number) has lower resistance and less voltage drop. Upgrading from 14 AWG to 12 AWG reduces resistance by 37% and voltage drop proportionally for the same current and distance.
What happens if voltage drop is too high?
Effects of high voltage drop: incandescent and LED lights dim, motors run hotter and less efficiently, appliances may malfunction, and resistance in the wire generates heat that reduces wire lifespan.