Voltage Drop Calculator
Determine the voltage drop for electrical cables running over long distances. Adjust conductor sizes, phases, and wire materials to ensure compliance with NEC standards.
Voltage Drop Calculator
Results
Vd = (√3 × L × I × R) / 1000Voltage Drop Calculations & Standards
Voltage drop occurs when the electrical impedance of a cable resists current flow. Over short distances, this is negligible. However, as the distance increases, the accumulative resistance results in a lower voltage at the load.
National Electrical Code (NEC) Sizing Standards
The NEC recommends the following maximum voltage drop values for electrical distribution:
- Branch Circuits: A maximum of 3% voltage drop from the branch-circuit overcurrent device to the farthest outlet.
- Feeder + Branch Combined: A maximum of 5% total voltage drop from the service entrance connection to the final load outlet.
Mathematical Formulas
The voltage drop is computed based on wire size resistance values:
Single-Phase Circuits:
Three-Phase Circuits:
Where:
* Vd = Voltage drop (Volts)
* L = Conductor length one-way (feet)
* I = Load current (Amps)
* R = Resistance of conductor (Ω/1000ft) based on NEC Chapter 9 Table 8
Step-by-Step Worked Example
Problem: Size voltage drop for a 3-phase 480V circuit running a 60A load over 200 feet using 6 AWG Copper wire ($R = 0.491\ \Omega$ per 1000 feet).
- Identify parameters: Voltage = 480 V, Phase = 3 (multiplier = $\sqrt3 \approx 1.732$), Current = 60 A, Length = 200 ft, Resistance = 0.491 $\Omega$/1k ft.
- Apply 3-Phase formula:
Vd = (1.732 * 200 * 60 * 0.491) / 1000 = 10.20 Volts. - Calculate percentage drop:
% Drop = (10.20V / 480V) * 100 = 2.13%. - Verify NEC Compliance: 2.13% is less than the 3% branch circuit limit. The installation complies.
Frequently Asked Questions (FAQs)
How does conductor temperature affect voltage drop?
Resistance increases as temperature rises. Conductor tables typically provide resistance at 75°C. In extremely hot environments or heavily loaded conduit runs, cable resistance rises, leading to higher voltage drop than calculated at room temperature.
What happens when voltage drop is too high?
Electrical appliances will pull more current to compensate for the lower voltage, causing overheating. Motors will lose torque and run hot. Sensitive microelectronics may crash or fail to boot entirely.
Why is the 3-phase multiplier √3 instead of 2?
In single-phase, current travels out and returns along two wires, doubling the distance. In a balanced three-phase system, phases are 120° apart and return paths cancel out, leading to the geometric multiplier of √3 (approximately 1.732) representing line-to-line electrical separation.