Voltage drop is the decrease of electrical potential along the path of a current flowing in an electrical circuit.
The Formula:
VD = (2 × L × R × I) / 1000
L = One-way distance (ft)
R = Resistance per 1000ft
I = Current (Amps)
NEC Standard: The National Electrical Code recommends a maximum voltage drop of 3% for branch circuits and 5% for the combined feeder and branch circuit.
Why it matters?
Excessive voltage drop can cause motors to run hot, lights to flicker, and digital equipment to malfunction.
The Variance: Reality vs. Math
Electrical math is precise, but real-world conditions introduce variance:
- Ambient Temperature: Wire resistance increases as it gets hotter. A run through a 140°F attic will have higher voltage drop than the same run in a 70°F basement.
- Connection Resistance: The formula assumes perfect continuity. Loose terminations or corroded splices add localized resistance that the math can't predict.
- Power Factor: For AC circuits, inductive loads (like large motors) have a power factor less than 1.0, which can increase the effective voltage drop compared to a simple resistive load (like a heater).