Voltage Drop Calculator
Excessive voltage drop causes equipment malfunction, motor overheating, and dimming lights. The NEC recommends no more than 3% voltage drop for branch circuits and 5% total for feeder plus branch. This calculator determines actual voltage drop for a given wire run.
Circuit Parameters
Based on NEC Chapter 9, Table 8 conductor resistance values at 75°C. NEC Article 210.19(A) recommends max 3% drop for branch circuits, 5% total (feeder + branch). This is a recommendation, not a requirement, except where specified by local code.
Voltage Drop Formula
Single Phase: VD = 2 × I × R × L / 1000
Three Phase: VD = √3 × I × R × L / 1000
Where I = current (amps), R = resistance (ohms per 1000 ft), L = one-way distance (ft).
NEC Voltage Drop Recommendations
| Circuit Type | Max Recommended Drop |
|---|---|
| Branch circuit (outlets, lights) | 3% |
| Feeder (panel to subpanel) | 3% |
| Total (feeder + branch combined) | 5% |
| Motor circuits | 3% at full load |
| Sensitive electronics | 2% or less |
Frequently Asked Questions
How do I fix excessive voltage drop?
Three options: (1) Increase wire size — going up one gauge roughly halves resistance. (2) Reduce circuit length — relocate the panel or subpanel closer. (3) Increase voltage — a 240V circuit has half the current of 120V for the same power, halving voltage drop.
Does voltage drop waste electricity?
Yes. Voltage drop means power is being dissipated as heat in the wire itself (I²R loss). A 5% voltage drop means roughly 5% of the power is wasted in the conductors.
References & Methodology
- National Fire Protection Association (NFPA), "NFPA 70: National Electrical Code (NEC)," 2023. — Chapter 9, Table 1 conduit fill percentages and voltage drop calculation methodology per Article 215.
- IEEE Std 141 (Red Book), "Recommended Practice for Electric Power Distribution for Industrial Plants," 2022. — Voltage drop limits and conductor sizing for branch circuits and feeders.
- National Electrical Manufacturers Association (NEMA), "Conduit and Raceway Standards," nema.org. — Conduit trade size dimensions and wire cross-sectional area data.
References
- 2017 National Electrical Code (NEC) as adopted by the Arizona Department of Fire, Building and Life
- 2020 National Electrical Code (NEC) as adopted by the Pennsylvania Department of Labor & Industr
- 2017 National Electrical Code as adopted by the Tennessee Department of Commerce and Insurance, Divi
- 2020 NEC as referenced by the Georgia Department of Community Affairs (DCA)
- 10 CFR Part 431 — Energy Efficiency Program for Certain Commercial and Industrial Equipment (eCFR)
- NFPA 70B Recommended Practice for Electrical Equipment Maintenance
- 2023 NEC as the state electrical code
- 2023 National Electrical Code as adopted