Voltage Drop Calculator
Calculate voltage drop in electrical circuits, analyze wire gauge requirements, and ensure NEC compliance for professional electrical installations.
Circuit Parameters
Environmental Factors
Voltage Drop Analysis
Final Voltage
At the load
Power Loss
Heat dissipation
NEC Compliance
3% branch circuit limit
Wire Resistance
Total circuit resistance
Recommendations
- • Voltage drop (10.6%) exceeds NEC 3% recommendation
- • Consider using 1 AWG wire to reduce voltage drop
- • Wire ampacity should be at least 125% of load current per NEC 210.20(A)
- • Voltage drop exceeds 5% - equipment may not operate properly
- • Always consult local electrical codes and a licensed electrician
- • Consider power factor correction if using reactive loads
How it works
Voltage drop is the voltage lost as current flows through a wire's resistance over a distance. Too much drop dims lights and strains motors, so codes limit it (commonly to 3%). It rises with current and wire length, and falls with thicker wire.
Voltage drop
Vdrop = I × R_wire (R_wire grows with length, shrinks with gauge)
- I
- current in amps
- R_wire
- total wire resistance (Ω), set by length and gauge
Worked example
- Current = 15 A
- Wire round-trip resistance ≈ 0.2 Ω
- Vdrop = 15 × 0.2
3 volts dropped — 2.5% on a 120 V circuit, just inside the 3% guideline.
Good to know
- Long runs need a thicker (lower-gauge) wire to keep the drop within limits.
- Account for the full round-trip length, both the supply and return conductors.
- The 3% recommendation for branch circuits keeps equipment running efficiently and safely.
Related Calculators
Frequently Asked Questions
What is voltage drop and why is it important?
Voltage drop is the reduction in voltage that occurs when current flows through wire resistance. Excessive voltage drop can cause equipment malfunction, reduced efficiency, and code violations. NEC recommends maximum 3% drop for branch circuits.
How do you calculate voltage drop?
For single-phase: VD = 2 × I × R × L. For three-phase: VD = √3 × I × R × L × cos(φ). Where I is current, R is resistance per foot, L is one-way length, and cos(φ) is power factor.
What wire gauge should I use to minimize voltage drop?
Wire gauge depends on current, distance, and acceptable voltage drop. Larger gauge (smaller AWG number) has lower resistance. Use the calculator to compare options and ensure NEC compliance.
What safety factor should I use?
Safety factors vary by application and standards. Common values range from 1.5 to 4. Always consult relevant codes.
How do I account for real conditions?
Consider temperature, material variations, manufacturing tolerances, and operating conditions. Use conservative estimates.
Should I round calculations?
Keep full precision during calculations and only round final results to avoid accumulating errors.
How often to verify calculations?
Critical calculations should be independently checked. Review when conditions change significantly.