⚡ Electrical Design Tool

Voltage Drop Calculator

Calculate voltage drop for any lighting or electrical circuit. Checks NEC compliance, recommends correct wire size, and flags unsafe drops before you pull wire.

NEC 210.19(A): Branch circuits ≤ 3% voltage drop
NEC 215.2(A): Feeders ≤ 3% drop
Combined max: 5% total feeder + branch
Quick presets
Power Source
System Voltage (V)
Phase
System Type
Cable
Wire Gauge (AWG)
Conductor Material
One-Way Run Length (ft)
Distance from panel to last fixture (one way)
Length Unit
Load
Load Current (A)
Total amps on this circuit
Or enter Watts (optional)
Auto-converts to amps at selected voltage
✓
GOOD
Voltage drop is within NEC limits.
Voltage Drop %
—
percent
—
Voltage Drop
—
volts lost
End Voltage
—
V at load
Min Wire Size
—
for ≤3% drop
Circuit diagram — voltage along the run
Recommendations
Calculation detail
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The calculators and tools on Formula Factory are provided for general guidance and informational purposes only. Results are estimates based on standard formulas and the values you enter — they do not constitute professional engineering, electrical, or architectural advice. Always verify calculations with a qualified professional before making decisions for any safety-critical, code-compliance, or commercial application. Formula Factory makes no representations or warranties as to the accuracy or completeness of any result, and accepts no liability for errors, omissions, or any outcomes arising from reliance on this information.