Voltage Drop Calculator

Voltage Drop Explained: What Every Electrician Needs to Know

Voltage drop is the reduction in voltage as electrical current flows through a conductor. Every wire has resistance, and that resistance converts some electrical energy into heat, reducing the voltage available at the load. Too much voltage drop causes dim lights, overheating motors, and tripped breakers.

The Voltage Drop Formula

VD = (2 × L × I × R) ÷ 1000

Where: L = one-way wire length (ft), I = current (amps), R = resistance per 1,000 ft (ohms). The factor of 2 accounts for the current flowing out and returning.

NEC Voltage Drop Recommendations

The National Electrical Code provides these guidelines (informational notes, not hard requirements):

• 3% maximum for branch circuits (NEC 210.19(A) Informational Note No. 4)
• 5% maximum for feeder + branch circuit combined (NEC 215.2(A) Informational Note No. 2)

Wire Resistance Table (Ω per 1,000 ft)

Common Scenarios

• Residential 20A circuit, 100 ft: 12 AWG copper = 3.3% drop at 120V. Consider upgrading to 10 AWG.
• Workshop 30A circuit, 150 ft: 10 AWG copper = 7.7% drop at 120V. Use 6 AWG or switch to 240V.
• Barn/outbuilding 50A, 200 ft: 6 AWG copper = 8.2% drop at 240V. Use 4 AWG or larger.

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