Cable Voltage Drop Calculator
Single-Phase Voltage Drop
Vd = 2 × I × R × L for single-phase circuits
Three-Phase Voltage Drop
Vd = √3 × I × R × L for three-phase circuits
Formula
Single-Phase: Vd = 2 × I × R × L | Three-Phase: Vd = √3 × I × R × L | Vd% = (Vd / Vsource) × 100 | R from NEC Chapter 9 Table 8
Frequently Asked Questions
What is the maximum allowable voltage drop per NEC?
NEC recommends (but does not mandate) a maximum of 3% voltage drop for branch circuits and 5% total for the combination of feeder and branch circuit. These are in NEC 210.19(A) and 215.2(A) Informational Notes. Many engineers treat these as requirements.
How do I calculate voltage drop for single-phase vs three-phase?
Single-phase: Vd = 2 × I × R × L (factor of 2 for round-trip). Three-phase: Vd = √3 × I × R × L (factor of √3 ≈ 1.732). Where I is current in amps, R is resistance per foot, and L is one-way distance in feet.
How can I reduce voltage drop?
Use larger wire (lower gauge number), shorten cable runs, increase voltage (e.g., 240V instead of 120V halves the current), or use copper instead of aluminum. For long runs, it's often more cost-effective to increase wire size than to suffer power losses.
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