Voltage Drop Calculator
Calculate voltage drop and check NEC compliance for DC, single-phase, and three-phase circuits.
Learn why voltage drop matters, how NEC limits apply, and practical tips for keeping your circuits compliant.
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Voltage Drop Limits
NEC 210.19/215.2(A) recommends maximum 3% voltage drop for branch circuits and 5% total (feeder + branch). IEC 60364-5-52 recommends ≤3% for lighting and ≤5% for other loads. This calculator uses the K-value method per NEC Chapter 9, Table 8 data at 75°C.
Frequently Asked Questions
What is an acceptable voltage drop?
NEC recommends maximum 3% voltage drop for branch circuits and 5% total for feeder plus branch circuits. For sensitive equipment like medical devices or data centers, tighter limits of 1-2% are common. Exceeding these limits can cause equipment malfunction, overheating, and energy waste.
How do I calculate voltage drop?
For single-phase or DC circuits: VD = (2 × K × I × L) / CM, where K is the resistivity constant (12.9 for copper, 21.2 for aluminum), I is current in amps, L is one-way length in feet, and CM is the conductor's circular mils from NEC Chapter 9 Table 8. For three-phase, replace 2 with √3 (1.732).
Does voltage drop affect LED lighting?
Yes. LED drivers are sensitive to voltage variations. While LEDs use less current than incandescent bulbs, voltage drop can cause flickering, reduced efficiency, or premature driver failure. Keep voltage drop under 3% for LED circuits.
What wire size for a 100-foot run at 20 amps?
For a 120V single-phase copper circuit at 20A over 100 feet: 12 AWG gives about 6.6% drop (exceeds NEC 3% recommendation). Use 10 AWG instead, which gives about 3.1% drop — within acceptable limits.
What is the K value for voltage drop?
K is the resistivity constant at 75°C. K = 12.9 for copper and K = 21.2 for aluminum. These represent the resistance in ohms per circular mil-foot of conductor length, per NEC standards.