1.1 Ohm's Law & Voltage Drop Calculations
Understand single-phase and 3-phase Ohm's law formulas and calculate loop resistance and conductor voltage drop under NEC Informational Note 210.19.
Key Blueprint Takeaways
- Voltage drop formula: Vd = (2 * K * I * L) / CM for single-phase circuits.
- NEC 210.19(A) Informational Note No. 4 recommends a maximum 3% drop on branch circuits and 5% total overall system drop.
- Always double the one-way distance to account for both conductors in a single-phase loop calculation.
Electrical theory serves as the foundation of the National Electrical Code. Ohm's law dictates that current is directly proportional to voltage and inversely proportional to resistance (I = V / R). Power calculations expand upon this relationship (P = V * I = I^2 * R = V^2 / R).
Voltage drop occurs as current flows through conductor resistance. Excessive voltage drop impairs motor operation, dims lighting, and increases thermal losses across raceways. The NEC recommends limiting voltage drop to 3% on branch circuits and 5% total across combined feeder and branch circuits.
A 120V single-phase branch circuit powers a continuous resistive heater drawing 16A located 150 feet away from the service panel. Using copper conductors with a resistance of 1.93 ohms per 1,000 feet, what is the approximate voltage drop across the circuit conductors and does it comply with the NEC Informational Note recommendation?