Voltage drop is the reduction in electrical potential along a conductor caused by its resistance. The NEC recommends branch-circuit voltage drop not exceed 3% and total feeder-plus-branch drop not exceed 5%. Exceeding these limits can cause motors to overheat, lights to dim, and electronics to malfunction. Enter voltage, current, distance, wire gauge, and phase to calculate the drop.
Voltage Drop Formula
- Single-phase: Vd = 2 x I x R x L / 1000, where I is current in amps, R is resistance in ohms per 1000 ft (from NEC Chapter 9 Table 8), and L is one-way distance in feet.
- Three-phase: Vd = sqrt(3) x I x R x L / 1000. The sqrt(3) factor (1.732) replaces the factor of 2 because the return path is shared across three conductors.
- Voltage drop percentage = Vd / Source Voltage x 100. End voltage = Source Voltage - Vd.
Wire Resistance by AWG Gauge
AWG (American Wire Gauge) numbers decrease as wire diameter increases. AWG 14 copper has 3.14 ohms per 1000 ft, AWG 12 has 1.98, AWG 10 has 1.24, and so on down to 4/0 at 0.0608 ohms per 1000 ft. Each 3-gauge decrease approximately doubles the cross-sectional area and halves the resistance. Selecting a wire one or two sizes larger than the minimum ampacity requirement is a common strategy to keep voltage drop within acceptable limits on long runs.
NEC Voltage Drop Recommendations
NEC Article 210.19(A) Informational Note No. 4 and Article 215.2(A) Informational Note No. 2 recommend a maximum of 3% voltage drop on branch circuits and 5% total (feeder plus branch). These are recommendations, not mandatory code requirements, but most inspectors and engineers treat them as de facto standards. For sensitive loads like medical equipment or data centers, tighter limits of 1-2% are common.
Practical Factors Not Modeled
- This calculator uses DC resistance values for copper conductors. AC circuits also experience reactance (inductive and capacitive), which increases effective impedance, especially in larger conductors and conduit runs.
- Temperature affects resistance: copper resistance increases roughly 0.4% per degree Celsius above 20 C.
- Aluminum conductors have about 1.6x the resistance of copper for the same gauge.
- Conductor fill, ambient temperature, and derating factors may further affect real-world performance.
FAQ
Q: Why is the distance doubled for single-phase voltage drop?
A: In a single-phase circuit, current flows out on the hot conductor and returns on the neutral, so the total conductor length is twice the one-way distance. Three-phase circuits use the sqrt(3) multiplier instead because the return current is distributed across three phases.
Q: What is the maximum allowable voltage drop per NEC?
A: The NEC recommends a maximum of 3% for branch circuits and 5% total (feeder plus branch). These are informational notes, not enforceable code requirements, but they are widely adopted as best practice.
Q: Does this calculator work for aluminum wire?
A: No. The built-in resistance values are for copper conductors only. Aluminum has approximately 1.6 times the resistance of copper for the same AWG size. For aluminum, multiply the resistance values by 1.6 or consult NEC Table 8 directly.