Voltage Drop With Reactance
Single- or three-phase voltage drop using NEC Chapter 9 Table 9 R and X per 1000 ft and the load power factor.
Example
You enter
- System voltage (V, line-to-line) 480
- Load current (A) 100
- One-way length (ft) 200
- R per 1000 ft (ohm; Table 9) 0.13
- X per 1000 ft (ohm; Table 9) 0.044
- Power factor (0.5-1.0) 0.85
- Phase three
You get
- Voltage drop (V) 4.63
- Percent drop 0.96 %
- Voltage at load (V) 475.4
Details, formula, and sources
With the 3% / 5% advisory band.
Vd = k * I * (R*cos(theta) + X*sin(theta)) * L / 1000, with k = 2 single-phase or sqrt(3) three-phase and theta = arccos(PF). R and X per 1000 ft are selected by conductor size and conduit material.
NEC 2023 (NFPA 70). Chapter 9 Table 9 (R and X per 1000 ft); 210.19(A) Note 4 (3% branch) and 215.2(A)(1) Note 2 (5% total) advisory bands.
Free read-only access at nfpa.org/freeaccess.
Estimate. AHJ and licensed electrician govern. Verify against the NEC edition adopted in your jurisdiction.
Field names used by the API: system_voltage_v, current_a, length_ft, r_ohm_per_kft, x_ohm_per_kft, power_factor, phase, drop_v, drop_percent, voltage_at_load_v
- Impedance values R and X per 1000 ft are entered from NEC Chapter 9 Table 9 for the conductor size and conduit material (steel raceway raises X)NEC 2023 Chapter 9 Table 9; the tile does not bundle the table
- Advisory band 3% on a branch, 5% total (branch + feeder) is advisory, not mandatoryNEC 2023 210.19(A) Note 4 and 215.2(A)(1) Note 2