NEC – Running Voltage Drop Calculation

The NEC standard specifies the equation for voltage drop calculation as follows

Step – I Calculate Impedance Zc from NEC Table 9

Zc = ( R * cosø ) + ( X * sinø )

Where:

R is the resistance value from table

X is the reactance value from table (ignores capacitance component)

cosø is the power factor of circuit (Load)

sinø = sin (arccos ø)

Zc is in W/1000ft or W/km

Note:
This calculation is based on the line to neutral voltage.

Step – II Calculate Running Voltage Drop

Vd = (C * Id * L * Zc) / 1000

Where:

• Vd = actual voltage drop, in volts. This is considered to be line to neutral.

• L = route length of circuit, in metres or feet

  Note: If L in Meters, then Zc is in W/km If L in Feet, then Zc is in W/1000ft Zc = impedance of cable, in W/1000ft or W/km Id = the current to be carried by the cable, in amperes. C = 2 if 1 phase (phase to neutral); 2 if single-phase (phase to phase); Ö3 if 3 phase