NEC – Voltage Drop Calculation for Non Table 9 Sizes

Table 9 does not cover all possible cable sizes or Multi/Single core types in the rating tables.

In order to cover the cables that are not covered in Table 9 the following equation should be used:

For Imperial

Vd = (C * Id * L * R ) / (1000 * CM)

Where:

Vd = actual voltage drop, in volts.

L = route length of circuit, in feet

R = 0.9 Ohms/Copper or 21.2 Ohms/Aluminium (resistance constants for a conductor that is 1 circular mil in diameter and 1 foot long at an operating temperature of 75°C.)

Id = the current to be carried by the cable, in amperes.

CM = k Circular-Mils (conductor wire size)

If the wire size is not in kcmil then it needs to be converted using lookup Table 8.

C = 2 if 1 phase (phase to neutral); 2 if single-phase (phase to phase); Ö3 if 3 phase

Note:
In the case of cables in parallel, the design current needs to be divided by 2.

For Metric

Vd = (C * Id * L * R) / (1000 * CM)

Where:

Vd = actual voltage drop, in volts.

L = route length of circuit, in meter

R = 2.9527 Ohms/Copper or 69.553 Ohms/Aluminium (resistance constants for a conductor that is 1 circular mil in diameter and 1m long at an operating temperature of 75°C.)

Id = the current to be carried by the cable, in amperes.

CM = k Circular-Mils (conductor wire size)

If the wire size is not in kcmil then it needs to be converted using lookup Table 8.

C = 2 if 1 phase (phase to neutral); 2 if single-phase (phase to phase); Ö3 if 3 phase

Note:
In the case of cables in parallel, the design current needs to be divided by 2.

The relevant voltage drop in % is equal to:

Vp% = (100 * Vd) / Vs

Where:

Vp% = permissible voltage drop on the circuit run, usually expressed as a percentage of the supply voltage.

Vs = supply voltage, in volts. Can be line or phase.