How do I calculate three-phase voltage drop?
Three-phase voltage drop uses Vd = (1.732 x K x I x L) / circular mils. K is 12.9 for copper and 21.2 for aluminum, I is the operating current, and L is the one-way length in feet. The 1.732 handles the three-phase factor where a single-phase circuit would use 2. Pull circular mils from Chapter 9 Table 8.
Last reviewed June 2026 · Answered by JourneymanIQ staff
Use the three-phase formula
Vd = (1.732 x K x I x L) / circular mils. The only change from single-phase is the 1.732 in place of the 2.
Plug the right values
K is 12.9 for copper, 21.2 for aluminum. I is the operating current, not the breaker rating. L is the one-way run in feet.
Read circular mils from the table
Chapter 9 Table 8 gives circular mils: 14 AWG is 4,110, 12 AWG is 6,530, 10 AWG is 10,380. Use the value for your conductor.
1.732 for three-phase, operating current not breaker rating, circular mils from Chapter 9 Table 8.
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