3 Phase Voltage Drop Calculator (NEC Table 8)

3 Phase Voltage Drop Calculator

Voltage drop for balanced three-phase feeders and branch circuits at 208, 240, 480 and 600 V. Enter the wire size, current and one-way length.

The three-phase voltage drop formula

VD = 1.732 × I × R × L ÷ 1000
VD% = VD ÷ V_L-L × 100

I is the line current in amps. R is the conductor resistance in ohms per 1000 ft at 75 °C from NEC Chapter 9 Table 8. L is the one-way length in feet. The answer is the drop in line-to-line voltage, so compare it with the line-to-line system voltage.

Worked examples

1. 480 V feeder, 1/0 copper

100 A over 250 ft with 1/0 copper (0.122 Ω/kft):

VD = 1.732 × 100 × 0.122 × 250 ÷ 1000 = 5.28 V
VD% = 5.28 ÷ 480 = 1.10%

Well within 3%. At 480 V, three-phase feeders can run a long way before voltage drop matters.

2. 208 V rooftop unit

A 40 A RTU, 150 ft from the panel, on 8 AWG copper (0.764 Ω/kft):

VD = 1.732 × 40 × 0.764 × 150 ÷ 1000 = 7.94 V → 3.82%

That is over 3%. Upsizing to 6 AWG (0.491 Ω/kft) gives 5.10 V, or 2.45%. If you upsize ungrounded conductors for voltage drop, NEC 250.122(B) requires the equipment grounding conductor to be increased in proportion.

3. 600 V feeder in aluminum (Canada)

150 A over 300 ft on 3/0 aluminum (0.126 Ω/kft):

VD = 1.732 × 150 × 0.126 × 300 ÷ 1000 = 9.82 V → 1.64%

Under CEC Rule 8-102 this is within the 3% feeder limit. For metric lengths, use the Canadian voltage drop calculator.

Why voltage matters so much

For the same load in kVA, current falls as voltage rises, and the allowed drop in volts rises too. Moving a load from 208 V to 480 V cuts the current by 57% and more than doubles the allowable drop, so percent drop falls by about 81%. This is the main reason large buildings distribute at 480 V or 600 V.

Sizing a three-phase feeder for voltage drop

Rearranging the formula gives the largest resistance you can accept:

R_max = (V × VD%_max) × 1000 ÷ (1.732 × I × L)

For the RTU above at 3%: (208 × 0.03 × 1000) ÷ (1.732 × 40 × 150) = 0.600 Ω/kft. The smallest copper size at or below that is 6 AWG (0.491). The wire size calculator does this automatically and also checks ampacity.

Motors and voltage drop

Motors draw five to seven times full-load current when starting, so running drop of 3% can become a 15–20% dip at start. That can stop contactors pulling in. For large motors on long runs, check starting drop too, or use a soft starter or VFD.

Related formulas

See the voltage drop formula guide for the K-factor (circular mil) method, and the single-phase voltage drop calculator for 120/240 V circuits.

Frequently asked questions

What is the formula for three-phase voltage drop?

VD = √3 × I × R × L ÷ 1000, where I is the line current, R is resistance in ohms per 1000 ft (NEC Chapter 9 Table 8) and L is the one-way length in feet. Divide by the line-to-line voltage for percent.

Why does three phase use 1.732 instead of 2?

In a single-phase circuit, current flows out on one conductor and back on the other, so both lengths count (factor 2). In a balanced three-phase circuit, the phase currents are 120° apart and the drop between lines is √3 times the drop in one conductor.

What voltage drop is allowed on a three-phase feeder?

NEC informational notes in 210.19 and 215.2 recommend 3% on a feeder or branch circuit and 5% total. In Canada, CEC Rule 8-102 makes 3% and 5% mandatory limits.

Does the calculator handle unbalanced loads?

No. It assumes a balanced load. For unbalanced loads, calculate the most heavily loaded phase as a single-phase circuit to neutral as a conservative check.

Does it include reactance?

It uses DC resistance from Table 8, which is accurate for conductors up to about 1/0 and slightly optimistic for large conductors in steel conduit. For long runs of large wire, NEC Chapter 9 Table 9 effective impedance gives a more precise answer.