Three-phase voltage drop looks simple when the load is balanced: use the line-to-line voltage, multiply current by conductor resistance and route length, then apply the square-root-of-three factor. Real projects are messier. A 480 V motor control center may be balanced enough for the standard formula. A 208Y/120 V commercial panel feeding kitchen receptacles, LED drivers, small motors, and point-of-sale equipment may have one phase carrying much more current than the other two. A 400 V IEC distribution board may meet ampacity rules but still leave a long final circuit with dimming LED drivers below their preferred input range. The calculation has to match the circuit you are actually building.

This guide is written for electricians, engineers, maintenance teams, and serious DIYers who use the voltage drop calculator but want to understand which inputs belong in the three-phase mode. It also shows when the better move is to check the heaviest single-phase leg, review neutral current, or compare copper and aluminum using the wire size calculator and the voltage unbalance calculator.

The code backdrop is familiar. The National Electrical Code gives commonly used voltage-drop design recommendations in informational notes around NEC 210.19(A) for branch circuits and NEC 215.2(A) for feeders. International work often follows the same engineering logic through IEC practices such as IEC 60364-5-52, where conductor size, grouping, installation method, operating temperature, and voltage drop are checked together rather than in isolation.

“For a balanced 3-phase motor feeder, the square-root-of-three formula is the right first pass. For a 4-wire wye panel with uneven 120 V or 277 V loads, I want to see the worst leg and the neutral checked before anyone signs off on conductor size.”
— Hommer Zhao, Technical Director

Start With the Circuit Type, Not the Formula

The most common mistake is choosing three-phase mode just because the panel is three-phase. The load itself controls the method. A 480 V, 3-phase, 45 A rooftop unit is a true line-to-line three-phase load. A 120 V receptacle on phase A and neutral is not. A 277 V lighting branch circuit from a 480Y/277 V panel is a single-phase line-to-neutral load even though it comes from a three-phase distribution system.

For a balanced three-phase load, voltage drop is commonly estimated as:

VD = 1.732 x K x I x D / CM

K is conductor resistivity, I is load current, D is one-way distance, and CM is circular mil area. Calculator tools often use resistance per 1000 ft instead, but the design logic is the same.

For a two-wire single-phase or DC circuit, the round-trip factor is 2 instead of 1.732. That difference is small enough to tempt shortcuts, but it becomes expensive on long feeders. On a 225 ft run at 80 A, a 15 percent formula error can be the difference between a comfortable 2.6 percent drop and a circuit that misses a 3 percent project limit.

Circuit/load	Use voltage	Method	Common check
480 V motor, 3-phase	480 V line-to-line	3-phase balanced	Running drop plus motor-starting dip
208Y/120 V receptacle	120 V line-to-neutral	Single-phase leg	Heaviest phase and neutral current
400 V IEC pump	400 V line-to-line	3-phase balanced	IEC 60364-5-52 installation method
480Y/277 V lighting	277 V line-to-neutral	Single-phase branch	Driver input range and harmonics
208 V panel feeder	208 V line-to-line plus leg checks	Hybrid review	Unbalance, neutral, farthest load

Example 1: Balanced 480 V Motor Feeder

Suppose a 480 V, 3-phase motor load draws 96 A at 0.88 power factor. The feeder route is 180 ft one way in copper conductors. The design target is 3 percent maximum running voltage drop, with a separate starting-voltage review because the equipment is a motor load covered by NEC Article 430.

If the first pass uses 2 AWG copper, a typical resistance value is about 0.156 ohm per 1000 ft at 75 C before temperature adjustment. The simplified resistance-only drop is approximately:

VD = 1.732 x 96 A x 180 ft x 0.156 / 1000 = 4.67 V

Percent drop = 4.67 V / 480 V x 100 = 0.97 percent

That running number looks comfortable. The next question is not whether 0.97 percent is acceptable; it is whether the same installation survives ampacity, temperature, terminal-rating, conduit-fill, short-circuit, and motor-starting checks. If the motor starter sees a 600 percent inrush current, the momentary drop could be roughly six times the running drop before motor impedance and source stiffness are considered. That is why the motor starting voltage drop calculator is a better tool for the start event.

“A feeder that drops only 1 percent at full-load current can still create a nuisance trip if the motor starter sees 5 to 7 times FLA and the upstream transformer is already soft. Running voltage drop and starting dip are related, but they are not the same check.”
— Hommer Zhao, Technical Director

Example 2: Unbalanced 208Y/120 V Commercial Panel

Now consider a small retail buildout. A 208Y/120 V, 3-phase, 4-wire feeder supplies a remote panel 140 ft from the service equipment. The load schedule shows 38 A on phase A, 24 A on phase B, and 18 A on phase C after realistic demand and diversity assumptions. The average phase current is 26.7 A, but that average does not represent the conductor serving phase A.

If the feeder calculation only uses 26.7 A at 208 V, the drop will look artificially low. The better field approach is to check the line-to-line feeder at the expected three-phase load and then check the worst line-to-neutral path at 120 V. A 38 A line-to-neutral load at 140 ft has much less voltage budget because 3 percent of 120 V is only 3.6 V. The same 3.6 V on a 208 V line-to-line load would be only 1.7 percent.

Neutral current also deserves a real look. With linear loads, the neutral carries the vector imbalance of the three phases. With many switch-mode power supplies, LED drivers, and other nonlinear loads, triplen harmonics can add in the neutral rather than cancel. NEC 310.15(E) addresses neutral conductors in ampacity adjustment, and designers should be cautious when the panel serves dense electronic loads. IEC projects face the same practical concern through IEC 60364-5-52 and local national annexes.

Field scenario

On a Q1 2026 tenant-improvement review, we saw a 208Y/120 V panel drawn with a neat 90 A diversified total. The phase schedule was not neat: 42 A, 31 A, and 19 A on the three legs. Re-running the voltage drop on the 42 A leg changed the recommendation from 8 AWG to 6 AWG copper for the longest branch path, and the contractor avoided low-voltage complaints at the farthest POS counter.

Balanced vs. Unbalanced Design Decisions

The table below is a practical decision guide. It is not a substitute for the project drawings, local code adoption, or the equipment manufacturer data sheet, but it helps decide which calculation should control the first wire-size pass.

Condition	Likely controlling input	Typical target	Design response
Balanced 3-phase motor	Full-load amps at line-to-line voltage	2-3 percent running	Check starting dip separately
Long 480 V feeder	Feeder demand current	2 percent feeder budget	Upsize conductor or shorten route
208Y/120 V mixed panel	Heaviest line-to-neutral leg	3 percent branch, 5 percent combined	Rebalance phases and verify neutral
LED lighting with drivers	Farthest driver input voltage	Often tighter than 3 percent	Split circuits or move transformer
Nonlinear electronic loads	Neutral and harmonic heating	Project-specific	Oversize neutral or separate circuits

“The best three-phase voltage-drop calculation is usually two calculations: one for the balanced feeder behavior and one for the worst line-to-neutral load that a real person will plug in at the end of the run.”
— Hommer Zhao, Technical Director

NEC and IEC Checks That Belong Beside the Math

Voltage drop does not replace code ampacity. NEC Table 310.16, conductor temperature limitations in NEC 110.14(C), adjustment factors in NEC 310.15(C)(1), and equipment-specific articles such as NEC 430 for motors still control the legal installation. The voltage-drop calculation only tells you whether the selected conductor is likely to deliver acceptable voltage at the load.

For IEC work, IEC 60364-5-52 makes the same point through installation method, grouping, ambient temperature, conductor material, and voltage drop. A cable clipped direct, a cable in thermal insulation, and a group of loaded circuits in a warm tray are not equivalent. The resistance used in the drop calculation should reflect operating temperature, not only a catalog value at 20 C.

Use line-to-line voltage for true 3-phase loads. A 400 V pump or 480 V air handler belongs in three-phase mode.
Use line-to-neutral voltage for one-phase loads. A 120 V receptacle or 277 V luminaire branch circuit is checked at that lower voltage.
Do not hide unbalance in averages. If phase A carries 40 A and phase C carries 18 A, the average is not the conductor current on phase A.
Document assumptions. Record voltage, current, distance, conductor material, temperature, power factor, and the target percentage so plan reviewers can follow the decision.

Practical calculator workflow

Run the feeder in three-phase mode first. Then run the farthest line-to-neutral branch at its actual voltage. If either result breaks the project limit, compare a larger conductor, a shorter route, a panel relocation, or a split-circuit layout before ordering wire.

FAQ: Three-Phase Voltage Drop

What voltage should I use for a three-phase voltage drop calculation?

Use the line-to-line voltage for a three-phase load: 208 V, 400 V, 415 V, or 480 V are common examples. For a line-to-neutral load on one phase, calculate that leg at the line-to-neutral voltage, such as 120 V on a 208Y/120 V system or 277 V on a 480Y/277 V system.

Why does the three-phase formula use 1.732?

The 1.732 factor is the square root of 3. It represents the phase relationship in a balanced three-phase circuit, where the line currents are 120 electrical degrees apart instead of sharing the same outbound-and-return path used in a single-phase two-wire calculation.

Is the NEC 3 percent voltage drop rule mandatory?

For general branch circuits and feeders, the common 3 percent branch-circuit and 5 percent combined recommendation appears in NEC informational notes, including NEC 210.19(A) and NEC 215.2(A). It is not usually a hard prescriptive rule, but many specifications and AHJs treat it as the expected design target.

How do I handle unbalanced three-phase voltage drop?

Do not rely only on the average three-phase current. Check the heaviest phase or calculate each line-to-neutral load path separately. On a 208Y/120 V panel with 38 A, 24 A, and 18 A single-phase loads, the 38 A leg usually controls conductor selection even though the average is only 26.7 A.

Does neutral current affect voltage drop?

Yes, when line-to-neutral loads are unbalanced or nonlinear. A shared neutral in a 3-phase, 4-wire wye system can carry imbalance current, and triplen harmonic current from electronic loads can add in the neutral. NEC 310.15(E) and IEC 60364-5-52 both matter when evaluating neutral loading.

When should I upsize a three-phase feeder for voltage drop?

Upsize when the calculated drop exceeds the project target, commonly 2 percent for a feeder on sensitive equipment or 3 percent for a general feeder. For example, a 120 A, 480 V, 3-phase load at 250 ft may push 1 AWG copper near the 3 percent range while 1/0 copper gives more operating margin.

Calculate Before You Pull Conductors

Before ordering copper or aluminum, run the load both ways when the system is mixed: balanced three-phase feeder, then the worst line-to-neutral branch. Use the calculator results with NEC 210.19, NEC 215.2, NEC 310.15, NEC 430, and IEC 60364-5-52 as a design record, not just a number on a screen.

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Three-Phase Voltage Drop Calculation: Balanced Loads, Neutral Current, and NEC/IEC Design Checks