電圧不平衡と電圧降下：三相モーターを過熱させない配線設計

Three-phase motor problems are often blamed on the motor first: weak bearings, poor winding insulation, a bad starter, or a questionable replacement part. In many field failures, the quieter cause is upstream. A feeder can be legal by ampacity, a breaker can be correctly sized under NEC Article 430, and the measured voltage can still be hostile to the motor because voltage drop and voltage unbalance arrive together.

TL;DR

• Check voltage drop and voltage unbalance under real motor load, not only at the empty disconnect.
• Keep three-phase voltage unbalance near 1% before assuming a motor can run at nameplate load.
• NEC 430 protects motor circuits; NEC 210.19 and 215.2 notes guide voltage-drop performance.
• IEC 60364-5-52 ties voltage drop to installation method, grouping, conductor material, and load type.

Voltage drop is the voltage lost in conductors and terminations as load current flows through resistance and reactance. Voltage unbalance is the percent difference between the average three-phase line voltage and the phase voltage farthest from that average. A three-phase induction motor is a rotating machine that converts a balanced three-phase supply into torque; when the supply becomes unbalanced, negative-sequence current produces heat instead of useful shaft work. Ampacity is the maximum current a conductor can carry under stated conditions without exceeding its temperature rating.

The code and standards context matters. The National Electrical Code gives motor-circuit rules in Article 430 and ampacity rules in Article 310. The familiar 3% branch and 5% total voltage-drop values are design recommendations in informational notes, not a substitute for the required overcurrent and conductor checks. International projects often reference the International Electrotechnical Commission through IEC 60364-5-52 for wiring-system voltage drop, grouping, installation method, and conductor material. For motor behavior, many engineers also recognize NEMA guidance on motor derating when voltage unbalance rises above 1%.

“On a motor feeder, 2.5% voltage drop can look acceptable until one phase is already 1.5% low from the utility or transformer. The motor sees the combination, not your spreadsheet boundary.”
— Hommer Zhao, Technical Director

The Field Sequence: Measure, Then Calculate

Start at the motor controller or disconnect with the motor running at normal load. Measure all three line-to-line voltages: A-B, B-C, and C-A. Do not rely only on the panelboard because a long feeder, a loose termination, or a single overloaded phase can change the picture at the motor. Enter the same circuit into the voltage unbalance calculator and the voltage drop calculator. The unbalance result tells you whether the supply quality is suitable for the motor; the voltage-drop result tells you whether the route and conductor choice are stealing too much voltage under load.

For a 480 V motor, suppose measured line voltages at the starter are 480 V, 472 V, and 488 V. The average is 480 V. The greatest deviation from average is 8 V, so voltage unbalance is 8 / 480 x 100 = 1.67%. That number is more important than the comfortable-looking average. A 1.67% voltage unbalance can produce much larger current unbalance in the motor windings, which means extra heating even when the full-load amps are close to nameplate.

Worked Example: 30 HP, 480 V Pump Motor at the End of a Feeder

Consider a 30 HP, 480 V three-phase pump motor located 260 feet from the distribution panel. The design current from NEC Table 430.250 is 40 A for a 30 HP, 460 V motor. The installer proposes 8 AWG copper in conduit because ampacity appears adequate after terminations and conditions are checked. Using a common 8 AWG copper resistance near 0.628 ohms per 1,000 feet and the three-phase voltage-drop factor:

Running voltage drop

VD = 1.732 x 40 A x 0.628 ohms/kft x 0.260 kft

VD = 11.3 V

Percent drop = 11.3 / 480 x 100 = 2.35%

A 2.35% running drop may fit the usual design target. Now add measured service voltage that is already unbalanced: 483 V, 475 V, and 488 V at the panel under similar plant load. The average is 482 V and the maximum deviation is 7 V, or 1.45%. If one leg also has a weaker termination at the starter, the motor may see both a lower average voltage and an unbalanced set of phase voltages. Upsizing the feeder to 6 AWG copper lowers running drop toward 1.5%, but it does not fix a utility, transformer tap, or termination imbalance. Both problems must be separated.

“When current unbalance is high, do not solve only with bigger wire. Bigger wire reduces impedance, but it will not correct an upstream single-phase load imbalance or a damaged contactor pole.”
— Hommer Zhao, Technical Director

Comparison Table: What the Numbers Mean

Condition at motor	Typical number	Risk	Practical response
Voltage drop only	2% to 3%	Reduced torque and lower terminal voltage	Upsize conductor or shorten route
Voltage unbalance	1% to 2%	High current unbalance and heat	Measure source, taps, contacts, and phase loads
Starting sag	10% to 15%	Slow acceleration and overload trips	Check starter type and starting current
Loose termination	One phase abnormal	Localized heat and phase loss risk	Thermal scan, torque check, repair hardware
Undersized feeder	Over 5% total	Weak motor performance and system losses	Recalculate with conductor and raceway conditions

Do the Code Check in the Right Order

First, size motor branch-circuit conductors under NEC 430.22 using the required percentage of motor full-load current. Second, select short-circuit and ground-fault protection under NEC 430.52. Third, confirm conductor ampacity using NEC 310.16, correction factors, adjustment factors, terminal temperature limits, and raceway fill where relevant. Only then should voltage drop be used to increase conductor size for performance. The wire size calculator and NEC standards guide are useful checks before you treat a voltage-drop answer as a final installation design.

For starting problems, run a separate check with the motor starting voltage drop calculator. A motor that draws 40 A at full load can draw 240 A during a 600% across-the-line start. The steady-state drop may be 2.4%, while the start sag is large enough to make the motor accelerate slowly or pull a contactor into chatter. That is not the same problem as voltage unbalance, but the symptoms can overlap.

“My field rule is simple: if a three-phase motor is overheating, I want loaded voltage readings, loaded current readings, and the feeder route length before I price a replacement motor.”
— Hommer Zhao, Technical Director

A Practical Troubleshooting Checklist

Measure line-to-line voltage at the motor while the motor is loaded.

Measure phase current and compare current unbalance with voltage unbalance.

Calculate steady-state voltage drop from actual route length and conductor size.

Check starting sag if the complaint is slow acceleration or nuisance tripping.

Inspect contactor poles, fuse clips, lugs, and disconnect blades for heat damage.

Review single-phase loads on the same transformer or service.

Common mistake

Do not average the three voltages and stop. A motor can have a good average voltage and still have damaging unbalance. For a real diagnostic example with complete calculations, compare the workflow against the site's calculation examples.

FAQ

How much voltage unbalance is too much for a three-phase motor?

Keep phase voltage unbalance below 1% where practical. Many NEMA MG 1 motor applications require derating above 1%, and a 3% unbalance can create roughly 18% to 30% current unbalance depending on motor and loading.

Does NEC 430 set a voltage-drop limit for motor feeders?

NEC 430 covers conductor sizing, overload protection, and short-circuit protection, but the common 3% branch-circuit and 5% total voltage-drop targets come from NEC 210.19(A)(1) and 215.2(A)(1) informational notes.

Should I calculate voltage drop with running current or starting current?

Check both. Use running current for steady-state voltage drop, then check starting current because a motor can draw 500% to 700% of full-load current during across-the-line starting.

Can one small phase-to-phase voltage difference overheat a motor?

Yes. On a 480 V system, readings of 480 V, 472 V, and 488 V average 480 V but create about 1.67% voltage unbalance, enough to justify checking current unbalance and motor derating.

How does IEC 60364 treat voltage drop for motor circuits?

IEC 60364-5-52 treats voltage drop as a design check tied to conductor material, installation method, grouping, and load behavior. Many IEC projects use 5% for power circuits, with tighter limits for sensitive loads.

What is the fastest field workflow for a suspect motor circuit?

Measure all three line-to-line voltages under load, calculate percent unbalance, run the route through a voltage-drop calculator, then compare conductor ampacity against NEC 310.16 and motor rules in NEC 430.

Run the motor circuit before you replace the motor

Use the calculators to compare voltage drop, unbalance, conductor size, and starting behavior with your measured field data. For a project-specific review or correction to a published example, send the route length, voltage readings, motor nameplate data, and conductor details through the contact page.

Contact Voltage Drop Calculator