เพาเวอร์แฟกเตอร์และรีแอกแตนซ์ในแรงดันตก: เมื่อดูแค่ความต้านทานสายไม่พอ

Power factor and reactance become important when an AC circuit is long enough, large enough, or inductive enough that plain conductor resistance no longer tells the whole story. A 20 amp receptacle branch circuit serving mostly electronics may be adequately screened with resistance and route length. A 225 amp 480Y/277V motor-control-center feeder, a 75 kVA transformer secondary, or a 208V compressor circuit with poor starting power factor deserves a fuller impedance calculation before conductor size is released.

We reviewed a real design scenario for a small manufacturing tenant where a 480V three-phase feeder ran about 310 feet one way to a remote machine panel. The first pass used only copper resistance and showed about 2.4% drop at 180A. After reactance and 0.82 load power factor were included, the practical operating drop moved close to 3.1%. That changed the design decision: the feeder was upsized one step and the installer avoided weak voltage at the machine panel when two motors and a control transformer were loaded together.

This article is for electricians, engineers, and careful DIYers using the calculator to decide when a simple voltage-drop check is good enough and when the AC formula needs power factor and conductor reactance. NEC 210.19(A)(1) and NEC 215.2(A)(1) informational notes point toward about 3% branch-circuit and 5% total feeder-plus-branch voltage drop for reasonable performance. IEC 60364-5-52 uses a different structure, but the same design habit applies: define the load, choose a safe conductor, then verify delivered voltage under real operating conditions.

TL;DR

Resistance-only voltage drop is acceptable for many short, high-power-factor circuits, but it can miss long AC feeder behavior.
Use impedance with power factor when motors, transformers, large feeders, or low power factor loads control the design.
NEC guidance still uses practical 3% branch and 5% total targets; IEC 60364-5-52 uses similar performance thinking.
For three-phase feeders, calculate with line current, route length, resistance, reactance, and the load power factor.

The design baseline in this article is anchored to power factor , electrical impedance , the National Electrical Code . Those references matter because code language, conductor physics, and equipment behavior usually fail in the same place: a circuit that was technically legal on paper but poorly optimized for the distance, load, or operating temperature in the field.

"For a 120V branch circuit at 12 amps and 60 feet, resistance dominates the decision. For a 480V feeder at 180 amps and 300 feet, I want resistance, reactance, and power factor in the same calculation before I sign off the wire size."
— Hommer Zhao, Technical Director

Resistance, Reactance, Impedance, and Power Factor Defined

Resistance is the part of conductor opposition that converts electrical energy into heat. Reactance is the AC opposition caused by magnetic and electric fields around conductors and equipment. Impedance is the combined AC opposition made from resistance and reactance. Power factor is the ratio between real power and apparent power, and it tells you how much of the current is aligned with useful work instead of circulating as reactive current.

For DC circuits, voltage drop is usually a resistance problem. For single-phase AC circuits with short runs and near-unity power factor, the resistance-only result is often a practical approximation. For long AC feeders, three-phase motor circuits, transformer secondaries, service conductors, and grouped conductors in metal raceway, reactance can become meaningful enough to affect conductor choice. The longer the run and the larger the current, the more important it is to stop treating AC like DC.

The common three-phase approximation is voltage drop equals square root of 3 times current times length times the expression R times power factor plus X times sine of the power-factor angle. In plain language, resistance and reactance do not contribute equally at every load. At 0.95 power factor, resistance drives most of the result. At 0.75 or 0.80 power factor, reactance contributes more strongly. Motor starting can be even more severe because locked-rotor current is high and starting power factor is often low.

Use resistance-only checks for simple screening. Short branch circuits, DC loads, heating loads, and high-power-factor lighting usually behave well enough for a resistance-based first pass.
Use impedance checks for inductive AC loads. Motors, transformers, welders, large HVAC equipment, UPS input feeders, and long three-phase runs should include power factor and reactance.
Keep NEC and IEC targets separate from equipment tolerance. A 3% branch or 5% total design target is useful, but some controls, drives, and motor starters need tighter delivered-voltage limits.
Document the assumption. Write whether the calculation used 1.0, 0.90, 0.85, or measured power factor so future troubleshooting has a real design basis.

Comparison Table: When to Use Resistance or Full AC Impedance

These examples show where a simple resistance calculation is usually adequate and where power factor plus reactance should be part of the voltage-drop review. Values are design-screening guidance; final calculations need actual conductor data, raceway configuration, temperature, and equipment instructions.

Circuit Type	Example Load	Typical Power Factor	Route Length	Simple Check Risk	Better Calculation Choice
Electric heater branch	240V / 24A resistance load	Near 1.00	55 ft one way	Low	Resistance-only check is usually enough
LED lighting row	277V / 14A driver load	0.90 to 0.98	180 ft one way	Moderate	Check resistance, then verify driver tolerance
Compressor branch	208V / 32A motor load	0.75 to 0.85 running	120 ft one way	High during start	Use impedance and starting-current review
MCC feeder	480V / 180A mixed motors	0.80 to 0.90	310 ft one way	High	Use three-phase impedance formula
Transformer secondary	208Y/120V / 225A panel	0.85 to 0.95	85 ft one way	Moderate to high	Check secondary conductor impedance and panel load mix
24V DC controls	2.5A solenoid circuit	Not applicable	45 ft one way	Percent drop is severe	Use DC round-trip resistance calculation

"A low power factor does not simply mean bad efficiency. It changes how the voltage-drop vector resolves. On motor feeders, using only resistance can make a 3% design look cleaner than it will measure in the field."
— Hommer Zhao, Technical Director

Example 1: 480V Three-Phase Feeder at 180A and 0.82 Power Factor

Assume a 480V three-phase feeder, 180A load current, 310 feet one-way route, copper conductors in raceway, and a measured or estimated 0.82 power factor. A resistance-only calculation may show the conductor is near a 3% target. Once reactance is included, the voltage-drop vector can move the result above the target because the reactive component aligns with the load current differently than a unity-power-factor heating load.

The design decision is not automatic upsizing in every case. First confirm the load current, route length, conductor material, raceway type, and actual equipment voltage range. Then compare the selected conductor with the next larger size. If the branch circuits downstream also need voltage-drop margin, keeping the feeder closer to 2% or 2.5% is often better than spending the full 3% at the feeder alone.

Example 2: 208V Compressor Circuit with Starting Sag

A 208V compressor drawing 32A running current on a 120-foot one-way branch can pass a running voltage-drop check and still start poorly. If the motor starts at six times running current, starting current can reach about 192A for a short time. NEC 430 allows motor-circuit rules that differ from ordinary branch circuits, but the motor still needs enough voltage at its terminals to accelerate.

At low starting power factor, reactance matters more than it does during steady operation. This is why motor-starting voltage drop should be reviewed separately from running voltage drop. A conductor that looks acceptable at 32A can cause contactor chatter, dimming, or overload trips when the starting event is modeled with realistic current and power factor.

Example 3: 277V Lighting Feeder with High-Power-Factor Drivers

A 277V lighting feeder serving high-power-factor LED drivers may not need the same level of reactance concern as a motor feeder, but it still needs a documented assumption. If the drivers are listed near 0.95 power factor and the route is 180 feet, a resistance-based calculation may be close enough for layout decisions. If the project uses long parallel runs, steel raceway, or older drivers with poorer power factor, a fuller AC check is safer.

The practical lesson is to use the load type to choose the calculation method. A calculator result is only as good as the assumptions entered. When the load includes power-factor data on the nameplate or submittal, use it instead of assuming every AC load behaves like a heater.

Common Mistakes with Power Factor Voltage Drop

Assuming unity power factor for every AC load

Motors, transformers, welders, and older lighting drivers often operate below 1.00 power factor. The voltage-drop result can change enough to affect conductor size.

Checking running current but ignoring starting current

Motor-starting current can be 5 to 7 times running current, and the starting power factor can be low. That brief sag can decide whether equipment starts cleanly.

Using feeder budget twice

If a feeder already uses 3%, the downstream branch circuit cannot casually use another 3% and still satisfy a 5% total design target.

Copying a resistance table without units

Ohms per 1000 feet, ohms per kilometer, one-way length, and round-trip length are easy to mix. Unit mistakes are often larger than the reactance correction.

A Practical Workflow for AC Voltage-Drop Decisions

Use this sequence when the circuit is long, inductive, three-phase, or expensive enough that a wrong conductor size would create rework.

1. Classify the load. Identify whether the load is resistive, electronic, motor, transformer, welder, HVAC, or mixed panel load. Record voltage, phase, amperes, duty cycle, and power factor.
2. Select a code-legal conductor first. Check NEC 310.16 ampacity, NEC 310.15 adjustment and correction, terminal temperature under NEC 110.14, and equipment-specific articles such as NEC 430 for motors.
3. Calculate voltage drop with the right model. Use resistance-only for simple high-power-factor loads. Use impedance with R, X, power factor, and sine of the power-factor angle for long AC feeders and motor circuits.
4. Compare feeder and branch totals. Keep the NEC informational-note target of about 3% branch and 5% total in view, or apply the stricter IEC/project voltage-drop limit if specified.
5. Run a sensitivity check. Compare the selected conductor with the next larger size at 0.95, 0.85, and 0.75 power factor if load data is uncertain. The comparison often reveals whether upsizing is cheap insurance.

FAQ

When does power factor matter for voltage drop?

Power factor matters most on long AC feeders, motor circuits, transformer secondaries, welders, UPS inputs, and loads below about 0.90 power factor. A 300 ft three-phase feeder at 180A should not be checked like a short 12A heater circuit.

What is the NEC voltage-drop limit for feeders?

NEC 215.2(A)(1) includes informational guidance commonly used as a 3% feeder or branch target and a 5% total feeder-plus-branch target. It is design guidance, not a universal mandatory rule, but it is a strong performance benchmark.

Does reactance matter on small residential branch circuits?

Usually not much. On a short 120V or 240V branch circuit with high-power-factor loads, conductor resistance normally controls. Reactance becomes more important as current, distance, conductor size, and inductive load content increase.

How should I handle motor-starting voltage drop?

Check running voltage drop and starting sag separately. A motor running at 32A may start near 160A to 224A depending on design, and the starting power factor can be much lower than running power factor.

Is IEC 60364 different from NEC voltage-drop practice?

IEC 60364-5-52 organizes conductor sizing, installation method, and voltage-drop review differently from the NEC, but both systems require a safe conductor and a delivered-voltage check. Project limits such as 3%, 4%, or 5% should be documented.

Which site tools should I use for power-factor voltage drop?

Start with the voltage drop calculator, compare conductor size on the wire size calculator, and use the three-phase calculator when the load is 208V, 400V, or 480V three-phase with a known power factor.

Need to Check a Long AC Feeder or Motor Circuit?

Use the voltage drop calculator to compare resistance-only and AC impedance assumptions, then contact the engineering team if the route includes motors, transformers, long three-phase feeders, or mixed NEC/IEC review requirements.

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