A common misconception when selecting and installing motors is assuming bigger is always better. Matching a motor correctly to the load ensures it runs more efficiently, reduces energy use, and cuts costs. Motors generally operate at peak efficiency when carrying 90% to 95% of their rated load. Just because the nameplate says "25 Hp" doesn’t mean the motor is delivering the full twenty-five horsepower in operation. The actual output depends on the load, and if the motor consistently runs at much lower horsepower, money is being wasted—it’s better to use a motor that’s properly sized.

The same principle applies to conductor sizing and the fuses or circuit breaker that power the motor. These are determined by the motor’s full load current rating, expected duty cycle, and other conditions. Oversizing conductors and breakers leads to unnecessary costs. Also, keep in mind that even with low horsepower demand, motors still draw a considerable amount of current. For instance, a motor running with no load at all still uses about 50% of its rated current.

When Replacing a Motor, Make Sure It Fits the Job

Whenever you replace a motor, it’s crucial to ensure the motor is properly matched to the application. Along with choosing the correct voltage, phase (single-phase or three-phase), design letter, and code letter, you also need to pick the right horsepower rating. If the motor has already been replaced before, or if it’s driving equipment like a pump or fan that wasn’t originally sized by the OEM as part of a complete system, you might not have the correct motor size. Taking basic voltage and current measurements to calculate your actual horsepower needs will help you run a more efficient setup.

This data is especially useful during an energy study. If your motor typically operates at 90% load or less for long periods, it may be an ideal candidate for a variable speed drive (VSD), which can deliver major energy savings. For example, if you can reduce the motor speed to 90% of its full-rated speed using a VSD, energy consumption drops to about 73% of what would be required at full speed. That’s another strong reason to know the true load demands of your equipment.

In other cases, the motor might actually be overloaded and drawing more current than its rating allows. Causes could include worn bearings, a misaligned shaft, poor maintenance, or simply too much load. No matter the reason, the outcome is the same: excessive heat in the windings. Heat breaks down insulation and is the number one cause of motor failure. While overload protection is typically set to trip at 115%–125% of the nameplate full load current, the heat generated during this period will still shorten the motor’s lifespan.

Determining Load Horsepower, Wiring, and Breaker Size for Safe and Efficient Installations

Power Quality

By Randy Barnett

JMBom iFlex™ Flexible Current Probe

Wrap a JMBom iFlex™ Flexible Current Probe around a single conductor, or position the jaws of a clamp meter around one conductor for measurement.

A common misconception in motor selection and installation is that the nameplate rating reflects the motor’s actual output. In reality, motors deliver load more efficiently and economically when correctly sized. The most efficient operating range for most motors is between 90% and 95% of their rated load. For example, a motor labeled “25 Hp” isn’t necessarily delivering twenty-five horsepower during operation—it may be producing much less depending on the actual load. If a motor consistently runs well below its rated horsepower, energy and money are being wasted. In such cases, replacing it with a properly sized motor is the smarter choice.

Conductor and breaker sizing must also be carefully considered. These are based on the motor’s full-load current rating, expected duty cycle, and other operating factors. Oversizing conductors or circuit breakers adds unnecessary cost. Keep in mind that even at light load conditions, motors still draw significant current. For instance, a motor operating without any load at all still consumes roughly 50% of its rated current.

When You Replace a Motor, Make Sure It Fits the Job

Whenever you replace a motor, it’s essential to match it to the application. In addition to selecting the correct voltage, phase type (single-phase or three-phase), design letter, and code letter, be sure to choose the correct horsepower rating. If the motor was previously replaced, or is running equipment like a fan or pump that wasn’t sized by the OEM as part of the system, the current motor might not be the right fit. Taking basic voltage and current measurements will help estimate your real horsepower needs and improve efficiency.

This information is also extremely valuable for energy studies. If a motor operates at 90% load or less for extended periods, it may be a strong candidate for a variable speed drive (VSD), which can generate major savings. For example, if the speed of a motor is reduced to 90% of its rated speed with a VSD, energy use drops to only about 73% of what would be required at full speed. This is yet another reason to know your true load requirements.

At the same time, motors can sometimes be overloaded—drawing more current than rated. This may be due to mechanical problems such as worn bearings, misaligned shafts, poor maintenance, or simply an excessive load. The result is excessive heat in the windings. Heat degrades insulation and is the leading cause of motor failure. Even though overload protection typically trips between 115% and 125% of the nameplate current rating, the heat generated during this period can still shorten motor life.

Determining Actual Motor Horsepower

As part of a preventive maintenance program, motor voltage and current should be measured and recorded regularly. You can estimate actual horsepower using the formula:

Horsepower (hp) = Voltage × Amperage × % Efficiency × Power Factor × 1.73 ÷ 746

Where:

• Voltage = average of the three line voltages: (A-B + A-C + B-C) ÷ 3
• Amperage = average of the three phase currents: (A + B + C) ÷ 3
• % Efficiency = motor efficiency listed on the nameplate
• Power Factor = ratio of real power (kW) to apparent power (kVA). If you don’t have a meter, use an estimated value of 0.85.
• 1.73 = constant used in three-phase calculations
• 746 = constant for converting watts to horsepower (746 watts = 1 Hp)

Example:

How much horsepower is a 25 Hp motor producing at 472 volts, drawing an average of 20 amps per phase, with 90% nameplate efficiency?

Horsepower = 472 V × 20 A × 0.90 × 0.85 × 1.73 ÷ 746

= 17 Hp

The fastest way to estimate motor horsepower is by measuring the motor’s current and voltage with a digital clamp meter, then applying a simple calculation:

Horsepower (hp) = Voltage × Amperage × % Efficiency × Power Factor × 1.73 ÷ 746

Always follow proper safety procedures for the application. Using remote display digital multimeters, such as the JMBom 381 Remote Display True RMS Clamp Meter, helps reduce exposure to dangerous voltages and minimizes time spent in arc-flash hazard zones.

For accurate results, it’s essential to use a true-RMS clamp meter. While motor current is often displayed directly on the panel of an adjustable speed drive, many other types of equipment require a meter that can deliver reliable readings even in the presence of harmonics and distorted waveforms.

Measuring Loads Beyond Motors

It’s just as important to record operating values for loads other than motors. Since horsepower isn’t calculated for these types of loads, you can still use the same procedure outlined in the sidebar “Use this formula to estimate motor horsepower”—but instead of determining horsepower, simply measure and record the current going to the load.

Examples include hermetic refrigerant motor-compressors in HVAC systems, lighting circuits, and heating elements. When troubleshooting issues such as breaker trips or equipment overheating, compare the rated-load current listed for hermetic compressors—or the current ratings provided for other equipment—with your measured values.

To properly size breakers and conductors for these loads, always follow the National Electrical Code® (NEC®), as well as the manufacturer’s instructions, equipment drawings, and local code requirements. While the NEC includes detailed rules for specific equipment types like motors and HVAC units, the general guideline is:

• Conductors and circuit breakers should be sized at 125% of the continuous load plus 100% of the non-continuous load.

Understanding Continuous Loads and Motor Sizing

A continuous load is defined as one where the maximum current is expected to flow for three hours or longer. When sizing conductors and breakers for motors, it’s important to use the appropriate NEC® tables for motor full-load amperage—not previously measured values or the motor’s nameplate data. Measured current helps determine the size of the load, but wiring and breaker sizing for motors must follow NEC full-load current tables, which list values based on motor phase, voltage, and horsepower. For non-motor loads, such as heating or lighting, you would instead use manufacturer ratings and measured values.

Example:

Consider a three-phase, 25-horsepower chilled water pump motor that is expected to run continuously at full load for more than three hours. According to NEC tables, the full-load current for a three-phase, 460-volt, 25-horsepower motor is 34 amps. The conductors must therefore be sized at:

34 A × 1.25 = 43 A (125% of full-load current).

From there, NEC ampacity tables are used to determine the actual wire size, taking into account insulation type, ambient temperature, and installation conditions.

For the overcurrent protective device, NEC Table 430.52 specifies that the maximum fuse or circuit breaker rating may range between 175% and 250% of the motor’s full-load current.

Because sizing depends on multiple factors, always consult the National Electrical Code® or a qualified electrician when determining conductor sizes, breaker ratings, and overload protection for motors. The same rules apply to hermetic refrigerant motor-compressors and other types of electrical equipment.

The Goal: Safe, Properly Sized Installations at Peak Efficiency

Field verification of motor horsepower is essential to confirm that the correct motor size has been applied. If the motor is oversized, consider either replacing it with a correctly sized unit or installing a variable-speed drive for improved efficiency.

Regularly measuring and recording current and voltage should also be part of a strong preventive maintenance program. When it comes to wiring and breaker sizing, always follow the National Electrical Code® (NEC®) requirements for the type of load.

Ultimately, the objective is straightforward: a safe, properly sized installation that delivers maximum efficiency and reliability.

Frequently Ask Questions

What is a motor load?

A motor load refers to the amount of current an electric motor draws while operating. It depends on the demand placed on the motor’s shaft. For a given motor, load can be estimated by looking at input power, current (amperage), or operating speed. Most motors are designed to run best between 50% and 100% of their rated load.

What is the load of a 25 hp motor?

According to NEC® tables, the full-load current of a three-phase, 460-volt, 25-horsepower motor is 34 amps. To size the conductors properly, this value is multiplied by 125%, giving 43 amps.

What are the different types of motor load?

There are three main categories of motor load:

• Constant Power – requires the same power at all speeds, while torque decreases as speed increases.
• Constant Torque – torque remains the same, but power increases with speed.
• Variable Torque – common in applications like fans and pumps, where torque changes with speed.

How do you check the motor load?

A tachometer can be used to measure the actual motor speed, which then allows the load to be calculated. A battery-powered stroboscopic tachometer is one of the safest, most convenient, and accurate tools for this task.

How can motor load be increased?

Adding size or weight to the driven equipment increases its inertia, which raises the torque demand on the motor. Using gearing is another effective method, as it reduces the reflected inertia on the motor by the square of the gear ratio.

How do you measure the load of a motor?

The difference between a motor’s synchronous speed and its actual speed is called slip. Slip is directly proportional to the motor’s load. By measuring the actual speed, you can accurately estimate how much load the motor is carrying.

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