Sizing motors right the first time

Determining load horsepower, wiring, and breaker size for safe and efficient installations.


Figure 1: Properly sizing motors for a given load results in driving loads more efficiently, saving energy, and saving dollars. Courtesy: Fluke“Let’s just oversize the motor and we can run it lightly loaded—that will save us some money and be easier on the motor.”

This is a false belief among some who select and install motors. Properly sizing motors for a given load results in driving loads more efficiently, saving energy, and saving dollars. Motors typically are most efficient when they are 90% to 95% loaded. Just because a motor says “25 hp” on the nameplate does not mean the motor is producing 25 hp as it operates. The motor may be producing quite a bit less depending on the load requirements. If the motor constantly runs at these reduced horsepower requirements, money is being wasted, and you should consider replacing it with a correctly sized motor.

It is also important to realize that even when lightly loaded, a motor still draws a relatively large amount of current. For example, a motor operating under no load at all still draws about 50% of its rated current.

Conductors and fuses or circuit breakers supplying that motor are sized according to the full load current rating of the motor, how often it is expected to operate, and other factors. Installing larger conductors and breakers than needed is a waste of money.

Match the motor to the job

When replacing motors, it is important to match the motor to the job. In addition to selecting the proper voltage, phase (3-phase or single-phase), design letter, and code letter, be sure to select the proper horsepower rating. If the motor has been replaced previously, or is running a pump, fan, or other equipment that was not sized as part of an entire system by the original equipment manufacturer (OEM), you may not be selecting the right size motor. Taking some basic voltage and current readings to estimate your own horsepower requirements will provide you with a more efficient system.

The information that you collect about the motor is valuable when conducting an energy study. If the motor runs at 90% of full load or less for extended periods, the application may be right for a variable speed drive and, thus, significant savings. For example, pump affinity laws indicate that reducing pump speed from full speed to 90% can reduce energy consumption to 73% of that required for full speed operation. Although VFDs can introduce additional losses, the potential savings may be enough to warrant further investigation. Just another reason to know the load requirements of your equipment!

In some cases the motor may be overloaded, drawing more than its rated current. Whether it is bad bearings, a misaligned shaft, other maintenance issues, or just excessive load on the motor, one detrimental effect is occurring for sure: Excessive heat is being produced in the windings. Heat deteriorates insulation and is the leading cause of motor failure. While properly sized and installed overloads will trip the motor at typically 115% to 125% of the full load current value on the nameplate, the heat developed during this time is sure to shorten motor life.

Figure 2: Specifying the right motor saves energy and money. Courtesy: Fluke

Determining actual motor horsepower

Motor running current and voltage values should be measured and recorded on a routine basis as part of a preventive maintenance program. The actual horsepower produced by a motor should be closely estimated in the field, as well. Energy losses, inefficiencies created by oversizing motors, or maintenance issues created by under-sizing motors are all good reasons to know how much horsepower the motor is attempting to produce.

The quickest method to closely estimate motor horsepower is to use a digital clamp meter to measure current and voltage to the motor, and then perform a simple calculation. Using the logged data and motor nameplate efficiency, suitable horsepower values can be obtained using this following formula:

hp = V x amp x % efficiency x power factor x 1.73/746

See the sidebar “How to estimate motor horsepower” for more detail. Be sure to follow the safe work practices appropriate to the specific application. With the availability of remote display digital multimeters, workers can reduce their exposure to lethal voltages and the arc-flash hazard zone.

Measuring loads other than motors

You also need to record operating values of loads other than motors. Since horsepower is not determined for loads other than motors, simply use the procedure outlined in the sidebar to measure and record the current value to the load. Examples of such loads might be hermetic refrigerant motor-compressors found in HVAC equipment, lighting loads, and heating elements. Rated-load current on hermetic refrigerant compressors and current ratings on other types of equipment need to be compared to measured values when you are dealing with breaker trips or overheating of equipment.

To determine the size of breakers and conductors needed to supply your load, refer to the National Electrical Code (NEC), manufacturers’ instructions, drawings, and any local code requirements. Though the NEC has specific rules for various types of equipment, such as motors and HVAC equipment, generally conductors and circuit breakers are sized at 125% of the continuous load plus 100% of the non-continuous load. A “continuous load” is one where the maximum current is expected to continue for 3 hours or more.

Figure 3: Using a variable speed drive with motors that run at 90% of full load or less for extended periods may give significant savings. Courtesy: FlukeOne important point: When sizing conductors and breakers for motors, use the appropriate table in the NEC for motor full-load amperage—not the value previously measured or motor nameplate information. The value previously measured is for helping to determine the size of the load. Sizing wires and breakers to supply a motor is based on code tables providing the full-load current values for specific phase, voltage, and horsepower motors. Manufacturers’ ratings and measured values are used for other than motor loads.

For example, a 3-phase, 25 hp chilled water pump motor is expected to operate at full load for 3 hours or more. The NEC tables indicate that the full load current of a 3-phase, 460 V, 25 hp motor is 34 amp. Therefore, the conductors supplying the motor must be sized at 34 x 1.25 = 43 amp (125% of 34 amp). Ampacity tables in the NEC are used to determine the actual conductor size based on insulation type, ambient temperatures, and other conditions. The maximum circuit breaker or fuse size for the motor is based on another NEC table, Table 430.52. The maximum value of this overcurrent protective device may range anywhere from 175% to 250% of the full load current. Always consult the NEC or a qualified electrician for exact sizes for motor wiring, fuses, and circuit breakers, as well as the overload protection requirements for motors. The same holds true for hermetic refrigerant motor-compressors and other types of electrical equipment.

Achieving the goal

The ultimate goal is to have a properly sized motor and safe installation, operating at maximum efficiency. You must determine motor horsepower in the field to verify that the right-size motor has been applied. If the motor is oversized, consider replacing the motor or installing a variable speed drive. Routinely measuring and recording current and voltage values is also an important part of a quality preventive maintenance program. Size the wiring and breakers for any type of load using the NEC. Remember, the goal is a properly sized and safe installation operating at maximum efficiency.

Barnett is director of Electrical Program Delivery at NTT Workforce Development Institute and a certified energy auditor through the Assn. of Energy Engineers. He has over 35 years of industrial electrical experience as a journeyman electrician, technical trainer, and author.

How to estimate motor horsepower

Example of estimating motor horsepower

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