HVAC Equipment Power Rating Calculations – Part One

In this course, we will learn the following:
  • How to select and size the over current protection device for HVAC equipment.
  • How to size the circuit wiring/cables for HVAC equipment.
  • How to select and size the disconnection means for HVAC equipment.
  • How to select and size the motor controller for HVAC equipment.

For the above items to be fully understood, you must review our previous courses for HVAC which were:


In our previous Course HVAC-1: An Introduction to Heating, Ventilation and Air Conditioning (HVAC) Systems,We explained that the HVAC system includes the following (2) major parts:
  1. Heating systems,
  2. Cooling systems.

1- Heating systems

Also, we classify the Heating systems to (2) types:

1.A Central Heating System: where the following Equipment used to generate heating:

  • Furnace,
  • Hot water & steam boiler,
  • Heat Pump,
  • In Floor or Under Floor Heating,
  • Electric Thermal Storage (ETS ) Unit.

1.B Local Heating System: where the following Equipment used to generate heating:

  • Portable electric heaters,
  • Built-in electric resistance heaters,
  • Electric resistance baseboard radiators,
  • Infrared heaters,
  • Fireplaces,
  • Wood stoves.

2- Cooling systems

A While the cooling systems are classified into (3) main categories:

2.A  Decentralized Systems (Individual Room Systems): This can be divided to three major types as follows:

  1. Mini-Split Cooling Units (ductless split Units),
  2. Window Units,
  3. Mini- Heat Pump.

2.B  Semi- centralized systems (packaged systems): This can be divided to two major types as follows: 

1- One piece systems (Unitary Packaged Systems): This is divided to:
  • Packaged Air Conditioners with Water Cooled Condenser.
  • Packaged Air Conditioners with Air Cooled Condensers.

2- Two pieces systems (ducted Split system).

2.C  Centralized systems (Central Hydronic systems): This can be divided to three major types as follows:

  1. Centralized Ducted “All – Air” Systems.
  2. Centralized Fluid Based Hydronic Systems.
  3. Combined (Hybrid) Water and Air Systems.

You can review the construction of all the above cooling systems/units in this previous course or to review the summary of Air-Conditioning Systems Types
. However, the following table will show a comparison between these configurations as follows:

Parts Consuming Power in HVAC Systems

In order to size the power ratings required for HVAC system we must now first which Parts consuming power in it. Generally the parts that consuming power in HVAC Systems are mainly (2) items:
  1. Motors,
  2. Heaters.

First Part: Motors

Both alternating current (ac) and direct current (dc) may be used to operate electric motors. Alternating current is most commonly used. Direct current motors are found in areas supplied with direct current only.

First: Direct Current and Universal Motors

Areas with dc power must use direct current motors in refrigeration. Generally, these motors are compound wound. However, Direct current may be used only on external-drive systems,
Direct current motors have a mechanical likeness to both capacitor-and repulsion-start induction motors

For more information about DC Motors you can review the following previous articles:

Second: Alternating Current Motors

The following alternating current motors are used to drive compressors. Alternating current motors may be Single-phase or Polyphase (two or three or four-phase). The AC motors for Air conditioning/refrigeration systems include:

  1. The external-drive motor,
  2. Hermetic motors,
  3. The condenser and evaporator fan motor.

 AC Motors in Air conditioning/refrigeration systems may be grouped into four general classifications according to use:

1- Motors to drive compressors:
  • External-drive (open belt-drive),
  • Hermetic direct-drive.

2- Motors to drive fans for:
  • Condensers,
  • Evaporators,
  • Air circulation,
  • Induced draft,
  • Forced draft.

3- Motors to drive pumps:
  • Condensate pumps,
  • Chilled water pumps,
  • Condenser water pumps,
  • lce-making machine water pumps,
  • Oil pumps.

4- Motors to drive miscellaneous devices:
  • Vending machines,
  • Automatic ice cube makers.

For more information about AC Motors you can review our previous Course Motor-1: An Introduction to Electrical Motors Basics.

Let's take more details about the Motors types used in Air conditioning /refrigeration systems as follows:

First: Types of Motors which drive the compressors

As we stated in above, there are (2) different types of motors are used for driving refrigeration compressors as follows:

1- External drive Motor (open belt-drive):

External drive Motor

  • They are conventional motors. The compressor is driven with one or more V-belts or by direct drive. Figure 4-73shows a sectional view of a typical motor used on small external drive compressor.
  • An external-drive (open) compressor is bolted together its crank shaft extends through the crankcase. The crankshaft is driven by a flywheel (pulley) and belt. It may also be driven directly by an electric motor or gas driven engine. A crankshaft seal is required where the crankshaft comes through the crankcase. External-drive compressors have been largely replaced by newer, more efficient designs like the Hermetic motors.

Types of The external-drive motor used on open, belt-driven compressors include:
  1. Repulsion-start, induction –run,
  2. Capacitor- start, induction –run,
  3. Capacitor- start Capacitor –run,
  4. Capacitor –run,
  5. Permanent split capacitor,
  6. induction polyphase.

2- Hermetic Motor:

Hermetic Motor

  • In hermetic compressors, the motor is mounted under the same dome as the compressor. These motors are lubricated by the oil carried in the refrigerant. Hermetic motors do not use brushes or open Points inside the dome. Arcing would cause pollution in both the oil and the refrigerant. This would lead to an electrical burn out. 
  • A special electrical starting device is located outside the dome. 

Types of Hermetic motors include:
  1. Capacitor- start, induction –run (CSIR),
  2. Capacitor- start Capacitor –run (CSCR),
  3. Capacitor –run,
  4. Permanent split capacitor (PSC),
  5. Induction two-phase and polyphase.

The Hermetic motors are either single-phase or polyphase as follows:

2.A Hermetic Single-phase Motor

Four types of single-phase induction motors are used:
  1. Split Phase (SP),
  2. Capacitor-start, induction-run (CSIR),
  3. Capacitor-start, capacitor-run (CSCR),
  4. Permanent split capacitor (PSC).

2.B Hermetic Polyphase Motor

  • Large hermetic compressors usually are driven by three-phase motors. Three-phase motors are available from 1/2 hP size and up. 
  • The buildings in which the unit is to be placed must be wired for three-phase service. Very few residences have three-phase electrical Power. However,most industries and some commercial buildings are wired for three-phase.

For more information about the External drive, Hermetic motors and different types of compressors, please review our previous article: Types of Motors/ Compressors in HVAC Systems.

  • Three-phase motor use contactors or motor starters. They don't have the usual starting relays.
  • Refrigeration motors of 1/100 hP to several hundred hp are in use. Motors as small as 1/20 hP are used to drive compressors.

Second: Types of Motors used to drive the fans

There are (5) different types of fans in the air conditioning/ refrigeration system as follows:

  1. Condensers fans,
  2. Evaporators fans,
  3. Air circulation fans,
  4. Induced draft fans,
  5. Forced draft fans.

The types of motors used to drive the above fans include:
  1. Split-phase motor,
  2. Shaded pole motor,
  3. Capacitor motor,
  4. Permanent split capacitor motor.

Today, the capacitor motor is used for most applications. It is one of the most popular motors for single phase hermetic units.

Third: Types of Motors used to drive the Pumps

1- For smaller pumps the following (2) types of motors can be used:
1.A Split-Phase Air Conditioner Motor
  • It is usually used in small pumps where the capacity is below 1 horsepower.

1.B Electronically Commutated Motors (ECM) for HVAC Centrifugal Pumps
  • Smaller centrifugal pumps may have an option of using an ECM or electronically commutated motor. ECM motors have permanent magnets normally produced using rare earth. Since we are not producing a magnetic field, the efficiency of the ECM is higher than an induction motor. 
  • The maximum ECM horsepower normally available today is around 5 HP. For this reason, ECM motors are normally only available on smaller pumps for smaller applications.  They are very prevalent in fractional horsepower pumps and fans.

2- For Large Pumps
  • The 3 phase induction motor will be used for large pumps, noting that these motors are essentially constant speed motors and lack any provisions for speed control when running on main power and are intrinsically unable to adjust to the changing load demands of the application. 
  • Therefore, suitable external equipment has to be used to control their speed which are called Variable frequency drives VFD.

Usually the pumps in HVAC/refrigeration are used in the following systems:
1- Condenser water system
  • where the hot water from the condenser is pumped to the cooling tower which is located a distance away from the condenser. The hot water is then cooled at the cooling tower before being circulated back to the condenser. This process is done repeatedly.

2- Chilled water system 
  • where the chilled water from the chillers are being pumped and circulated to the various sections of a building before being used to cooled the space. The chilled water system may have the following pumps:

  1. Primary pump flows water through the evaporator side of the chiller
  2. Secondary pump flows water through AHUs.

3- Hot water system
  • where the hot water from the boiler is circulated to the heat transfer units and back.

4- Condensate system
  • where condensate pump is used on evaporator drains and furnaces when condensate is to be raised above Drain pan level. A condensate pump may be used to provide a Positive method of removing the condensate. The pump is mounted on the drain and is self priming. It uses about 10W and operates continuously.
  • It is also used for pumping the slightly acidic condensate produced by high efficiency gas furnaces.
  • Therefore, there are two intakes. One is for air conditioning condensate drain line, and one is for the Furnace.

Parts consuming power as per used unit/system

First: Parts consuming power in Mini-Split & Window Units are:

  1. The compressor- Motor,
  2. Condenser Cooling Fan Motor,
  3. Cooling Fan or Blower or evaporator or air handler Motor,
  4. Electric heater (existing only in mini-heat pump units).

Second: Parts consuming power in all packaged units are:

  1. The compressor- Motor,
  2. Condenser Cooling Fan Motor,
  3. Cooling Fan or Blower or evaporator or air handler Motor,
  4. Electric heater.

Third: Parts consuming power in centralized systems:

  1. Chillers compressors,
  2. AHU compressors (in case of Roof Top Units RTU),
  3. Primary Pumps,
  4. Secondary pumps,
  5. AHU- Return air fan,
  6. AHU - Forced draught fan,
  7. FCU – fan,
  8. Cooling towers- fan,
  9. Furnace blower,
  10. Boiler blower,
  11. AHU - Electric Heating,
  12. FCU - Electric Heating,
  13. Boiler - Electric Heating.

Motor Nameplate - Air Conditioner Motor Applications 

Reading a motor’s nameplate can often be challenging. Nameplates differ according to manufacturer, and a variety of factors, such as dirt and damage, can hide crucial information needed to size a Variable Frequency Drive (VFD), make a repair, find a replacement, correct the power factor, purchase parts, or do almost anything with the motor. But even a clean nameplate can be confusing with all of the abbreviations and codes used in such a small space. The following data is designed to help you understand the crucial information contained on your motor’s nameplate:

Motor’s Nameplate

1- Phase (PH): 
  • A power system can be either single-phase or poly-phase. Single-phase power is most commonly found in homes, rural areas and in small commercial establishments. A poly-phase power system consists of 2 or more alternating currents of equal frequency and amplitude but offset from each other by a phase angle. 
  • For motors, an advantage of three-phase power is simpler construction which requires less maintenance. Also, a more powerful machine can be built into a smaller frame and will generally operate at a higher efficiency than single-phase motors of the same rating. Make sure the correct phase is connected to the motor.

2- Voltage (Volts), Current Capacity & Service Factor Amperage: 
2.A Voltage (Volts):
  • This should match exactly. If the supply voltage to the motor is too high, it will cause the lifetime of the motor to drop as local hot spot on the windings may occur. Eventually the motor will overheat and burn. If the supply voltage is lower than the lowest operation voltage, the current drawn will increase causing it to overheat. This condition also cause the lifetime of the motor to drop.
  • The bellow image lists motor nameplate voltages and provides the best match to distribution system voltages and meets current motor design practices:
Nameplate Voltage Ratings

2.B Current Capacity of the Motor
  • It will specify the FLA and LRA. The FLA or full-load amperage is the current drawn by the motor when running at full load. For example, a 2HP motor may draw a current of 24A at 120V AC and 12A at 230V AC in a single-phase system.
  • The LRA or locked-rotor amperage is the current drawn when the motor is starting and the rotor has yet to turn. This current is usually 5-7 times higher than the FLA.

2.C Service Factor Amperage or SFA 
  • It may be printed on the plate or in the data sheet of the motor. This specifies the reserve horsepower that enables it to operate above the nameplate horsepower without causing damage to the motor. However, the lifetime of the motor will be reduced compared to if it is operated at full load.

3- Horsepower (HP): 
  • Horsepower take into account how fast the motor shaft is turned. Turning the shaft rapidly requires more horsepower than turning it slowly. Thus, horsepower is a measure of the rate at which work is done. Very small motors are often rated in watts.

4- Physical Size/Frame (FR): 
  • Motor dimension is reflected by the “frame” size number. This number reflects the same mounting and shaft information between different manufacturers in order to be consistent.

5- Speed (RPM): 
  • It is the revolution per minute of the motor when running at full load.

6- Frequency (Hz): 
  • Frequency is crucial to the operation of the motor. The frequency is either 50 Hz or 60 Hz. In the United States, the frequency is 60 Hz. If the power supply frequency is 60 Hz, make sure the motor is rated at 60 Hz or else the speed of the motor may be different from its rated speed.

7- Service Factor (SF): 
  • It is defined as the permissible amount of overload a motor will handle within defined temperature limits. When voltage and frequency are maintained at nameplate rated values, the motor may be overloaded up to the horsepower obtained by multiplying the rated horsepower by the service factor shown on the nameplate. However, locked-rotor torque, locked-rotor current and breakdown torque are unchanged. 
  • NEMA has defined service factor values for standard poly-phase drip-proof, 60 Hz motors as shown in the following table:
NEMA Service Factor

8- Type: the motor will be one of the following types:
  1. Shaded Pole - Small direct-drive fans and blowers
  2. PSC - Direct-drive fans and blowers
  3. Split-Phase - Belt-drive and direct-drive fans and blowers, small tools, centrifugal pumps, and appliances
  4. Capacitor-Start - Pumps, compressors, tools, conveyors, farm equipment, and industrial ventilators
  5. 3-Phase - Applications where 3-Phase power is available

9- Enclosure (Encl.): the enclosure will be one of the following types:
  1. ODP/Open - Clean, dry, non hazardous environments
  2. Enclosed TEFC/TENV - Dirty, moist, non hazardous environments
  3. Hazardous - Designed for use in hazardous environments as defined by National Electric Code (NEC) classifications. NEC Class and Group are designated on UL Hazardous Location nameplate mounted on the motor.

10- Duty Cycle: 
  • It describes the energization / de-energization, and load variations, with respect to time for any given application. Duty cycle applications may be divided into three basic classifications:

  1. Continuous duty,
  2. Intermittent duty,
  3. Varying duty.

11- Bearing Type: 
  • Bearings, mounted on the shaft, support the rotor and allow it to turn. Not all bearings are suitable for every application; a universal, all-purpose bearing does not exist. The choice of bearing arrangement is based on the following qualities:

  1. Load carrying capacity in the axial and radial direction. 
  2. Overspeed and duration. 
  3. Rotating speed. 
  4. Bearing life. 
Bearing Type

  • The size of the bearing to be used is initially selected on the basis of its load carrying capacity, in relation to the load to be carried, and the requirements regarding its life and reliability. Other factors must also be taken into consideration, such as operating temperature, dirty and dusty environmental conditions, and vibration and shocks affecting bearings in running and resting conditions.

12- Thermal Protection:
  • Insulation codes are designated in order of their thermal capabilities by A, B, F, and H. The higher the designated Code letter, the greater the heat capability. For instance, a class “F” insulation has a longer nominal life at a given operating temperature than a class “B”.

Other Motor Nameplate information

Other information that can be found also in the motor name plate is as follows:
1- Model Number of the Motor
  • It is useful when you need to refer to the characteristics of the motor and when ordering a replacement parts or new motor. if blank, the motor is custom.

2- Power Factor
  • Power factor is indicated on the nameplate as either “PF” or “P .F” or cos φ. Power factor is an expression of the ratio of active power (W) to apparent power (VA) expressed as a percentage.

3- Efficiency
  • The percentage of the input power that is actually converted to work output from the motor shaft.

4- Torque speed curve

  • The design letter indicates the shape of the torque speed curve.

5- Temperature rise
  • The temperature rise is usually specified in degrees Celsius. Most motors are designed to operate in a maximum ambient temperature of 40°C. If the ambient temperature exceeds 40°C, the motor may need to be modified to compensate for the increase in total temperature. The temperature rise is the result of heat generated by motor losses during operation as follows:

  1. At no-load, friction in the bearings, core losses (eddy current and hysteresis losses), and stator I2R losses contribute to temperature rise.
  2. At full-load, additional losses which cause heating are rotor I2R losses and stray load losses. (NOTE: I = current in amps and R = resistance of the stator or rotor in ohms).

  • Since current increases with an increase in motor load and under locked-rotor, temperature rise will be significantly higher under these conditions. Therefore, applications requiring frequent starting and/or frequent overloads may require special motors to compensate for the increase in total temperature.

6- Seasonal energy efficiency ratio (SEER)
  • This ratio is obtained by dividing the total cooling that the equipment is able to provide over the entire season (Btu) over the total energy in Watt-hours it will consume (Wh).

In below image, you can find all the abbreviations that can exist on the motor name plate:

Abbreviations used in Motor Listings

In the next article, we will explain in details the HVAC system units/ratings and the famous motors used in HVAC systems. So, please keep following.

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