HVAC Equipment Power Rating Calculations – Part Seven

Today, we will explain how to size the Branch Circuit Overload Protection for Air Conditioning and Refrigeration Equipment and Provisions for Room Air Conditioners as Per Article 440.


The National Electrical Code (NEC) And HVAC Systems – Part Five


Fifth: Branch Circuit Overload Protection



  • In Article "HVAC Equipment Power Rating Calculations – Part Six", we explained that as a general rule, the over-current device rating shall not exceed the ampacity of a conductor. As stated in 240.4, conductors, other than flexible cords, flexible cables and fixture wires shall be protected against over-current in accordance with their ampacities specified in 310.15, unless otherwise permitted or required in 240.4(A) through (G). 
  • This means that the over-current device rating shall be equal to or less than the conductor ampacity. But for motors, the motor branch-circuit over-current protective devices are permitted to be sized much higher than the rated ampacity of the motor branch circuit conductors and overload protection must be used to protect motor branch-circuit conductors from overloads.






  • Overload protection is used for protection against excessive heating due to motor overload and failure to start.
  • The requirements for overload protection provided in NEC can be broken down into two categories as follows:

  1. Protection for the motor-compressor as per 440.52(A),
  2. Protection for the controllers, disconnecting means and circuit conductors as per 440.52(B).


Notes:
  • The manufacturers of hermetic systems supply the correct protection, conductor sizing, and other information on the equipment nameplate. Use this information when wiring an air conditioner or other hermetic motor equipment.
  • We most often attempt to provide both categories of protection mentioned above with a single device, which is allowed according to 440.52(B). This is not always possible, however, and we have to provide two devices to accomplish both jobs.




Protection for the motor-compressor as per 440.52(A)

  • Overload protection for motor-compressors is very similar to overload protection for standard motors. 
  •  The methods for providing overload protection for the motor-compressors are listed as follows:

  1. A separate overload relay,
  2. A thermally protected motor-compressor,
  3. A fuse or inverse time circuit breaker,
  4. A special protective system.

  • All Motor-Compressors and Equipment on 15- or 20-Ampere Branch Circuits will follow the same rules as per 440.52(A). Noting that the rating of the attachment plug and receptacle or cord connector (for Cord- and Attachment-Plug-Connected Motor Compressors and Equipment) shall not exceed 20 amperes at 125 volts or 15 amperes at 250 volts.



Protection for the controllers, disconnecting means and circuit conductors as per 440.52(B)

The methods for providing overload protection for the conductors, controller and disconnecting means are listed as follows:
  1. An overload relay,
  2. A thermally protected motor-compressor,
  3. A fuse or inverse time circuit breaker,
  4. A special protective system.



The Methods for Overload Protection 

As explained above, we have (4) methods of overload protection as follows:


1- An Overload Relay


  • A separate overload relay may be used to provide overload protection. The relay must be selected to trip at no more than 140% of the motor-compressor rated-load current.
  • Unlike a standard motor, the NEC does not list any conditions for adjusting the relay to a higher setting if the motor start-up current trips the overload. 
  • NEC 440.53 also requires the overload relay to be provided with ground-fault and short-circuit protection. This is accomplished by placing the overload relay in the circuit after the short-circuit, ground-fault protective device.



2- A thermally protected motor-compressor

External thermal protector

  • Hermetic compressor motors are protected from overheating by thermal protectors built into or mounted in contact with the compressor motor.
  • Many motor compressors today have over load protection built into the motor-compressor that will not permit a continuous current in excess of 156 percent of the marked rated-load current or branch-circuit selection current.
  • The NEC allows this type of overload protection and requires no further protection if the nameplate of the equipment is marked “Thermally Protected.”


Thermally Protected



3- A fuse or inverse time circuit breaker

  • The inverse time circuit breaker (Thermal-Magnetic Type) can provide overload protection, ground-fault protection and short-circuit protection for circuits at the same time. Also, many types of fuses can provide the same protection.
  • The NEC allows one of these type devices to be used as the overload protection for the motor compressor under the following conditions:

  1. The breaker or fuse is also the short-circuit and ground-fault protective device for the circuit.
  2. The device is rated for no more than 125% of the motor compressor rated load current.
  3. The device has sufficient time delay to allow the motor to start and accelerate its load without tripping.
  4. The motor compressor or the equipment is marked with the maximum branch-circuit fuse or inverse time circuit breaker rating.
  5. The fuse or inverse time circuit breaker is rated no greater than the rating marked on the equipment.

  
Notes: 
  • This is the method most often selected, especially when dealing with residential AC units. All of the protection can be accomplished with a single circuit breaker.
  • Most AC equipment has nameplate marked with the maximum allowable over-current device. In accordance with 110.3 this marking must be followed exactly and the markings must be taken literally as follows:

  1. If the marking is “Maximum Fuse Size”, then only fuses may be used.
  2. If the marking is “Maximum HCAR Circuit Breaker Size”, then only HCAR breakers may be used.
  3. If the marking is “Maximum Fuse Size or HCAR Circuit Breaker Size”, then only fuses or HCAR breakers may be used.
  4. If the marking is “Maximum Fuse or Inverse Time Circuit Breaker Size”, then only fuses or Inverse Time Circuit Breaker may be used.

 
Maximum Fuse Size or HCAR Circuit Breaker Size

4- A special protective system 
  • This method is most often used on large commercial air conditioning systems where the overload protection is provided by a system of sensors and relays included in the control circuits provided by the manufacturer.  
  • The NEC allows using a special protective system as the overload protection under the following conditions:


  1. This special protective system must open the control circuits and stop current to the motor compressor in the event of an overload.
  2. This special protective system will not permit a continuous current in excess of 156 percent of the marked rated-load current or branch circuit selection current.



Summary Of Sizing Steps for Branch Circuit Overload Protection
 for Air Conditioning and Refrigeration Equipment


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Option#1
When using A separate overload relay, The relay must be selected to trip at no more than 140% of the motor-compressor rated-load current.
Option#2
When using a Thermal Protector, it must not permit a continuous current in excess of 156 percent of the marked rated-load current or branch-circuit selection current.
Option#3
When using A fuse or inverse time circuit breaker, The device must be rated for no more than 125% of the motor compressor rated load current.
Option#4
When the manufacturer uses a special protective system, it must not permit a continuous current in excess of 156 percent of the marked rated-load current or branch circuit selection current.
Notes:
  • The manufacturers of hermetic systems supply the correct protection, conductor sizing, and other information on the equipment nameplate. Use this information when wiring an air conditioner or other hermetic motor equipment.
  • All Motor-Compressors and Equipment on 15- or 20-Ampere Branch Circuits will follow the same rules as per 440.52(A). Noting that the rating of the attachment plug and receptacle or cord connector (for Cord- and Attachment-Plug-Connected Motor Compressors and Equipment) shall not exceed 20 amperes at 125 volts or
    15 amperes at 250 volts.
  • 15 amperes at 250 volts.
  • The NEC allows this type of overload protection and requires no further protection if the nameplate of the equipment is marked “Thermally Protected.”
  • Most AC equipment has nameplate marked with the maximum allowable over-current device. In accordance with 110.3 this marking must be followed exactly and the markings must be taken literally as follows:

  1. If the marking is “Maximum Fuse Size”, then only fuses may be used.
  2. If the marking is “Maximum HCAR Circuit Breaker Size”, then only HCAR breakers may be used.
  3. If the marking is “Maximum Fuse Size or HCAR Circuit Breaker Size”, then only fuses or HCAR breakers may be used.
  4. If the marking is “Maximum Fuse or Inverse Time Circuit Breaker Size”, then only fuses or Inverse Time Circuit Breaker may be used.



Sixth: Provisions for Room Air Conditioners



  • A room air conditioner (with or without provisions for heating) shall be considered any AC appliance of the air cooled window, console or in wall type that is installed in the conditioned room and that has a hermetic refrigerant motor compressor.
  • Room air conditioners aren’t just for residential use. They are also used for commercial and industrial applications like guard shacks, construction trailers, chemical storage sheds, and industrial control panels.
  • These below rules will be applied only for the AC units rated less than 40A, 250V and must be single phase.  
  • If AC the unit is rated 3-phase or rated over 250 volts shall be directly connected to a wiring method recognized in NEC Chapter 3 and the below rules will not be applied.



1- Sizing Branch-Circuit Short-Circuit and Ground-Fault Protection

  • Equipment that starts and operates on a 15- or 20- ampere 120-volt, or 15-ampere 208- or 240-volt single-phase branch circuit, shall be permitted to be protected by the 15- or 20-ampere over‐ current device protecting the branch circuit, [440.22(B) exception#1].
  • The nameplate marking of cord-and-plug-connected equipment rated not greater than 250 volts, single-phase, such as household refrigerators and freezers, drinking water coolers, and beverage dispensers, shall be used in determining the branch circuit requirements, and each unit shall be considered as a single motor unless the nameplate is marked otherwise [440.22(B) exception#2].


2- Sizing Branch Circuit Conductors


  • The NEC 440.62 has two requirements for the branch circuits that supply room air conditioners as follows:

  1. If a room air conditioner is the only load on the circuit, the circuit must be rated for 125% of the marked rating of the air conditioner.
  2. If the room air conditioner is to be supplied from a standard receptacle circuit and other loads may be supplied from the circuit, the branch circuit must be rated at least 200% of the air conditioners marked rating.

  • Also, NEC 440.62(A) lists the requirements for considering a room air conditioner to be a single motor unit if the unit meets the following requirements:

  1. It must be cord and plug connected,
  2. I must have a rating of 250 volts or less and 40 amperes or less,
  3. The nameplate must list a total rated current for the equipment as opposed to individual ratings for the motors,
  4. The branch circuit, short-circuit and ground-fault protective device must not be rated higher than the ampacity of the branch circuit conductors or the receptacle rating, whichever is less.


Note:
  • For the air conditioning unit which is designed to do more than one job (such as air conditioning and heating) or which is designed with more than one motor (such as a motor-compressor and a fan blower motor for the heat) will be considered as a single motor unit if the above requirements were verified.


3- Sizing Disconnecting Means


  •  As per NEC 440.63, an attachment plug and receptacle can be used as the sole disconnecting means as long as one of two conditions is met:

  1. The manual controls for the AC unit are readily accessible and located no more than 1.8 m (6 feet) of the floor.
  2. An approved manually operable switch is installed in a readily accessible location that is in sight of the AC unit.



4- Protection Device for the Cord


  • As per NEC 440.65, the manufacturer must install one of the following protection devices:

  1. Leakage-current detector-interruptor (LCDI),
  2. Arc-fault circuit interrupter (AFCI),
  3. Heat detecting circuit interrupter (HDCI).

  • The protection device shall be an integral part of the attachment plug or be located in the power supply cord within 300 mm (12 in.) of the attachment plug. 



Summary of Sizing Steps for Provisions for Room Air Conditioners


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Sizing Branch-Circuit Short-Circuit and Ground-Fault Protection
Equipment that starts and operates on a 15- or 20- ampere 120-volt, or 15-ampere 208- or 240-volt single-phase branch circuit, shall be permitted to be protected by the 15- or 20-ampere over‐ current device protecting the branch circuit, [440.22(B) exception#1].
The nameplate marking of cord-and-plug-connected equipment rated not greater than 250 volts, single-phase, such as household refrigerators and freezers, drinking water coolers, and beverage dispensers, shall be used in determining the branch circuit requirements, and each unit shall be considered as a single motor unless the nameplate is marked otherwise [440.22(B) exception#2].
Sizing Branch Circuit Conductors
If a room air conditioner is the only load on the circuit, the circuit must be rated for 125% of the marked rating of the air conditioner.
If the room air conditioner is to be supplied from a standard receptacle circuit and other loads may be supplied from the circuit, the branch circuit must be rated at least 200% of the air conditioners marked rating.
Sizing Disconnecting Means
an attachment plug and receptacle can be used as the sole disconnecting means as long as one of two conditions is met:
  1. The manual controls for the AC unit are readily accessible and located no more than 1.8 m (6 feet) of the floor.
  2. An approved manually operable switch is installed in a readily accessible location that is in sight of the AC unit.
Protection Device For the Cord
As per NEC 440.65, the manufacturer must install one of the following protection devices:
  1. Leakage-current detector-interruptor (LCDI),
  2. Arc-fault circuit interrupter (AFCI),
  3. Heat detecting circuit interrupter (HDCI).

The protection device shall be an integral part of the attachment plug or be located in the power supply cord within 300 mm (12 in.) of the attachment plug. 
Notes:
  • These below rules will be applied only for the AC units rated less than 40A, 250V and must be single phase.  
  • If AC the unit is rated 3-phase or rated over 250 volts shall be directly connected to a wiring method recognized in NEC Chapter 3 and the below rules will not be applied.

In the next article, we will list all the summary tables for sizing calculations for Air Conditioning and Refrigeration Equipment as Per NEC Article 440. So, please keep following.
The previous and related articles are listed in below table:
Subject Of Previous Article
Article
Summary of heating and cooling systems,
Parts Consuming Power in HVAC Systems,
Types of motors used in HVAC Systems,
Types of pumps used in HVAC/refrigeration,
Parts consuming power as per used unit/system,
Motor Nameplate for Air Conditioner Motor Applications.
HVAC Equipment Power Rating Calculations – Part One 
The Common Types Of Motors Used In HVAC Industry
First: the Common Types of Motors Used in HVAC Industry to drive Compressors

1- Single-Phase Hermetic Motors:

1.1 Split Phase (SP),
1.2 Capacitor-Start, Induction-Run (CSIR),
1.3 Capacitor-Start, Capacitor-Run (CSCR),
1.4 Permanent Split Capacitor (PSC),
2- Poly-Phase Hermetic Motors.
Second: the Common Types of Motors Used in HVAC Industry to drive Fans
1- Shaded-Pole Motors
HVAC System Units And Ratings
Energy Conversions In Air Conditioning / Refrigeration Systems

 HVAC Equipment Power Rating Calculations – Part Two

Difference between Service, Feeder and Branch circuit load calculation
First: The HVAC System Contribution in Service/Feeder Load Calculations
First: NEC Standard Method
Second: NEC Optional Calculation Method
First: For Single Dwelling Units
Second: Multifamily Dwelling
Third: Two Family Dwelling (That are Supplied By a Single Feeder)
Forth: Existing Dwelling Unit
 HVAC Equipment Power Rating Calculations – Part Three


NEC Code & the Hermetic Refrigerant Motor-Compressors
Sizing Calculations for Air Conditioning and Refrigeration Equipment as Per Article 440:
First: Sizing Calculations for Disconnecting Means:
Case# 1: The load type is a single hermetic motor only as per 440.12(A),
Case# 2: The load type is a Combination loads as per 440.12(B).
 HVAC Equipment Power Rating Calculations – Part Four


The Minimum Circuit Ampacity (MCA)
Maximum Over-Current Protection (MOP) or (MOCP)
Second: Sizing calculation for Branch-Circuit Short-Circuit and Ground-Fault Protection Devices
Case# 1: The load type is a single hermetic motor only as per 440.22(A),
Case# 2: The load type is a Combination loads as per 440.22(B).
Case# 2A: Where a hermetic refrigerant motor-compressor is the largest load,
Case# 2B: Where a hermetic refrigerant motor-compressor is not the largest.
HVAC Equipment Power Rating Calculations – Part Five
Third: Sizing Branch Circuit Conductors ‎Case# 1: The load type is a single hermetic motor only as per 440.32,‎
Special Case: For a Wye-Start, Delta-Run Connected Motor-Compressor
Case# 2: The load type is a Combination loads as per 440.33.‎
Fourth: Sizing Controllers ‎
Case# 1: Controller Serving Motor Compressor only as per 440.41(A),‎
Case# 2: Controller Serving More Than One Load as per 440.41(B).‎

 HVAC Equipment Power Rating Calculations – Part Six



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