HVAC Equipment Power Rating Calculations – Part Four



In the previous article "The NationalElectrical Code (NEC) and HVAC Systems – Part One", we differentiate between the service, feeder and branch circuit load calculations and we explain the contribution of HVAC system in the service and feeder calculations. Today we will explain the branch circuit load calculations for HVAC system loads.

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


Introduction



Motors for air conditioning and refrigeration equipment are special-purpose motors called “hermetic,” the definition of Hermetic Refrigerant Motor-Compressor as in ARTICLE 100-DEFINITIONS is as follows:

Hermetic Refrigerant Motor-Compressor: 
A combination consisting of a compressor and motor, both of which are enclosed in the same housing, with no external shaft or shaft seals, with the motor operating in the refrigerant.




NEC Code & the Hermetic Refrigerant Motor-Compressors

  • Article 440- Air-Conditioning and Refrigerating Equipment is used for sizing disconnecting means, Branch-Circuit Short-Circuit and Ground-Fault Protection, branch circuit conductors and branch circuit overload protection for Hermetic Refrigerant Motor-Compressors in Parts II, III, IV, and VI, respectively while Part V provides requirements for the controllers of Motor-Compressors.
  • Article 440- Air-Conditioning and Refrigerating Equipment supplements or amends all other rules and articles in the NEC code as indicated in below table:


Article
Section (or Part)
Rule
Article 210
Branch Circuits
Part I. General Provisions

Table 210.3 Specific-Purpose Branch Circuits
Branch Circuits requirements for Air-conditioning and refrigerating equipment shall be according to 440.6, 440.31, 440.32.
Article 220 Branch-Circuit, Feeder, and Service Load Calculations

Part I. General Provisions

Table 220.3 Specific-Purpose Calculation References
Air-conditioning and refrigerating equipment, branch circuit conductor sizing will be found in Article 440 Part IV.

Part II. Branch-Circuit Load Calculations
220.18 Maximum Loads
(A) Motor-Operated and Combination Loads.
Where a circuit supplies only air-conditioning equipment, refrigerating equipment, or both, Article 440 shall apply.
Part III. Feeder and Service Load Calculations
220.50 Motors.
For hermetic refrigerant motor-compressors loads shall be calculated in accordance with 440.6.
Article 240 Overcurrent Protection

Part I. General

Table 240.3 Other Articles
Air-conditioning and refrigerating equipment shall be protected against overcurrent in accordance with article 440.
Part I. General
240.4 Protection of Conductors.

Table 240.4(G) Specific Conductor Applications
Overcurrent protection for Air-conditioning and
refrigeration equipment circuit conductors shall be permitted to be provided as referenced in Article 440, Parts III, VI.
Article 409
Industrial Control Panels
Part I. General

Table 409.3 Other Articles
industrial control panels that contain branch circuits
for specific loads or components, or are for control of Air-conditioning and refrigerating equipment shall be constructed and installed in accordance with Article 440
Article 422 Appliances

Part I. General
422.3 Other Articles.
The requirements of Article 440 shall apply to the installation of appliances containing a hermetic refrigerant motor compressor(s)
Part I. General
422.11 Overcurrent Protection
(G) Motor-Operated Appliances.
Hermetic refrigerant motor-compressors in air-conditioning or refrigerating equipment shall be provided with overload protection in accordance with Part VI of Article 440.
Where appliance overcurrent protective devices that are separate from the appliance are required, data for selection of these devices shall be marked on the appliance. The minimum marking shall be that specified in 440.4.
Article 424
Fixed Electric Space-Heating Equipment
Part I. General

424.2 Other Articles.
Fixed electric space-heating equipment incorporating a hermetic refrigerant motor-compressor shall also comply with Article 440.
Article 425
Fixed Resistance and Electrode Industrial
Process Heating Equipment
Part I. General
425.2 Other Articles.
Fixed industrial process heating equipment incorporating a hermetic refrigerant motor-compressor shall also comply with Article 440.
Article 430
Motors, Motor Circuits, and Controllers
Part I. General
430.1 Scope.
Informational Note No. 1
Air-conditioning and refrigerating equipment are covered in Article 440.
430.5 Other Articles.
Table 430.5 Other Articles
Motors and controllers for Air-conditioning and refrigerating equipment shall comply with Article 440.
Part V. Motor Feeder Short-Circuit and Ground-Fault Protection
430.62 Rating or Setting — Motor Load.
A feeder supplying a specific fixed motor load(s) and consisting of conductor sizes based on 430.24 shall be provided with a protective device having a rating or setting not greater than the largest rating or setting of the branch circuit short-circuit and ground-fault protective device for any motor supplied by the feeder [based on the maximum permitted value for the specific type of a protective device in accordance with 430.52, or 440.22(A) for hermetic refrigerant motor compressors], plus the sum of the full-load currents of the other motors of the group.

430.63 Rating or Setting — Motor Load and Other Load(s).
Where a feeder supplies a motor load and other load(s), the feeder protective device shall have a rating not less than that required for the sum of the other load(s) plus the following:
For a single hermetic refrigerant motor-compressor, the rating permitted by 440.22.

  • As per 440.3 (D), Hermetic refrigerant motor compressors, circuits, controllers, and equipment shall also comply with other applicable articles shown in Table 440.3(D) based on the equipment or the occupancy as follows:



  • Article 440 is not applicable to some Air-Conditioning and Refrigerating Equipment as in the following table:


Article
Section (or Part)
Rule
Article 440- Air-Conditioning and Refrigerating Equipment
440.3 (B)
The rules of Articles 422, 424, or 430, as applicable, shall apply to air-conditioning and refrigerating equipment that does not incorporate a hermetic refrigerant motor-compressor. This equipment includes devices that employ refrigeration compressors driven by conventional motors, furnaces with air-conditioning evaporator coils installed, fan-coil units, remote forced air-cooled condensers, remote commercial refrigerators, and so forth.
440.3 (C)
Equipment such as room air conditioners, household refrigerators and freezers, drinking water coolers, and beverage dispensers shall be considered appliances, and the provisions of Article 422 shall also apply.





Important Definitions

Understanding the following definitions is important when making sizing calculations or installing air conditioners, heat pumps, and other equipment using hermetic refrigerant motor-compressors.

1- Locked Rotor Amperes (LRA):

  • Locked Rotor Current, LRC is the steady-state current taken from the power line with the rotor locked (stopped) and with the rated voltage and frequency applied. Steady-state means that the motor starts at rest or zero speed.
  • The manufacturer is required to furnish the locked rotor current on the compressor nameplate and/or the unit nameplate. One of the reasons the manufacturer is required to supply this information is that the motors in the units are not produced in standard horsepower ratings. This information is needed when sizing the disconnecting means, particularly when a switch is used, as the switches are rated in horsepower.


2- Rated Load-Current (RLA or RLC):

  • The RLA is determined by the manufacturer of the hermetic refrigerant motor-compressor through testing at rated refrigerant pressure, temperature conditions, and voltage.
  • In most instances, the RLA is at least equal to 64.1% of the hermetic refrigerant motor-compressor’s maximum continuous current (MCC).


3- Minimum Branch-Circuit Selection Current (MBCSC):

  • Some hermetic refrigerant motor-compressors are designed to operate continuously at currents greater than 156% of the RLA. 
  • In such cases, the unit’s nameplate is marked with “Branch Circuit Selection Current” or “Minimum Supply Circuit Conductor Ampacity.” The MBCSC will be no less than at least 64.1% of the MCC rating of the hermetic refrigerant motor-compressor.




Sizing Calculations for Air Conditioning and Refrigeration Equipment as Per Article 440



We will explain the following Sizing calculations:

  1. Disconnecting Means,
  2. Branch-Circuit Short-Circuit and Ground-Fault Protection Devices,
  3. Branch Circuit Conductors,
  4. Branch Circuit Overload Protection,
  5. Motor Controllers.



First: Sizing Calculations for Disconnecting Means


Disconnecting Means:  
  • A device, or group of devices, or other means by which the conductors of a circuit can be disconnected from their source of supply.

Disconnecting Means for a Window air conditioner
To know the types of disconnecting means for air conditioning equipment, please review the following article " Electrical Rules and Calculations for Air-Conditioning Systems – Part Two "
When sizing the Disconnecting means for air conditioning or refrigeration system equipment, different cases apply, depending on the type of load as follows:
  • 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).





Case# 1: The load type is a single hermetic motor only as per 440.12(A),


In this case:
The ampere rating of the disconnecting means ≥ 115% of the compressors rated load current or branch-circuit selection current, whichever is greater
With the following condition:
The horsepower rating of the disconnecting means must be checked to be sure that it is capable of safely disconnecting the full amount of the motor locked-rotor current. So,
The horsepower rating of the disconnecting means must be the equivalent horsepower rating of the hermetic motor.
The equivalent horsepower rating of the hermetic motor can be obtained by taking the larger horsepower value from the following two values:
  • Value#1: Select the equivalent horsepower rating From NEC Tables 430.248, 430.249 or 430.250 using the greater of either the rated load current or the branch circuit selection current.
  • Value#2: Select the equivalent horsepower rating from NEC Tables 430.251(A) and 430.251(B) using the locked-rotor current.

Note: 
  • if the value falls between two horsepower ratings in a table, round up to the larger HP.

For example, the locked-rotor current equals 182 amperes. Checking NEC Table
430.251(B) , in below image, in the 208-volt column for the conversion of locked-rotor current to horsepower, we find that an LRA of 182 amperes falls between a 10-horsepower and a 15-horsepowermotor. Therefore, we consider our example to be equivalent to a 15-horsepower motor.

Important note:
  • if the above condition is verified and in case of using a non-fused disconnecting means, we can use a non-fused disconnecting means with ampere rating < 115% of the specified current. [440.12(A)(1) exception].















Case# 2: The load type is a Combination loads as per 440.12(B)



In this case, the combined load may be:

  1. Two or more hermetic motor-compressors or
  2. One or more hermetic motor-compressors with other motors or loads (such as fans, heaters, solenoids, and coils,)


Note: for calculation purposes, the combined load will be considered as a single equipment (single motor).
In this case,
The ampere rating of the disconnecting means ≥ 115% of the sum of all of the individual loads at rated load conditions
With the following condition:
The horsepower rating of the disconnecting means must be checked to be sure that it is capable of safely disconnecting the sum of all of the individual loads locked-rotor currents. So,
The horsepower rating of the disconnecting means must be the equivalent horsepower rating of all individual loads as a single motor.
The equivalent horsepower rating of all individual loads as a single motor can be obtained by taking the larger horsepower value from the following two values:
  • Value#1: Select The equivalent horsepower rating of all individual loads From NEC Tables 430.248, 430.249 or 430.250 using the sum of all of the individual loads at rated load conditions, where:

The sum of all of the individual loads at rated load conditions = motor-compressor rated-load current(s) or branch-circuit selection current(s), whichever is greater + fan /blower motors rated-load current(s) + full load currents of other motors + the rating in amperes of other loads
  • Value#2: Select the equivalent horsepower rating from NEC Tables 430.251(A) and 430.251(B) using the sum of all of the individual loads locked-rotor currents, where:

The sum of all of the individual loads locked-rotor currents = motor-compressor locked-rotor current + fan /blower motors locked-rotor current(s) + locked-rotor currents of other motors + the rating in amperes of other loads
Note
  • if the value falls between two horsepower ratings in a table, round up to the larger HP.

Important note
  • if the above condition is verified and in case of using a non-fused disconnecting means, we can use a non-fused disconnecting means with ampere rating < 115% of the sum of all of the individual loads at rated load conditions. [440.12(B)(2) exception].




General Rules for Disconnecting Means

  • For small motor-compressors not having the locked-rotor current marked on the nameplate, or for small motors not covered by Table 430.247, Table 430.248, Table 430.249, or Table 430.250, the locked-rotor current shall be assumed to be six times the rated-load current.
  • Every disconnecting means that is in the motor-compressor circuit between the point of attachment of the feeder and the point of connection at the motor must be rated according to cases#1 or 2 in above.
  • Where the rated-load or locked-rotor current as determined above would indicate a disconnecting means rated in excess of 100 hp, the disconnecting means shall be permitted to be a general-use or isolating switch where plainly marked “Do not operate under load.” [430.109(E)].
  • An attachment plug and receptacle can serve as the disconnecting means for cord-connected room air conditioners, household refrigerators and freezers, drinking water coolers, and beverage dispensers [440.13]. ). However, this is true only if the manual controls on the room air conditioner are readily accessible and within 6 ft of the floor, or a readily accessible disconnecting means (e.g., that cord connector) is within sight from the room air conditioner.
  • You must provide single-phase, cord-and-plug-connected room air conditioners with a factory-installed leakage current detector or with an AFCI [440.65]. 
  • If we have of the compressor and condenser motor name plates, then the FLC ratings on the nameplate or the nameplate branch circuit selection current of the compressor and condenser motor, whichever is greater, must be used at 115 percent to size the disconnecting means.


  

Summary Of Sizing Steps For Disconnecting Means For
Single Hermetic Motor Only


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Step#1
Select AR of disconnecting means ≥ 115% of the compressors rated load current or branch-circuit selection current, whichever is greater.
Step#2
Calculate the greater value A or B of equivalent horsepower rating for the selected disconnecting means in Step#1.
A
Use NEC Tables 430.248, 430.249 or 430.250 using the greater of either the rated load current or the branch circuit selection current.
B
Use NEC Tables 430.251(A) and 430.251(B) using the locked-rotor current.
Step#3
The Selected disconnecting means from step#1 must have HP rating ≥ the equivalent horsepower rating from step#2. If not, repeat step#1 with higher value of AR.
Notes:
  • In A or B, if the current value falls between two horsepower ratings in a table, round up to the larger HP.
  • Step#2 must be verified to be sure that The horsepower rating of the disconnecting means is capable of safely disconnecting the full amount of the motor locked-rotor current.
  • In case of using a non-fused disconnecting means, non-fused disconnect means AR < 115% of the specified current.
  • AR: Ampere Rating



Summary Of Sizing Steps For Disconnecting Means For
Combination loads


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Step#1
Select AR of disconnecting means ≥ 115% of the sum of all of the individual loads at rated load conditions.
Step#2
Calculate the greater value A or B of equivalent horsepower rating for the selected disconnecting means in Step#1.
A
Use NEC Tables 430.248, 430.249 or 430.250 using the sum of all of the individual loads at rated load conditions, where:

The sum of all of the individual loads at rated load conditions = motor-compressor rated-load current(s) or branch-circuit selection current(s), whichever is greater + fan /blower motors rated-load current(s) + full load currents of other motors + the rating in amperes of other loads
B
Use NEC Tables 430.251(A) and 430.251(B) using the sum of all of the individual loads locked-rotor currents, where:

The sum of all of the individual loads locked-rotor currents = motor-compressor locked-rotor current + fan /blower motors locked-rotor current(s) + locked-rotor currents of other motors + the rating in amperes of other loads.
Step#3
The Selected disconnecting means from step#1 must have HP rating ≥ the equivalent horsepower rating from step#2. If not, repeat step#1 with higher value of AR.
Notes:
  • In A or B, if the current value falls between two horsepower ratings in a table, round up to the larger HP.
  • Step#2 must be verified to be sure that The horsepower rating of the disconnecting means is capable of safely disconnecting the sum of all of the individual loads locked-rotor currents.
  • In case of using a non-fused disconnecting means, non-fused disconnect means AR < 115% of the sum of all of the individual loads at rated load conditions.
  • AR: Ampere Rating



In the next article, we will explain in details the sizing calculations for:
  • Branch-Circuit Short-Circuit and Ground-Fault Protection Devices,
  • Branch Circuit Conductors,
  • Branch Circuit Overload Protection.

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.


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


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






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