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 specialpurpose motors called
“hermetic,” the definition of Hermetic
Refrigerant MotorCompressor as in ARTICLE 100DEFINITIONS is as follows:
Hermetic
Refrigerant MotorCompressor:
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 MotorCompressors

Important Definitions
Understanding the following
definitions is important when making sizing calculations or installing air
conditioners, heat pumps, and other equipment using hermetic refrigerant
motorcompressors.
1 Locked Rotor Amperes (LRA):
2 Rated LoadCurrent (RLA or RLC):
3 Minimum BranchCircuit Selection Current (MBCSC):

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

We will explain the following Sizing calculations:

First:
Sizing Calculations for Disconnecting Means

Disconnecting Means:
To know the types of disconnecting means for air
conditioning equipment, please review the following article " Electrical Rules and Calculations for AirConditioning 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),
In this case:
The
ampere rating of the disconnecting means ≥ 115% of the compressors rated load
current or branchcircuit 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 lockedrotor 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:
Note:
For example, the
lockedrotor current equals 182 amperes. Checking NEC Table
430.251(B) , in below image, in the 208volt column for the conversion of lockedrotor current to horsepower, we find that an LRA of 182 amperes falls between a 10horsepower and a 15horsepowermotor. Therefore, we consider our example to be equivalent to a 15horsepower motor.
Important
note:

Case# 2: The load type is a Combination loads as per
440.12(B)
In this case, the combined load may be:
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 lockedrotor 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:
The sum of all of the individual loads at rated
load conditions = motorcompressor ratedload current(s) or branchcircuit
selection current(s), whichever is greater + fan /blower motors ratedload
current(s) + full load currents of other motors + the rating in amperes of
other loads
The sum of all of the individual loads lockedrotor
currents = motorcompressor lockedrotor current + fan /blower motors
lockedrotor current(s) + lockedrotor currents of other motors + the rating
in amperes of other loads
Note:
Important note:

General Rules for 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 branchcircuit
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 lockedrotor 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:

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 = motorcompressor
ratedload current(s) or branchcircuit selection current(s), whichever is
greater + fan /blower motors ratedload 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
lockedrotor currents, where:
The sum of
all of the individual loads lockedrotor currents = motorcompressor
lockedrotor current + fan /blower motors lockedrotor current(s) +
lockedrotor 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
the next article, we will explain in details the sizing calculations for:
 BranchCircuit
ShortCircuit and GroundFault 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
SinglePhase Hermetic Motors:
1.1 Split Phase (SP),
1.2 CapacitorStart, InductionRun (CSIR), 1.3 CapacitorStart, CapacitorRun (CSCR), 1.4 Permanent Split Capacitor (PSC),
2
PolyPhase Hermetic Motors.
Second:
the Common Types of Motors Used in HVAC Industry to drive Fans
1 ShadedPole 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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