In the previous article "The National Electrical Code (NEC) and HVAC Systems – Part Two", We
indicated that Sizing Calculations for Air Conditioning
and Refrigeration Equipment as Per Article 440 will include
Sizing calculations for:
 Disconnecting Means,
 BranchCircuit
ShortCircuit and GroundFault Protection Devices,
 Branch Circuit Conductors,
 Branch Circuit Overload Protection,
 Motor Controllers.
We explained the
first calculation; Disconnecting Means in
this previous article. Today, we will continue explaining other sizing
calculations for BranchCircuit
ShortCircuit and GroundFault Protection Devices for Air Conditioning
and Refrigeration Equipment as Per Article 440.
The National
Electrical Code (NEC) And HVAC Systems – Part Three

MCA
and MOP

the
manufacturers are supplying nameplates on heir HVAC equipment which indicate
important information, some of this information are necessary in sizing
calculations of BranchCircuit ShortCircuit and GroundFault Protection
Devices, Branch Circuit Conductors and Branch Circuit Overload Protection,
which are:

The Minimum Circuit Ampacity (MCA)
MCA
is the highest steadystate electrical current that the air conditioning
equipment should see when operating correctly. It is needed to
guarantee that the wiring will not overheat under all operating conditions
for the life of air conditioning equipment. The wire size takes into account
the normal current draw, ageing of components and anticipated faults.
MCA = 1.25 x [Motor Rated Current +
Heater Current] + 100% Other Loads (Amps)
The
MCA calculation is dependent on the types of loads in the air conditioner.
Different calculations are used for direct expansion and chilled water units
as follows:
For direct
expansion units:
MCA = 1.25 x [Motor Rated Current + Heater
Current] + 100% Other Loads (Amps)
For chilled
water units:
MCA = 1.25 x Sum of all loads Rated Current
Notes:
Example#1:
(2 nos.) 40 Ton Compressors RLA = 52 Amps each (4 nos.) Condenser Fans RLA = 1.5 Amps each
Calculate MCA?
Solution:
MCA = (52 x 1.25) + 52 + 1.5 + 1.5 + 1.5 + 1.5 = 123 Amps

The below table gives a very good summary of
both MOP & MCA:
Example#2:
Calculate the MCA &
MOP?
In this example, the
correction factor for the humidifier is zero. The heater and humidifier do not
run at the same time, so only one is used in the calculations. The heater
rating is used because it is higher than the humidifier rating.

Second:
Sizing calculation for BranchCircuit ShortCircuit and GroundFault
Protection Devices

Shortcircuit and groundfault protection cannot exceed the
nameplate ratings. If the equipment does not have a nameplate specifying the
size and type of protection device, depending on the load type, you can use
the below calculation rules.
When making the sizing calculation for protection against overcurrent due to ShortCircuit and GroundFault 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
440.22(A)
The shortcircuit and groundfault protection device for
motorcompressor must be capable of carrying the starting current of the
motor.
So, you initially size the short circuit and ground fault OCPD
AR as follows:
The short circuit and ground fault OCPD AR ≤ 175%
of the motorcompressor ratedload current or
branchcircuit selection current, which‐ ever is greater.
But, if you reached the 175% threshold but the OCPD
can’t carry the starting current of the motor compressor, you can use the
next size larger OCPD. However, this OCPD can’t exceed 225% of the motor
compressor current rating. In this case, The rule become as follows:
175% of the motorcompressor
ratedload current or branchcircuit selection current, which‐ ever is
greater ≤ the short circuit and ground fault OCPD AR ≤ 225%
of the motorcompressor ratedload current or
branchcircuit selection current, which‐ ever is greater.
How to check
that the OCPD
(overcurrent protective device) AR with will not trip while motor compressor
starting?
Note:
Example#3:
Size the overcurrent protection device for a 24A
motorcompressor on a 240V circuit?
Solution:
24A x 1.75 = 42A, next size down protection = 40A.
If a 40A protection device is not capable of carrying the
starting current, you can size the protection device up to 225 percent of the
equipment load current rating (24A x 2.25 = 54A, next size down 50A).

Case# 2: The load type is a Combination loads as per 440.22(B).
In this case, the combined load may be:
Note:
In this case, Size the equipment shortcircuit and groundfault
protection with the "largest load" method according to the
following cases:
Note
for the largest load method:

Case# A: Where a hermetic
refrigerant motorcompressor is the largest load
In this case:
the rating of the branchcircuit shortcircuit and
groundfault protective device = 175% of the
largest motorcompressor ratedload current or branchcircuit selection
current, which‐ ever is greater + the sum of the ratedload current or
branchcircuit selection current, whichever is greater, of the other motor
compressor(s) + the ratings of the other loads.
But if the OCPD can’t carry
the starting current of the motor compressor, you can use the Maximum OverCurrent
Protection (MOP) calculation:
the Maximum OverCurrent Protection (MOP) = 225%
of the largest motorcompressor
ratedload current or branchcircuit selection current, which‐ ever is
greater + the sum of the ratedload current or branchcircuit selection
current, whichever is greater, of the other motor compressor(s) +
the ratings of the other loads.
Example#4:
Size the overcurrent protection device for a 20.2 A
motorcompressor with condenser fan motor 3.2A and evaporative fan motor 3.2
A on a 208V circuit?
Solution:
The rating of the
branchcircuit shortcircuit and groundfault protective device = 20.2 x 175% +3.2+3.2
= 41.75 A
Next size down protection = 40A.
If
a 40A protection device is not capable of carrying the starting current, you the Maximum OverCurrent
Protection (MOP) calculation:
The Maximum OverCurrent Protection (MOP) = (20.2A x 225% + 3.2+3.2) = 51.85A,
Next size down protection = 50A.

Case# B: Where a hermetic
refrigerant motorcompressor is not the largest
In this case:
the
rating of the branchcircuit shortcircuit and groundfault protective device
= the ratedload current or branch circuit selection
current, whichever is greater, rating(s) for the motorcompressor(s) + the
ratedload current For other motor loads not exceeding 800A + the ratedload
current for other loads
But if the OCPD can’t carry
the starting current of the motor compressor, you can use the Maximum OverCurrent
Protection (MOP) calculation:
the Maximum OverCurrent Protection (MOP) = 225%
of the largest Load rated current or
branchcircuit selection current , whichever is greater
+ the sum of the ratedload current or branchcircuit selection current,
whichever is greater, of the motor compressor(s) +
the ratings of the other loads.
Important notes for MOP value:

General
notes for sizing calculation for BranchCircuit
ShortCircuit and GroundFault Protection
Devices

Summary
Of Sizing Steps For BranchCircuit
ShortCircuit and GroundFault Protection Devices For Single Hermetic Motor Only


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Step#1

Size the short circuit and
ground fault OCPD AR ≤ 175% of the motorcompressor ratedload current or branchcircuit
selection current, which‐ ever is greater.

Step#2

Compare the time current curves for both the
motor and the circuit breaker:
If there is intersection, go to step#3,
If not, the OCPD
AR is adequate (end of sizing calculations).

Step#3

Size the short circuit and
ground fault OCPD AR ≤ 225% of the motorcompressor ratedload current or branchcircuit selection
current, which‐ ever is greater.

Notes:

Summary
Of Sizing Steps For BranchCircuit
ShortCircuit and GroundFault Protection Devices For Combination Loads


www.ElectricalKnowhow.com


Case# A: Where a hermetic
refrigerant motorcompressor is the largest load


Step#1

OPCD AR = 175% of the largest motorcompressor ratedload current or branchcircuit selection current, which‐ ever is greater + the sum of the ratedload current or branchcircuit selection current, whichever is greater, of the other motor compressor(s) + the ratings of the other loads. 
Step#2

Compare the time current curves for both the
motor and the circuit breaker:
If there is intersection, go to step#3,
If not, the OCPD AR is
adequate (end of sizing calculations).

Step#3

the Maximum OverCurrent
Protection (MOP) = 225% of the largest
motorcompressor ratedload current or branchcircuit selection current,
which‐ ever is greater + the sum of the ratedload current or branchcircuit
selection current, whichever is greater, of the other motor
compressor(s) + the ratings of the other loads.

Case# B: Where a hermetic
refrigerant motorcompressor is not the largest


Step#1

OPCD AR = the ratedload current or branch circuit selection current,
whichever is greater, rating(s) for the motorcompressor(s) + the ratedload
current For other motor loads not exceeding 800A + the ratedload current for
other loads

Step#2

Compare the time current curves for both the
motor and the circuit breaker:
If there is intersection, go to step#3,
If not, the OCPD AR is
adequate (end of sizing calculations).

Step#3

the Maximum OverCurrent Protection (MOP) = 225% of the largest Load rated current or branchcircuit selection current , whichever is greater + the sum of the ratedload current or branchcircuit selection current, whichever is greater, of the motor compressor(s) + the ratings of the other loads. 
Notes:

In
the next article, we will explain in details the sizing calculations for:
 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

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


NEC Code & the
Hermetic Refrigerant MotorCompressors
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

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