HVAC Equipment Power Rating Calculations – Part Five ~ Electrical Knowhow

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,
Branch-Circuit Short-Circuit and Ground-Fault 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 Branch-Circuit Short-Circuit and Ground-Fault 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 Branch-Circuit Short-Circuit and Ground-Fault Protection Devices, Branch Circuit Conductors and Branch Circuit Overload Protection, which are:

MCA: Minimum Circuit Ampacity,
MOP (Sometimes MOCP): Maximum Over-Current Protection.

The Minimum Circuit Ampacity (MCA)

MCA is the highest steady-state 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 (Amps)

Notes:

Only the rating of one compressor is multiplied by 125% and the rating of the other compressor has no correction factor.
The rated load current (RLA) is Rated Load Amps, a "calculated" number, based on UL mandated bench testing procedures. It is not the same as Full Load Amps (FLA). RLA represents the actual expected compressor draw at given operating pressures and temperatures.

Example#1:

A Condensing Unit 460V/3-phase with the following components:
(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

Maximum Over-Current Protection (MOP) or (MOCP)

It is a calculated value that determines the maximum size of the over-current protection device (fuse or breaker) required to properly protect the equipment under anticipated fault conditions. The MOP takes into account startup surges and component ageing.

MOP = [2.25 x (Rated Current of Largest Motor)] + (Other Motor Loads) + (All Heater Loads)

Notes:

If the MOP value is not an even multiple of 5, and if the MOP value is not a standard value, the MOP is rounded down to the nearest standard size.
If the MOP is less than the MCA, the MOP is made equal to the MCA and then rounded up to the nearest standard size. In other words, the MOP shall not be less than the MCA.
If the MOP is less than 15, it shall be rounded up to 15 amps. This is the minimum size of fuse or circuit breaker permitted by [NEC 440.22(A) exception].
The rating of the largest motor is multiplied by 2.25 regardless of whether it is a compressor, fan or pump.
Unlike the MCA, the calculation of the MOP is less dependent on the types of loads in the air conditioner. The same calculation is used for direct expansion and chilled water units.
Most manufacturers of the equipment do all the calculations and publish them in both the equipment literature and on the unit data plate so we don’t have to calculate them.
The M in MCA stands for Minimum but The M in MOCP stands for Maximum
The normal rules for calculating the MOP require it to be larger than the MCA. MCA can be larger than the MOP in one application only where The InRow RP chilled water units are used. These air conditioners have three identical fans, three identical heaters, a humidifier and a condensate pump. There is no one predominating load. The MOP calculation takes 225% of the rating of one component plus the ratings of the other seven. However, the MCA is calculated as 125% of the rating of all of the components. The result is an MCA larger than the MOP.

InRow RP chilled water units

Standard sizes for fuses and fixed trip circuit breakers, per NEC 240.6, are 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 110, 125, 150, 175, 200, 225, 250, 300, 350, 400, 450, 500, 600, 700, 800, 1000, 1200, 1600, 2000, 2500, 3000, 4000 5000, and 6000 amps.

The below table gives a very good summary of both MOP & MCA:

Value		Determined by	Rules / guidelines	Purpose	Comment
Maximum rating for over-current protection devices	MOP	Electrical load to be carried by conductors and safety margin	Calculations per UL 1995 Section 36.15	Make sure that the over-current protection (circuit breaker or fuse) will open before the conductors in the circuit have too much current through them	"Maximum" refers to the largest circuit breaker that can be used and still be safe.
Minimum Circuit Ampacity	MCA	Electrical load to be carried by conductors and safety margin	Calculations per UL 1995 Section 36.14	Make sure that wires within the unit and the wires going to the unit can carry the amount of current being drawn by the electrical components in the unit.	"Minimum" refers to the smallest conductor that can be used and still be safe.

Example#2:

An air conditioner with two compressors, two fans, electric reheat and a humidifier with the following ratings:

Load	Rating
Compressor 1	34.3
Compressor 1	34.3
Fan Motor 1	13.4
Fan Motor 1	10.0
Fan Motor 1	10.0
Heater 1	25.0
Heater 1	12.5
Humidifier	12.0

Calculate the MCA & MOP?

Solution:

load	rating	MCA Correction Factor	MCA Rating	MOP Correction Factor	MOP Rating
Compressor 1	34.3	125%	42.9	225%	77.2
Compressor 2	34.3	100%	34.3	100%	34.3
Fan Motor 1	13.4	100%	13.4	100%	13.4
Fan Motor 2	10.0	100%	10.0	100%	10.0
Fan Motor 3	10.0	100%	10.0	100%	10.0
Heater 1	25.0	125%	31.2	100%	25.0
Heater 2	12.5	100%	12.5	100%	12.5
Humidifier	12.0	0	0	0	0
Total			154.3		182.4 Down size to 175

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 Branch-Circuit Short-Circuit and Ground-Fault Protection Devices

Short-circuit and ground-fault 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 over-current due to Short-Circuit and Ground-Fault 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),
Case# 2: The load type is a Combination loads as per 440.22(B).

Case# 1: The load type is a single hermetic motor only as per 440.22(A)

The short-circuit and ground-fault protection device for motor-compressor 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 motor-compressor rated-load current or branch-circuit 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 motor-compressor rated-load current or branch-circuit selection current, which‐ ever is greater ≤ the short circuit and ground fault OCPD AR ≤ 225% of the motor-compressor rated-load current or branch-circuit selection current, which‐ ever is greater.

How to check that the OCPD (over-current protective device) AR with will not trip while motor compressor starting?

If we have a compressor-motor with the above nameplate with FLA @ 460 V =31 A, the size of OPCD will be =1.75 x 31 = 54.25 A
Down size OPCD to standard size = 50 A.
The inrush current for this 25 HP motor @ 460 V = 183 A from table 430.251(B)

From 430.52 exceptions (1) and (2), Instantaneous Trip Circuit Breaker which also is known as motor-circuit protectors (MCPs), Where the setting specified in Table 430.52 is not sufficient for the starting current of the motor, the setting of an instantaneous trip circuit breaker shall be permitted to be increased for Design B energy-efficient motors no more than 1700 percent of full-load motor current. This is done usually to let the transient peak current of the motor not tripping the circuit breaker, so the motor can be started.
The transient peak current = 31x17 = 527 A max value from 430.52 exception (2)
In this case the time current curve for the motor will be like this (not to scale):

the time current curve for the motor

And to be sure that transient peak current of the motor not tripping the circuit breaker, we usually compare the current time curves for both the motor and the circuit breaker.
For 50 A breaker the time current curves will be as follows:

the time current curves of 50 A breaker and motor

Note:

there is intersection between the two curves which means that the 50 A breaker will trip and will not permit the transient peak current of the motor to pass.

In this time we will use another Instantaneous Trip Circuit Breaker with higher AR ( i.e. 225% of FLA) as follows:
the new size of OPCD will be =2.25 x 31 = 69.75 A
Size OPCD to standard size = 70 A.
In this case, the current time curves for both the motor and the circuit breaker will be as follows (not to scale):

Note:

in this case there is no intersection between the two curves and the 70 A breaker will not be tripped by the transient peak current of the motor. And the motor can start.

Example#3:

Size the over-current protection device for a 24A motor-compressor on a 240V circuit?

Solution:

Size the branch-circuit protection according to [440.22(A)].

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:

Two or more hermetic motor-compressors or
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, Size the equipment short-circuit and ground-fault protection with the "largest load" method according to the following cases:

Case# A: Where a hermetic refrigerant motor-compressor is the largest load,
Case# B: Where a hermetic refrigerant motor-compressor is not the largest.

Note for the largest load method:

If we have many air conditioner compressors with the same rating and this rating is considered to be the largest motors load, then apply the largest load rule for one of them only and others will be at rated load current like other loads. [440.7].

Case# A: Where a hermetic refrigerant motor-compressor is the largest load

In this case:

the rating of the branch-circuit short-circuit and ground-fault protective device = 175% of the largest motor-compressor rated-load current or branch-circuit selection current, which‐ ever is greater + the sum of the rated-load current or branch-circuit 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 Over-Current Protection (MOP) calculation:

the Maximum Over-Current Protection (MOP) = 225% of the largest motor-compressor rated-load current or branch-circuit selection current, which‐ ever is greater + the sum of the rated-load current or branch-circuit selection current, whichever is greater, of the other motor compressor(s) + the ratings of the other loads.

Example#4:

Size the over-current protection device for a 20.2 A motor-compressor with condenser fan motor 3.2A and evaporative fan motor 3.2 A on a 208V circuit?

Solution:

The rating of the branch-circuit short-circuit and ground-fault 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 Over-Current Protection (MOP) calculation:

The Maximum Over-Current Protection (MOP) = (20.2A x 225% + 3.2+3.2) = 51.85A,

Next size down protection = 50A.

Case# B: Where a hermetic refrigerant motor-compressor is not the largest

In this case:

the rating of the branch-circuit short-circuit and ground-fault protective device = the rated-load current or branch circuit selection current, whichever is greater, rating(s) for the motor-compressor(s) + the rated-load current For other motor loads not exceeding 800A + the rated-load current for other loads

But if the OCPD can’t carry the starting current of the motor compressor, you can use the Maximum Over-Current Protection (MOP) calculation:

the Maximum Over-Current Protection (MOP) = 225% of the largest Load rated current or branch-circuit selection current , whichever is greater + the sum of the rated-load current or branch-circuit selection current, whichever is greater, of the motor compressor(s) + the ratings of the other loads.

Important notes for MOP value:

If the MOP value is not an even multiple of 5, and if the MOP value is not a standard value, the MOP is rounded down to the nearest standard size.
If the MOP is less than the MCA, the MOP is made equal to the MCA and then rounded up to the nearest standard fuse size, the MOP shall not be less than the MCA.
If the MOP is less than 15, it shall be rounded up to 15 amps. This is the minimum size of fuse or circuit breaker permitted by [NEC 440.22(A) exception].
Minimum Circuit Amps (MCA) is a calculated value that specifies the minimum main power wire size.

General notes for sizing calculation for Branch-Circuit Short-Circuit and Ground-Fault Protection Devices

Air conditioning and refrigeration system motors are considered a single machine, even if the motors are remotely located from each other [440.8].
You must provide separate overload protection for the motor [440.52(A)] and over-current protection for the conductors [440.52(B)].
If the equipment contains more than one hermetic refrigerant motor compressor, or a hermetic refrigerant motor compressor and other loads, the equipment must have a visible nameplate containing the maximum rating of the branch circuit short circuit and ground fault OCPD. The manufacturer calculates these ratings per [440.22] and [440.32]. But don't use a protective device that exceeds the manufacturer's values [440.22(C)].
If the equipment nameplate specifies "Maximum Fuse Size," use a one-time or dual-element fuse. If the nameplate specifies "HACR Circuit Breaker," use an HACR-rated circuit breaker [110.3(B)]. see below image.

The rating of the branch-circuit short-circuit and ground fault protective device shall not be required to be less than 15 amperes [440.22(A) exception].
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].

Summary Of Sizing Steps For Branch-Circuit Short-Circuit and Ground-Fault 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 motor-compressor rated-load current or branch-circuit 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 motor-compressor rated-load current or branch-circuit selection current, which‐ ever is greater.
Notes: AR: Ampere Rating, OCPD: Over-Current Protective Device, The short-circuit and ground-fault protection device for motor-compressor must be capable of carrying the starting current of the motor. The inrush current for motors is calculated from table NEC 430.251(B) @ the operational voltage. for Design B energy-efficient motors, the peak transient current equal 1700 percent of full-load motor current as per NEC 430.52 exception (2). You must provide separate overload protection for the motor [440.52(A)] and over-current protection for the conductors [440.52(B)]. Don't use a protective device that exceeds the manufacturer's values [440.22(C)]. If the equipment nameplate specifies "Maximum Fuse Size," use a one-time or dual-element fuse. If the nameplate specifies "HACR Circuit Breaker," use an HACR-rated circuit breaker [110.3(B)]. The rating of the branch-circuit short-circuit and ground fault protective device shall not be required to be less than 15 amperes [440.22(A) exception]. 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].

Summary Of Sizing Steps For Branch-Circuit Short-Circuit and Ground-Fault Protection Devices For Combination Loads
www.Electrical-Knowhow.com
Case# A: Where a hermetic refrigerant motor-compressor is the largest load
Step#1	OPCD AR = 175% of the largest motor-compressor rated-load current or branch-circuit selection current, which‐ ever is greater + the sum of the rated-load current or branch-circuit 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 Over-Current Protection (MOP) = 225% of the largest motor-compressor rated-load current or branch-circuit selection current, which‐ ever is greater + the sum of the rated-load current or branch-circuit selection current, whichever is greater, of the other motor compressor(s) + the ratings of the other loads.
Case# B: Where a hermetic refrigerant motor-compressor is not the largest
Step#1	OPCD AR = the rated-load current or branch circuit selection current, whichever is greater, rating(s) for the motor-compressor(s) + the rated-load current For other motor loads not exceeding 800A + the rated-load 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 Over-Current Protection (MOP) = 225% of the largest Load rated current or branch-circuit selection current , whichever is greater + the sum of the rated-load current or branch-circuit selection current, whichever is greater, of the motor compressor(s) + the ratings of the other loads.
Notes: AR: Ampere Rating, OCPD: Over-Current Protective Device, MOP: Maximum Over-Current Protection, MCA: Minimum Circuit Amps, If the MOP value is not an even multiple of 5, and if the MOP value is not a standard value, the MOP is rounded down to the nearest standard size. If the MOP is less than the MCA, the MOP is made equal to the MCA and then rounded up to the nearest standard fuse size, the MOP shall not be less than the MCA. If the MOP is less than 15, it shall be rounded up to 15 amps. This is the minimum size of fuse or circuit breaker permitted by [NEC 440.22(A) exception]. The short-circuit and ground-fault protection device for motor-compressor must be capable of carrying the starting current of the motor. The inrush current for motors is calculated from table NEC 430.251(B) @ the operational voltage. For Design B energy-efficient motors, the peak transient current equal 1700 percent of full-load motor current as per NEC 430.52 exception (2). You must provide separate overload protection for the motor [440.52(A)] and over-current protection for the conductors [440.52(B)]. Don't use a protective device that exceeds the manufacturer's values [440.22(C)]. If the equipment nameplate specifies "Maximum Fuse Size," use a one-time or dual-element fuse. If the nameplate specifies "HACR Circuit Breaker," use an HACR-rated circuit breaker [110.3(B)]. The rating of the branch-circuit short-circuit and ground fault protective device shall not be required to be less than 15 amperes [440.22(A) exception]. 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].

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.	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

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