Today, we will explain how to size the branch circuit conductors, controllers and overload protection for Air Conditioning and Refrigeration Equipment as Per Article 440.
The National
Electrical Code (NEC) And HVAC Systems – Part Four

Third:
Sizing Branch Circuit Conductors

When making the sizing calculation for branch circuit conductors 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 NEC 440.32
Size
these branchcircuit conductors no smaller than the minimum circuit ampacity on
the equipment nameplate i.e. If the equipment is marked with minimum circuit
ampacity (MCA), use this value to
size the branch circuit conductors.
If
the equipment is not marked with minimum circuit ampacity, size the
conductors per [NEC 440.32] as follows:
Branch circuit conductor ampacity ≥ 125% of the motor compressor
ratedload current or the branch circuit selection current, whichever is
greater
Example#1:
What
size 75°C conductor and overcurrent device do you need for an 18A motor
compressor?
Solution:
Step#1: Determine the Branch circuit conductor ampacity from [NEC
440.32]:
Branch
circuit conductor ampacity = 18A × 1.25 = 22.50A
Step#2: Determine the Branch circuit conductor size from [Table
310.15(B)(16)]:
from above, 12
AWG, rated 25A at 75°C
Step#3: Determine the branchcircuit protection [240.6(A)
and 440.22(A)]:
The
branchcircuit protection = 18A × 1.75 = 31.50A,
Next
size down = 30A
Suppose
the 30A OCPD can’t carry the starting current, so, you decide to size the
OCPD up to 225% of the equipment load current rating. That works out as:
The
branchcircuit protection = 18A × 2.25 = 40.50A,
Next
size down = 40A.
Note: You
can use a 30A or 40A OCPD to protect a 12 AWG conductor for an
airconditioning circuit.
Note for above example#1:

Special Case: For a WyeStart, DeltaRun Connected
MotorCompressor
The branchcircuit conductors
ampacity between the controller and the motorcompressor = 72 % of either the
motorcompressor ratedload current or the branchcircuit selection current,
whichever is greater.
Why derating branchcircuit
conductors ampacity between the controller and the motorcompressor by 72%
for wyestart, deltarun connected motorcompressor?
So, the Reduction in phase current
in delta (running) = 58%
To calculate the branchcircuit
conductors ampacity in this case, we multiply the current by 125% or 1.25,
then 1.25 x 58% of the current = 0.72 = 72% of the current value.
Example#2:
A
wyestart, delta run motor compressor with 751 A rated load current.
Calculate its Branch circuit conductor ampacity?
Solution:
Determine the Branch circuit conductor ampacity from [NEC
440.32]:
Since
this motor compressor is a wyestart, delta run
So,
the Branch circuit conductor ampacity = 0.72 x 731 A = 526.32 A

Case# 2: The load type is a Combination loads as per NEC 440.33.
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, Size the branch circuit conductors ampacity with the "largest
load" method according to [NEC 440.34 and 440.35] as follows:
Branch circuit conductor ampacity = 125% of the largest
motorcompressor ratedload current or branchcircuit selection current,
which‐ ever is greater + the sum of the ratedload current of the other motor
+ the ratings of the other loads.

Notes for Sizing branch circuit conductors

Summary
Of Sizing Steps For BranchCircuit Conductors
For
Single Hermetic Motor Only


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Step#1

If the equipment is marked with minimum circuit ampacity
(MCA), use this value to size the branch circuit
conductors. If not, go to Step#2.

Step#2

Size the branch circuit
conductors ampacity ≥ 125% of the motor compressor
ratedload current or the branch circuit selection current, whichever is
greater

Special Case

For a WyeStart, DeltaRun Connected
MotorCompressor:
The branchcircuit conductors
ampacity between the controller and the motorcompressor = 72 % of either the
motorcompressor ratedload current or the branchcircuit selection current,
whichever is greater.

Notes:

Summary
Of Sizing Steps For BranchCircuit Conductors
For
Combination Loads


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Step#1

If the
equipment is marked with minimum circuit ampacity (MCA), use this value to size the branch circuit conductors. If not, go
to Step#2.

Step#2

Size
Branch circuit conductor ampacity = 125% of the largest motorcompressor
ratedload current or branchcircuit selection current, which‐ ever is
greater + the sum of the ratedload current of the other motor + the ratings
of the other loads.

Notes:

Fourth:
Sizing Controllers

When making the sizing calculation for MotorCompressor Controllers for air conditioning or refrigeration system equipment, different cases apply, depending on the type of load as follows:

Case# 1: Controller Serving Motor Compressor only as per
440.41(A)
The controller must have a continuous duty FLC ≥
the nameplate rated current or branch circuit selection current (whichever is
larger),
And also,
The controller must have a LRC ≥ the locked rotor
current of the compressor

Summary
Of Sizing Steps For Controller Serving Motor Compressor
only


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Step#1

The
controller must have a continuous duty FLC ≥ the nameplate rated current or
branch circuit selection current (whichever is larger),

Step#2

And also,
The controller must
have a LRC ≥ the locked rotor current of the compressor .

Notes:

Summary
Of Sizing Steps For Controller Serving More Than One Load


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Step#1

The ampere rating of the Controller ≥ 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

Select Controller
with HP rating ≥ the equivalent horsepower rating from step#1. If not, repeat
step#1 with higher value of AR.

Notes:

In
the next article, we will explain in details the sizing calculations for:
 Branch Circuit Overload Protection,
 Provisions for Room Air Conditioners.
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).


The Minimum Circuit Ampacity (MCA)
Maximum OverCurrent Protection (MOP) or
(MOCP)
Second: Sizing
calculation for BranchCircuit ShortCircuit and GroundFault 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 motorcompressor is
the largest load,
Case# 2B: Where
a hermetic refrigerant motorcompressor is not the largest.

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