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.
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The National
Electrical Code (NEC) And HVAC Systems – Part Four
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Third:
Sizing Branch Circuit Conductors
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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:
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Case# 1: The load type is a single hermetic motor only as per NEC 440.32
Size
these branch-circuit 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
rated-load current or the branch circuit selection current, whichever is
greater
Example#1:
What
size 75°C conductor and over-current 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 branch-circuit protection [240.6(A)
and 440.22(A)]:
The
branch-circuit 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
branch-circuit 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
air-conditioning circuit.
Note for above example#1:
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Special Case: For a Wye-Start, Delta-Run Connected
Motor-Compressor
The branch-circuit conductors
ampacity between the controller and the motor-compressor = 72 % of either the
motor-compressor rated-load current or the branch-circuit selection current,
whichever is greater.
Why derating branch-circuit
conductors ampacity between the controller and the motor-compressor by 72%
for wye-start, delta-run connected motor-compressor?
So, the Reduction in phase current
in delta (running) = 58%
To calculate the branch-circuit
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
wye-start, 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 wye-start, delta run
So,
the Branch circuit conductor ampacity = 0.72 x 731 A = 526.32 A
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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
motor-compressor rated-load current or branch-circuit selection current,
which‐ ever is greater + the sum of the rated-load current of the other motor
+ the ratings of the other loads.
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Notes for Sizing branch circuit conductors
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Summary
Of Sizing Steps For Branch-Circuit Conductors
For
Single Hermetic Motor Only
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www.Electrical-Knowhow.com
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Step#1
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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.
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Step#2
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Size the branch circuit
conductors ampacity ≥ 125% of the motor compressor
rated-load current or the branch circuit selection current, whichever is
greater
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Special Case
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For a Wye-Start, Delta-Run Connected
Motor-Compressor:
The branch-circuit conductors
ampacity between the controller and the motor-compressor = 72 % of either the
motor-compressor rated-load current or the branch-circuit selection current,
whichever is greater.
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Notes:
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Summary
Of Sizing Steps For Branch-Circuit Conductors
For
Combination Loads
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www.Electrical-Knowhow.com
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Step#1
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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.
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Step#2
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Size
Branch circuit conductor ampacity = 125% of the largest motor-compressor
rated-load current or branch-circuit selection current, which‐ ever is
greater + the sum of the rated-load current of the other motor + the ratings
of the other loads.
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Notes:
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Fourth:
Sizing Controllers
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When making the sizing calculation for Motor-Compressor Controllers for air conditioning or refrigeration system equipment, different cases apply, depending on the type of load as follows:
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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
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Case# 2: Controller Serving More Than One Load as per 440.41(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).
Where the controller serves a hermetic motor-compressor(s)
plus other loads, the controller rating is determined according to 440.12(B),
in much the same manner as determining the disconnecting means rating.
In this case,
The ampere rating of the Controller ≥ 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 Controller 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#A: 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#B: 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.
Example#3:
A
motor-compressor has a marked FLC of 55 amperes and a branch circuit
selection current of 58.6 amperes and a locked rotor current of 216 amperes.
The motor-compressor is a 208V three-phase motor. What is the minimum sized
horsepower rated controller that is required for this installation?
Solution:
Step#1:
Determine
which is greater, the FLC or branch circuit selection current.
58.6 is
greater than 55. Therefore we will use the branch circuit selection current.
Step#2:
Apply the
branch circuit selection current to Table 430.250. We will use 430.150
because the motor is a three phase motor.
Find the
column for 208V at the top of Table 430.150. Use the Induction-type half of the
Table. Now slide finger down the column until you come to a value equal to or
greater than 58.6.
You will
stop at 59.4. Now slide your finger (or use a straight edge) across to the
left hand column and read the equivalent horsepower. In this case it is 20
hp.
Step#3:
Apply the
locked rotor current to Table 430.251(B). We will use Table 430.251(B)
because our motor compressor is three-phase.
Find 208V
at the top of the Table.
Slide your
finger down the column until you come to a value that is equal to or greater
than 216 amperes. You will stop at 257.
Now slide
your finger across to the left hand column and read the equivalent
horsepower. In this case 15 hp.
Step#4:
The motor
controller must have a horsepower rating of at least as much as the highest value
obtained in steps 2 and 3.
In this
case the motor controller must be at least 20 hp.
Note:
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Summary
Of Sizing Steps For Controller Serving Motor Compressor
only
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www.Electrical-Knowhow.com
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Step#1
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The
controller must have a continuous duty FLC ≥ the nameplate rated current or
branch circuit selection current (whichever is larger),
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Step#2
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And also,
The controller must
have a LRC ≥ the locked rotor current of the compressor .
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Notes:
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Summary
Of Sizing Steps For Controller Serving More Than One Load
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www.Electrical-Knowhow.com
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Step#1
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The ampere rating of the Controller ≥ 115% of the sum of all
of the individual loads at rated load conditions
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Step#2
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Calculate the
greater value A or B of equivalent horsepower rating for the selected disconnecting means
in Step#1.
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A
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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
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B
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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.
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Step#3
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Select Controller
with HP rating ≥ the equivalent horsepower rating from step#1. If not, repeat
step#1 with higher value of AR.
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Notes:
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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:
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Subject Of Previous
Article
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Article
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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.
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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
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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
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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).
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The Minimum Circuit Ampacity (MCA)
Maximum Over-Current Protection (MOP) or
(MOCP)
Second: Sizing
calculation for Branch-Circuit Short-Circuit and Ground-Fault 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 motor-compressor is
the largest load,
Case# 2B: Where
a hermetic refrigerant motor-compressor is not the largest.
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