Today, we will explain how to size the Branch Circuit Overload Protection for Air Conditioning and Refrigeration Equipment and Provisions for Room Air Conditioners as Per Article 440.
The National
Electrical Code (NEC) And HVAC Systems – Part Five
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Fifth:
Branch Circuit Overload Protection
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- In Article "HVAC Equipment Power Rating Calculations – Part Six", we explained that as a general rule, the over-current device
rating shall not exceed the ampacity of a conductor. As stated in 240.4, conductors,
other than flexible cords, flexible cables and fixture wires shall be
protected against over-current in accordance with their ampacities specified
in 310.15, unless otherwise permitted or required in 240.4(A) through (G).
- This means that the over-current device rating shall be equal to or less than
the conductor ampacity. But for motors, the motor branch-circuit over-current protective
devices are permitted to be sized much higher than the rated ampacity of the
motor branch circuit conductors and overload protection must be used
to protect motor
branch-circuit conductors from overloads.
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- Overload
protection is used for protection against excessive heating due to motor
overload and failure to start.
- The
requirements for overload protection provided in NEC can be broken down into two
categories as follows:
- Protection
for the motor-compressor as per 440.52(A),
- Protection
for the controllers, disconnecting means and circuit conductors as per
440.52(B).
Notes:
- The manufacturers of hermetic systems supply the correct
protection, conductor sizing, and other information on the equipment
nameplate. Use this information when wiring an air conditioner or other
hermetic motor equipment.
- We
most often attempt to provide both categories of protection mentioned above with
a single device, which is allowed according to 440.52(B). This is not always
possible, however, and we have to provide two devices to accomplish both
jobs.
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Protection for the motor-compressor as per 440.52(A)
- Overload
protection for motor-compressors is very similar to overload protection for
standard motors.
- The methods
for providing overload protection for the motor-compressors are listed as
follows:
- A separate
overload relay,
- A thermally
protected motor-compressor,
- A fuse or
inverse time circuit breaker,
- A special
protective system.
- All
Motor-Compressors and Equipment on 15- or 20-Ampere Branch Circuits will
follow the same rules as per 440.52(A). Noting that the
rating of the attachment plug and receptacle or cord connector (for Cord- and
Attachment-Plug-Connected Motor Compressors and Equipment) shall not exceed
20 amperes at 125 volts or 15 amperes at 250 volts.
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Protection for the controllers, disconnecting means and circuit
conductors as per 440.52(B)
The methods
for providing overload protection for the conductors, controller and
disconnecting means are listed as follows:
- An overload
relay,
- A thermally
protected motor-compressor,
- A fuse or
inverse time circuit breaker,
- A special
protective system.
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The Methods for Overload Protection
As explained
above, we have (4) methods of overload protection as follows:
1- An Overload Relay
- A separate
overload relay may be used to provide overload protection. The relay must be
selected to trip at no more than 140% of the motor-compressor rated-load
current.
- Unlike a
standard motor, the NEC does not list any conditions for adjusting the relay
to a higher setting if the motor start-up current trips the overload.
- NEC 440.53
also requires the overload relay to be provided with ground-fault and
short-circuit protection. This is accomplished by placing the overload relay
in the circuit after the short-circuit, ground-fault protective device.
2- A thermally protected motor-compressor
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External thermal protector |
- Hermetic compressor motors are protected from
overheating by thermal protectors built into or mounted in contact with the
compressor motor.
- Many motor compressors today have over load protection built
into the motor-compressor that will not permit a continuous current in excess
of 156 percent of the marked rated-load current or branch-circuit selection
current.
- The NEC
allows this type of overload protection and requires no further protection if
the nameplate of the equipment is marked “Thermally Protected.”
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Thermally Protected |
3- A fuse or inverse time circuit breaker
- The inverse
time circuit breaker (Thermal-Magnetic Type) can provide overload protection,
ground-fault protection and short-circuit protection for circuits at the same
time. Also, many types of fuses can provide the same protection.
- The NEC
allows one of these type devices to be used as the overload protection for
the motor compressor under the following conditions:
- The breaker
or fuse is also the short-circuit and ground-fault protective device for the
circuit.
- The device
is rated for no more than 125% of the motor compressor rated load current.
- The device
has sufficient time delay to allow the motor to start and accelerate its load
without tripping.
- The motor
compressor or the equipment is marked with the maximum branch-circuit fuse or
inverse time circuit breaker rating.
- The fuse or
inverse time circuit breaker is rated no greater than the rating marked on
the equipment.
Notes:
- This is the
method most often selected, especially when dealing with residential AC
units. All of the protection can be accomplished with a single circuit
breaker.
- Most AC
equipment has nameplate marked with the maximum allowable over-current
device. In accordance with 110.3 this marking must be followed exactly and
the markings must be taken literally as follows:
- If the
marking is “Maximum Fuse Size”, then only fuses may be used.
- If the
marking is “Maximum HCAR Circuit Breaker Size”, then only HCAR breakers may
be used.
- If the
marking is “Maximum Fuse Size or HCAR Circuit Breaker Size”, then only fuses
or HCAR breakers may be used.
- If the
marking is “Maximum Fuse or Inverse Time Circuit Breaker Size”, then only
fuses or Inverse Time Circuit Breaker may be used.
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Maximum Fuse Size or HCAR Circuit Breaker Size |
4- A special protective system
- This method is most often
used on large commercial air conditioning systems where the overload
protection is provided by a system of sensors and relays included in the
control circuits provided by the manufacturer.
- The NEC allows using a
special protective system as the overload protection under the following
conditions:
- This special protective system must open the control circuits and stop current to the motor compressor in the event of an overload.
- This special protective system will not permit a continuous current in excess of 156 percent of the marked rated-load current or branch circuit selection current.
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Summary
Of Sizing Steps for Branch Circuit
Overload Protection
for Air Conditioning and Refrigeration Equipment
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www.Electrical-Knowhow.com
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Option#1
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When using A
separate overload relay, The relay must be selected to trip at no more than
140% of the motor-compressor rated-load current.
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Option#2
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When
using a Thermal Protector, it must not permit a continuous current in excess
of 156 percent of the marked rated-load current or branch-circuit selection
current.
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Option#3
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When using A
fuse or inverse time circuit breaker, The device must be rated for no more
than 125% of the motor compressor rated load current.
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Option#4
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When
the manufacturer uses a special protective system, it must not
permit a continuous current in excess of 156 percent of the marked rated-load
current or branch circuit selection current.
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Notes:
- The manufacturers of hermetic systems supply the correct
protection, conductor sizing, and other information on the equipment
nameplate. Use this information when wiring an air conditioner or other
hermetic motor equipment.
- All Motor-Compressors and Equipment on 15- or 20-Ampere Branch
Circuits will follow the same rules as per 440.52(A). Noting that the
rating of the attachment plug and receptacle or cord connector (for Cord- and
Attachment-Plug-Connected Motor Compressors and Equipment) shall not exceed
20 amperes at 125 volts or
15 amperes at 250 volts.
-
15 amperes at 250 volts.
- The NEC
allows this type of overload protection and requires no further protection if
the nameplate of the equipment is marked “Thermally Protected.”
- Most AC
equipment has nameplate marked with the maximum allowable over-current
device. In accordance with 110.3 this marking must be followed exactly and
the markings must be taken literally as follows:
- If the
marking is “Maximum Fuse Size”, then only fuses may be used.
- If the
marking is “Maximum HCAR Circuit Breaker Size”, then only HCAR breakers may
be used.
- If the
marking is “Maximum Fuse Size or HCAR Circuit Breaker Size”, then only fuses
or HCAR breakers may be used.
- If the
marking is “Maximum Fuse or Inverse Time Circuit Breaker Size”, then only
fuses or Inverse Time Circuit Breaker may be used.
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Sixth: Provisions for
Room Air Conditioners
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- A
room air conditioner (with or without provisions for heating)
shall be considered any AC appliance of the air cooled window, console or in
wall type that is installed in the conditioned room and that has a hermetic
refrigerant motor compressor.
- Room air conditioners aren’t just for residential
use. They are also used for commercial and industrial applications like guard
shacks, construction trailers, chemical storage sheds, and industrial control
panels.
- These
below rules will be applied only for the AC units rated less than 40A, 250V
and must be single phase.
- If
AC the unit is rated 3-phase or rated over 250 volts shall be directly connected
to a wiring method recognized in NEC Chapter 3 and the below rules
will not be applied.
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1- Sizing Branch-Circuit
Short-Circuit and Ground-Fault Protection
- 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].
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2- Sizing Branch Circuit Conductors
- The NEC 440.62 has two
requirements for the branch circuits that supply room air conditioners as
follows:
- If
a room air conditioner is the only load on the circuit, the circuit must be
rated for 125% of the marked rating of the air conditioner.
- If
the room air conditioner is to be supplied from a standard receptacle circuit
and other loads may be supplied from the circuit, the branch circuit must be
rated at least 200% of the air conditioners marked rating.
- Also,
NEC 440.62(A) lists the requirements for considering a room air conditioner
to be a single motor unit if the unit meets the following requirements:
- It
must be cord and plug connected,
- I
must have a rating of 250 volts or less and 40 amperes or less,
- The
nameplate must list a total rated current for the equipment as opposed to
individual ratings for the motors,
- The
branch circuit, short-circuit and ground-fault protective device must not be
rated higher than the ampacity of the branch circuit conductors or the
receptacle rating, whichever is less.
Note:
- For
the air conditioning unit which is designed to do more than one job (such as air
conditioning and heating) or which is designed with more than one motor (such
as a motor-compressor and a fan blower motor for the heat) will be considered
as a single motor unit if the above requirements were verified.
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3- Sizing Disconnecting Means
- As per NEC 440.63,
an attachment plug and receptacle can be used as the sole disconnecting means
as long as one of two conditions is met:
- The
manual controls for the AC unit are readily accessible and located no more
than 1.8 m (6 feet) of the floor.
- An
approved manually operable switch is installed in a readily accessible
location that is in sight of the AC unit.
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4- Protection Device for the Cord
- As
per NEC 440.65, the manufacturer must install one of the following protection
devices:
- Leakage-current
detector-interruptor (LCDI),
-
Arc-fault circuit interrupter (AFCI),
-
Heat detecting circuit interrupter (HDCI).
-
The protection device shall be an integral part of the attachment plug or be
located in the power supply cord within 300 mm (12 in.) of the attachment
plug.
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Summary of
Sizing Steps for Provisions for Room Air Conditioners
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www.Electrical-Knowhow.com
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Sizing
Branch-Circuit Short-Circuit and Ground-Fault Protection
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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].
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Sizing Branch Circuit
Conductors
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If
a room air conditioner is the only load on the circuit, the circuit must be
rated for 125% of the marked rating of the air conditioner.
If
the room air conditioner is to be supplied from a standard receptacle circuit
and other loads may be supplied from the circuit, the branch circuit must be
rated at least 200% of the air conditioners marked rating.
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Sizing
Disconnecting Means
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an
attachment plug and receptacle can be used as the sole disconnecting means as
long as one of two conditions is met:
- The
manual controls for the AC unit are readily accessible and located no more
than 1.8 m (6 feet) of the floor.
- An
approved manually operable switch is installed in a readily accessible
location that is in sight of the AC unit.
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Protection
Device For the Cord
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As
per NEC 440.65, the manufacturer must install one of the following protection
devices:
- Leakage-current
detector-interruptor (LCDI),
-
Arc-fault circuit interrupter (AFCI),
-
Heat detecting circuit interrupter (HDCI).
The protection device shall be an integral part of the attachment plug or be
located in the power supply cord within 300 mm (12 in.) of the attachment
plug.
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Notes:
- These
below rules will be applied only for the AC units rated less than 40A, 250V
and must be single phase.
- If
AC the unit is rated 3-phase or rated over 250 volts shall be directly connected
to a wiring method recognized in NEC Chapter 3 and the below rules
will not be applied.
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In
the next article, we will list all the summary tables for sizing calculations for Air
Conditioning and Refrigeration Equipment as Per NEC Article 440. So,
please keep following.
The previous and related articles
are listed in below table:
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
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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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HVAC Equipment Power Rating Calculations – Part Three
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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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HVAC Equipment Power Rating Calculations – Part Four
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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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Third: Sizing Branch Circuit Conductors Case# 1: The load type is a single hermetic motor only as
per 440.32,
Special Case:
For a Wye-Start, Delta-Run Connected Motor-Compressor
Case# 2: The load type is a Combination loads as per
440.33.
Fourth: Sizing Controllers
Case# 1: Controller Serving Motor Compressor only as per 440.41(A),
Case# 2: Controller Serving More Than One Load as per 440.41(B).
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HVAC Equipment Power Rating Calculations – Part Six |
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