Topic

Article









I explained the Electrical Wiring Diagrams for Air Conditioning Systems equipment in the following Articles:
Topic

Article



The electrical
wiring diagrams for:


The electrical
wiring diagrams for:

Today, I will explain how to size Disconnecting Means used for Air Conditioning Systems.
Sizing of Disconnecting means
for Air conditioning equipment

Important Definitions
1.1 Full Load Current FLC (or FLA):
Note:
Motor running current ≤ Motor Full
Load Current FLC
1.2 Locked Rotor Current LRC (or LRA):
Locked Rotor Current, LRC is the steadystate current taken from the
power line with the rotor locked (stopped) and with the rated
voltage and frequency applied. Steadystate means that the motor starts at rest
or zero speed.
Why the motor
Locked Rotor Current LRC is important?
1 Knowing a motor's locked rotor amps (LRA) and acceleration time helps in selecting upstream breakers. We make sure the circuit breaker or fuse can supply the locked rotor amps long enough to get the motor load up to speed and that it will trip if locked rotor amps continues beyond the allowable stall time.
Note:
Compare the LRA of a motor to the instantaneous time current curve
(not the continuous current rating) for a protective device to see if the
motor will be allowed to start without causing a nuisance trip.
2 Locked Rotor Current is a good approximation of the short circuit
current that a motor will add to a short circuit on the power system. When a
short occurs, the motor acts like an induction generator and delivers current
to the fault until the energy stored in the motor's magnetic field and
spinning rotor is dissipated. The current does not last long, but it must be
accounted for in short circuit calculations.

Introduction
As we stated before in Article Electrical
Rules and Calculations for AirConditioning Systems – Part Two that NEC Article 440 is in
addition to or amendatory of the provisions of Article 430 and other
applicable articles. Many requirements — for example, for disconnecting
means, controllers, single or group installations and sizing of conductors—
are the same as or similar to those applied in Article 430. So, Article 440
must be applied in conjunction with Article 430.
So, let’s first study the
disconnecting means sizing requirements as required by NEC Article 430.

First: Sizing of
Disconnecting means for Motor as per NEC Article 430

Rule#1: Disconnecting Means Ampere Rating for Motors
AR _{dis} = 1.15 x I _{FL} of motor

Table 430.247 
Table 430.248 
Table 430.249 
Table 430.250 
Example#1:
A 50 HP Motor,
3Ã˜, 460 V, Design B load. Determine the size of the disconnect required for this motor.
Solution:
Get the fullload
current (I_{FL}) equivalent to the horsepower rating of each motor
from Table 430.250.
I_{FL} = 65 A
Apply Rule#1 for
calculating Ampere Rating of disconnecting means AR _{dis }as follows:
AR _{dis} = 1.15 x I _{FL} of motor
= 1.15 x 65 A = 74.75 A
So, the
Ampere Rating of the disconnect must be at least 74.75 amps.

Determining Disconnecting
Means equivalent horsepower Rating in case of Motors

Exceptions to Rule#1: Disconnecting Means Ampere
Rating for Motors
Exception#1:
A listed unfused
motorcircuit switch having a horsepower rating not less than the motor
horsepower shall be permitted to have an ampere rating less than 115 percent
of the fullload current rating of the motor.
Exception#2:
Disconnecting
means for a torque motor shall have an ampere rating of at least 115 percent
of the motor nameplate current.

Rule#2: Sizing of disconnect means for Motors in Combination with other Loads
For Combination Loads
which operate simultaneously by a single disconnecting means and have one of
the following combinations:
The rating of the
disconnecting means shall be determined based on the rating of the combined
load which can be calculated as in the following (2) cases:
Case#1: If Motors
ratings are given in horsepower
Step#1: Get the
fullload current (I_{FL}) equivalent to the horsepower rating of
each motor from Table 430.247, Table 430.248, Table 430.249, or Table
430.250.
Step#2: Calculate Equivalent fullload current
for the combined load (I _{CFL}) as follows:
Equivalent fullload current
for the combined load = Î£ all full load currents,
including resistance loads
I _{CFL }= Î£ I_{FL} of
motors + Î£ I_{FL} of resistance loads
Step#3: Calculate the
Min. ampere rating of disconnect switch (AR _{dis.min} ) based on Equivalent fullload current
for the combined load (I _{CFL }) as follows:
Min. ampere
rating of disconnect means = 1.15 x Equivalent fullload current for the
combined load
AR _{dis.min} = 1.15 x I _{CFL}
Step#4: Get the
lockedrotor current (I _{LR}) equivalent to the horsepower rating of
each motor from Table 430.251(A) or Table 430.251(B). (see note for small
motors in below)
Step#5: Calculate Equivalent lockedrotor
current for the combined load (I _{CLR}) as follows:
Equivalent lockedrotor
current for the combined load = Î£ all Locked Rotor
currents, including resistance loads
I _{CLR = }Î£ I _{LR}
of motors + Î£ I_{FL} of resistance loads
Step#6: Select the
disconnect means horsepower rating (HPR _{dis}) that correspond to Equivalent lockedrotor
current (I _{CLR}) for the combined load
from Table 430.251(A) or Table 430.251(B).
Step#7: get full load
current from Table 430.247, Table 430.248, Table 430.249, or Table 430.250
that correspond to the disconnect means horsepower rating (HPR _{dis})
of step#6
Step#8: calculate
selected disconnect Ampere rating (AR _{dis. Selected}) as follows:
AR _{dis. Selected} = 1.15 x I _{FL}
from step#7
Step#9: compare between
disconnect means ampere rating (AR _{dis. Selected}) from step#8
and Min. ampere rating of disconnect means
(AR _{dis.min} ) from step#3 as follows:
Note:
Where two or more
motors or other loads cannot be started simultaneously, then Equivalent lockedrotor
current for the combined load (I _{CLR}) will be calculated as
follows:
I _{CLR = }Î£ I _{LR}
of motors that can be started simultaneously + Î£ I_{FL} of other
concurrent loads
Case#2: If Motors ratings are given in Ampere
The Ampere Rating of the disconnecting means will be as follows:
AR _{dis} ≥ 1.15 x I _{CFL}
Where:
I _{CFL }= Î£ I_{FL} of
motors + Î£ I_{FL} of resistance loads
Exception:
A listed nonfused
motorcircuit switch having a horsepower rating equal to or greater than the
equivalent horsepower of the combined loads, determined by Case#1, shall be
permitted to have an ampere rating less than 115 percent of the sum of all
currents at the fullload condition.

Table 430.251(A) 
Table 430.251(B) 
Important Note for small motors
For small motors not covered
by Table 430.247, Table 430.248, Table 430.249, or Table 430.250, the
lockedrotor current (I _{LR}) shall
be assumed to be six times the fullload current (I _{FL}).
I _{LR} = 6 I _{FL}
Note:

Second: Sizing of
Disconnecting means for Air Conditioning Equipment as per NEC Article 440

Introduction
Which current value should be used in determining the Ampere
Rating of the disconnect means for Hermetic Refrigerant MotorCompressor and
fans?
We have three ratings can be marked on the nameplate of air
conditioning equipment including Hermetic Refrigerant MotorCompressor and
fans which are:
The used rating in determining the Ampere Rating of the
disconnect means for Hermetic Refrigerant MotorCompressor and fans will be as
follows:

Rule#1: Disconnecting Means Ampere Rating for Hermetic Refrigerant
MotorCompressor
As per NEC 440.12(A)(1),
For a hermetic refrigerant motor compressor, the ampere rating of the
disconnecting means must be at least 115% of the nameplate ratedload current
or branch circuit selection current (whichever is greater).
AR _{dis}
= 1.15 x Î£ the greatest value from nameplate hermetic refrigerant
motorcompressor ratedload current(s) or branchcircuit selection current(s)

Determining Disconnecting
Means equivalent horsepower Rating in case of Hermetic Refrigerant
MotorCompressor

Exceptions to Rule#1: Disconnecting Means Ampere Rating for Hermetic Refrigerant
MotorCompressor
A listed unfused motor circuit switch,
without fuseholders, having a horsepower rating not less than the equivalent
horsepower determined in accordance with 440.12(A)(2)explained in above,
shall be permitted to have an ampere rating less than 115 percent of the
specified current.

Rule#2: Sizing of disconnect means for hermetic refrigerant
motorcompressors in Combination with other Loads
As per NEC 440.12(B), For Combination Loads
which operate simultaneously by a single disconnecting means and have one of
the following combinations:
The rating of the
disconnecting means shall be determined based on the rating of the combined
load which can be calculated as in the following (2) cases:
Case#1: If Motors
ratings are given in horsepower
Step#1: Get the
fullload currents of all loads as follows:
A For motors and
loads other than a hermetic refrigerant motor compressor, and fan or
blower motors:
Get the fullload current (I _{FL}) equivalent
to the horsepower rating of each motor from Table 430.248, Table 430.249, or
Table 430.250.
Then sum these
motors full load currents with other full load currents of resistance loads
to get a combined full load current (I _{CFL1}) as follows:
I _{CFL1 }= Î£ I_{FL} of motors + Î£ I_{FL} of
resistance loads
B For hermetic refrigerant motor compressor (s):
I _{FLHRMC} = Î£ the greatest value from nameplate hermetic
refrigerant motorcompressor ratedload current(s) or branchcircuit
selection current(s)
C For fan or blower motors:
I _{FLFAN} = Î£ The nameplate full load currents of
fan(s)
Step#2: Calculate Equivalent Total fullload
current for the combined load (I _{FLTOTAL}) as follows:
I FLTOTAL = I _{CFL1} + I _{FLHRMC} + I _{FLFAN}
Step#3: Calculate the
Min. ampere rating of disconnect switch (AR _{dis.min} ) based on Equivalent Total fullload
current for the combined load (I _{FLTOTAL}) as follows:
Min. ampere
rating of disconnect means = 1.15 x Equivalent fullload current for the
combined load
AR _{dis.min} = 1.15 x I _{FLTOTAL}
Step#4: Get the lockedrotor
currents of all loads as follows:
A For motors and
loads other than a hermetic refrigerant motor compressor, and fan or
blower motors:
Get the
lockedrotor current (I _{LR}) equivalent to the horsepower rating of
each motor from Table 430.251(A) or Table 430.251(B). (see note for small
motors in below)
Then sum these
motors lockedrotor currents with other full load currents of resistance
loads to get a combined lockedrotor current current (I _{CLR1}) as
follows:
I _{CLR1 }=_{ }Î£ I _{LR}
of motors + Î£ I_{FL} of resistance loads
B For hermetic refrigerant motor compressor (s):
I _{LRHRMC} = Î£ Nameplate Locked rotor current of
hermetic refrigerant motorcompressor (s)
C For fan or blower motors:
I _{LRFAN} = Î£ The nameplate Locked rotor currents of
fan(s)
Step#5: Calculate Equivalent Total lockedrotor
current for the combined load (I_{LRTOTAL})
as follows:
I_{LRTOTAL} = I _{CLR1} + I _{LRHRMC}
+ I _{LRFAN}
Step#6: Select the
disconnect means horsepower rating (HPR _{dis}) that correspond to Equivalent total lockedrotor
current (I_{LRTOTAL}) for the
combined load from Table 430.251(A) or Table
430.251(B).
Step#7: get full load
current from Table 430.248, Table 430.249, or Table 430.250 that correspond
to the disconnect means horsepower rating (HPR _{dis}) of step#6.
Step#8: calculate
selected disconnect Ampere rating (AR _{dis. Selected}) as follows:
AR _{dis. Selected} = 1.15 x I _{FL}
from step#7
Step#9: compare between
disconnect means ampere rating (AR _{dis. Selected}) from step#8
and Min. ampere rating of disconnect
means (AR _{dis.min} ) from step#3 as follows:
Note:
Where two or more
motors or other loads such as resistance heaters, or both, cannot be started simultaneously, then Equivalent
lockedrotor current for the combined load (I_{LRTOTAL})
will be calculated as follows:
I_{LRTOTAL} = current of simultaneously motors or fans loads + current of
nonsimultaneously motors or fans loads + Î£ I_{FL}
of other concurrent loads
Where:
current of simultaneously motors or fans loads = Î£ I _{LR}
of motors( other than hermetic refrigerant motorcompressor or fan) that can be started simultaneously + Î£ Nameplate
Locked rotor current of hermetic refrigerant motorcompressor (s) that can be started simultaneously + Î£ The
nameplate Locked rotor currents of fan(s) that can
be started simultaneously
current of nonsimultaneously motors or fans loads = Î£ the greatest value from nameplate of non simultaneously hermetic refrigerant
motorcompressor ratedload current(s) or branchcircuit selection current(s)
+ Î£ The nameplate full load currents of non
simultaneously fan(s)
Case#2: If Motors ratings are given in Ampere
The Ampere Rating of the disconnect means will be as follows:
AR _{dis} ≥ 1.15 x I _{FLTOTAL}
Where:
I _{FLTOTAL} = I _{CFL1} + I _{FLHRMC}
+ I _{FLFAN}
Exception:
A listed unfused motor
circuit switch, without fuseholders, having a horsepower rating not less than
the equivalent horsepower determined by case#1 shall be permitted to have an
ampere rating less than 115 percent of the sum of all currents.

Important Note for small motors
For small motor compressors
not having the lockedrotor current marked on the nameplate, or for small
motors not covered by Table 430.248, Table 430.249, or Table 430.250, the
lockedrotor current (I _{LR}) shall
be assumed to be six times the fullload current (I _{FL}).
I _{LR} = 6 I _{FL}
Note:

Important Note for Disconnecting Means Rated in Excess of 100
Horsepower
Where the ratedload or
lockedrotor current as determined by rules#1 or 2 would indicate a
disconnecting means rated in excess of 100 hp, the disconnecting means shall
be permitted to be a generaluse or isolating switch where plainly marked “Do
not operate under load.”

In the next Article, I will explain Types and Sizing of MotorCompressor Controllers. So, please keep following.