Electrical Rules and Calculations for Air-Conditioning Systems – Part Five

I explained some of the Electrical Rules and Calculations for Air-Conditioning Systems in the following Articles:

Topic	Article
Introduction for Air-Conditioning Systems Types, Introduction for Types of Motors/Compressors used in Air-Conditioning Systems.	Electrical Rules and Calculations for Air-Conditioning Systems – Part One
Types of Disconnecting Means for Air-Conditioning Systems	Electrical Rules and Calculations for Air-Conditioning Systems – Part Two
identification and Marking of Disconnecting Means for Air-Conditioning Systems	Electrical Rules and Calculations for Air-Conditioning Systems – Part Three
where to locate Disconnecting Means used for Air Conditioning Systems	Electrical Rules and Calculations for Air-Conditioning Systems – Part Four

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

Topic	Article
Importance of Electrical Wiring for Air Conditioning Systems. How to get the Electrical Wiring for Air Conditioning systems? Types of Electrical Wiring Diagrams For Air Conditioning Systems. How to read Electrical Wiring Diagrams?	Electrical Wiring Diagrams for Air Conditioning Systems – Part One
The electrical wiring diagrams for: Window air conditioning units, Split air conditioning units, Multi-Split air conditioning units, Mini-heat pumps, Split Packaged units, Unitary Packaged units.	Electrical Wiring Diagrams for Air Conditioning Systems – Part Two
The electrical wiring diagrams for: Chillers, Air Handling Units, Fans, and Pumps.	Electrical Wiring Diagrams for Air Conditioning Systems – Part Three

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): Full load current is the current drawn by a motor at its rated voltage, speed, frequency and horsepower. The term Full load current FLC is equivalent to the term Rated load current RLC. Note: Where there is change in the motor voltage, speed, frequency and horsepower from the rated values, the current drawn by the motor in this case will be called the motor running current where: Motor running current ≤ Motor Full Load Current FLC 1.2 Locked Rotor Current LRC (or LRA): Locked Rotor Current, LRC is the steady-state current taken from the power line with the rotor locked (stopped) and with the rated voltage and frequency applied. Steady-state 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 Air-Conditioning 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 As per NEC 430.110 Ampere, The disconnecting means for motor circuits rated 600 volts, nominal, or less shall have an ampere rating (AR dis) not less than 115 percent of the full-load current rating of the motor. AR dis = 1.15 x I FL of motor The full-load current (IFL) equivalent to the horsepower rating of motor is obtained from Table 430.247, Table 430.248, Table 430.249, or Table 430.250. In case the horsepower rating of the motor do not correspond to the horsepower ratings shown in Table 430.247, Table 430.248, Table 430.249, or Table 430.250, the horsepower rating corresponding to the next higher value shall be selected.

Table 430.247

Table 430.248

Table 430.249

Table 430.250

Example#1: A 50 HP Motor, 3Ø, 460 V, Design B load. Determine the size of the disconnect required for this motor. Solution: Get the full-load current (IFL) equivalent to the horsepower rating of each motor from Table 430.250. IFL = 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 To determine the equivalent horsepower Rating of the Disconnecting Means, the horsepower rating shall be selected from Table 430.247, Table 430.248, Table 430.249, or Table 430.250 corresponding to the Disconnecting Means Ampere Rating, and also the horsepower rating from Table 430.251(A) or Table 430.251(B) corresponding to the locked-rotor current. In case the Full Load current and locked-rotor current do not correspond to the currents shown in Table 430.247, Table 430.248, Table 430.249, or Table 430.250, Table 430.251(A), or Table 430.251(B), the equivalent horsepower rating corresponding to the next higher value shall be selected. In case different horsepower ratings are obtained when applying these tables, a horsepower rating at least equal to the larger of the values obtained shall be selected.

Exceptions to Rule#1: Disconnecting Means Ampere Rating for Motors Exception#1: A listed unfused motor-circuit 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 full-load 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: Two or more motors are used together, One or more motors are used in combination with other loads, such as resistance heaters. 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 full-load current (IFL) 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 full-load current for the combined load (I CFL) as follows: Equivalent full-load current for the combined load = Σ all full load currents, including resistance loads I CFL = Σ IFL of motors + Σ IFL of resistance loads Step#3: Calculate the Min. ampere rating of disconnect switch (AR dis.min ) based on Equivalent full-load current for the combined load (I CFL ) as follows: Min. ampere rating of disconnect means = 1.15 x Equivalent full-load current for the combined load AR dis.min = 1.15 x I CFL Step#4: Get the locked-rotor 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 locked-rotor current for the combined load (I CLR) as follows: Equivalent locked-rotor current for the combined load = Σ all Locked Rotor currents, including resistance loads I CLR = Σ I LR of motors + Σ IFL of resistance loads Step#6: Select the disconnect means horsepower rating (HPR dis) that correspond to Equivalent locked-rotor 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: if (AR dis. Selected) from step#8 ≥ (AR dis.min) from step#3, then the horse power rating of the disconnect means will be (HPR dis) from step#6. if (AR dis. Selected) from step#8 <(AR dis.min) from step#3, repeat steps #7,8 and 9 with next higher value of (HPR dis) obtained from step#6. Note: Where two or more motors or other loads cannot be started simultaneously, then Equivalent locked-rotor current for the combined load (I CLR) will be calculated as follows: I CLR = Σ I LR of motors that can be started simultaneously + Σ IFL 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 = Σ IFL of motors + Σ IFL of resistance loads Exception: A listed non-fused motor-circuit 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 full-load 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 locked-rotor current (I LR) shall be assumed to be six times the full-load current (I FL). I LR = 6 I FL Note: A general-use switch, circuit breaker, molded case switch (non-automatic circuit interrupter), or attachment plug and receptacle used as a disconnecting means must have an ampere rating of not less than 115 percent of the motor full-load current. Listed circuit breakers and molded case switches are tested under overload conditions at six times their rating, to cover motor circuit applications, and are suitable for use as a motor disconnecting means.

Example#2: A combined load consists of one 5-hp, one 3-hp, and two 1⁄2-hp motors, plus a 10-kW, all rated 240 volts, 3-phase. All motors are Design B motors. Determine the size of the disconnect required for this combination load. Solution: Step#1: Get the full-load current (IFL) equivalent to the horsepower rating of each motor from Table 430.250. Load Table Used IFL (in Ampere) 5-hp motor 430.250 15.2 3-hp motor 430.250 9.6 1⁄2-hp motor 430.250 2.2 1⁄2-hp motor 430.250 2.2 10-kW motor (10 x1000) / ((240 x1.732) = 24.1 Step#1 Step#2: Calculate Equivalent full-load current for the combined load (I CFL) I CFL = Σ IFL of motors + Σ IFL of resistance loads I CFL = 15.2 + 9.6 + 2.2 + 2.2 + 24.1 = 53.3 A Step#3: Calculate the Min. ampere rating of disconnect switch (AR dis.min ) based on Equivalent full-load current for the combined load (I CFL ) AR dis.min = 1.15 x I CFL = 1.15 x 53.3 = 61.3 A Step#4: Get the locked-rotor current (I LR) equivalent to the horsepower rating of each motor from Table 430.251(B). Load Table Used ILR (in Ampere) 5-hp motor 430.251(B) 92 3-hp motor 430.251(B) 64 1⁄2-hp motor 430.251(B) 20 1⁄2-hp motor 430.251(B) 20 10-kW motor (10 x1000) / ((240 x1.732) = 24.1 Step#4 Step#5: Calculate Equivalent locked-rotor current for the combined load (I CLR) I CLR = Σ I LR of motors + Σ IFL of resistance loads I CLR = 92 + 64 + 20 + 20 + 24.1 = 220.1 A Step#6: Select the disconnect means horsepower rating (HPR dis) that correspond to Equivalent locked-rotor current (I CLR) for the combined load from Table 430.251(B). For  LRA = I CLR = 220.1 A at 230 V, A 15-hp general-purpose or heavy-duty switch is suitable which has LRA = 232 A ≥ 220.1 A Step#6 Step#7: get full load current from Table 430.250 that correspond to the disconnect means horsepower rating (HPR dis) of step#6 A 15-hp general-purpose or heavy-duty switch will has a full load current = 42 A Step#7 Step#8: calculate selected disconnect Ampere rating (AR dis. Selected) as follows: AR dis. Selected = 1.15 x I FL from step#7 = 1.15 x 42 = 48.3 A 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: Since (AR dis. Selected) from step#8 = 48.3 A < (AR dis.min) from step#3 = 61.3A Then go to the next higher value of (HPR dis) obtained from step#6 which is 20 HP which has full load current = 54 A and its (AR dis. Selected) =  1.15 x 54 = 62.1 A > (AR dis.min) from step#3 = 61.3A Step#9 So, a 20-hp general-purpose or heavy-duty switch is suitable for this load combination.

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 Motor-Compressor and fans? We have three ratings can be marked on the nameplate of air conditioning equipment including Hermetic Refrigerant Motor-Compressor and fans which are: The nameplate rated-load current or nameplate full load current, The nameplate branch-circuit selection current (BCSC), The nameplate horsepower rating. The used rating in determining the Ampere Rating of the disconnect means for Hermetic Refrigerant Motor-Compressor and fans will be as follows: As per NEC 440.6 (A), the rated-load current marked on the nameplate of the equipment in which the motor-compressor is employed shall be used in determining the rating or ampacity of the disconnecting means, But if no rated-load current is shown on the equipment nameplate, the rated-load current shown on the compressor nameplate shall be used. As per NEC 440.2, When a branch-circuit selection current (BCSC) is marked on a nameplate, it must be used instead of the rated-load current to determine the size/Ratings of the disconnecting means. The value of branch-circuit selection current is always greater than the marked rated-load current. As per NEC 440.6 (B), For multimotor equipment employing a shaded-pole or permanent split-capacitor-type fan or blower motor, the full-load current for such motor marked on the nameplate of the equipment in which the fan or blower motor is employed shall be used instead of the horsepower rating to determine the ampacity or rating of the disconnecting means. This marking on the equipment nameplate shall not be less than the current marked on the fan or blower motor nameplate.

Rule#1: Disconnecting Means Ampere Rating for Hermetic Refrigerant Motor-Compressor 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 rated-load current or branch circuit selection current (whichever is greater). AR dis = 1.15 x Σ the greatest value from nameplate hermetic refrigerant motor-compressor rated-load current(s) or branch-circuit selection current(s)

Determining Disconnecting Means equivalent horsepower Rating in case of Hermetic Refrigerant Motor-Compressor As per NEC 440.12(A)(2), To determine the equivalent horsepower Rating of the Disconnecting Means, the horsepower rating shall be selected from Table 430.248, Table 430.249, or Table 430.250 corresponding to the rated load current or branch-circuit selection current, whichever is greater, and also the horsepower rating from Table 430.251(A) or Table 430.251(B) corresponding to the locked-rotor current. In case the nameplate rated-load current or branch-circuit selection current and locked-rotor current do not correspond to the currents shown in Table 430.248, Table 430.249, Table 430.250, Table 430.251(A), or Table 430.251(B), the horsepower rating corresponding to the next higher value shall be selected. In case different horsepower ratings are obtained when applying these tables, a horsepower rating at least equal to the larger of the values obtained shall be selected.

Exceptions to Rule#1: Disconnecting Means Ampere Rating for Hermetic Refrigerant Motor-Compressor  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 motor-compressors 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: Two or more hermetic refrigerant motor-compressors are used together, One or more hermetic refrigerant motor-compressor with other motors or loads such as resistance loads. 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 full-load 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 full-load 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 CFL-1) as follows: I CFL-1 =  Σ IFL of motors + Σ IFL of resistance loads B- For  hermetic refrigerant motor- compressor (s): I FL-HRMC = Σ the greatest value from nameplate hermetic refrigerant motor-compressor rated-load current(s) or branch-circuit selection current(s) C- For fan or blower motors: I FL-FAN = Σ The nameplate full load currents of fan(s) Step#2: Calculate Equivalent Total full-load current for the combined load (I FL-TOTAL) as follows: I FL-TOTAL = I CFL-1 + I FL-HRMC + I FL-FAN Step#3: Calculate the Min. ampere rating of disconnect switch (AR dis.min ) based on Equivalent Total full-load current for the combined load (I FL-TOTAL) as follows: Min. ampere rating of disconnect means = 1.15 x Equivalent full-load current for the combined load AR dis.min = 1.15 x I FL-TOTAL Step#4: Get the locked-rotor 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 locked-rotor 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 locked-rotor currents with other full load currents of resistance loads to get a combined locked-rotor current current (I CLR-1) as follows: I CLR-1 = Σ I LR of motors + Σ IFL of resistance loads B- For  hermetic refrigerant motor- compressor (s): I LR-HRMC = Σ Nameplate Locked rotor current of hermetic refrigerant motor-compressor (s) C- For fan or blower motors: I LR-FAN = Σ The nameplate Locked rotor currents of fan(s) Step#5: Calculate Equivalent Total locked-rotor current for the combined load (ILR-TOTAL) as follows: ILR-TOTAL = I CLR-1 + I LR-HRMC + I LR-FAN Step#6: Select the disconnect means horsepower rating (HPR dis) that correspond to Equivalent total locked-rotor current (ILR-TOTAL) 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: if (AR dis. Selected) from step#8 ≥ (AR dis.min) from step#3, then the horse power rating of the disconnect means will be (HPR dis) from step#6. if (AR dis. Selected) from step#8 <(AR dis.min) from step#3, repeat steps #7,8 and 9 with next higher value of (HPR dis) obtained from step#6. Note: Where two or more motors or other loads such as resistance heaters, or both, cannot be started simultaneously, then Equivalent locked-rotor current for the combined load (ILR-TOTAL) will be calculated as follows: ILR-TOTAL = current of simultaneously motors or fans loads + current of non-simultaneously motors or fans loads + Σ IFL of other concurrent loads Where: current of simultaneously motors or fans loads = Σ I LR of motors( other than hermetic refrigerant motor-compressor or fan) that can be started simultaneously + Σ Nameplate Locked rotor current of hermetic refrigerant motor-compressor (s) that can be started simultaneously + Σ The nameplate Locked rotor currents of fan(s) that can be started simultaneously current of non-simultaneously motors or fans loads =  Σ the greatest value from nameplate of non- simultaneously hermetic refrigerant motor-compressor rated-load current(s) or branch-circuit selection current(s) + Σ 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 FL-TOTAL Where: I FL-TOTAL = I CFL-1 + I FL-HRMC + I FL-FAN 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 locked-rotor current marked on the nameplate, or for small motors not covered by Table 430.248, Table 430.249, or Table 430.250, the locked-rotor current (I LR) shall be assumed to be six times the full-load current (I FL). I LR = 6 I FL Note: A general-use switch, circuit breaker, molded case switch (non-automatic circuit interrupter), or attachment plug and receptacle used as a disconnecting means must have an ampere rating of not less than 115 percent of the motor full-load current. Listed circuit breakers and molded case switches are tested under overload conditions at six times their rating, to cover motor circuit applications, and are suitable for use as a motor disconnecting means.

Important Note for Disconnecting Means Rated in Excess of 100 Horsepower Where the rated-load or locked-rotor 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 general-use or isolating switch where plainly marked “Do not operate under load.”

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

 

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