### 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. Types of Disconnecting Means for Air-Conditioning Systems identification and Marking of Disconnecting Means for Air-Conditioning Systems where to locate Disconnecting Means used for Air Conditioning Systems

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? 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. The electrical wiring diagrams for: Chillers, Air Handling Units, Fans, and Pumps.

Today, I will explain how to size Disconnecting Means used for Air Conditioning Systems.

 Sizing of Disconnecting means for Air conditioning equipment

 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.

 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

 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.

 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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