HVAC Equipment Power Rating Calculations – Part Six

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.

 The National Electrical Code (NEC) And HVAC Systems – Part Four

 Third: Sizing Branch Circuit Conductors

 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: Case# 1: The load type is a single hermetic motor only as per 440.32, Case# 2: The load type is a Combination loads as per 440.33.

 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: As a general rule, the overcurrent 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 overcurrent in accordance with their ampacities specified in 310.15, unless otherwise permitted or required in 240.4(A) through (G). This means that the overcurrent device rating shall be equal to or less than the conductor ampacity. But for motors, the motor branch-circuit overcurrent 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.

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?

 Connection Voltage Relation Current Relation Reduction in Phase Current Star (starting) VL = √3 × VPh IPh = IL 0 Delta (running) VL = VPh IPh = IL / √3 1/√3 = 58%

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

 Case# 2: The load type is a Combination loads as per NEC 440.33. In this case, the combined load may be: Two or more hermetic motor-compressors or One or more hermetic motor-compressors with other motors or loads (such as fans, heaters, solenoids, and coils,) 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.

 Notes for Sizing branch circuit conductors  The size of conductors for electric motor-driven air-conditioning and refrigerating equipment (including hermetic motor compressors) shall be selected from Table 310.15(B)(16) through Table 310.15(B)(19) or calculated in accordance with 310.15 as applicable [NEC 440.6]. You can review these tables in these articles: All of the derating factors in 310.15 apply to this ampacity and so do the termination limitations of 110.14. You can review the derating factors in this article: Correction Factors Affecting Conductor Ampacity Tables For a hermetic refrigerant motor-compressor, 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 branch circuit conductors. Where no rated-load current is shown on the equipment nameplate, the rated-load current shown on the compressor nameplate shall be used. [NEC 440.6(A)]  Where so marked, the branch-circuit selection current shall be used instead of the rated-load current to determine the rating or ampacity of the branch-circuit conductors. [NEC 440.6(A) 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. [NEC 440.6(A) Exception.2] For multi-motor 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 branch-circuit conductors, This marking on the equipment nameplate shall not be less than the current marked on the fan or blower motor nameplate. [NEC 440.6(B)] The amapacity of the conductor may never be lower than the “Minimum Circuit Ampacity” marked on the nameplate of combination equipment as required by 440.4(B).

Summary Of Sizing Steps For Branch-Circuit Conductors
For Single Hermetic Motor Only

www.Electrical-Knowhow.com
Step#1

Step#2

Size the branch circuit conductors ampacity ≥ 125% of the motor compressor rated-load current or the branch circuit selection current, whichever is greater

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.
Notes:
• Size these branch-circuit conductors no smaller than the minimum circuit ampacity on the equipment nameplate if existing.
• 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.
• The size of conductors for electric motor-driven air-conditioning and refrigerating equipment (including hermetic motor compressors) shall be selected from Table 310.15(B)(16) through Table 310.15(B)(19) or calculated in accordance with 310.15 as applicable [NEC 440.6].
• All of the derating factors in 310.15 apply to this ampacity and so do the termination limitations of 110.14.
• For a hermetic refrigerant motor-compressor, 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 branch circuit conductors. Where no rated-load current is shown on the equipment nameplate, the rated-load current shown on the compressor nameplate shall be used. [NEC 440.6(A)]
•  Where so marked, the branch-circuit selection current shall be used instead of the rated-load current to determine the rating or ampacity of the branch-circuit conductors. [NEC 440.6(A) 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. [NEC 440.6(A) Exception.2]

 Fourth: Sizing Controllers

 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: 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).

 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

Case# 2: Controller Serving More Than One Load as per 440.41(B).

In this case, the combined load may be:

1. Two or more hermetic motor-compressors or
2. One or more hermetic motor-compressors with other motors or loads (such as fans, heaters, solenoids, and coils,)

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:

 Table 430.250

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:

 Table 430.251(B)

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:
• A controller shall be marked with the manufacturer’s name, trademark, or symbol; identifying designation; voltage; phase; full-load and locked-rotor current (or horsepower) rating; and other data as may be needed to properly indicate the motor-compressor for which it is suitable (NEC 440.5).

 Summary Of Sizing Steps For Controller Serving Motor Compressor only www.Electrical-Knowhow.com Step#1 The controller must have a continuous duty FLC ≥ the nameplate rated current or branch circuit selection current (whichever is larger), Step#2 And also, The controller must have a LRC ≥ the locked rotor current of the compressor . Notes: FLC: full-load current LRC: locked rotor current A controller shall be marked with the full-load and locked-rotor current (or horsepower) rating (NEC 440.5). If the selected controller is marked in only horsepower. We are directed to use the motor compressors nameplate FLC and locked rotor current and search in Tables 430.248-250 for FLC and 430.251(A) or (B) for LRC for the equivalent horsepower. The result that gives us the higher equivalent horsepower is the one we will use to size the controller.

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:
Subject Of Previous Article
Article
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.

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
HVAC System Units And Ratings
Energy Conversions In Air Conditioning / Refrigeration Systems

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

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).
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.

 Back To