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:
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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:
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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?
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:
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
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Summary Of Sizing Steps For Branch-Circuit Conductors For Single Hermetic Motor Only | |
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Step#1 | |
Step#2 | |
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:
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Summary Of Sizing Steps For Branch-Circuit Conductors For Combination Loads | |
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Step#1 | If the equipment is marked with minimum circuit ampacity (MCA), use this value to size the branch circuit conductors. If not, go to Step#2. |
Step#2 | Size 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:
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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:
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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:
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:
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:
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:
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Summary Of Sizing Steps For Controller Serving Motor Compressor only | |
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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:
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Summary Of Sizing Steps For Controller Serving More Than One Load | |
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Step#1 | The ampere rating of the Controller ≥ 115% of the sum of all of the individual loads at rated load conditions |
Step#2 | Calculate the greater value A or B of equivalent horsepower rating for the selected disconnecting means in Step#1. |
A | Use 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 |
B | Use 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. |
Step#3 | Select Controller with HP rating ≥ the equivalent horsepower rating from step#1. If not, repeat step#1 with higher value of AR. |
Notes:
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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. | HVAC Equipment Power Rating Calculations – Part One |
The Common Types Of Motors Used In HVAC Industry First: the Common Types of Motors Used in HVAC Industry to drive Compressors
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 | 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 | HVAC Equipment Power Rating Calculations – Part Three |
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). | HVAC Equipment Power Rating Calculations – Part Four |
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. | HVAC Equipment Power Rating Calculations – Part Five |