Air conditioning and refrigeration equipment

The Copper Kettle's two new rooftop units were craned onto the roof on a windy Tuesday, and Dana climbed up with Tyler to read the nameplates before they ran the circuits. Tyler reached for the motor tables out of habit, and Dana stopped him for the second time that month. Air-conditioning and refrigerating equipment with hermetic compressors has its own article, Article 440, and the nameplate on the unit, not the motor tables, sets the circuit. The unit's manufacturer has already done the motor calculation and printed the results: minimum circuit ampacity and maximum overcurrent protection. The electrician's job is to read those numbers correctly and meet the disconnect and receptacle rules around the unit.

This lesson covers Article 440: why hermetic compressors are treated differently, the rated-load current and branch-circuit selection current, how the minimum circuit ampacity and maximum overcurrent protection on the nameplate are used, the disconnect rules, the service receptacle requirement, and room air conditioners. It ends with how Article 440 equipment is added to feeder calculations.

Why hermetic compressors are different

In a hermetic refrigerant motor-compressor, the motor and compressor are sealed together inside a welded shell and cooled by the refrigerant itself. The motor's current depends on the refrigerant pressures, which depend on the weather and the load on the system, so a horsepower rating does not describe it well. Article 440 replaces horsepower with currents marked by the manufacturer. Section 440.3 explains how Article 440 relates to Article 430: Article 440 adds to or amends the motor rules for equipment with a hermetic compressor, and Article 430 still applies to the unit's other motors, such as fans, where Article 440 does not say otherwise.

Rated-load current and branch-circuit selection current

Two currents on an Article 440 nameplate matter. The rated-load current (RLA) is the current of the compressor when it runs at its rated load, determined by the manufacturer. Some compressors also carry a branch-circuit selection current (BCSC), which is higher than the rated-load current and is marked when the compressor's protective device allows it to run at a higher continuous current. Section 440.4 and the related definitions require the selection current, where marked, to be used instead of the rated-load current when sizing the conductors, the controller, the disconnect and the short-circuit protection. On an exam, if both currents appear, use the selection current.

The unit nameplate also lists the fan motors' full-load currents, and for packaged equipment it lists two results that combine everything: the minimum circuit ampacity (MCA) and the maximum overcurrent protective device rating (MOCP, sometimes printed as maximum fuse or maximum breaker size). Section 440.4(B) requires these markings on multimotor and combination-load equipment, and the installer uses them directly.

How the manufacturer's numbers are calculated

The minimum circuit ampacity follows the same idea as 430.24 for several motors. Section 440.33 sets the conductor ampacity for a circuit supplying a compressor with other loads at 125% of the largest motor-compressor current (rated-load or selection current, whichever is larger), plus the other motor and load currents. For a single compressor alone, 440.32 sets the conductor ampacity at 125% of the rated-load current or selection current, whichever is greater. Knowing this lets you check a nameplate, and exam questions sometimes ask you to calculate the minimum circuit ampacity from the component currents.

The maximum overcurrent protection follows 440.22. For a single compressor, the branch-circuit short-circuit and ground-fault device may not exceed 175% of the rated-load or selection current, and it may be increased to not more than 225% where the 175% device will not carry the starting current. For equipment with several motors, the device is based on the largest compressor's current at those percentages plus the other loads. Manufacturers do this calculation and round according to the article's rules; the result is the MOCP on the nameplate. Section 440.4(B) and 110.3(B) make that marked value a limit: the installed fuse or breaker may not exceed it.

Choosing the conductor and breaker in practice

With MCA 43.3 amperes and MOCP 70 amperes, Dana's circuit choice was straightforward. The conductors must have an ampacity of at least 43.3 amperes at the terminal temperature rating, so a conductor with a 75 °C ampacity of about 50 amperes works if the unit's terminals are rated 75 °C. The breaker may be anything up to 70 amperes that will carry the starting current, and must be of the type the nameplate allows; many nameplates say "maximum fuse or HACR breaker", and the installed device must match that wording. Ray usually installs the MOCP value or one size below it to avoid nuisance trips. A 70-ampere breaker on conductors rated about 50 amperes is permitted here for the same reason as in Lesson 3: the compressor's own protection handles overload.

Two exam traps hide in this step. The first is sizing the conductor to the MOCP instead of the MCA; the conductor needs only the MCA. The second is installing a breaker larger than the MOCP because a calculation suggested it; the nameplate limit governs.

The disconnecting means

Section 440.14 requires a disconnect within sight from and readily accessible from the air-conditioning or refrigerating equipment. The disconnect may be mounted on or within the equipment, but it must not be located on panels designed to allow access to the equipment, and it must not obscure the nameplate. Some exceptions exist, including one for equipment essential to an industrial process where a lockable disconnect elsewhere is used under qualified supervision. For a rooftop unit, the common answer is a fused or unfused disconnect mounted beside the unit or on its frame where the manufacturer allows.

Section 440.12 sizes the disconnect. For a hermetic compressor, the ampere rating must be at least 115% of the rated-load current or selection current, whichever is greater. A horsepower-rated switch needs an equivalent horsepower, which 440.12 tells you to find from the motor tables using the rated-load current and the locked-rotor current, taking the larger horsepower. For equipment with several motors and other loads, the ratings are combined.

The service receptacle

Section 210.63 requires a 125-volt, single-phase, 15- or 20-ampere receptacle within 25 feet of heating, air-conditioning and refrigeration equipment, located at an accessible location on the same level as the equipment. On a roof, that means a receptacle on the roof within 25 feet of the units, and 210.8 requires it to have ground-fault circuit-interrupter protection because it is outdoors. The receptacle may not be connected to the load side of the equipment's disconnect. The requirement exists so a service technician can plug in tools and a recovery machine without running cords from the building.

Room air conditioners and other cord-connected equipment

Part VII of Article 440 covers room air conditioners, the window and through-wall units common in apartments and small offices. For cord-and-plug-connected units, 440.62 treats the unit as a single motor unit when it meets the stated conditions, and 440.62(B) and (C) limit the unit's rating relative to the branch circuit: a room air conditioner alone on a branch circuit may not exceed 80% of the circuit rating, and where lighting or other appliances share the circuit, it may not exceed 50%. The attachment plug may serve as the disconnect. Section 440.65 requires the cord to have leakage-current detection and interruption or arc-fault protection, which is why room air conditioner plugs have the large rectangular head with a test button.

Feeder calculations with Article 440 equipment

When rooftop units and condensing units are added to a feeder calculation, Article 220 and Article 440 work together. For load calculations, the equipment is counted at its nameplate values. For feeder conductors serving several pieces of equipment, the method parallels 430.24: the largest motor-compressor's current at 125% plus the other loads. Exam problems usually give the MCA of each unit, and a common simplification is to add the largest unit's compressor at 125% and everything else at 100%; read the question to see which data it wants you to use.

def mca(compressor_rla, others, bcsc=None):
    big = max(compressor_rla, bcsc or 0)
    return round(1.25 * big + sum(others), 1)

def limits(rla):
    return round(1.75 * rla, 1), round(2.25 * rla, 1)

rla, fans = 28.0, [1.9, 6.4]
print("Copper Kettle RTU minimum circuit ampacity:", mca(rla, fans), "A")
print("Compressor protection limits (175%, 225%):", limits(rla), "A; nameplate MOCP governs: 70 A")
print("Disconnect, compressor alone:", round(1.15 * rla, 1), "A; with fans:", round(1.15 * (rla + sum(fans)), 1), "A")
print("Room unit limit on a 15 A circuit:", 0.80 * 15, "A alone,", 0.50 * 15, "A with other loads")

Copper Kettle RTU minimum circuit ampacity: 43.3 A
Compressor protection limits (175%, 225%): (49.0, 63.0) A; nameplate MOCP governs: 70 A
Disconnect, compressor alone: 32.2 A; with fans: 41.7 A
Room unit limit on a 15 A circuit: 12.0 A alone, 7.5 A with other loads

Breaker types and two units on one feeder

Many equipment nameplates say "maximum fuse or HACR breaker". HACR stands for heating, air-conditioning and refrigeration, and a breaker listed with that marking has been tested for group motor applications in that equipment. Most modern molded-case breakers carry the marking, but the installer must check, because 110.3(B) requires the equipment to be installed according to its listing and labeling. When the nameplate says "maximum fuse" only, a breaker may not be used for that unit.