Air Conditioning A/C System Operating Controls & Overcurrent Protection InspectAPedia® -
Air conditioning system operation controls: thermostats, zone dampers, electrical switches
Air conditioning safety switches
Rules of thumb for sizing air conditioner fuses or circuit breakers
This article explains the basic air conditioning system operating controls, air conditioning operating defects,
and both normal and abnormal air conditioner operating temperatures and how to measure them.
We also review the basic air conditioning safety switches.
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Also see A/C - HEAT PUMP CONTROLS & SWITCHES and THERMOSTATS. This chapter is part of our more extensive air conditioning inspection, diagnosis, & repair document which
describes the inspection, diagnosis, and repair of residential air conditioning systems (A/C systems) for home buyers, owners, and
home inspectors.
If your air conditioning or heat pump system has lost its cooling capacity or won't start see REPAIR GUIDE for AIR CONDITIONERS. see How to determine the cooling capacity of air conditioning equipment if the system seems to be working but is inadequate to cool your building.Contact us to suggest text changes and additions and, if you wish, to receive online listing and credit for that contribution.
OPERATING CONTROLS - Air Conditioning System Operating Controls
Basic air conditioning inspection and inspection report information for A/C controls:
Thermostats: The air conditioning system is operated by thermostat in the living area. If multiple air handling units and compressors are installed
you should find a thermostat for each area served by that equipment. See THERMOSTATS for details about thermostatic controls. If your Air conditioner or heat pump won't turn on or off properly this is a good place to start. Also see A/C - HEAT PUMP CONTROLS & SWITCHES
Air conditioning system zone dampers: Some air conditioning system designs may use
a single air handler and compressor, but may add zone dampers in the duct work to provide individual "zoning" of cool air
distribution. In this case each zone thermostat both calls for the system blower/compressor to operate and also causes
a motorized zone damper to open to direct cool air to a particular portion of the building. An ordinary home inspection
is unlikely to address proper operation of motorized zone dampers. See ZONE DAMPER CONTROLS for details about automatic and manual heating and air conditioning air zone controls.
Manual duct dampers may also be present in duct work to
manually balance air distribution among building areas. Be sure to look for these when diagnosing poor cool air delivery to an area.)
Also see our note below about the presence of multiple switches and controls.
See ZONE DAMPER CONTROLS for details about automatic and manual heating and air conditioning air zone controls.
Electrical switches for air conditioning systems will often be found as follows: (some of these may be absent on some systems). For details see A/C - HEAT PUMP CONTROLS & SWITCHES
Electrical panel circuit breakers or fuses will be provided separately to control the air handler (blower system) circuit and
the compressor/condenser circuit. Of these the compressor is usually supplied by a 240V circuit and the air handler by a 120V circuit.
Compressor safety shut off switch outside at the compressor/condenser. The switch may be a circuit breaker, fuse, or a simple "pull-out"
disconnect located close to the compressor.
Air handler service switch inside on or close to the air handler unit itself
Air handler blower compartment safety switch: a safety interlock that will turn off electricity to the air handler or blower unit if the blower compartment door is not securely shut. If your air conditioner blower will not start this switch and the blower compartment doors should be checked. See BLOWER FAN OPERATION & TESTING
Air conditioner/heat pump contactor relay switch: turns on high amp drawing equipment such as the compressor motor. See CONTACTOR RELAY DIAGNOSIS & REPAIR
Air conditioner or heat pump pressure control safety switch: turns off the system at excessive refrigerant pressure and in some systems at too-low pressure. Also used in automotive air conditioning. See A/C - HEAT PUMP CONTROLS & SWITCHES
SAFETY CONTROLS - Air Conditioning Automatic Safety Controls - Cooling System Fuse or Circuit Breaker Size Requirements
Electric Power Controls - Safety Disconnects for Air Conditioners
Safety disconnects should installed outside next to the compressor/condenser unit and are often also installed
next to or mounted on the air handler/blower unit.
If you cannot find an outside electrical disconnect one should be installed. These controls are recommended for safety
to reduce the temptation to open the cabinet and work on the equipment with power on.
Working on electrically "live"
cooling equipment risks both shock and mechanical injury such as being cut by the fan if the motor starts
unexpectedly. Safety shutoffs are required for new equipment.
How to Specify the Breaker or Fuse Size for Air Conditioning Circuits
Our photograph of a modern circuit breaker panel (left) shows where your search for the air conditioning or heat pump system main circuit breakers would typically begin. Look for two control circuits for the air conditioner or heat pump system that will typically include:
A circuit, 120V or 240V serving the indoor air handler
A circuit, usually 240V, serving the outdoor compressor/condenser unit.
The Amperage Rating of safety disconnects and A/C or Heat Pump circuit breakers
The safety switch on newer equipment
may be a simple pull-out fuse-block type power disconnect, leaving circuit protection to be provided only at the circuit breaker
or fuse for the A/C circuit where it originates in the electrical panel. Where the actual overcurrent protection is provided
(at older circuit breakers used as auxiliary safety disconnects at the equipment, and at the main panel at the origin of the
cooling circuit for the compressor/condenser unit)
electrical overload protection size (circuit breaker or fuse amperage rating)
for modern A/C equipment is specified by the manufacturer.
The Maximum Fuse or HACR type Breaker: specifies the maximum overcurrent protection or MOP to be used to protect
the equipment. The permitted ampacity of the equipment electrical circuit protection (fuse or circuit breaker amps)
expressed as MOP or Maximum Overcurrent Protection.
If MOP is specified, the breaker or fuse protecting the equipment should match this number.
As we explained at the beginning of this document, a hermetic compressor draws varying amounts of current (measured in amps) as its internal
pressure changes during operation. We said that current draw is higher when starting the motor than when the system is in
steady state operation.
Current draw is highest if the motor
is starting against its highest back pressure such as if the air conditioning system has been turned off and then back on in the middle of operation.
Because fusing an air conditioning
compressor at the minimum level can result in blown fuses or tripped breakers during these intervals of heavy current draw during compressor
startup, compressors are either protected by a slow-blow fuse or a somewhat larger than minimum circuit breaker.]
Rules of thumb for over sizing air conditioning system breakers or fuses:
On some older equipment
MOP is not specified. Only when MOP has not been specified can the overcurrent protection required be
determined by alternative means such as [RLA OR BCSC whichever is greater x 175%],
or if the compressor keeps tripping that device or blowing that fuse, RLA x 225% might be used.
The National Electrical Code (NEC) specifies the degree to which a breaker or fuse may exceed the RLA.
[For example, if the MOP or fuse size is specified by the manufacturer to be 40 amps, then a 40 amp breaker must be
installed with no increase or change in that rating.]
Multiple switches are often present on cooling systems. As we reminded in the previous chapter, if the
air conditioning system won't run, before requesting a service
call check all of the switches as well as the thermostat for proper settings.
Air Conditioner Fuse or Circuit Breaker Size Details
Generally, what is the ampacity we see in the field when inspecting an air conditioning compressor circuit?
When the air conditioning system is running, if you measured the amperage, it would be roughly 80% of the RLA.
The breaker size is typically about 125% of the total of the compressor RLA and the condenser fan FLA (full load amperage).
We are referring here to the main circuit breaker that controls the air conditioner compressor/condenser unit - a switch that is typically located in the main electrical panel or in a sub-panel serving the air conditioning or heat pump equipment.
Our photo at left shows a different switch: an outside service switch that incorporates a circuit breaker next to the compressor/condenser.
This circuit is for use by the service technician and because it is downstream of the wire bringing power to the compressor/condenser unit, it is not protecting that wire from an overcurrent. While both of these circuit circuit breakers must be properly served, don't confuse their role nor their location.
The rationale is that the circuit breaker protecting the air conditioner compressor unit should trip in the event of a
locked rotor [the revolving axle of a compressor motor, for example] or some significant electrical event,
but should not trip during start up loads which, as we know can be significantly higher than the RLA momentarily [as the
compressor motor draws higher amperage to get itself started].
Why can we put an "oversized" fuse or circuit breaker on an air conditioning compressor circuit?
An air conditioning electrical circuit is different than a general household circuit in that we have a known current load.
[There is
only one device connected to the air conditioning electrical circuit, and we can read its operating characteristics.]
We are not worried about an overload situation where people plug several appliances into receptacles on a single circuit.
Generally speaking, the amperage draw is fine or is way too big.
Code Citatin: Section E3602.10 of the IRC says,
Branch circuits for air conditioning and heat pump equipment. The ampacity of the conductors supplying a multi motor and combination load equipment shall not be less than the minimum circuit and capacity marked on the equipment.
The branch-circuit overcurrent device rating shall be the size and type marked on the appliance and shall be listed for the specific purpose." In short, do what it says on the data plate.
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Additional technical contributors & reference sources for this article are listed below.
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Books & Articles on Building & Environmental Inspection, Testing, Diagnosis, & Repair
Our recommended books about building & mechanical systems design, inspection, problem diagnosis, and repair, and about indoor environment and IAQ testing, diagnosis, and cleanup are at the InspectAPedia Bookstore. Also see our Book Reviews - InspectAPedia.
The Home Reference Book - the Encyclopedia of Homes, Carson Dunlop & Associates, Toronto, Ontario, 2010, $69.00 U.S., is available from Carson Dunlop. The Home Reference Book is a bound volume of more than 450 illustrated pages that assist home inspectors and home owners in the inspection and detection of problems on buildings. The text is intended as a reference guide to help building owners operate and maintain their home effectively. InspectAPedia.com ® author/editor Daniel Friedman is a contributing author. Field inspection worksheets are included at the back of the volume.
"Air Conditioning & Refrigeration I & II", BOCES Education, Warren Hilliard (instructor), Poughkeepsie, New York, May - July 1982, [classroom notes from air conditioning and refrigeration maintenance and repair course attended by the website author]
Carson Dunlop, Associates, Toronto, have provided us with (and we recommend)
Carson Dunlop Weldon & Associates' Technical Reference Guide to manufacturer's model and serial number information for heating and cooling equipment ($69.00 U.S.).
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