Condensate tray overflow switch (C) D Friedman Air Conditioning A/C System & Heat Pump Operating & Safety Controls

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Air conditioner & heat pump controls & switches:

This article explains the function, location, identification & use of all air conditioning & heat pump system operating controls. Photos and text help you to find & recognize each of these controls and the text explains what the control does.

What are all of the controls and switches found on A/C & heat pump systems, where is each control found, what does it do, and which controls should be used by the building occupants. Air conditioning safety switches.

Rules of thumb for sizing air conditioner fuses or circuit breakers.

Examples of "hidden" or hard-to-find switches or controls on heating & cooling systems.

We include links to detailed diagnosis & repair articles related to the various HVACR controls & swtiches. We also review the basic air conditioning safety switches, contactors, relays, refrigerant metering devices, motor overolad switches, relays, resets.

Green links show where you are. © Copyright 2017, All Rights Reserved.

List & Purpose of All of the Air Conditioning & Heat Pump System Operating & Safety Controls

Thermostat Cool Off Heat switch (C) D Friedman

This chapter is part of our 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.

[Click to enlarge any image]

If your air conditioning or heat pump system has lost its cooling capacity or won't start see REPAIR GUIDE for AIR CONDITIONERS.

See COOLING CAPACITY, RATED 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.

Also see A/C - HEAT PUMP CONTROLS & SWITCHES where we list all Air Conditioning & Heat Pump System Controls & Switches, and see THERMOSTATS.

Basic air conditioning inspection and inspection report information for A/C controls:

Duct damper manual (C) D Friedman

Electrical switch on heat or A/C (C) D Friedman

SAFETY CONTROLS - Air Conditioning Automatic Safety Controls - Cooling System Fuse or Circuit Breaker Size Requirements

Photograph of the outdoor safety switch for an air conditioning residential system

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 at your compressor/condenser unit, 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

Electrical panel circuit breakers (C) D Friedman

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:

  1. A circuit, 120V or 240V serving the indoor air handler
  2. 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 protectthe 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

Photograph of the outdoor safety switch for an air conditioning residential system, shown here with the cover openGenerally, 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.

Thanks to Alan Carson for these details.


Continue reading at THERMOSTATS, HEATING / COOLING or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.




Or see COOLING & HEATING CONTROL & SWITCH INDEX - index to all controls

Or see DIAGNOSTIC GUIDES A/C / HEAT PUMP - suggestions for diagnosing compressor or condenser fan and coil problems that can mean intermittent or totally lost cooling capacity of your system.

Or see these

Air Conditioning & Heat Pump Controls Articles

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