25 Feb 2014

In your home, 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 A/C disconnects 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 Air Conditioning circuit where it originates in the electrical panel.

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.

A hermetic compressor draws varying amounts of current (measured in amps) as its internal pressure changes during operation. The 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.

Inspecting an air conditioning fuse compressor circuit breaker size?                                         

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.

Some homeowners have an older or different switch: an outside service switch that incorporates a circuit breaker next to the compressor/condenser.

This circuit is for use and should be diagnosed by a licensed electrician in Pasadena 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 breakers must be properly served, don’t confuse their role or 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 easily.

Homeowners and electricians alike should be conscious of overloading situations. Plugging in several appliances into receptacles on a single circuit or maintaining appropriate load at the Air conditioning circuit are important.

For additional information regarding overloading electrical circuits or Air conditioning branch circuits contact a licensed electrician in Pasadena at United Plumbing Heating Air & Electric.

Code Citation: 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”.