Electrical Wiring in the Home/AC Unit


I am A State Certified Electrician and I pretty much broke the golden rule. In troubleshooting a bad circuit I found that the circuit had lost it's return (neutral). At the Main Service Panel I found the return partially burned off and just sitting in the Panel unconnected. I did not turn off the main breaker and attempted to install the wire while all was hot. Ofcourse there was a arc and the AC unit which is apparently on the same circuit that I was working on, wouldn't come on. I checked the power to the unit and it is getting it's desired voltages. I have been a electrician for over 25 years but I have always passed on working on AC units so I am without much knowledge. Is there some type of trip or overcurrent protection inside the compartment of the unit? I am thinking either I have screwed these peoples Unit up or I have throw that trip circuit.

A split system AC has the high voltage feeding the outdoor unit (220 volts) and it has an indoor unit that supplies the low voltage to the outdoor unit.

Did you check the low voltage feeding the unit?

The outdoor unit always has the 220 volts present (unless the breaker or disconnect is off) at the contactor. Some units use a single pole contactor and one leg feeds the contactor and the other goes directly to the compressor.

The indoor unit transforms it's 110 volts to 24 volts and sends it to the thermostat. The thermostat controls the low voltage to the outdoor unit contactor.

When a call for cooling takes place the thermostat sends 24 volts to the "Y" terminal and that energizes the contactor at the outdoor unit which completes the circuit for the high voltage.

I will walk you through a way to troubleshoot the problem:

Make sure the thermostat is on and calling for cooling. If the display is messed up or unresponsive you have to stop and pull the cover. Look for a blown fuse or burnt wiring.
Check for 24 volts between "R" (red# and "C" #common).
If there is no "C" then you need to go to the indoor blower unit and check to see if you are getting 24 volts from the transformer.

If you are getting 24 volts from the transformer you need to the check the circuit board inside the indoor unit. It is fused protected.

If you are getting 24 volts on the "R" and "C" at the circuit board but the thermostat is not working you should consider that the thermostat may have got ruined.

Okay, lets say you do have the 24 volts at the "R" and "C" at the thermostat. Put the thermostat into cooling mode and check for voltage between "Y" and "C".
Note: most newer t-stats have a delay that may be up to 5 minutes so you need to wait until the thermostat actually calls for cooling.

If you get the 24 volts from the t-stat through "Y" then you should go out to the outdoor unit and check the terminals on the side of the contactor for 24 volts.
The contactor is really just a relay and the coil is fed with 24 volts and that closes the contacts for the high voltage.

If you are getting 24 volts on the coil of the contactor and the contactor is not engaging then you need to turn both the low voltage and high voltage off. Pull the wires off the coil of the contactor and perform an ohm reading on the coil.
The other thing to look for is the contacts on the contactor could be pitted so bad they won't connect. I have even seen a bug in between the contacts.

If the contactor is energized and the contacts are closed then you need to do a sound check.

But before we go there try what I outlined and let me know.  
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Electrical Wiring in the Home

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Greg Hughes


Troubleshooting electrical including appliances and HVAC.


34 years of experience.

Both college and tech schools in this field.

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