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How Come Home Air Conditioner Fuses Blow?

Similar questions answered on this page: Blown fuses on air conditioner

  • How come air conditioning fuses blow?

  • How come air conditioning fuses fail?

  • Why do the fuses constantly blow?

The most likely reasons for your fuses to fail on your air conditioning system:

  • The fuses are the wrong type. Air conditioner fuses are slo-blow or time delay fuses. The amount of electricity it takes to start the compressor is normally 3-4 times what the running amperage is. A 3 ton 30 amp circuit will usually draw about 87 amps on start up for a few seconds at most. A standard fuse will wear out rather quickly. (The picture shows copper tubing being used as fuses. This is one of the most dangerous things that a person do. The fuses are blowing for a reason. Bypassing the safety means an extreme fire danger)

  • The fuses are the wrong size. The fuses need to be sized for the wires serving the air conditioner and they need to be sized to the larger load detailed on the air conditioning nameplate. In the past, most A/C systems were labeled with a minimum circuit breaker or fuse size and a maximum circuit breaker or fuse size. To save money, many contractors always used the smallest wire size possible. This is fine when the units are new, but as they age and the wire starts to corrode a little the amperage draw increases and so does the heat it generates causing the fuses to blow or fail.

  • Bad disconnect box. If the terminals are discolored or the push in connection are discolored, warped or corroded it is time to replace the disconnect box. These poor quality contacts create heat and heat is the number one reason for fuses to pop, not excessive amperage.

  • Aluminum wire connections. If the wire connections are aluminum or the wire itself is aluminum the connections need to be made with a corrosion stopping electrical grease, typically Noalox. Failure to use this electrical grease can cause heat buildup and premature failure.

  • Short Cycling of the Compressor. Short cycling of the compressor is when the unit is running normally and then is turned off only to quickly turn back on within a minute or so. This causes great damage to the compressor and can cause a locked rotor on the compressor. Locked rotor amperage is the absolute fullest load that the air conditioner can handle and it exceeds the rating of the fuses considerably. Fuses and circuit breakers will normal fails to open within 10 seconds of operation. This can be caused by a register blowing on the thermostat. A short in the thermostat wire, relay chatter, relay failure or even somebody playing with the thermostat.

  • Electrical problem in the air conditioning unit itself. A short circuit, failed capacitor, broken wire, shorted windings, fan motor failure and even a time delay relay failure can cause an overload situation leading to blown fuses on the air conditioner.

  • Freon Overcharge. If there is too much Freon in your air conditioning system the load on the compressor increases dramatically and the performance of your air conditioning system suffers. More Freon is bad, not good. Excessive amperage draw, poor performance, shortened lifespan and blown fuses are a symptom of a Freon overcharge.

  • Low Voltage to the Disconnect. Usually low voltage situation occur during long electrical runs, electrical runs through the attic or over the top of the roof and electrical runs with wire nuts or other connections in their run. Under sized wiring can also cause this problem. This problem can only be diagnosed when the unit is running. The National Electric Code has us electricians (Yes, I am a licensed Electrician) sizing the wire for a maximum of 5% voltage drop. The longer the run, the bigger the wire. Some HVAC manufactures specify no more than a 2% voltage drop. This can only be measured when the unit is operated versus the nonoperational voltage. We take three readings, one before we start the air conditioning unit and the next as the unit starts and the third when the unit is under full operational load. If the voltage drops unacceptably at any time the wire must be repaired or replaced.

So, when you call your local Heating and Air Conditioning Company and ask them how much it is going to cost to replace your fuses and they tell you that they will have to look at the unit, these problems I have detailed on this page are what they are thinking of. Fuses are almost never the problem. Fuses are a safety mechanism stopping the operation of the air conditioner because something is wrong. Most of the time, the problem is merely a bad electrical connection that costs a few pennies to fix.


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