Blowing Low Voltage Fuse Breaker Trip

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Blowing Low Voltage Fuse Breaker Trip | Control Wiring Troubleshooting - I’ve got a 5 amp fuse that keeps popping on a circuit board. The voltage out of the transformer reads 26.3 volts. I am still popping fuses and reading over 24 volts. Can you help?

Okay, I can give you some of the basics to help you but first, we need to clear a few things up. You seem to think the voltage is what is causing the fuse to blow. First, it is not voltage that causes fuses or breakers to blow or trip. Secondly, it is amperage. Finally, it is likely you have a dead short from what you are describing but let’s go through the basics first:

Blowing Low Voltage Fuse Breaker Trip | Control Wiring Troubleshooting

What is the purpose of the fuse or breaker in a control circuit? The purpose of the fuse or breaker is to protect the control board and the wiring in case of over amperage. Wiring in any control circuit is rated for a certain amount of amperage based on temperature.

Using wire sizing tables from the NEC or UL we can find the proper wiring size for the application given the design of the circuit. Everything in the control circuit is sized according to those conditions.

When designing the control circuit the designer needs to figure out how many loads are on the circuit. This will determine wire size and transformer size.

Circuit Design, Limitations, and VA Ratings

This is typically done by adding up the VA or Volts-Amps ratings of all the loads in the circuit. That typically determines the limitations of the circuit. However, in this case, the limitation would the 5 amp fuse on the circuit board.

So if all the loads in the circuit were energized all at the same time then you cannot exceed 5 amps otherwise you blow the fuse. In most cases, for the typical HVAC systems in residential that provides heating and cooling 5 amps is more than sufficient.

Transformers are rated by there VA rating and you never want to exceed the VA rating on the transformer. If you are getting 26.3 volts from the 24-volt transformer you are okay. Lastly, plus or minus 10% is the generally accepted tolerance for most electrical components including transformers.

Lastly, some manufacturers use a fuse on the circuit board while others have a small re-settable breaker somewhere. Changing the fuse is usually easy but resetting the breaker is even better since it does not require you to purchase fuses every time you blow one. Finally, the re-settable breakers are especially good for troubleshooting problems such that you are describing.

Fuse On the Circuit Board Blowing

In your case, you likely have a dead short somewhere in the wiring or a bad load that is going short to ground causing the fuse to blow. Furthermore, I would remove all the control wires from the board. Make sure you have a good fuse and then reattach them one at a time.

Furthermore, when the fuse blows you will know which wire is causing the problem. Or which wire has the dead short or bad load. Then it is a simple matter of tracing it out to make sure the wire is okay. If the wire is fine with no dead shorts, then you look at the load. An example of this would be a relay or contactor.

Relays and contactors have coils in them that are energized through the wire. If the coil gets a dead short in it then there is your problem. Replace the relay or contactor and problem solved.

Blowing Low Voltage Fuse Breaker Trip | Conclusion

I’ve seen many things cause fuses to blow and breakers to trip. Among them are:

Weed eaters cutting through the control wiring at the condenser
Bad coils in relays and contactors as noted above
Some driving a nail in the wall and hitting the thermostat control wiring
Squirrels and other rodents/animals chewing on the wire
Finally, not properly dressing the wire for terminations at the thermostat and control board

And it goes on and on. Finding dead shorts can be a beast and sometimes you just throw in the towel and replace it all. Lastly, I hope that helps and wish you luck. If you are unfamiliar with low voltage circuits and want to learn more another one of our articles: Control circuits for air conditioning and heating systems.