This is a post about electrical theory, but not about low voltage electronics, instead this post is about HVAC.

Recently NorthRidgeFix posted a video about their HVAC system; the 20 amp breaker was tripping. This was resolved by replacing the 20 amp breaker with a 30 amp breaker. I wish to provide the following suggestion:

The typical HVAC compressor motor will not draw more than 20 amps unless the compressor capacitor is failing. A failing capacitor will cause the compressor to draw more amps and overheat, thus shortening its lifespan. In such a scenario, increasing from a 20 amp breaker to a 30 amp breaker only kicks the can down the road and may result in premature compressor failure.

I would recommend to verify that the compressor capacitor and also fan motor capacitor are correctly sized per the motor nameplate ratings, and I would also recommend to verify that the capacitors are still within their rated operating tolerance.

HVAC capacitors can fail after only a couple years of use, especially the low quality "budget" capacitors sold at Lowes and Home Depot and sold on Amazon and eBay. HVAC capacitors can fail simply from sitting outside for some time, unused, in an HVAC unit that is exposed to the heat of the sun. I would recommend to inspect these capacitors annually. Typically, the physically larger and heavier capacitors will have a longer service life and will have a longer warranty.

The code requirements for motor overload protection can be complicated and difficult even for master electricians to understand. The standard HVAC setup involves a breaker in the main panel (typically 30, 40, or 50 amps) feeding a dedicated HVAC circuit, with a disconnect device (breaker or fuse) located at the other end of the HVAC circuit, right where the HVAC unit is connected. The HVAC unit should connect directly to the dedicated HVAC disconnect device, and the dedicated HVAC disconnect should be fed by the dedicated HVAC circuit.  Ideally there will not be more than one appliance connected to the dedicated circuit (hence the term 'dedicated.')

It is important to understand that circuit protection is not the same thing as motor overload protection. A motor overload protection device typically opens the circuit at much less amps than a circuit protection device. It may very well be permitted (or not?) by code to install and use a motor overload protection breaker or fuse of 20 amps as the HVAC disconnect on a circuit that is protected by a 30 amp, 40 amp, or 50 amp breaker.

Imagine that hypothetically, you would install a 20 amp motor overload breaker inside the HVAC unit, wired directly to the compressor motor, and imagine that you would install a 5 amp motor overload breaker inside the HVAC unit, wired directly to the fan motor. These motor overload breakers installed inside the HVAC unit would do nothing to change the characteristics and behavior of the 30 amp, 40 amp, or 50 amp circuit to which the HVAC unit was connected. You should think of the HVAC disconnect device as if it is part of the HVAC unit, located inside the HVAC unit. Even if the HVAC's motor overload disconnect is physically located immediately outside the HVAC unit, it is still the same, electrically, as if it was located inside the HVAC unit's cabinet. I hope this hypothetical explanation helps.

For most HVAC compressor motors, a 20 amps or 25 amps fuse or breaker would provide an appropriate level of motor overload protection. Yes, the typical HVAC compressor nameplate will specify a 30 amp circuit, even though it does not typically draw anywhere near 30 amps... A rational analysis can only conclude that you should not size your motor overload protection according to the ratings information on the appliance nameplate, instead, you should size your motor overload protection according to the ratings information on the motor nameplate. 

If the HVAC manufacturer and the HVAC service technician tell you to use a 30 Amp fuse or breaker in your HVAC disconnect box, even though your HVAC compressor motor only draws 15 Amps, well, that just means they can sell you a new HVAC system sooner rather than later... 

If your HVAC compressor is drawing 15 amps, and your HVAC fan motor is drawing 2 amps, then you will achieve maximum protection for your motor if you use a 20 amp motor overload breaker...

Of course, if a hypothetical appliance has a 15 amp motor and also has a 15 amp heating element, then you will never be able to connect the entire appliance to a 20 amp motor overload breaker or fuse. Instead, the motor overload breaker or fuse will need to be connected directly to the motor, separately from the heating element. But in the case of most HVAC units, the compressor motor has by far the largest amp draw, while the electronics and the fan motor amp draw are negligible by comparrison. Therefore, for an HVAC unit, we can get away with placing the compressor motor overload protection outside the unit, at the HVAC disconnect device.

Yes, a 20 amp breaker or fuse does not provide good motor overload protection for the HVAC fan motor that is only drawing 2 amps, but that's okay, fan motors are relatively cheap and easy to replace, and anyway, the fan motor capacitor is less likely to go bad than the compressor capacitor. It is most important to provide optmial overload protection for the compressor motor, because replacing an HVAC compressor can cost thousands of dollars and can result in hours or even days of down time.

I do not know if it allowed by electrical code to connect a 20 amp disconnect breaker or fuse to a circuit that is protected by a 30 amp or 40 amp or 50 amp breaker, even when the appliance nameplate specifies a 30 amp circuit.  I'm not a master electrician. I also don't care if it's allowed by code, because it is obviously safe to do this. Replacing an HVAC compressor is expensive as hell. Every time, I'm going to install in the HVAC disconnect the smallest rated motor overload protection device that I can get away with, because I want to minimize the overheating of my compressor motor when the capacitor eventually fails. Nearly all HVAC capacitors eventually fail.

Maybe the electrical code prefers for the motor overload protection breaker or fuse to be located at the HVAC disconnect, up on the roof, instead of being installed in the main breaker panel. But obviously, for troubleshooting purposes, it is a lot easier if the motor overload breaker is located in the main breaker panel where it is readily accessible. It is obviously perfectly safe to protect a 50 amp circuit with a 20 amp breaker.  Of course it is only practical to protect a 50 amp circuit with a 20 amp breaker if the appliances plugged into the circuit are actually drawing less than 20 amps. And as I stated above, the typical HVAC compressor motor will not draw more than 20 amps unless the compressor capacitor is failing.

One last point:  Yes, starting current for a fan motor and compressor motor can be very high, but if you look at the published trip curves of breakers (for example, Square D QO breaker trip curves are published) you will see that breakers will tolerate a whole lot of momentary starting current before they trip.

I hope that these suggestions are helpful!

PS - Remember that failure to speak out against fascism is an invitation...