The point of this article is to give you a full understanding of the role fuses, overloads, and circuit breakers play in the protection of HVAC/R equipment. If you skim-read or jump to conclusions, you will be tempted to argue. Be patient; if you want to understand, you will need to read all the way through and possibly even watch the videos at the end. This topic is WIDELY misunderstood, so the odds are that you will likely think I'm crazy when you first read it. Do your own detailed research once you get to the end if you still dispute what is contained here.

There are a few topics in HVAC/R that get widely confused and result in a lot of misinformation because of the similarity of the concepts. If you have two terms with SIMILAR meanings but get used interchangeably, you can reach completely logical-sounding (though totally incorrect) conclusions.

For example, a tech could say a particular circuit is reading “no ohms to ground,” and by that, he could mean zero ohms, or he could mean the meter is reading OL, which means infinite ohms.

In the same way, I often hear people say that something is “shorted,” and what they really mean is that it's not working or something inexplicable is happening. So, let's define some terms, starting with one of the most often confused. Here is the dictionary definition:

Short Circuit

In a device, an electrical circuit of lower resistance than that of a normal circuit, typically resulting from the unintended contact of components and consequent accidental diversion of the current.

When a professional uses the term short or short circuit, they can mean an electrical path with lower resistance than designed, or they can also mean any unintended path.

For example, if two conductors in a cable are compromised and touching one another, a tech will often say that they are shorted even if there is not a low resistance overall in the circuit.

Because of this, the term “short” has become broad and must be used carefully.

Overload

To place too much a load on

Overload protection

Overload protection is a protection against a running overcurrent that would cause overheating of the protected equipment

Overload protection deals with higher current resulting from too much current being pulled by a load. When the compressor goes out on overload after one second because it is locked, that is an example of overload.

When a condenser fan goes off after running with a blade with too steep of a pitch, that is an example of overload. An overload condition in a motor is dealt with by the overload inside the motor, not by the overcurrent protection/circuit breaker/fuses. In the case of motor loads specifically, if the overload were to fail, the overcurrent protection would usually break the circuit eventually, but that is not its primary design function in most cases. Again, I recommend reading THIS from Siemens to clear things up.

NEC Guidelines and Installation Practices

When manufacturers write their system specs and print their equipment labels, they use the National Electrical Code (NEC) guidelines. They refer to articles 430 and 440 of the code to calculate the required minimum conductor size and maximum overcurrent size. That is how they come up with the MOCP, or max breaker/fuse size, and the MCA (or minimum circuit ampacity/conductor size) required.

Here is some manufacturer electrical data from a Carrier 25HCC condensing unit:

Notice the maximum breaker or fuse is 40 amps on the 4-ton, and the MCA is 26.1 with an allowable wire size of  #10 on an assembly rated at 75°C and a 60°C circuit.

Yep.

On this system, it is perfectly acceptable by the National Electrical Code and the manufacturer to run #10 wire and a 40 amp breaker so long as the wire run is a properly rated copper conductor under 70 feet (on this specific unit because of the spec shown above). Wire length impacts voltage drop, which is only addressed as a suggestion in the code but is clearly laid out by the manufacturer either directly or based on the minimum voltage if you do a voltage drop calculation. In this case, the voltage must be 197v or above.

That is because the circuit breaker or fuse provides overcurrent protection (as well as some backup overload protection), and the motor overloads provide overload protection.

Now, if you would like, you are allowed to put in a lower-rated breaker than the max so long as you don't go below the MCA rating because the breaker itself also needs to handle the rated capacity. Just be aware that the lower you go, the more likely you will be to have nuisance tripping.

You can also install a larger wire if you like; just be aware that the equipment lugs may not be rated to hold/connect the larger wire in some cases. Also, keep in mind that you must also upsize the grounding conductor when arbitrarily upsizing the current-carrying conductors.

If you would like some more discussion on the topic, you can see the videos I did on this HERE, HERE, HERE, and HERE.

Finally, before you start leaving comments about what is written, please watch the two videos below. If you watch these videos, read the reference material, and STILL think that what I'm saying is false in some way, feel free to join in the conversation.

—Bryan

P.S. – Mike Holt (shown below) is a friend of mine and is considered the authority on NEC and electrical training in the US. I highly recommend watching his video covering many of the topics in this tech tip: