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Air Conditioners Part I — oversized fuses can be costly

Watch size of compressor fuses to avoid expensive repairs

I am finding oversized fuses at air conditioner compressors more frequently lately. I tell people replacing those $10 fuses may save their $1,500 compressor (and some get mad at me?).

Let me explain why the fuses are there. If Mr. Contractor is working on an air conditioner compressor, he needs to be able to shut the power off. One place to do this is at the electrical panel, by turning off the breaker. But what if Mr. Homeowner comes home, not knowing Mrs. Homeowner called for Mr. Contractor. Mr. Homeowner turns down the thermostat and nothing happens. So he goes to the electrical panel and says “Aha! Here’s the problem” and flips the breaker on. Mr. Contractor who had both hands inside the air conditioner just got a new hairdo! And Mr. Homeowner may lose all his hair if Mr. Contractor can catch him.

So if the electrical panel is not nearby and in sight of the compressor there needs to be an electrical disconnect box near the compressor. Most, especially older disconnect boxes, contain fuses. With or without fuses, the box will have a small switch that Mr. Contractor can pull out to shut off power to the compressor, thus preventing Mr. Homeowner from trying to electrocute him.

The compressor should have a data label stating the maximum overcurrent protection device (meaning fuses or breakers). Let’s use 40 amp for this example. That means that either the fuses or the breaker have to be 40 amp or less, but not both.

In the old days, like before you could ask your smart phone who will win the next election, electricians would usually install 50 or 60 amp breakers in the panel for the compressor. This was because A) compressors weren’t very efficient and most needed that size, and B) it was up to the air conditioner contractor to install the proper size fuses in the disconnect box.

The average life for an air conditioner compressor around here is about 20 years. So Mr. Homeowner is having a new compressor installed at his 20-year-old house. New compressors are much more efficient. The data tag states the maximum size fuses should be 30 amp, and there are 40 amp fuses installed (and a 50 amp breaker in the panel).

My report comment states either the fuses or the breaker need to be changed, the fuses are less expensive and much easier to install. And this is a very “strong” comment. I was attending a class once where two Lennox engineers were presenting. I raised my hand and said I often find the fuses are 10 or even just 5 amp oversized — is this really a problem?

Both presenters yelled “YES!”, rudely waking up some of the attendees. They said if the tag says maximum 40 amp, it means 40 amp, not 45 amp. If the fuses are oversized and the air conditioner fails, it could cause much more expensive damage. And it could also void the manufacturer’s warranty.

So why would anyone be unhappy about a recommendation for a $15 improvement that could save them $1,500 or more? I don’t know either. I’m guessing these are the same people that drive the 400 horsepower twin turbo cars at 10 mph under the limit in the left lane, then get mad at you for passing them on the right. Don’t get me started on drivers — I don’t know if it’s just me but there seem to be more poor drivers. A poor driver, of course, is anyone that wants to drive 5 mph faster or slower than I do. I wrote in this column 10 years ago, and it’s still true today, It’s not a good thing if you can’t see the driver’s head over the headrest in the car ahead of you.

Back to compressors. In newer homes it is more common to find the disconnect box at the compressor does not have fuses. It just has the pull-out switch so Mr. Contractor can shut off the power. In this case the breaker in the electrical panel is the only overcurrent protection device, and must be the size specified on the compressor. And we sometimes run into the same problem — newer compressors are more efficient so the maximum size breaker is often smaller. Breakers are a little more expensive, $25 to $50.

So if you’re having your air conditioner replaced you might want to ask if they checked the size of the breaker in the panel.