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For years, my A/C condensing unit has blown a fuse once, maybe twice a year. It's 27 years old now and blew a few fuses last spring. Now it blows a fuse every few days. So, I replaced a bad running capacitor on the compressor and replaced a leaking capacitor on the condenser fan motor, still blows fuses every few days. Dangdest thing is that it has a very respectable amperage reading - 20 amps on each leg (it's 220v) even when it's very hot (105 degrees outside) and the fuse only blows in the evening when the temperature has gone down for the day! I thought that maybe it blew a fuse when cycling off and on, so, I tried just operating the A/C manually (turning it off when the inside temp was comfortable and on when I wanted some cooling) - still blows a fuse.

I now wonder if the problem is with the fuse box or power supply. It is typical that after a day or two of replacing a fuse, the new fuse darkens - it still works, but can get quite dark. A blown fuse will be really dark and the nice clean copper is pretty scorched. I cleaned off the contacting surfaces of the fusebox and tightened the wires down.

I'm now suspicious of a possible occasional voltage drop in our power supply. Wouldn't that spike the amps to my A/C unit and burn a fuse? A few weeks ago the power company installed a box to monitor power quality right outside my house. When I asked them what they were doing they informed me about monitoring the power and then asked me if I noticed lights flickering and so on.

I thought I might monitor voltage last night to see if I might catch a drop (wishful thinking I guess). But anyway, just using a 120v receptacle, the volts were 120.0 volts to begin with, 118.8 volts when the A/C came on, went down to 117.4 volts, then ranged between those last two numbers 'till I turned the A/C off an hour later. Then the reading was 120.9. First thing in the morning, the voltage was 123.0 volts So, there's quite a swing in voltage - I imagine that's normal, but I don't know. I'm up a hill, at the end of the power line in a city area.

Here's the particulars of my A/C condenser - it's a 1985 Sears 3.5 ton (NCABA42AB01) - full load on condenser= 21.5 amps time delay fuse called for is 30 amps and that's the fuse I keep replacing.

Anybody have any ideas - poltergeist, maybe?

Update: It just blew a fuse again - mid day this time. I noticed that the fuse box was really hot - I measured it with an infrared thermometer - 220 degrees F! The condenser unit was buzzing, only one fuse blew - the other looks fine. I'm thinking I need to replace the fused disconnect.

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Which fuse is blowing? There are 4 common locations for fuses: main fuse panel for air handler, main fuse panel for condenser, local disconnect for air handler, or local disconnect for condenser. –  longneck Sep 9 '13 at 14:08
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1 Answer

Long-term darkening of a fuse on a high-draw circuit like an AC is not unusual. However, darkening as quickly as you have described means the fuse is operating right at its limit for too long.

The first thing I would do is call an electrician and have the fuse replaced with a breaker. At least that way you don't have to touch a hot fuse to replace it. They may also find a problem with the wiring (corrosion or a break) that could explain your problem.

Next is I would buy/borrow a multimeter that has an amp clamp on it. Measure the current used by various parts of your system (blower motor, compressor, condenser fan) as well as the main power feed to find out how much current you are pulling. This may help you identify which component is causing the problem.

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Thanks for the response. Using an amp clamp I find that draw is normal - right around 20 amps on both legs, but the fuse block is around 220 degrees F - hot! I figure I should replace the fused disconnect and go from there. The existing fuse block is about thirty years old. Wish I could go with breakers, but disconnects for AC don't seem to come with 'em. –  John Sep 9 '13 at 14:42
    
You don't need a fused disconnect. If the condenser unit is protected upstream by a dedicated fuse or breaker, you can just use a simple disconnect. –  longneck Sep 9 '13 at 14:49
    
Also, have you tried measuring the voltage of both legs at the disconnect and between the 2 legs? –  longneck Sep 9 '13 at 14:50
    
The fuse blowing is at the local disconnect for condenser. –  John Sep 9 '13 at 14:56
    
Hey, you're right, longneck. It looks like my father in law installed a fused disconnect downstream from the main breaker panel. Funny thing - the breakers are 30 amp and the fuses are 30 amp time delay. If my reasoning is right, the breakers should have tripped before the time delay fuses. This means that the problem is with the fuse block - it's probably corroded somewhere and heating things up destroying fuses faster and faster! Make sense? –  John Sep 9 '13 at 15:17
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