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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, 2013 at 14:08

6 Answers 6

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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, 2013 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, 2013 at 14:49
  • Also, have you tried measuring the voltage of both legs at the disconnect and between the 2 legs?
    – longneck
    Sep 9, 2013 at 14:50
  • The fuse blowing is at the local disconnect for condenser.
    – John
    Sep 9, 2013 at 14:56
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    Because I wanted to know if the fuse block was corroded, rather than just replace things, I pulled it out of the disconnect box. Sure enough, contacts were bad - oxidized, pitted and coated with a black, plastic stuff! I polished everything with emery cloth, put it back together and it's running much cooler now - fuse block isn't heating up the metal box. I'll consider this a temporary fix, put in a new breaker in the main and a non-fused disconnect. Thanks for your help longneck!
    – John
    Sep 9, 2013 at 21:33
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Your using the wrong style fuses. Use a time delay fuse , which is for motors. Hence FNR vs FLNR. FNR is made with a time delay which allows for the large burst of current to start the motor. FLNR does not and absorbs the burst which is burning it out the fuse faster. Also check the sticker on the side sticker and make sure your buying the right amp.

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I would say look for a loose wire or a wire that is partially bare that is touching metal.check your ground wire and make sure that you have a good connection.

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It could be the relay that switches from start leg on the cap to the run leg on the cap, if the relay contacts are badly burnt it stays on the start leg, 5 volt drop is nothing I have worked in many areas that there is only 112V/ 224V with the motor starting and not having the phase corrction it cant make speed draws a little more power and also usually dosent cool as well (other than a loose wire connection or the fuse holders are loose that will also cause additional heat) just my .02 worth , Ed B- licensed General Journeyman electrician & universal EPA 608 & 609 CFC licenses, but I am still learning after 30 years would be nice to hear what your resolution is

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I had the same problem on a newly built home. The new AC unit kept blowing fuses. I disconnected the low voltage ground wire and no more blown fuses. The unit seems to work fine now.

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defective electrical disconnect / just replace it

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  • Can you elaborate on how a defective disconnect could cause this issue?
    – Tester101
    Sep 24, 2015 at 9:49

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