View Full Version : AC Blown Fuses at outdoor disconnect

06-26-2007, 04:09 PM
Hello all,

I have my AC comp. with an outdoor disconnect (picture attached), and the darned fuses keep blowing. Have had problems on and off for a few years, every few weeks or months it would blow. They are the time delay FNR-20 fuses meant for motor protection. Well, today I've gone through about 8 fuses, it blows every 30 minutes or so. Only the replacements go, so I am thinking that I probably have a bad batch of fuses. I even changed locations with the one that hasn't gone, and still, the replacement goes, not the old tried and true that has been in there working fine for about a year. I think it has something to do with dirty power here in the house, the fuse often goes after a slight light flicker. Flippo construction is a couple blocks away working on undergrounding of electrical lines, so my guess is that it is sorta Flippo's fault. So I am wondering, can I replace this outdoor device with a breaker type enclosure? Also, the box states 30 Amp, but I have two 20Amp fuses in it, this doesn't see right. Shouldn't I have two 15 Amp, or am I mistaken and should have two 30 Amp? The Comp.'s CB size at the main panel is 40 Amp. Thanks for the advice.

06-26-2007, 06:14 PM
On the side of the AC unit will be a name plate that has Minimum Circuit Ampacity xx amps and Maximum Fuse Size xx amps. The xx will be a number.

If the unit calls for a 20 amp fuse and the 20 amp fuses are blowing it is time to call a AC technician.

06-26-2007, 06:23 PM
That fused disconnect is designed for a MAXIMUM of 30A, but if your A/C unit specifies a smaller fuse, you can use it. You just can't use a bigger fuse than either the compressor unit specifies, or the box is rated for. For 240vac, you need each side to have the same size fuse.

If that doesn't fix it, your compressor may be shot...happened to me last year. replaced the whole unit since a compressor was nearly as much.

Under really heavy power consumption times, like now when parts of the country are in a heat wave, they lower the voltage some (brown out). This will cause the compressor to have to work harder and draw more current. You might check your voltage going to the compressor.

06-26-2007, 06:59 PM
Jim, thanks for the note. Actually, I do notice it happening more frequently during hot times, figured as much what you said but you have helped to confirm it. I will check the voltage at the comp to see.

I also think that the company doing the work is causing mini brown outs all around town. I blew fuse after fuse this morning and afternoon, and I know they all stop work promptly at 4:00. Since I replaced the last fuse (same group of fuses that were blowing too) I haven't had a problem, and that is the hottest part of the day.

I've been trying to put off buying a new unit for at least another year (said the same thing last year too :rolleyes: ) so I may need to suck it up. I just ran a new lineset over the winter to accomodate a larger unit, as well as a larger condensate drain from the attic, so I guess I am as ready as ever.

I'm just worried I will miss the sound of a helicopter taking off or a group of Harleys rolling into town when my AC starts. The thing is about 20 years old I believe, a Sears unit. I can't remember the SEER off hand, but lord knows the thing is not efficient.

Thanks again to both of you for the input.

06-26-2007, 07:18 PM
Here is a quick shot of the plate from the side of the compressor. It looks like no matter what a 20 Amp is under sized. Am I correct or just badly misreading things? Should I up it to 30 since the disconnect will support it?

06-26-2007, 08:01 PM
Should I up it to 30 since the disconnect will support it?

Yes indeed

06-26-2007, 08:13 PM
I'm surprised that it didn't blow fuses all of the time, especially if the voltage was a little low. It does say up to, what, 45A breaker?

Do yourself a favor, get a new one. They are significantly more efficient. Depending on your electric rates, you might pay for the thing in 3-4 years, depending also on the length of your cooling season. Unfortuneately, though, you'll probably need to replace the evaporator, too to gain the max efficiency as it is unlikely that the refrigerants are compatible.

06-26-2007, 08:44 PM
If the fuses are borderline as seems to be the case, during hot weather with multiple starts you will be more likely to blow fuses, due to increased ambient and heating from multiple starts. Mike had it right on installing larger fuses.

06-26-2007, 10:32 PM
20A fuses are definitely underrated, the slowblow must have been saving you. But I question why there were 20A fuses in the disconnect? Can you see what gauge the wire is?

06-27-2007, 03:01 AM
The minimum fuse size is 26.9 and you are surprised that you are blowing 20 amp fuses???? Hello, you need to install 30 amp fuses now. If you continue to have nuisance tripping with the 30 amp fuses, you can increase up to 45 amps, with a new fuse box. Do you need a new compressor? Depends on how energy conscious you are. There may be other stuff that will save you more money per capital outlay.

06-27-2007, 03:24 AM
Thanks again everyone for the input/info.

My house is a very old one, 121 years old now, and some previous owners were a little less than smart when it came to maintaining it. My wife and I are renovating the entire place, one area at a time.

My guess is that in the past someone had a fuse go and just went ahead and replaced it with another fuse lying around, not the proper size. I will up the size of the fuse now and see what that does for me in the short term.

As I said before, over the winter I got the inside of the house ready for a much improved HVAC system, which included running PEX from the basement into the attic in anticipation of installing a boiler and hyro coil for 2nd floor heating and then retrofitting an radiant on the first floor. I figured when I installed the hydro-coil in teh attic for the 2nd floor heating, I would also do a new compressor outdoors and also a new handler in. Right now I have an electric furnace heating the 2nd floor and gas forced air on the first. The electic furnace is a beast as far as energy usage goes but barely heats at all. That is our #1 energy user. So replacing both the outdoor unit and inside as well will probably save us a lot in the long term.

Broke my collarbone about a month ago so a lot of projects are on hold. I guess its a good time to start doing research on the best options for the new unit while I heal. The surgery I had to have on my shoulder to fix the bone was about $10,000, glad I have insurance, but I sure wish they would cover a new AC unit too :)

Thanks again all.

06-28-2007, 09:02 PM
you should have fallen on the AC ;)

09-11-2009, 06:07 PM
I have a similar situation. AC/Heat on the roof. Fuses blow after a couple of hours. Have been replacing fuses with FRN-R-40. But since the original post I see that the min. circuit amps 49.0 and max fuse amps 60.0. Could the reason the fuses are blowing is because they are below rated at 45 amps?
The electrician at the local hardware store recommends FRN-R-60 amp fuses.
What do you all think?


09-11-2009, 06:23 PM
Check that the fuse connections are tight.

Check your 240v supply: Measure the voltage at an unused elec. dryer outlet and turn on an elec. wall oven or all burners of an elec. cooktop.
If the voltage rises more than 0.4vac when you turn off the load you have a bad connection at, or upstream of the panel.

Given the age of your house you might want to check the neutral connection and some outlets; for this you need a 120v 10A load [toaster or hair dryer] and the DVM from above. For more accuracy you also should know if you have #12 or #14 wire, aluminum or copper.

I'd also get a clamp-on ammeter and check the unit's current draw.

The "trip curve" of the fuse has to match the unit's current draw.

On average, for a 20 yr old HVAC unit you have a 50% chance of reaching 35 YO.

09-12-2009, 08:54 AM
Looks like the 60 amp fuses are holding. I'm in Sacramento, CA so now is not a good time to be shopping for a new AC, what with it being 100+.
I will probably replace it in the fall. The AC tech also said that our returns were too small and that contributes to an overload.


09-12-2009, 03:59 PM
Important factors for your application are:

1) The wires feeding the condensing unit should be no smaller than 10awg copper.

2) The fuses should be no larger than 45A.

3) The Breaker feeding the circuit should be a 40 or 45A HACR type.

Changing to 60A fuses would be a incorrect and a safety problem.

09-12-2009, 10:10 PM
Guess I'm confused. :confused::confused::confused:
The minimum circuit amps. is 49.0 and the max fuse amps is 60.0
The margin seems to be 'tween the 2. Where would 60 amps be a safety issue?
The AC is working fine now and hasn't blown anymore. So it seems to me that the solution has been found. Am I missing something?


09-13-2009, 11:37 AM
minimum circuit amps. is 49.0 and the max fuse amps is 60.0
Where would 60 amps be a safety issue?
The AC is working fine now
So it seems to me that the solution has been found
Probably it hasn't.
Just check that 60A doesn't exceed the ampacity of the cable that connects your unit to the supply. The cable insulation temperature rise above ambient at 60A will be 1.5x that of 49A so its life may be shortened.
The high current draw may also indicate an impending failure which may call for prompt action, not so much for personnel/property safety as heading off ruining your costly compressor.

A clamp-on ammeter will pay for itself the first time you use it troubleshoot something successfully. Having one for general house/auto use is not a bad idea, but get one that can do AC and DC.

Amps is what makes things go; volts is just to force the unwilling amps into places they wouldn't normally go.

09-13-2009, 09:29 PM
Guess I'm confused. :confused::confused::confused:
The minimum circuit amps. is 49.0 and the max fuse amps is 60.0
The margin seems to be 'tween the 2. Where would 60 amps be a safety issue?
The AC is working fine now and hasn't blown anymore. So it seems to me that the solution has been found. Am I missing something?


Most likley you're fine. I think your post is being confused with the OP.
MCA=size of wire.
Max Overcurrent=size of fuse or breaker.
What guage wire is feeding your compressor?

09-15-2009, 06:08 PM
Thanks Action Dave!
I recently was told that there usually are only 1 of 2 fuses on outside AC:

35amp or 60amp.

Is that true?

09-15-2009, 06:32 PM
No .