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Please forgive the novice question... one of the two 10A/250V fuses blew on the disconnect to my AC compressor unit (photo 1). I went to the big box store and they only had 20A and 30A fuses so I got the 20A and replaced the one blown 10A fuse with the 20A, so now there's a 10A and a 20A on the dual line disconnect. They're both Class RK5.

The unit is running again fine but is this okay long-term? Or should I go out and immediately find a matching, 10A fuse? I included photos of the fuses themselves and the inside of the disconnect for reference as well.

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    The specification on the unit is what you go by. Imagine that ten amps is what is called for, and is what you should use. Using a bigger fuse can cause a safety defect in the unit(fire). Fuses usually don't blow for the fun of it.
    – crip659
    Jun 7 '21 at 12:57
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    A short circuit that develops 100A should trip a 10A in around 0.1 seconds, a 30A should trip around 40 seconds. That could be the difference between replacing a motor or completely being destroyed by fire. ferrazshawmutsales.com/pdfs/TRTRS.pdf Jun 7 '21 at 13:15
  • I added a pic of the info from the compressor. It says Max fuse amps is 30. Do you have additional thoughts based on this info?
    – mikeLdub
    Jun 7 '21 at 14:00
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    Based on the edit, you should probably replace the other 10 with a matching 20 and call it a day. If it operated for (some unspecified period of time, but I assume it's far from new) on 10's, 20's are likely adequate even though 30's are allowed.
    – Ecnerwal
    Jun 7 '21 at 14:12
  • Whatever you do, be sure the fuse unit gets properly seated. I recently had one of those boxes melt. Furnace repair dude found nothing wrong with the unit the fuses were protecting. He suspects the fuse unit blades didn't get lubed (dielectric grease) and fully inserted when a fuse was replaced. Loose connections will overheat. Haven't had a problem since, so he's probably correct.
    – dougp
    Jun 7 '21 at 22:09
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The minimum circuit amps is 17.8 and the max is 30.

It's amazing that the 10 amp fuse did not blow sooner.

Replace both fuses with a fuse that is between 17.8 and 30 amps.

You could wait until the second 10 amp fuse blows if you're not interested in spending the money at this time.

If the A/C unit blows the new higher capacity fuses then it's time to get the A/C unit checked out.

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First off, check the name plate on the compressor for the minimum and maximum breaker/fuse size. A 10 Amp fuse seems low to me. Also check the breaker size in the main panel. If your unit does, in fact, call for 10 Amp fuses, you should get a replacement 10 Amp fuse pronto. Actually, replace them both because the one could have blown from fatigue. Based on new information from your edit, I'd replace both fuses with 30 Amp fuses. Normally, you wouldn't need a fused disconnect there, just your breaker in the panel sized for the AC. The installer probably had one laying around so you got stuck with it.

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  • Thanks, I also thought it was low. I added a photo of the info from the compressor and it says 30amp and the breaker is 30amp. Any further thoughts based on this?
    – mikeLdub
    Jun 7 '21 at 13:58
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    @mikeLdub I'd replace both fuses with 30 Amp fuses. Normally, you wouldn't need a fused disconnect there, just your breaker in the panel sized for the AC. The installer probably had one laying around so you got stuck with it.
    – JACK
    Jun 7 '21 at 16:26
  • @JACK Correct, if the panel breaker is HACR rated. I bought this glorified light switch for my new A/C install instead of fuses =) homedepot.com/p/…
    – MonkeyZeus
    Jun 7 '21 at 16:50
  • @MonkeyZeus You're getting fancy... mine has two "U" shaped pieces of copper in a bakelite housing. :-)
    – JACK
    Jun 7 '21 at 17:11
  • @JACK Fancy? I beg to differ! The glorified light switch was cheaper than any of the pull-out styles: fusible and non-fusible.
    – MonkeyZeus
    Jun 7 '21 at 17:14

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