12 amp a/c (2001) blows 15a type sl fuses?

all originally built 1979.
circuit: circuit used only for a/c (single outlet). runs thru 20 breaker in panel , then 15 aml type sl fuse on each line (black, white) in nearby fuse box. then about 25 to 30 (possibly in concrete floor) to a/c putlet.
i've noticed original 20a breaker has been disconn, and rewired to new 20a breaker in lowest positoin in panel, suggesting the original breaker failed.
a/c: originally slightly larger a/c. when choosing 2001 replacement (physically smaller) i think it had slightly larger cooling capacity.
this are "window" a/c installed per instuctions into original wall framing, siding, etc. 12 amp sticker.
it seems to blow fuses only in this drastic heat wave. after the a/c has been running overnight (much longer than 6 hours, possibly only after 10+ hours). two recent blows occurred only after cool, 10pm and midnight.
How compressor was cycling is unknown to me. a/c is decently qiuet, and no one was expecting fuse blow, so no one was trying to listen . How a/c may have been (or not) cycling depends on thermostat setting. and thermostat is just a numbered knob :)
i've noticed the brigtht solder-like coating seen thru "window" of *both* fuses turns oxidized looking gray. only one fuse blows, so i assume both got very hot, then one blows becuse sligthly more susceptible, or hit by additional tiny transient, or whatever chose that fuse.
only once did i *witness* the a/c stop due to fuse blow. unfortunately i don't know if the fuse had blown maybe 10 or 15 min before anyone noticed a/c had stopped.
the fuses were warm (not hot) the cover plate of fuse box was warm.
Box is 4 octagonal or 4 square (more likely but can't recall for sure). box might be deeper than standard but i don't think so (again as i recall from when invesigating earlier weird fuse behavior, not incl this post)
cover is flat, but with fuse sockets and flip cover over each socket. socket partly recessed into box, but hte covers also protrude from wall obnoxiously (approx 1 inch). hopefully that's enough descriptuon (if relevant) because i can't find pic on google images. :)
the sl sockets may be adapters, but the fuses screw in tightly.
any suggestions where to look?
have questions about facts or evidence missing from my description?
i see fuses in addition to breaker for a/c compressors so i assume the sl fuses ("slow blow") are economical method to give different type wiring protection than the regular household panel breaker (not designed for motorstart circuits).
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On Tue, 25 Jul 2006 12:56:03 -0500, " snipped-for-privacy@EhOhEll.Net"

A 12 amp system on a cool day will pull 14 amps on a very hot day. And during start up, it will temporarily pull 20 amps.
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maximum official (?) temps in the news: 113 F i guess not quite record high.
this place cooks due to aged ceiling insulation. (condo complex. i have to write letters, politics, etc) probably reaches ten (15?) degrees F more than out door temps.
fortunately the wall a/c is shaded rather well.

that might be a reason why the older a/c eventually broke the original 20a breaker.
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snipped-for-privacy@EhOhEll.Net wrote:

For a wall A/C plugged into (I assume) a wall outlet, don't ken the reason for the fuse box after the circuit breaker.
Is the wiring from the fuse box to the outlet only 14 instead of 12 or (hopefully not!) is the whole circuit 14? If it isn't all at least 12, that needs to be fixed and then once it were verified it was, then the a 20 A fuse would be ok although I'd be real tempted to dump it entirely.
As someone noted and you alluded to, starting current is higher than running and who knows how they rated the unit--often times those aren't very good.
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wrote:

yes, one round grounded plug for the circuit.

no. i looked at that during the earlier problem (earlier problem: one fuse was allowing small current. very weird. i was testing across screws of the outlet, and of the fuseholder/sockets. also removed panel cover and saw the breaker wiring. so i saw all wiring). the wiring is 12

umm yeah. i wonder if an a/c can begin(?) to fail and seem to be operating correctly yet somehow be drawing more current.
i wondered, but don't think likely, if wiring could deteriorate or even have been damaged *without* reacting more normally, such as shorting betwen conductors (obvious total failure)
the weasly, in between, occasional symptoms are puzzling.
thanks for the ideas.
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snipped-for-privacy@EhOhEll.Net wrote:

Look for slightly loose connections in the fuse box. These can have higher than expected resistance and can heat up under current draw and transmit that heat through conduction to the fuse itself, increasing the chance that it will blow at a lower transmitted current than its rating.
Try opening the breaker after the A/C has been running for a while and then quickly feeling around inside the fusebox to see if anything feels unusually warm. PLEASE make sure you open the CORRECT breaker.
I've had several occurances of shits and giggles with a couple of the GE fused disconnect switches on my two heat pump systems. The switch contacts will conduct fine for a year or so, then start to develop higher than expected resistance which creates heat that gets conducted to the cartridge fuse's end caps and eventually causes a fuse link to melt.
Jeff
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(W1BSV + Brass Rat \'57 EE)
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Jeff Wisnia wrote:

...
...
Good thought but I'm still at a loss for the reason for a 15A fuse inline w/ a 20A breaker on a single outlet circuit for a small wall-mounted A/C. 15A for a 12A-rated device just isn't much overhead even w/o anything else going on. If OP is right and all is 12 ga wiring and is only the single outlet, I see no reason couldn't safefly use a 20A fuse (after checking for the above suggestion and for any other loose connections or signs of overheating, etc.
Is also possible that perhaps the base of the fuse sockets themselves (I'm assuming the screw-in type???) might be corroded, especially if box is in a basement???
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dpb wrote:

Yes, if all the wiring is 12 gauge, you could use 20 amp fuses. But, check the receptical too, there should be a current rating marked on it, and while I'm not a code mavin, if the socket is marked 15 amps it's prolly a no-no to have larger than 15 amp fuses ahead of it, huh?
But, you may find that the fusebox sockets will only accept 15 amp fuses. IIRC some of those screw in ceramic/glass fuses (Weren't they made by Buss and called Fusestats?) had different size threads so you could only install in the size that the sockets were made for.
However, if the receptical IS good for 20 amps I can't think of a reason why you shouldn't just eliminate the fuses by removing their sockets and replace them with well made splices with compression cnnectors or wirenuts, located inside the fusebox. The breaker is all you should need to protect the wiring and receptical.
Jeff
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i would use a clamp on ampmeter to check current draw, there might be a problem.
i too agree lose contacts or poor fuse holder can cause this. overheating fuses from mechanical problems can cause all sorts of fuse blows
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Jeff Wisnia wrote:

If it is a single receptacle, which is probably what is described, true. Presumably it could be changed to 20A.
The description is a fuse in the black and the white wire. If this is a 230V circuit (2 pole) that is OK. if 120V the white wire should not be fused.

Yea - Fusestats. They are type S. I don't know what a type SL is. It is possible to remove the Fusestat adapters.

If the receptacle is 20A I agree, unless the A/C unit is marked to require a 15A circuit or fuses - not likely for a plug in unit. Fuses can operate faster than a circuit breaker, but I wouldn't bet on plug fuses to be faster.
Circuits, in general, are derated to 80% for continuous operation. Continuous is over 3 hours. My understanding is that a 100A fuse, when mounted on an old 'live front' open panel board, had a rating of 100A continuous. When mounted in an enclosure, the continuous rating became 80% or 80A because heat buildup over time would cause the fuse to open at a lower load current. Your 15A fuse may open at 12A (80%) after 3 hours and be operating correctly.
bud--
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I think it more likely that it's the wrong fuse type.
A 12A device on a 15A circuit is right on the 80% point. Which _should_ be okay. If the whole circuit is 12 guage, upping the fuse to 20A is safe, but it still may not solve the problem completely.
Motors are _large_ startup surges (2-3 times or more the continuous rating), and compressors are particularly bad at this. In some cases, I'd expect older ACs to have their startup surge _increase_ over time. Lots of hard-start compressor cycling (because of hot weather) just causes the startup surge over and over, and a fuse may degrade (indeed a breaker may too).
If these are ordinary glass fuses, I strongly recommend replacing them with "fusetron" (aka "time delay") fuses. This is what you're supposed to use with motors, they're much better at withstanding startup surges.
Note that _some_ (not all by any means) heavy motor installations (eg: some fixed A/Cs) need both a circuit breaker in the panel, and a fuse block right on or next to the equipment. The breaker is there to protect the wiring and is the routine code requirement. The fuse is there based on the manufacturer's installation instructions (_not_ code), because the fuse "mimics" the motor's thermal characteristics, and is intended to protect the _motor_ from overheating in service (not fault conditions, but more of a matter of exceeding the rated duty cycle - which may be happening with our current heat wave).
Most motors have their own thermal protection built in. But some don't, and they're the ones that need the fuses.
Check the installation manual.
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Chris Lewis wrote: ...

...
But OP said this is a new wall-mounted, plug in unit. That would be _extremely_ unlikely for such a unit to need a protective fuse. I agree fully for a fixed unit and at the time the original unit was installed it may have needed it.
As you note, checking the installation manual would be _a_good_thing_ (tm)....
And, while I didn't say it explicitly the wall outlet definitely should be a 20A rated unit if were to go to a 20A fuse/breaker. Although again, if they're not presently, the slo-blo should help and might get thru the problem. I suspect, too, the supposition that the hot weather duty cycle is the culprit is the culprit...
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True, I wasn't sure of that, so I played it safe ;-)
If it's a plugin type, it doesn't need the fuse block at all. If it did need a fuse block, you couldn't get the AC UL approved.

At least in the US, reasonably recent standard wall receptacles will be rated for 20A even if the outlet is a standard 15A pattern (rather than 20A) because the code permits them to be so used.
Similarly, if the A/C is UL approved, it'll be okay _as_is_ on a 20A circuit for the same reason.
That said, on a high load/surging receptacle, it's not a bad idea to change it to a spec-grade unit. Beefier, better wire termination.
[Deteriorating connections and overly long feed wires can cause this sort of problem with high load motors. Tends to prolong and accentuate startup surge.]

If they're not currently, I'm pretty convinced that a slo-blow/TDL fuse would solve the problem.

Indeed. Everything's hot. Including the A/Cs.
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Chris Lewis wrote:

I think 15A duplex receptacles have been rated for 20A total for both for a real long time. It would probalby be hard to make a duplex outlet safe for only 15A total.
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As long as the NEC has permitted 20A general purpose outlet circuits.
Which will be at least 20 years.

It's likely that _some_ Canadian manufactured receptacles (where 20A general purpose circuits have only been permitted for the past 5-ish years) might not do well on 20A, because the code didn't permit it, and the receptacles did not need to be officially rated for it.
However, commodity electrical manufacturing being what it is, virtually all receptacles used in Canada were made to meet US specification anyway. Until recently, CSA only cared about 15A. Now they care about 20A.
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