Why two fuses for a 5 ton residential AC?

snipped-for-privacy@noplace.org fired this volley in news: snipped-for-privacy@4ax.com:

compressor

In most areas, the fuses are redundant. The important part is that there be a manual disconnect visible from the unit (for safety in servicing). Those same disconnects are available fused or not fused, and it may be your electrician just had that one available.

LLoyd

A positive disconnect at (or "in direct view" maybe) the rotating machine is required by many/most codes.... and a hell of a good idea.

Many older ones had fuses. If the breakers are correctly sized for the wiring & AC unit; I'm not aware of any reason for more fuses. Maybe the main panel was upgraded at some point.

Replace the disconnect with an unfused one.

It is the original 200 amp SquareD service. I'm in Calif under the Uniform Electrical Code and UBC.

I'm sure the cable is adequate gauge. It is about 50 feet run. What size would be minimal? Best?

To replace the disconnect with an unfused one would involve the power company radio-disconnect box (for economy summer rates).

Would wrapping the old fuse with tin foil work? A small copper plate would be nicer - don't think I have any. How about a piece of tin from a can?

The positive disconnect - changing box is clearly an 'unattractive' idea. I understand the reason for it. In case of a fire - could an insurance claim be denied because of altering unneeded fuses?

Mike

Before going to all that work, you might try cutting power to the box and carefully cleaning and tightening the fuseholders. Make them and the fuses shiny with fine sandpaper. A poor connection can generate enough heat to blow the fuse even though the fuse rating hasn't been exceeded.

My 5-ton unit draws 19 amps per leg. So, in principle, #10 wire, or in some places, even #12 wire, would theoretically suffice. Proly #8 would make most people feel better. And, of course, the wire needs to be sized to the breaker.

I've had compressors in that same unit draw **40 A** per leg.... goodgawd.....

Why are you having to replace a 60 A fuse, which is in series with a 50A breaker? Is it overheating, as per Jim?

I wouldn't do aluminum foil. Use a copper rod. But, if the fuse is overheating, a copper rod could overheat as well.

In my case the fuse clips were ok, it was the switch contacts which got grotty with age, developed a bit too much resistance and the heat from the current flowing through that resistance got conducted through the nearby fuse clip and fried the fuses, usually opening the circuit by melting the solder joining the fuse link to the inside of an end cap and thoroughly charring the fibre fuse body so the fuse fell apart when I went to pul it out of the clips.

That kept happening in a fused disconnect n atticin the attic feeding our second floor heat pump, where high currents flowed when the auxillary resistance heaters kicked on.

I'd clean everything up real nice, smear KoprShield on contacting surfaces and still a fuse would crap out a year or so later.

I changed out the entire fused disconnect and things were ok for 2-3 years and the same s**te started again.

That's when I threw out the fused disconnect and replaced it with an unfused "pull out" disconnect. Haven't had any trouble with it in years.

Jeff

The cable is 6-3 with ground UL 600 Volt. I take that to be 6 ga. and very adequate.

This 'cleaning and tightening the fuse holders' was my original problem - years ago. My last fuse has lasted years. I still like to be rid of the fuses that appear to not be needed.

Would a thin wall 3/4inch OD piece of copper tubing be able to carry the current? How about a piece of 1/2inch (3/4"OD) galv iron pipe (my personal preference-grin) or do I really need the 3/4inch copper rod?

Mike

Why? Do they lock the disconnect?

Steel pipe would be really bad idea, ditto foil. I'd SWAG that thickwall copper pipe would work but by them, you'd have been better to replace the disconnect.

Howsomever--insurance companies seeming to be looking for _any_ excuse not to pay off on a claim, I'd play safe and continue replacing the fuses or replace he whole disconnect with a commercial UL listed one.

Cheap insurance insurance.

Can't you just wire across the fuse holders using the same gauge wire as the supply? Usually those fittings have room for two wires in the screw terminal.

Dave

Are you really hellbent on having a war with your insurance company? Remember, they're in the business of collecting premiums, not paying claims. Even though you & I both know that the pipe will carry the current, it's not UL approved, and the insurer is sure to use it as an excuse not to pay.

Reall, changing the box should be pretty simple, especially if you can just get the same model without the fuse holders. Just turn off the breaker at the main panel, and you're looking at waht, four wire connections and two pipe connections?

Just sayin' is all.

Hey PV - its a troll. Feeding into peoples knowledge to keep em going

- as you say, how come the circuit breaker doesnt trip first - their usually faster than slo-blo fuses. And for all the rest of his post - well, theres a limit to how far you can go with a troll.

Andrew VK3BFA

Not so - I let idiots (apparently) install this many years ago. Finally got some answers.

Why the 50amp does not trip first - it is clearly not a perfect world. M

On Mon, 13 Jul 2009 16:37:12 +0000 (UTC), the infamous David Lesher scrawled the following:

Maybe that's what the guy/electrician had in the truck at the time.

Right. The one I put in (saving myself $450 from the cost of the installation) was a simple disconnect. I installed the run for the condenser, the main unit, and the lights in the attic. Total: about $95, including the 250' spool of 12/3 romex, 30' of 10/3 romex, the disconnect, three lamp sockets, breakers for those, plus breakers and L6-20 sockets/plugs for three 240v circuits for the gar^H^H^Hshop. Oh, add 50' of 12/3 cord in that price for the bandsaw and dust collector.

-- Mistrust the man who finds everything good, the man who finds everything evil, and still more the man who is indifferent to everything. -- Johann K. Lavater

On Mon, 13 Jul 2009 10:15:14 -0700, the infamous Jim Stewart scrawled the following:

Erm, wouldn't the added resistance drop the voltage, resulting in the circuit exceed the current limits, Jim? Just a thought.

Is it heat, current, or are both considered the norm in a corrupted connection?

-- Mistrust the man who finds everything good, the man who finds everything evil, and still more the man who is indifferent to everything. -- Johann K. Lavater

Mike

Must be Air Conditioning season again.

So the first thing to realize is the difference in the functions of the breaker and the fuses you seem to have. The 50amp breaker is to protect the wire from the breaker panel to the AC unit itself. Or in your case the disconnect. The disconnect happens to be fused. Not uncommon. Perhaps this is all the installer had. There does need to be a disconnect in view of the AC unit. Dafety thing. Fused or not.

The breaker does need to be sized according to the capacity of the wire. A 50 amp breaker is not the correct size for #10 wire. #10 wire is usually rated at 30 amp but there is a safety factor of 20% so the maximun continuous load should not be over 24 amps. And breakers should not be oversized. If they are the breaker will not trip when the wire is overloaded. And there are breakers and breakers. Some breakers are for motors and their usually high starting currents. A good electrician should be able to straighten this situation out.

Now for the fuses. As with breaker there are fuses and fuses. Are you using the proper type of fuse? From you posting it seems to me that you need the lag type of fuse. There are several different types. Voltage ratings for fuses are important also. The installation literature for the AC unit should specify what type of fuse in the disconnect or possibility specify no fusing is needed. And fuses do get tired. But certainly not yearly. Usually fuses with AC units connected fail because the compresser or condenser fan motors are getting tired. Start to overheat and internal wear. Or a big one is dirty coils. Both condenser and evaporator. Have you checked the motor amps lately? Still in spec with the original readings taken during installation? 60 amp lag fuses should be large for a 5 ton. Particularly the newer higher SEER units. Mine are 30 amp but my SEER rating is 19. Again a service call from a real AC man should get to the heart of the fuse problem.

Don't as some suggested cheat or short out the fuse block. This is asking to die or burn or both.

Hope this helps. Bob AZ

Well, taking you at face value, and assuming you have some electrical knowledge......

1.Check that the fuses are indeed slow blow. 2.Check that the circuit breaker is indeed 50 amps - and is working. if they are as you state, the laws of physics dont apply in your particular case....

Corrosion/wear and tear/loose contacts in the fuse unit - could be - weirder things have happened - does the thing blow at switch on? (it shouldn't, slo-blo fuses are designed to handle starting loads) - stick your hand on the box, does it get warm, ditto the switch, then check it out as described by others.

And even thinking of replacing fuses with foil/copper rods etc is absolutely insane, hence my comment you were a troll - its what you would expect in the 3rd world, not where you live. The mind boggles at even thinking of doing such a grossly unsafe thing.

Otherwise, if nothing works, a prayer to whatever god you worship is your only option.....(or pay an electrician to check it out)

Andrew VK3BFA

Read the nameplate label on the air conditioining condensing unit really carefully - see what it says. Every word on there has meaning, and you have to do a bit of parsing. If it has to have fuses, the call out will not say anything about circuit breakers.

It will call for a Minimum Circuit Ampacity, and that's the minimum feeder wire and circuit breaker at the main panel.

A bit bigger on the wire ampacity is always better, look at the Locked Rotor Amps to see what the start current is - usually 10X the Full Load Amps. That's why the lights dim, it's drawing 10X current for that first half second as the motor spins up. Larger wire means lower voltage drop and a faster start, less strain on the motor windings, the compressor lives longer.

But when they call for a 'Maximum Fuse or Circuit Breaker Amps' or 'Maximum Fuse Amps' on the nameplate, you stick to that to protect the motor from a burn-out. And the Locked Rotor Amps is why you need to use time-delay fuses and/or a circuit breaker witrh a HVAC-R time delay built in.

If the nameplate calls for a "Minimum Fuse Size" it has to have a set of fuses somewhere in the feed line for protection. If the label calls for "Minimum Fuse or Circuit Breaker Size" you do not need a fused disconnect, just a safety switch within sight of the unit for servicing.

There is a good reason for the duplication - While a breaker might fail to open fast enough to protect the compressor motor if it doesn't start properly (bad start capacitor, high head pressure, etc.), the odds are much better that a set of fuses is going to open when needed.

On the flip side, the fuses are probably going to blow from old age after 10 years or so, due to all those start surges slowly weakening the elements. Them's the breaks.

The unit will probably need servicing after 10 years anyways, if you know how and are willing to make some stiff investments in education (including a refrigerant technician license) and specialized equipment you can do this yourself.

If you don't want to drop a few grand on gear and spend some time to learn how to do it yourself, you are far better off calling someone to service it anyways.

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Well, taking you at face value, and assuming you have some electrical knowledge......

1.Check that the fuses are indeed slow blow. 2.Check that the circuit breaker is indeed 50 amps - and is working. if they are as you state, the laws of physics dont apply in your particular case....

Corrosion/wear and tear/loose contacts in the fuse unit - could be - weirder things have happened - does the thing blow at switch on? (it shouldn't, slo-blo fuses are designed to handle starting loads) - stick your hand on the box, does it get warm, ditto the switch, then check it out as described by others.

And even thinking of replacing fuses with foil/copper rods etc is absolutely insane, hence my comment you were a troll - its what you would expect in the 3rd world, not where you live. The mind boggles at even thinking of doing such a grossly unsafe thing. =======================================

Well, this is sort of the thrust of this thread -- given the up-stream 50 A breakers, why would copper rod be so unsafe? Or a wire jump?

The only thing I can think of is that oddball heating/connection issue -- which a screw-type wire jump would sidestep, anyway.