Home AC stopped working last night...

[Snip's]

My house has two AC units... One for the seldom used upstairs and the downstairs unit for the main ground floor...
Went and checked the main circuit breaker box in the garage and the dedicated AC breaker is rated at 40 amps and had not tripped... I set the breaker to off...
Went outside and opened the cover on the disconnect box that feeds the AC unit... Inside were two 50 amp time delay cartridge fuses...
Pulled both fuses and one tested bad...
This one...


AC unit was installed in 2021...
AC Panel info...


Here are my questions...

A) Shouldn't the main breaker @ 40 amps tripped have before the 50 amp time delay fuse in the disconnect box? Or is age of the breaker and fuse having an effect?
B) Not sure if there is a benefit to having the circuit that feeds this AC using both a 40 Amp breaker and a 50 Amp time delay... My upstairs AC only has a non-fused disconnect box and a 20 Amp panel breaker...
C) AC panel notes minimum circuit Amps are 29... Seems like the 40 Amp breaker and the 50 Amp time delay fuse are quite a bit higher than they should be...
D) Should I just go with the same 50 Amp time delay fuse in the disconnect box and call it a day
E) Or... Replace the time delay fuse with a lower amp rating? 30 Amp, 35 Amp, 40 Amp, 45 Amp, 50 Amp are all available in time delay...
F) And lastly, is a "Fused" disconnect box really needed if its protected at the breaker panel?

Ideas / suggestions / recommendations would be appreciated...

Wife is getting irritable, so I'll be heading out soon to get the required stuff...

 

[OccupantRJ]

Were those rust spots on the fuse contacting the fuse clips? What do the fuse clips look like? It takes a good jolt to blow the fuse. my concern would be what went on in the unit. Look and see if the electrical contactor has visible issues or whether the wires feeding it are chafed. Sometimes contactors fall apart and short out.
if you have a VOM check from the output fuse clip to the unit frame to see if the wiring is shorted to ground.

 

[Snip's]

One thing I just noticed.... On the blown fuse there are some arc / burn marks...
I'm wondering if this was caused by corrosion due to outside elements creating bad connection with a higher resistance leading to blowing the fuse... The non-blown fuse does not have these marks...

 

[Snip's]

Shouldn't the 40 Amp in the panel blown before the 50 Amp time delay in the disconnect?

I'll get pictures of the disconnect contacts...

 

[Bert_]

Put in a new 50 amp fuse like it calls for and see what happens.

Why would you want to put in a smaller fuse? Do you like randomly blowing fuses for no reason? It's a motor, you need to account for startup current.

 

[Wrench97]

Is that disconnect from 2021 like the A/C unit or older?

 

[brewchief]

Slap a new 50 in and call it a day, check the clips that hold the fuse and make sure they grip it tightly, if they are loose it will get hot there and blow.

 

[Snip's]

Is that disconnect from 2021 like the A/C unit or older?

No... Older... Disconnect is from the old AC that was replaced...

 

[Snip's]

Fuse clips were tight in the disconnect...
No corrosion present in the disconnect...



While it was open I checked for any loose screws... all were tight...

Next up I'll be pulling the AC cover and inspecting the contactor...

 

[Snip's]

Slap a new 50 in and call it a day, check the clips that hold the fuse and make sure they grip it tightly, if they are loose it will get hot there and blow.

This was my plan to go with a 50 Amp replacement...
But wanted to ask more questions from the hive...

 

[mike93lx]

Fuses and circuit breakers trip differently. They aren't just an switch that hits as soon as the rated current is reached.

It says max fuse on the nameplate, so you need a fuse. Replace with a 50 and go from there

 

[Reborn]

As Bert suggested, your 40amp breaker is for "continuous" current. Your startup amps will be spiking up, and the fuses are presumably more useful for that.

Also, fuses wear out rather quickly, so I wouldn't overthink it. I've replaced quite a few in my time, and I'm fairly young. Last time I had a repair guy out and he said they are a wear item that last 3-5 years, for what that's worth.

 

[mattthemuppet]

Slow blow fuses are used when there’s a large start up/ in rush current, like a compressor starting up. Stops the fuse from blowing with the 2-3x start up current. I’d replace the fuse and see what happens

 

[Wrench97]

Check the contactor for burnt contacts, bad connections, check for a short to ground on the power wires on the load side of the contactor with a ohm meter, if all good replace the fuse listen to how it starts up.

 

[Snip's]

Fuses and circuit breakers trip differently. They aren't just an switch that hits as soon as the rated current is reached.

It says max fuse on the nameplate, so you need a fuse. Replace with a 50 and go from there

All is good now...
Contactor looked fine...
Bought 2 new 50 Amp time delay fuses @ $26.00 total and a second "backup set"...
Installed fuses, flipped the circuit breaker on... adjusted the thermostat...
Heard the contactor snap... After a few minutes the heater in the compressor reached temperature and kicked on...
AC is working now... Wife is happy... No service call needed for the fix...
And apparently fuses are considered a "wear item"... This, I did not know...
They must wait and go bad during severe Heat Domes, in particular...

Thanks all...

 

[Wrench97]

All is good now...
Contactor looked fine...
Bought 2 new 50 Amp time delay fuses @ $26.00 total and a second "backup set"...
Installed fuses, flipped the circuit breaker on... adjusted the thermostat...
Heard the contactor snap... After a few minutes the heater in the compressor reached temperature and kicked on...
AC is working now... Wife is happy... No service call needed for the fix...
And apparently fuses are considered a "wear item"... This, I did not know...
They must wait and go bad during severe Heat Domes, in particular...

Thanks all...

Times of high load.

 

[Snip's]

Times of high load.

Agreed...
I'm still surprised the 40 Amp breaker didn't trip before the 50 Amp time delay...
Maybe the 50 Amp fuse had aged to something less than what was needed to trip at 40 Amps... Don't know...

 

[Wrench97]

I think the arcing on the side of it is the story, possible low voltage in the high demand peak time and a failing connection caused it to fail.

 

[wyliesdiesels]

My house has two AC units... One for the seldom used upstairs and the downstairs unit for the main ground floor...
Went and checked the main circuit breaker box in the garage and the dedicated AC breaker is rated at 40 amps and had not tripped... I set the breaker to off...
Went outside and opened the cover on the disconnect box that feeds the AC unit... Inside were two 50 amp time delay cartridge fuses...
Pulled both fuses and one tested bad...
This one...

that one has arc marks on it. there was a bad connection there.

Here are my questions...

A) Shouldn't the main breaker @ 40 amps tripped have before the 50 amp time delay fuse in the disconnect box? Or is age of the breaker and fuse having an effect?

nope. totally dependent on the trip curve of the breaker and fuses as well as the nature of the fault.

B) Not sure if there is a benefit to having the circuit that feeds this AC using both a 40 Amp breaker and a 50 Amp time delay... My upstairs AC only has a non-fused disconnect box and a 20 Amp panel breaker...

fuses are required per the nameplate. max fuse and breaker size is 50a but that doesnt mean you cant go lower.

I would check the nameplate on the upstairs unit and make sure it doesnt need fuses as well

C) AC panel notes minimum circuit Amps are 29... Seems like the 40 Amp breaker and the 50 Amp time delay fuse are quite a bit higher than they should be...

nope again. you size wire to the MCA on the label, in your case #10 wire, and you size the fuse and breaker to to the MOCP on the label. but since it says max, you can go lower.

the reason the fuses and breakers are rated for higher current than the wire is because of in-rush current on startup which can be 4x-8x FLA. look for the LRA- locked rotor amps, rating on the label

D) Should I just go with the same 50 Amp time delay fuse in the disconnect box and call it a day
E) Or... Replace the time delay fuse with a lower amp rating? 30 Amp, 35 Amp, 40 Amp, 45 Amp, 50 Amp are all available in time delay...

stick with the 50a fuses

F) And lastly, is a "Fused" disconnect box really needed if its protected at the breaker panel?

yes because the label calls for fuses

 

[wyliesdiesels]

All is good now...
Contactor looked fine...
Bought 2 new 50 Amp time delay fuses @ $26.00 total and a second "backup set"...
Installed fuses, flipped the circuit breaker on... adjusted the thermostat...
Heard the contactor snap... After a few minutes the heater in the compressor reached temperature and kicked on...
AC is working now... Wife is happy... No service call needed for the fix...
And apparently fuses are considered a "wear item"... This, I did not know...
They must wait and go bad during severe Heat Domes, in particular...

Thanks all...

what heater are you referring to?

 

[brewchief]

Fuses and circuit breakers trip differently. They aren't just an switch that hits as soon as the rated current is reached.

It says max fuse on the nameplate, so you need a fuse. Replace with a 50 and go from there

It says max fuse 50 then the next box over says max breaker 50 so either is fine.

 

[Snip's]

what heater are you referring to?

Scroll compressor crankcase heater...

 

[Snip's]

It says max fuse 50 then the next box over says max breaker 50 so either is fine.

But in my case, both were installed... So does the sticker imply both are required or one or the other?

 

[Wrench97]

One or the other but you still need a disconnect by the equipment.
It's not unusual to see both on commercial equipment.

 

[Innovate1]

Scroll compressor crankcase heater...

I'm not sure exactly how the control for the heater works but I seriously doubt it should be coming on in your location this time of year. It should be plenty warm. They are meant for cold weather. Likely just a startup delay to prevent the compressor from staring against short cycle pressure.

 

[Snip's]

One or the other but you still need a disconnect by the equipment.
It's not unusual to see both on commercial equipment.

Yes, both AC units have disconnects by the equipment (code requirement)...
However, my seldom used upstairs AC unit disconnect does not have the time delay fuse, only 20 Amp panel breaker...
Was thinking of changing out that disconnect to one with time delay fuses...

 

[wyliesdiesels]

Yes, both AC units have disconnects by the equipment (code requirement)...
However, my seldom used upstairs AC unit disconnect does not have the time delay fuse, only 20 Amp panel breaker...
Was thinking of changing out that disconnect to one with time delay fuses...

check the nameplate on the unit to see if its required. may not be

 

[reader2580]

Slap a new 50 in and call it a day, check the clips that hold the fuse and make sure they grip it tightly, if they are loose it will get hot there and blow.

It is not considered good practice to just throw a new fuse in and let it go. A good HVAC person will try to figure out why the fuse blew first. (Many HVAC people are not good, or their boss/owner forces them to take shortcuts and do the minimum.)

 

[Bert_]

It is not considered good practice to just throw a new fuse in and let it go. A good HVAC person will try to figure out why the fuse blew first. (Many HVAC people are not good, or their boss/owner forces them to take shortcuts and do the minimum.)

You worry when it blows the second time.

Little fuses like this 50 are cheap enough I'll encourage people to put one in and see what happens. If somebody's paying for a service call then I'll pull out the amp clamp.

 

[sparky 1971]

You worry when it blows the second time.

Little fuses like this 50 are cheap enough I'll encourage people to put one in and see what happens. If somebody's paying for a service call then I'll pull out the amp clamp.

I've shown up with a fist full of fuses more than once but usually it's a freak deal and a new fuse fixes the problem. There are also the times where one fuse won't let enough smoke out, gotta sacrifice a couple of them to figure out what's wrong.

 

[dscheidt]

Agreed...
I'm still surprised the 40 Amp breaker didn't trip before the 50 Amp time delay...
Maybe the 50 Amp fuse had aged to something less than what was needed to trip at 40 Amps... Don't know...

No. Fuses do not age. (well, except from corrosion.) Their current carrying capacity doesn't change over time. they do not wear out, except by blowing on over current.
They're just a piece of wire or solder that melts, and possibly a spring (and for a 50A cartridge, probably some sand to quench the arc), there are no moving parts to wear out. Whoever told you they're a wear item is wrong, it's pretty common to find the original fuses in 50 year old disconnects. It could be modern disconnects are junk and don't have good tension and arc, but that's not the fuse wearing out, that's **** quality fuse holders.

The fuse blew because the trip curves are different. a 40A breaker will pass 50A for quite some time. I'm too lazy to look at a trip curve, but a couple minutes at 50A, and several seconds at 80A, while the fuse will blow within a second at 80A.

 

[Innovate1]

No. Fuses do not age. (well, except from corrosion.) Their current carrying capacity doesn't change over time. they do not wear out, except by blowing on over current.
They're just a piece of wire or solder that melts, and possibly a spring (and for a 50A cartridge, probably some sand to quench the arc), there are no moving parts to wear out. Whoever told you they're a wear item is wrong, it's pretty common to find the original fuses in 50 year old disconnects. It could be modern disconnects are junk and don't have good tension and arc, but that's not the fuse wearing out, that's **** quality fuse holders.

The fuse blew because the trip curves are different. a 40A breaker will pass 50A for quite some time. I'm too lazy to look at a trip curve, but a couple minutes at 50A, and several seconds at 80A, while the fuse will blow within a second at 80A.

Gotta disagree. In most cases it isn't an issues but in high inrush applications like motors and rectifier/bus capacitor front ends for other equipment the high current can cause some stress and flex on the element and this can fatigue and eventually fail.

 

[sparky 1971]

Gotta disagree. In most cases it isn't an issues but in high inrush applications like motors and rectifier/bus capacitor front ends for other equipment the high current can cause some stress and flex on the element and this can fatigue and eventually fail.

I agree. It's not an every day thing but I've been doing service type work for the last 24 of my 30 year career and there have quite a few calls where changing a fuse took care of the problem. The only explanation I could come up with was that the fuse was worn out. It's usually been on motors but there have been exceptions to that rule also.