tl;dr - Would we expect an amp drop between the SEU-AL before the disconnect and the copper going to the AC? If not expected, does this indicate an issue? Would AL wire stress a breaker more?

After having the fourth HVAC tech come out to see why my breaker is tripping, he determined that the actual AC seemed to be working correctly and then proceeded to start over checking everything in detail.

While I had visually checked the breaker (with case off), I did not remove it because I didn't want to risk mistakes. The Tech took the breaker out and when he held it, he said it was way too hot; it also stayed hot to the touch for several minutes after being replaced. I had already purchased a replacement breaker just in case, so he switched it and it was not causing an issue. From the breaker to the external disconnect box for the AC is 6 AWG SEU-AL wiring.

He took amp readings at the breaker box and then outside and was getting slightly different readings. He then took readings right at the disconnect with the wires next to them and he would observe the difference pretty much at the same time of about 5 amps.

He called a few people including his boss and an electrician with which he works. While he never saw the amps at the AC condenser ever go over the limit on the name card, nor did he see the amps on the breaker go above their trip value, his conclusion was that possibly the AL wire was in some way beating on the breaker (if the breaker just didn't go bad on its own). He checked inrush and running amps and didn't see the amp draw going too high.

We had the discussion about how copper is better, but they probably went with AL to save money, its possible safety implications (though this isn't a 70s house and this was to a disconnect, not a device), etc. For more discussions about AL being used in this situation, see this question. His final recommendation was that he didn't see anything immediately dangerous or broken, but to check/replace the breaker every few years in case there is something about this system that is causing the breaker to go bad. Maybe there is something bad with the wire in a finished part of the basement through which it travels on its run.

My questions therefore are:

  1. Would we expect that, essentially right next to each other, the amps on the AL would be like 17, while the copper would be 12? (See photo below for what I am talking about). Should the amps be consistent across the circuit or will it change based on the conductor / wiring

  2. Would using SEU-AL from the breaker to the disconnect cause more stress to the breaker? Does this make sense or is it more likely just a bad breaker, a surge happened at some point, etc.

  3. In reading about the history and code around Alumninum, I saw several things about the importance of properly torqueing the connections. The Tech seemed to really tighten everything up by hand when he saw a screw, but does this mean something else / what do I check?


Original photo was labeled wrong.

Since I have a clamp meter now, I took my own readings at photos. I will also add that while trying to clamp each running wire, when I tried to clamp the two on the left, I couldn't do it while running because the wire would move and spark at where it was screwed down on the disconnect. When the AC guy was here, there were also sparks. I am not sure if that is expected some times or also something to consider. I compared the disconnect to my other AC system and you could see the other one was nice and clean and this one had singe marks; though of course when plugging the other disconnect back in I also didn't get it quite right and it sparked.

As for the photos. I took the photos of the two on the right without disconnecting because I could clamp. For the two on the left, I pulled the disconnect each time before clamping to then get the reading. I took a few readings and the numbers were about the same on each wire. I was not sure if I needed to check something on the ground wires, but I didn't see a good way to get them. I also learned that there are amp ratings on the disconnects themselves, these are 60amp boxes if that makes a difference.

Is the box replacement a good step to consider? Is this something I can do myself if it just disconnect the power? or should I call an electrician at this point?

Is there usually some play where the disconnect connects the wires or should they be in so tight then when I move them around to clamp they should not have moved at all.

The theory is that the amps are going somewhere via the disconnect? It is not something on the AC unit otherwise it would still read the same at the disconnect, the drop would be on some split leg later. Is it possible it is just going to ground? i.e., the problem is probably "at the disconnect" or the "disconnect itself"?

Photos: Wiring labels Wires Labeled

Close up of all connections, anything obviously visually wrong? box connections

Right most (from breaker), 15.4:

Right most (from breaker)

Right center (to AC), 11.07:

Right center (to AC),

Left center, 15.17:

Left center

Left most, 15.28:

Left most

second reading, right most 15.23:

second reading, right most 15.23

second reading, right center 10.89:

enter image description here

disconnect prongs: Something that is just going to happen with pulling it in and out so many times? Or something to be concerned about. The disconnect on my other AC was perfect until I put it back it after trying to get a comparison photo and it was the same - no issues with the other unit

singe marks on the disconnect

Update 2

Based on some comments, etc. below I redid my reading today. I ensured the clamp was fully closed. Guess what my readings were on the AC line, ~15 amp. I unclamped a tiny bit, and guess what ~10 amps. Thanks for the people looking carefully at the photos! I guess the HVAC tech must have done the same thing and not noticed, so when I got the same readings on my own I didn't assume a problem with the measurements.

What an unnecessary roller coaster ride, but at least something that didn't make sense was resolved with something that did make sense.

I am still concerned about the disconnect given how beat up it is and added a few more photos. I see some burns, but probably from the arcing/sparking after the fact? I will also invest in a torque driver (cheaper than another service call leading to nonsense) and ensure everything is tight.

At the end of the day, maybe it really was just a bad breaker? Haven't had another trip since we changed it, but will need to let is run and take over the cover again to feel if it gets hot.

Photos of the readings as of a few minutes ago ensuring the clamp was done correctly:

Center-right to AC:

enter image description here

I also went from the right outside to right center ensuring the clamp was correct and got pretty close readings. Perhaps since nothing is wrong now I can assume its all okay?

For good measure photos up close of the disconnect:

screws at wires

disconnect pulled out

  • 2
    I would examine the disconnect. 5 amp difference will be caused by loose/dirty/corroded connections.
    – crip659
    Commented Jun 10 at 16:24
  • 1
    You can play around with numbers in a voltage drop calculator but for 17A (or even quite a bit more) at 240V (or even at 120V) this is going to be well under 1% drop on 6 AWG Aluminum, unless the A/C is a long ways from the house. In other words, it ain't the cable. Which means the disconnect (likely based on information so far) or some other connection or the breaker (if the problem goes away after the breaker swap). Commented Jun 10 at 16:54
  • You have pointed to the other side of the line as "copper to AC" - the two red wires are one Red AL to disconnect, one Red copper to AC, and the two black wires are one Black AL to disconnect, one Black copper to AC. If the 12A is on black and the 17A is on red, 5A from the red side is going to ground somewhere, but possibly NOT at the disconnect. Given there isn't a neutral, that's a fault to ground someplace. It's dissipating 1200W, so it should be pretty hot.
    – Ecnerwal
    Commented Jun 10 at 16:55
  • @Ecnerwal - I may have gotten it wrong with the wires; all I know it he clamped different wires, and I saw he didn't have two at a time; also watched him take reading near the contactor, etc.
    – HelpEric
    Commented Jun 10 at 18:58
  • @Ecnerwal If there's a fault to ground, it is likely only 600W being turned into heat (as each hot wire is 120V from ground, and two symmetrical leaks to ground would be awfully improbable) - but either way, enough power to eventually melt the disconnect and set it on fire. My money is on measurement error.
    – nobody
    Commented Jun 10 at 21:17

3 Answers 3


I'm sorry that you've not had a good experience with your HVAC tech. It's impossible for us across the internet to sort out all the interpretations and translations that may have occurred here -- there could be some combination of the tech watering down the explanations he offered to you, or he himself not entirely understanding the electric circuits, or even simply misunderstandings between what you heard vs what he intended to communicate. There are a few things related here that cause an electrical engineer to scratch his head and say "the way it's been told doesn't make any sense."

Copper and aluminum wire both are suitable conductors. It's critical to ensure that the terminals and connectors that are touching the aluminum are rated for such and are installed correctly, but there's nothing fundamentally wrong with using either one or both copper and aluminum in a circuit.

As Ecnerwal explained, currents entering (these are positive by convention) and exiting (negative by convention) always sum to zero. If ever current seems to be going missing it's either a measurement error or a current path was overlooked. It seems there's a good chance either or both of these things may have happened here.

Heating in an electrical device that isn't supposed to get hot always indicates a problem. Often it's a poor connection. A circuit breaker has three connections: from the breaker to the buss bar, from the circuit wire to the breaker terminal, and the switch contacts inside the breaker. Poor contact at any of these places could cause heating, which can affect the trip point of the breaker and can also cause permanent damage.

I'd replace the breaker (I see you already did). Inspect the buss bar and the wire ends for damage and ensure the new breaker is an approved brand and type as listed on the label on the panel itself. Ensure the breaker seats properly when installed and feels secure, and attach the conductors to the breaker terminals with a torque wrench set as indicated by the labeling on the breaker or panel.

If the problems continue, get an electrician out there (but maybe not the one this tech had called on the phone..). HVAC techs know all about refrigerant, air flow, and fuel gas, but an electrician who knows about wiring and motors is the specialist needed for this kind of problem.

Measurement & sparking

Four of your photos show measurements around 15 amps (note: the meter shows precision down to 0.01 amp, but we don't know how accurate it is). Two show measurement around 11 amps. Look closely at the area where the clamp meets in each of those photos -- notice that in the two that show around 11 amps, the jaws of the clamp are not properly closed. This is likely to cause measurement error. Try it again with the clamps fully closed.

Sparking should never occur at a screw terminal. 100% of the time this indicates the connection is loose and wants to start a fire. It needs to be torqued properly to eliminate the sparking; if this isn't possible then replace the defective or damaged parts.

It's normal for sparking to occur when the disconnect plug is inserted or removed under load. Pitting on the tips of the blades is evidence this has occurred. There's one that looks uncommonly bad, though, and I wonder whether that terminal may have become loose and the disconnect should be replaced.

  • @greg_hill thanks "new breaker is an approved brand and type as listed on the label on the panel itself" --> yes, same exact brand and model as before. He wasn't 100% himself which is why he called some people. "something, something, townhomes are known for using AL in this case" and the manufacturer tech support asked in the beginning when he called them if it was AL, and indicated this is a "known bad somehow". I'll get an electrician next time.
    – HelpEric
    Commented Jun 10 at 19:02
  • 1
    @HelpEric The "same exact brand and model as before" is not necessarily be the same thing as "an approved brand and type as listed on the label on the panel itself". If the wrong breaker type was installed in the panel before, replacing it with the same wrong type will not fix the issue. Non-electricians (such as HVAC installers) are notorious for using whatever breaker they have handy rather than what is actually compatible with the panel. Commented Jun 11 at 16:03
  • @StephenC.Steel All the breakers are the same brand, square d, and were installed when the house was built 5 years ago (and presumably passed inspection, but who knows). The sticker on the inside if the panel says "Schneider Square D" as well. Something else I should check?
    – HelpEric
    Commented Jun 11 at 16:39
  • No, it sounds like the breaker type is fine, since it matches the type required by the panel. This wasn't clear from your first statement. Commented Jun 11 at 20:05
  • ^^ This. The readings shown cannot possibly are correct. It is simply not possible for there to be different amounts of current on the two load wires exiting the disconnect. The partially-closed jaws of the clamp meter will cause erroneous low readings.
    – nobody
    Commented Jun 11 at 22:44

The "expected amp difference" is 0.000000000000 A. Current in = current out always. If current in appears not to equal current out, current is going out somewhere you haven't measured; or there's a measurement error.

If 17 A goes in and 12 A comes out, 5A is going somewhere other than to the AC unit. If it's not going out the wires to the AC it's either going to ground at the disconnect or going to the other line at the disconnect. You'd find out which by measuring current on the other line and current on the ground. Current on the ground should always be 0 in a system not suffering a fault.

Since neither of those things is supposed to happen, that would indicate a bad disconnect.

If the 12A measurement is not being made at the disconnect, there could be an issue between the disconnect and the point where it is being measured.

Amperage being the same in and out of a connector has nothing to do with the metal of the wires.

Torquing connections refers to finding the correct torque in the documentation for the item with the connection, and using a torque-measuring screwdriver or wrench to tighten the connector screw to that value, neither more nor less.

Side Note: The way to prevent damage to the disconnect blades and jaws is to use the breaker to shut the circuit off, then use the disconnect to ensure that nobody can turn it on while you are not looking at the breaker. That's what they are made to do, and why they are in sight of the outdoor unit - they are not really intended to be a switch. And in general the AC will be happier if you turn it off from the thermostat before you flip the breaker off.

  • thanks for the continuing support. Let me see if I can just go take a bunch of readings on my own on each/every wire and maybe some better photos. I just tried to snap while he was working for notes.
    – HelpEric
    Commented Jun 10 at 19:03
  • 3
    It shouldn't be hard to find a 1200W power sink. Or 600. Maybe if you have eg a 25W compressor on a 30A breaker, then maybe a 5A partial short somewhere, sometimes becomes 6A, 7A, etc .... something is drawing current and heating up and popping the breaker. There's nothing wrong with the breaker and nothing wrong with aluminum. They are both doing their jobs. If your guy can't find a 1200W power sink and suggests changing breakers as routine maintenance, I thiink you should at least try someone else. Focus on the first paragraph here. Power doesn't vanish., it gets used.
    – jay613
    Commented Jun 10 at 21:55
  • 5
    5 amps doesn't go missing "queitly", either there was a measurement error ("pretty much the same time" may not be good enough, really for such comparisions you want two meters so you can measure both at the same time), there was a load that is not accounted for, or something is getting bloody hot. Commented Jun 11 at 11:28
  • 2
    As suggested in Harper's answer, you can put the clamp over both red wires (or both black wires) and see if the reading is zero, or not.
    – Ecnerwal
    Commented Jun 12 at 2:29
  • 1
    @Ecnerwal turn the breaker off to pamper the disconnect? If I ever find an electrician who does that, I'll hire him to do everything. Every trade, I don't care. I'll f*ing ADOPT him.
    – jay613
    Commented Jun 12 at 11:44

Would we expect an amp drop between the SEU-AL before the disconnect and the copper going to the AC?

An impossibility due to the laws of physics. Electrons move in a circuit, in a functional circle. Current MUST be equal at all points in the circuit. Your samples are misreads: current is changing in between samples.

The only exception is if you have a ground fault, with the difference in current going to the earth or a ground wire somehow. You can't clamp the dirt, but you can clamp the ground wire.

You can test this definitively by taping both conductors to each other so they are next to each other, and putting the clamp around both of them. Equal and opposite currents will cancel out the magnetic fields and read net zero. If you see a nonzero reading, then either that's a reading error or you have a ground fault to that degree. In fact this is exactly how a GFCI works.

The Tech took the breaker out and when he held it, he said it was way too hot; it also stayed hot to the touch for several minutes after being replaced.

There are 2 reasons that will happen: a contact problem where the breaker clips onto the bus stab, almost always caused by misusing a breaker type not listed on the panel's sticker, OR, prior damage to the bus stab owing to misusing a breaker type not listed on the panel's sticker.

OR, a contact problem at the breaker's terminal screws, caused by the installer not using a torque screwdriver to tighten the screw to the stated specification.

Both are dreadfully common in HVAC installs, because 95% of HVAC installers find both rules dreadfully inconvenient, along with "pulling permits" and "only a licensed electrician can do work on another person's home".

Well, I'm disregarding a third reason: the panel being in direct sun. But this will make all breakers equally hot.

I had already purchased a replacement breaker

But how did you know which type of breaker to get? Most people look at the breaker not the panel.

We had the discussion about how copper is better,

It's not. Those lugs on that disconnect? Guess what they're made of. Most lug connectors are aluminum-body because thermal expansion differences work favorably when an aluminum lug envelops a copper wire. And what did they do in the 1960s when using small branch circuit aluminum wire? That's right. Used it on device terminals made entirely of copper/brass.

And science identified the other big culprit in those failures - Incorrect screw torque.

If an aluminum wire is going to have a problem, this will be obvious due to wire damage at a terminal. You can always pull the wire out, inspect for burning and pitting, and if none found, put it back in and re-torque with a torque screwdriver.

The real problem is when troubleshooting these things, aluminum becomes a "red herring" that distracts from the true cause.

I couldn't do it while running because the wire would move and spark at where it was screwed down on the disconnect

Wow, nobody has been using a torque screwdriver, have they? This is a showstopper right here. If moving a wire around causes this, that connection has already failed, and must be re-made.

disconnect prongs: Something that is just going to happen with pulling it in and out so many times?

Arcing on the end is what happens when you pull it under load. That is normal and I wouldn't worry unless I saw arcing on the sides.

Is the box replacement a good step to consider?

The disconnect needs to have each wire pulled off it, inspected carefully for arcing damage, put back on, and torqued to spec. Replacing the disconnect would be an expensive way to do that, and as such that might fix it, making it seem like the disconnect was at fault.

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