Well, it's good that he deferred to an electrician, because he doesn't sound like he has a clue of how to diagnose things and shouldn't have even removed the screws from the panel. Unless he removed the wires from the equipment at both ends, he's not going to tell with his meter if the wire is shorted or not.

The usual way wires get shorted when nobody has been working on them in my experience is that they brush up against the edge of some sheet metal somewhere or pass thorugh a sheet metal hole without the appropriate bushing (or the bushing has fallen out or rotted away).

The first thing I would do would be to carefully inspect everything, first to make sure your AC hack's diagnosis is correct and then to see if I could find the point of damage. If the point of damage is in some accessible place, you may be able to repair it by just adding a junction box to cut out the damaged section of wire. On the other hand, you may end up pulling a new cable.

The base of the matter is to get a good electrician in to diagnose it.

Thank you Ron for your helpful comment.

As you pointed out, I am trying to make a decision after having another electrician thoroughly diagnose from beginning since the first electrician started from the founding of the AC personnel. Maximum-diagnosis, minimum-repair, as someone noted in one of postings.

The cable for AHU running on attic floor was separated from other cables for lights and receptacles, and seems directly coming from wall close to main service panel with couple of cable TV cables. I hope that reduce drywall work needed. The wires seems not stapled and loosely laid on attic floor over many joists and beams. Could stepping on joist with power cable on it cause short trouble like this?

Also, I found the the previous owner did some improvement around garage, very likely by himself, install a receptacle with 4 outlets on wall (with two 20 Amp breakers added in main svc panel for this) when he built a kind of man-cave in 3rd car garage compartment. And he installed small AC unit for the compartment, very likely, by himself again considering the quality of finish.

Maybe opening the garage drywall would give a clear answer or more helpful clues for this trouble and potential hazards. I'll update after another electrician's visit.

Thank you Ron for your comment again, and future comments in advance.

Kihong

I doubt just stepping on the wires (where they cross the rafters) is going to damage them and unless they're pulled tight across a wood framing member that probably isn't going to damage.

Depending on whether this space is "accessible" or not determines if just laying the cable over the rafters is OK. Accessible for attics means there are permanent stairs or ladders leading to the space.

I'm still leading to the thing passing through some metal somewhere.

Hi, Ron.

It seems the cause of trouble was found. Close to your guess, a Staple.

Today, I had another electrician for 2nd opinion. And after his visit to attic, he said the wire is ok and AHU is the cause. Wired! So we went to main service panel and found the wire is open, no short! Then I ask him to try breaker test and the breaker tripped! Wired again! And he mentioned the receptacle might be a cause and asked me pay $$$ for further tracking.

So, I paid him for today's visit and he left.

I went to the attic, and unwired the receptacle and did short test on the two troubled wires. Open! Wired the receptacle. Then I found a staple right under the junction box and check the wire under the staple. The jacket of wire was not damaged but the wires feel little play. So, I wiggle the wire a little bit while two test probes on the wires. Open-short-open-short-... Ah! I set the wire in open position and went to the main service panel and tested breaker. It stayed!

I think the wires were damaged during one of previous works, maybe someone bent it too hard during work, the house is 20+ yrs old, but anyhow managed to maintain the contact until 3 days ago. I'll call home warranty company to arrange a work to fix it tomorrow. No tear down of drywall!

However, I still curious about one thing. Why the tester showed 'Open' while the breaker tripped due to short between two wires under the staple? Maybe they have different ways of working on short condition. Maybe someone have answer.

Anyway, thank you Ron again for your comments when I desperately needed help. Hope you have a wonderful holidays.

Kihong

Would this be an AFCI breaker by any chance (does it have a test button on it)?

That is not an AFCI breaker, just a general one. But, I am very happy with the heater back. The 3rd electrician I had was very careful and through. Right before closing the main service panel cover, he checked every breaker and found one was loose and already have a half burned contact. So we replaced it too.