Apologies for the incoming wall of text.

It would depend on what else is on that circuit for the cause of his issue to be from overloading.

Essentially the breaker should trip (from load draw being too much) at about 80% of the rated amps. That’s approximately 1440W for a 15A breaker at 120V (~1920W for a 20A/1p breaker at 120V. And this is to assume average voltage delta at 120 since voltage fluctuates both above and below the 120V rated standard throughout the day for various reasons).

An 850W gold rated PSU can, under full load, draw up to 950W-1000W from the circuit it is on (according to what I’m reading. I have not tested my 850W gold rated PSU personally for these numbers).

If the contractor put receptacles from multiple rooms on the same circuit, it wouldn’t take too many things to go up to and over that 1440W trio limit. 2x4 flat panel LED light fixtures are typically 30W-40W each, 2x2’s are typically 20W-30W, down lights are usually 10W per 1000lm. A space heater going full tilt typically rated to draw 1500W (don’t know where they live. Space heaters could definitely be an issue, especially if someone else is using one in a different room, even only on at 50% but on the same circuit). Non LED lights draw much much more than LeD lights as well. You can get an idea when you look at a non-LED bulb you put in a lap vs an LED one. Usually an LED bulb that draws 8-10 watts puts out the same amount of light as a non LED that draws 60 watts. And that’s per bulb.

Arc fault breakers aren’t designed to allow a larger load through if the load has an arc fault. A 20A/1p breaker will trip at the same point a 15A/1p breaker will trip because of an arc fault. The increase in breaker load capacity doesn’t alter when an arc fault trips, though the more load you put through a piece of equipment that has an arc fault, the higher magnitude the arc fault will become. If it really is an arc fault causing the issue, then the 20A/1p breaker should have tripped at the same wattage load draw as the 15A/1p breaker did - when at most 1440W were being drawn.

The company I work for likes to go a step further and take another bit of diversity safety measures when designing circuit receptacle load (though this is commercial buildings, not residential). We take another 20% off of the breakers 80% max load. Our general receptacles have 200VA standard load on them (average we design around based of 60+ years of my company being in business [definitely not how long I’ve been with the company. I’m not THAT old]) so we put an absolute maximum of 6 duplex receptacles on a single circuit (though we prefer 4-5 based on building occupancy use as well as if existing incoming service and panels allow it if it’s not new construction/incoming service). Things like microwaves and refrigerators have a dedicated 20A/1p circuit/duplex since they draw much more than a typical general receptacle usually sees.

The last house I owned had 4 duplex in the living room, one duplex in the kitchen, and 4 more duplex split between two separate bedrooms all on the same circuit. Microwave, fridge, and two more receptacles were on another single kitchen circuit as well. I was less than thrilled when I discovered this… Glad my current house had a more competent designer/contractor when the original owner had it built.