From the point of view of CT sizing, your guideline probably makes sense, even assuming that PVAndy is correct, though certain large loads may reasonably and predictably draw more than 50% of the rated current of the circuit they are on (I am thinking of HVAC systems and electric stoves and/or ovens).

In terms of the rating of double breakers, the waterheatertimer.org link that you cite does seem more definitive, but there is one remaining inconsistency that had already struck me, and that turns up explicitly further down on that page, where there is another question and answer:

Question:

If you load a panel up with only single pole breakers can you put 200 amps on each leg?

Answer: No.

200 amps on each leg would be 400 total amps. **If you draw 400 Amps, the main breaker will trip**. {emphasis mine} A 200 Amp 120/240Volt panel has maximum 200 Amp, no matter if loads are 120Volts or 240Volts.

A 200 Amp panel has a main breaker rated (or, at least labelled) at 200 Amps - or, at least, mine does and have done in the past. By the logic of PVAndy's statements and of the originally cited answer on the waterheatertimer.org page, this should presumably mean 200 Amps **per 'side'** (since the main breaker is a double breaker), so 400 Amps (if perfectly balanced) would not necessarily trip it. I am fairly sure, however, that a panel's main breaker does trip if the **total** current through it is more than its rating. Would PVAndy like to comment on this apparent inconsistency?

I don't think its an inconsistency, I think its a misunderstanding of how Single Phase (often called split phase) circuits work. You may find this interesting as it appears to be technically correct

https://en.wikipedia.org/wiki/Split-phase_electric_powerIn a 3 wire 240 V single phase circuit, as I tried to show in some of my earlier crude diagrams There are 3 wires, L1 (Line 1), L2 (Line 2) & N (Neutral)

L1 200 A Breaker --------|

120V Loads

N ---------------------------|

120 V Loads

L2 200A Breaker----------|

Remember L1 & L2 are 180 Degrees out of phase with each other, so that the potential (voltage) between them is 240V (nominal)

If the 120V loads on L1 & L2 are Balanced (equal) there will be no neutral current, the Neutral only carries the unbalanced current.

A 200A Panel can supply 2 x 200A at 120V (400A of 120V loads) or 200A of 240V loads (remembering that the actual loading by code should never exceed 80% )

Regarding the question of a 50A 2 pole breaker being used at lower current. A good example of where this is not the case is an electric range with all burners &the oven on. Also a electric water heater.

The primary type circuits I supervise the design of daily are PV Inverter where we backfeed existing main panels. The majority of those residential inverters are backfeed into 2 pole 240V breakers running at 80% of max amperage. They use a Neutral that carries no current but serves as a voltage reference.

Also keep in mind that a breaker trip curve varies quite a bit. Breakers don't actually trip at rated current. In the event of what is called a Bolt Fault (dead short circuit) a breaker will trip very quickly. All breakers and panels have what is called an AIC rating. This is the max current the breaker is rated to trip with a bold fault. (if a Bolt Fault exceed the AIC Rating of a panel, it may explode into molten metal) . Typically for residential equipment it is 10,000 Amps. The Bolt Fault Current is determined by the impedance of the utility transformer, wire size and material, and secondary line length.

Short circuit analysis is a very complex subject ( the reference text I use is over 400 pages) If you want some interesting reading Google Short Circuit Coordination Study

Andy