Originally Posted by
swhatIdo
That's the rationale, nonetheless. The main breaker will trip on overload, but not always. So, in order not to make a habit of testing the main, most inspectors will err on the side of avoiding the overload in the first place. The easiest, and really most common, way to do this is to prohibit the use of tandems.
Because 15A and 20A circuits are what we use most, and what we add the most, these become the biggest culprit in overloads. No one really has a need to fill a panel with 2pole 50's--there isn't that much consumer equip that requires that kind of connection. Extra 15A and 20A ckts are another matter. We have TVs in every room now, computers, phone chargers, Playstations, DVD players, fans, heaters, hairdryers...it all adds up quick.
We used to say that no one ever utilizes all ckts to max rating at the same time, so we didn't worry that our 100A panel had a 50 for the dryer, a 50 for the A/C, a 20 for the fridge and 6 15A general ckts, far exceeding 100A by fuse rating, but we are becoming less and less able to say that.
They know we'll use the hell out of them and might burn our houses down in the event that main doesn't trip(some breakers were notorious for this--FPE). Hence tandems are frowned upon.
Don't get me wrong, if you put in a pool and your panel was full, I'd install a tandem, maybe even 2, but after that I'll suggest a small subpanel.