Hypothetical situations / queries ... I find them annoying.
Two (2) Main breaker spaces -- per se -- may not mean what you think.
Aluminum bussed, residential panels, commonly use two (2) breaker spaces for one (1) Main breaker. This cuts the ampacity -- per pole-slot -- down in half.
This style of design is seen all over -- typically with the Main located in the center of the bus -- in a back-fed scheme All-in-One.
It consumes four (4) poles -- all ganged together -- under ONE breaker assembly.
We're off down a merry road because the test question is hypothetical -- with no actual hardware to inspect -- and likely no photo, either.
As phrased, "two main breaker spaces" can easily be taken other ways, as well. For the industry also sell short bus, high amp panels -- even All-in-Ones -- that are designed for surface mount retrofit requirements.
These permit a 'heavy-up' that doesn't disturb the old work... which becomes a sub-panel chained under this new Service. (Say 125A 240V old Service ==> 125A 240V sub-panel... with the heavy up being a 200A 240V Service. The extra capacity is then shunted off to one massive load (hot tub? barn? workshop?) or to a second sub-panel elsewhere on the structure. (home extension/ rooms added, etc.)
Then the query boils down to what is a proper design limit for this scheme.
And as noted above, it can be taken to mean a split-bus scheme.
This was particularly popular generations ago.
One such running scheme would feed all big loads from the top split bus. AND then a 'intra-panel feeder' would run from the top down to the bottom split section.
The Service would be protected by Big Fuses... Then the big (2-pole) loads would be breakered with modest devices... in the top of the split bus -- limited to the six throw rule -- including one two-pole breaker protecting/ controlling the lighting/receptacle loads -- in their multitude -- running out of the bottom of the split bus. These would not be limited by the six throw rule -- since a master breaker was installed directly ahead -- in the upper split bus.
This convoluted scheme was popular because it stayed away from high powered circuit breakers -- which were very pricy at the time.
(Even today, high power breakers get pricy, fast.)
Also, in the early days of C/B introduction, many AHJ still insisted on fuses at the Service MAIN.
These economics still exist. Notice how many commercial spaces are protected by T fuses in a NEMA3R big box -- while the interior panels are stuffed silly with branch C/Bs.
Again, this is what happens with hypotheticals: the original (test question) writer (not the original poster, here) invariably leaves out all of the real world details in his hypothetical -- because the NEMA universe is vast -- and it never entered his head that other gear fits his short description. (A specific piece of gear is never used in a hypothetical. One is left to imagine all of the possibilities -- which just go on and on.)
Some textbook hypotheticals have no correct answer -- and deliberately so. They're written to trigger a class discussion, moderated by the instructor, covering all of the different NEMA engineering schemes.
Hypotheticals also remind us of civilian descriptions of field wiring and circuit faults... for invariably they don't know what they're talking about. So when the service tech pulls up, it's "surprise, surprise."
Last edited by Tesla; 10/29/14 04:06 PM.