For those unaware:
The industry does manufacture "reducing lugs"... fittings that have a female opening suitable for bumped up wire... with a male protrusion that fits into the factory issue panel lug.
They are an elegant solution as compared to trying to stuff a tap-whip into the target panel.
They are so uncommon that you'll have to ask for them and wait. They are not normally available straight off the shelf.
Additionally, some manufacturers have trick lugs that can be bought as a field installed kit. They achieve the same result.
Sometimes it's just cheaper -- and quicker -- to up-size the target bus and leave many positions blank.
One reason to up-size conductors is to shift from copper to aluminum. The price spread -- for a long feeder -- is so great that it justifies a bump in pipe and panel bussing.
I had one boss who took twenty-years to figure that out. He saved $6,000 -- net -- on one set of feeders -- and that was before the big run up in metals prices.
As the price of juice is politically structured to only go up, the last thing one should design for is parasitic losses. For most retail consumers, a 3% voltage drop is now un-economic.
The copper industry made exactly this point years and years ago. Bumping up from #12 to #10 commonly pays for itself in less than 30 months -- even quicker in California. (Astonishing!) The one assumption is that the conductors pass current most of the business day.
The NEC is the wrong standard to design to. It wastes too much energy. Hence, one should use it as merely a minimum standard. All recent D-B contracts I've seen specify that the EC must be at least a full bump up from NEC minimums -- for every homerun conductor. (ie #10 is an absolute minimum)
