[diag - XLR4/cig/DC power plug]
We use a Canon 4-pin XLR connector. This is actually a broadcast standard for 12v power; p1 is used for 0v, p4 for 12v. XLR connectors are rated at 10A per pin, so for amplifiers which take higher currents we connect p1 & p2, and p3 & p4. The have the advantage that they cannot be connected incorrectly, and also lock together, avoiding cables falling out. I use at least 15A mains cable for power cables, as being easily obtainable; these lead to some voltage drops in longer lengths, but like a lot of things, it's very much a compromise between efficiency and practicality.
The car cigarette lighter connector is often used on 12v equipment, often built in (in particular power supplies, phone chargers and the like); they are are also useful for compatibility, allowing equipment to be used in a car if necessary. We don't fit them as standard on batteries or elsewhere as power outlets, as if left live, if something metal falls into them they could easily short out the system. We use 4 pin XLR adapters to cigarette lighter outlets where required.
2.1mm DC plugs - a lot of domestic equipment uses small DC power plugs, often the 2.1mm centre pin size, positive centre. It's best to make sure everything has a standard connection; some items use a nagative centre connection. It's easy enough to modify the equipment to the same standard, adding an internal voltage regulator where necessary.
[diag - XLR 3/jacks/phono]
3 pin XLR