Like most words we use in the US definitions can be in the eye of the beholder. I will ultimately differ to Bob Nuckolls. Allhis diagrams seem to use BUS, not BUSS. I also checked some Aircraft manuals form my previous life. For newer I used the Airbus A320 manual, they were all BUS. Just to make sure that no overseas infulence came into play I used my Convair 580 manual form 1976. again BUS. I don't know where the BUSS came from but it is defiantly in the minority.
What started this was when a friend of mine had his panel built by a very reputable firm and they used BUSS to label the avionics and main Bus/ Buss. In my every ending desire to copy/steal other peoples ideas i immediately changed mine to BUSS on the drawings. I have now changed it back.
At the end of the day all this is really good for is a argument over an adult beverage at Oshkosh. That what makes this fun.