I'm reaching out to all the brain power on this thread.

Does anyone know the history and where (if one exists) for the official definition of "Line Side" and "Load Side"?

Countless standards such as the NFPA 70 (NEC), 70E, IEEE 1584, C37.20.2, and even electric equipment uses the terms throughout the text and most everyone who's worked in the electrical trade for length of time understands what they mean. But for the life of me, I can't find where it originated from or if there's an official definition somewhere.

However we have GE switchgear, Magne-Blast breakers and ground trucks (4.16 and 13.5 kV) that uses the term "Bus Side" and "Line Side". And as you would guess, the "BUS Side" is the supply side where as the "Line Side" is the output side to the load. Yes, the word "Line" when used with GE medium voltage switchgear is 180 degrees opposite of what we've been taught throughout our careers.

Any ideas out there?

I don't have any insight on the history, but have a comment on the subject. In a radial system, "line side" is ambiguous, since the line in question could be coming from a source or leading to a load. Suggest using "source" and "load" side for these systems. Networked systems, where flow could be from multiple directions, should use "bus" and "line" sides.

I know that it doesn't match the usual definitions, but to me, it makes sense to call the side connected to the switchgear or panelboard bus the Bus Side. The side connected to wires, either source or load, call Line Side.

To add to confusion we have a piece of switchgear with six breakers. Two connections to utility transformers, one to a generator, two used as feeder breakers with a tie breaker between two sections of bus. I don't recall how the switchgear manufacture labelled the connections for the breaker, but now I want to go look the next time we have it down for maintenance.

@Tarbaby
We too have very similar switchgear fed from many different sources, a main feed, alternate feed, emergency diesel generator feed, tie breaker and now even a secondary emergency diesel generator feed. So to your point, which side is "line" and and which side is "load" is pretty subjective depending on the line up of your equipment, switching procedure and which source is heating your bus up.