If any of the loads are feeding things that have electronics of any sort, they likely now have what are called "Switch Mode Power Supplies " (SMPS), which create harmonics. SMPSs will be in PCs, PLCs, monitors, instrumentation, general 24VDC power supplies, electronic ballasts, VFDs, servo amplifiers, DC drives, etc. etc. etc.. Odd order harmonics that the SMPS create will go back into the source and circulate in the neutrals. Even order harmonics cancel each other out, and in 3 phase equipment, odd orders divisible by 3 cancel each other out, leaving only the odd order non-triplen harmonics to circulate. But in single phase circuits that are unbalanced, there is no cancellation of 3rd order harmonics so they are additive too. It can get nasty.
That by the way is one reason why the code changed years ago from allowing the neutral wires being allowed to be one size smaller than the line wires. Neutral conductors were actually over heating inside of walls. When I worked at Boeing years ago and we put a lot of CAD workstations into older buildings that just had draftsmen in them before, all the newly added computer power supplies caused the neutrals to over heat. We could see the routing in the walls by the brown discoloration of the white paint. it also caused a few transformers to melt down... that's what triggered them to investigate and fix the problem. After that, no more CAD stations (or PCs of any kind) were allowed in older buildings until they were rewired.
