Butt splice electrical symbol
I don't often draw wiring diagrams (that's for the MechE's to figure out), but when I do, I'll usually draw boxes around every type of union to explain EXACTLY what I want. You only need to do it once per junction type, but if you have different types, make sure they look different (color, shape, and line type, if you can).

This isn't mine, but something like this:



(Image source: Equipment Quality Standards - Electrical, from California Dept. of Transportation (Caltrans))