I've installed a few Reliance Pro/tran generator panels with 6, 8 or ten circuit capabilities. These come prewired with a whip designed to attach into the existing load center and splice in there. The problem I've had is depending on the existing panel, I'm pretty sure I'm exceding 75% cross sectional fill when I splice all the wires. Can any one explain exactly how to measure that? The same as box fill, I assume. I use a junction box on new installation, but on existing systems, its tough. Any ideas or experiences?