Great question — answering as the UphillTech team:
A bathroom overhead light position commonly has 3 separate loads in it: Overhead light Fan Heat lamp or air blast heater A perfect cable source The electrician who installed this did an excellent job future-proofing, going out of his way to use the rarely-seen 12/4 or 14/4 cable. This is perfect for a bathroom overhead light, because it provides all the wires you'll ever need, in one cable: black, red and blue are "hot" wires for all 3 of the above loads. White is a shared neutral, and ground is ground of course. You can choose colors as you please, or as the bathroom switches are already wired. You can tighten the wire-nut not-so-tight to hold wires together for testing, or pop the cover...
Full answer by Harper - Reinstate Monica: https://diy.stackexchange.com/a/108155 (CC BY-SA 3.0).
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