Why is my voltage low when I connect power to fuse boc?

Hello, sounds like maybe we're missing some of the story here. I'm not sure why anyone would purposely disconnect the power wire from the fuse box. If you are saying that when the battery is disconnected, it's reading 12.5 volts, but when attached it's reading 5 volts -- I could share with you that a battery and it's reference voltage with no load may very well be at 12.5 volts -- but then drop to 5 when a load such as the ignition being in the "on" position occurs. You have not really tested the battery in an appropriate way to determine if it's good or bad. Have the battery professionally tested, not just for reference voltage, but the ability to sustain a load, and cranking amp output. www.repairpal.com/repair-shops