When adding additional electrical devices, one of the most commonly mentioned products is the "Add-a-Circuit" or "Add-a-Fuse" or "Fuse-Tap." They're great little things, found here for reference: Mini Fuse Circuit Tap | Fuses & Fuse Holders | Installation Supplies | Installation/Power Supplies/Misc | Super Bright LEDs
However, it seems that there is confusion about how to properly use them. Heck, even the manufacturers don't provide tech data on these. We have a few electrical experts here, so I thought I'd pose a a few questions, both for my benefit and that of others. Perhaps this is all over-thinking, but I hate to assume things, especially with electricity.
Now, as I understand it, the primary purpose of a fuse is not to protect a device. Rather, it is to protect the wire between the power source and device from excessive load, correct? I've always been told the 80% rule: fuse rating < 80% of wire gauge limit, and actual load < 80% of fuse rating. Given adequate wire size, always use the smallest fuse that will support the item being powered.
Say I want to power a device rated at 4 amps. I decide to use an Add-a-Circuit and select my Wiper fuse, because it's a non-critical device, which is rated at 10 amps. I would always retain the original fuse size in the first slot, because going less could jeopardize the original circuit. The second slot in the Add-a-Circuit would in this case be a 5 amp fuse, because 80% of 5 amps is 4 amps, and this is the smallest fuse to support.
Here are my questions for the electrically-minded folks:
1. How should one properly select which fuse location to tap for a particular application (aside from switched v constant)? I.e. when would you use or not use a 20 amp spare versus a 10 amp circuit.
2. Would the above scenario exceed the design spec of the original circuit? In other words, if I increase the load of the original circuit by 4 amps (even with my secondary fuse), am I overloading the original circuit?
3. Does the Add-a-Circuit act independently of the source location or do they work in tandem with loads and limits?
For those who know, please chime in!