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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.

Scenario
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!
 

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I did want to explain this, because there is confusion about this little gizmo.
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.

Choosing a location of fuse slot is up to the needs of the person. If it's radio related use the radio fuse for example, if it's dome light related use the dome fuse and so on.

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?

All slots are rated at at least double the fuse that's in it. What this means is if you're tapping on a 20 amp fuse you should be safe with a 15 amp in the piggyback slot.



3. Does the Add-a-Circuit act independently of the source location or do they work in tandem with loads and limits?

Not sure I understand this question, but if you mean that if primary fuse blows does it affect piggyback power the answer is "NO".

Now, one question you didn't ask is about polarity. When you install the fuse tap (or "Add a circuit" as you call it) If it is upside down the primary fuse will be in series with the piggy back. Right side up and the fuses will act independantly from each other.

These 2 slots are the ones that should have the 12 volts once installed when empty of fuses

.
 

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Discussion Starter #3
So, question 3, clear as mud, yeah? That was the one I was having the most difficulty articulating. Let me put it another way. Hypothetical: say I want to tap fuse 28, rated at 2 amps. Could I add an accessory there (say, something with a 8 amp load) and fuse the piggyback side at 10 amps?

Perhaps I just have a fundamental misunderstanding of what's going on here. I'm just trying to figure out if the fuse panel is nothing more than a source or if I'm adding onto the existing load when I tap something.
 

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Never recommended to upsize the piggyback fuse. Because current might be drawn off of a relay or accessory switch. Like I said most circuits can handle double the rated fuse. You are limited by the main circuit fuse. For instance the "BCM" fuse block has 3 main supply fuses one (50 amp) of which is in the trunk and 2 others (30 & 40) coming from the underhood fuseblock.

 

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Always measure the inline DC current and fuse accordingly. For instance my footwell and cup holder LEDs drew 400ma so I fused with 2 amp fuse because it was the lowest value I could find.

---------- Post added at 12:43 PM ---------- Previous post was at 12:29 PM ----------

Maybe this is the appropriate place for this chart. It list color to amperage of fuses. BCM has mini fuses highlighted in yellow.

 
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