A question on Add-a-Circuit fuse tap amps
 

I'm adding an oil pressure gauge. I want to tap it off of cigarette lighter fuse (15 Amp) by the steering column. Question is, when I use a circuit extension, what amperages should I use in each slot? Should the total add up to 15, like 7.5+7.5 or 10+5? Or 15 for each fuse, totaling to 30 Amp?

The fuse in the fuse box for the cigarette lighter is there to protect the wire, not the load being powered by that wire.

If your gauge comes with a fuse that is smaller in current rating than the cigarette lighter fuse, then use that fuse in line with your gauge when you tap off of the lighter circuit.

Never EVER replace a fuse with one that is rated at a higher current or you risk causing an electrical fire when that wire melts because the fuse didn't blow instead.

Just to make sure I understand you....

Let's say my accessory is 10 A, and the cigarette lighter is 15A. Does that mean I should keep the 15A fuse in the lower slot and put a 10A in the upper slot of the circuit extender? I thought the fuses were connected in parallel and that the fuse amperages would add up...? No...?

You are correct on the fuse placement part, the 15A fuse should still protect the cig lighter circuit and the 10A fuse protecting the new circuit. If either the new load or cig lighter circuit began drawing too much current due to a fault of some kind in the wiring or the load itself, you want the associated fuse to pop, not the wiring from the fuse to the load.

On the other side of the fuses (fuse box side) you will see an increase in current draw due to the additional new load but that side is designed to supply lots of current.

I was under the impression that they're powered by AC, so polarity wouldn't matter. I guess I was wrong! How do I tell which end is + or -?

Fuses do not have polarity. You can't install them backwards. As such, there is no "positive" or "negative" side.

When he's talking about "sides" he's referring to sides of the circuit (i.e. pre- or post- fuse). Not the connections of the fuse itself. He's mentioning that because fuses protect only the wires that come after them, not before.

Your battery supplies 12v DC. Virtually all non-ignition voltages in cars are based on 12V DC, and the wiring is sized to carry whatever current (amperage) each circuit requires safely. Fuses are selected to carry slightly less current than the wiring, so that they blow before the wire in the case of an overdraw and protect the wiring. Fuses, switches, relays, etc., are always on the positive side of the circuit. Nothing should interrupt flow to ground on the negative side. As the above poster said, fuses don't have polarity, just as single wire does not have polarity. A fuse is just a wire designed to melt if current above a specific amount passes through it.

AC current is not used in vehicular wiring, typically. AC current from a DC battery source would require the use of an inverter.

Unless you install an inverter in your car, there is no USABLE alternating current available for you to connect to in there, it's all 12-Volts DC. (There is alternating current in some circuits like inside your alternator, inside your head unit and factory amplifier to power speakers, inside certain sensors, etc., but none of those are meant to power any aftermarket devices that you might install.)

I'm a bit confused on what you're asking. WHAT are you thinking is powered by AC? The fuse? The fuse doesn't have a plus side or a minus side, as @sharpsicle stated above, fuses are non-polar.

The fuse is simply a safety device and does not draw any power, it only limits it by blowing if the circuit that it is protecting exceeds its current rating.

Connecting your new circuit as you have shown in the diagram that you have provided should work just fine. There is another fuse in line with the main power wire to the fuse box to protect that circuit. But....keep the new fuse as close to the fuse box where you're tapping into as possible, and protect that wire between the fuse box and your new fuse from chaffing against any grounded metal surfaces and you should be fine.

**Edit: Studying your diagram a bit more, use the original 15A cigarette lighter fuse in the bottom slot and your new fuse in the top slot.**

This is professional advice.

Your diagram is correct and as others have said, fuses are not polarity conscious. That may not be the case for the load item, the gauge for instance. If the gauge is polarity conscious, + to the fused wire and - to ground.

a bit off topic, but a number of vehicles do in fact use negative switching. but the 86 isn't one of them.

How do I tell which direction I should insert the fuse tap? Is there any marking on the fuse box?

Most reliable way is to pull the fuse you're going to replace and take a multimeter to each side's connection and ground. The side that reads 12v with the fuse out is your "In" from your diagram.

So let me preface this by saying, use a meter to verify your connections.

Here's my fuse box with several taps (gauges, switches, and a rig to get 24/7 power to the cig outlet in the glovebox, blanked out to avoid confusing anyone)

My gauge is powered by the tap closest to the "bolt". There's an empty spot there that only gets power when the ignition is on. If you use this empty slot, just put 1 fuse in the tap, the one that feeds the wire. (the top blue spot in your diagram)