Connecting Fuse Tap to another Fuse location

Question

This question is similar to can i split off another circuit from fuse box to run power to cigarette lighter?, however I want to know the mechanics of mismatching amperage when it comes to fuse tapping:

Let's say I have 2 fuses in my vehicle, a 20 and 30 amp fuse:

And I want the Cigarette lighter to have power only when the radio has power, so I connect them via a Fuse Tap:

How exactly will amperage be shared among a fuse tap?

If the amplifier asks for 25 amps, will this blow the cigarette lighter's fuse? Or will each appliance receive exactly what it asks for?

Conversely, if I connected this fuse tap and cigarette lighter instead to a lower amperage, like 10, what would happen if the cigarette lighter asked for 15 amps?

Is this safe or should I specifically look for another fuse with the same 20 amps?

Answer 1

A fuse is simply a thin bit of wire (or other conductive material) that is designed to fail at a particular amperage in order to protect other elements in the circuit. The fuse doesn't regulate or provide amperage to the circuit. The base of the fuse tap pictured is protected based on whatever fuses you put in it (or it's stated max amperage).

However there is a problem with this device particularly if you use it with higher amperage as you have stated you intend to. Your radio and cigarette lighter will be protected by the fuses, but the wires feeding into the fuse block will be subject to a maximum of 50A instead of the designed 30A max. This means that if those wires are only capable of carrying 40A before overheating they could overheat and cause an electrical fire.

I am not familiar enough with the wiring leading into the fuseblock to tell you if they are typically sufficient and it likely varies from one car model to another.

Answer 2

Or will each appliance receive exactly what it asks for?

Yes

Conversely, if I connected this fuse tap and cigarette lighter instead to a lower amperage, like 10, what would happen if the cigarette lighter asked for 15 amps?

It does not ask, it takes, so it would burn the wire supplying the 10 amps, unless it is fuse protected somewhere else.

Safest bet is to connect the supply for both directly to the battery, installing fuses at the battery, then use a relay as suggested by Mike to control when those accessories get power.

Answer 3

Ignore all of this stuff about electronics "asking for" current. The fuse tap is adding a circuit to the fuse box. That's why it contains two fuses. The bottom fuse goes in line with the existing circuit, and the top fuse goes in line with the new circuit that comes off of the fuse tap.

In your diagram, the radio amplifier circuit is protected by a 30-amp fuse before, and a 30-amp fuse after. This is fine.

Your added circuit is protected by a 20-amp fuse. As long as the wire on that circuit, and the fuse tap pigtail itself, are adequate for 20 amps, and as long as the bus feeding the relay-switched side of this fuse box is adequate for 20 more amps than it was seeing before, this is also fine.