Adding a circuit


Hey, i was looking at wiring some leds into my car, possibly on the dom light. I am just starting an electrical apprenticship so i understand kinda how this stuff works so i was just wondering this, if i tap into that circuit, or any other circuit, how do i know that i am not going to pull to many amps away from the circuit i am tapping into?


JMHO, but the only way a circuit should be tapped into would be if the item being added is a very low current draw one; say an additional interior light, etc. or anything with a very low current draw (say 1 amp or so).

If it’s an item that has a substantial current draw (say 3-10 amps depending on exactly what you have in mind) then you should use a relay instead and use the existing circuit to trigger the relay.

A relay requires practically no current to trigger it and can prevent some nightmarish problems from occurring.


Unless it is a very low drain, it likely will be more total than it should as a circuit should be protected (fused) not much over maximum operating load.

You could check the wire size at the dome light as it should be the smallest on the circuit and see what kind am amps the light is drawing, then check the amp draw requirement of whatever you are planning to add to the circuit and see if the tables show that as an overload.

Was this on an apprenticship test?


Car electrical is a voltage-based system. If you tap into a line that supplies 12V to anything, you’ll get 12V to the component your hooking up. The danger is the amp pull. The car system will provide 12V to all circuits off the primary tap, and adjust the amps to compensate for any additional draw.

If you tap a component that draws 30 amps into a circuit that is fused to 10 amps, a blown fuse will happen. If you re-fuse a circuit for the 30 amp draw, you’ll have an electrical fire, since the wires were sized for a 10 amp draw. They cannot safely transfer 30 amps, and the circuit will melt and burn.

If you plan on tapping into any circuits, check everything running off that circuit to insure your not overloading the amp pull of that circuit. Case in point: Adding a stereo to a car. The stereo is fused with a 15 amp fuse on the battery lead. I cannot tap into the cigarette lighter/dome light circuit, because that circuit is fused for 10 amps, and that 10 amp pull has to include the dome lights and cigarette lighter. Bumping the circuit fuse to 20 amp or 25 amp is a STRICT DON’T DO IT. My choice has been to run a new circuit from a battery tap or the battery itself. I ALWAYS use a fused connector right at the tap or battery connection. Then run that under the dash for the stereo connection.


You want to know the amperage of the proposed circuit. When you get 12 VDC LEDs from Radio Shack, they are marked with their operating amperage. If you ground EACH LED, each LED will draw that rated amperage. If you run them in series, the amperage will still be the same as one LED; power will be down.

Errata: I meant to say, the current through one item, in a series circuit, will be the same as the current in the whole circuit.
If you run only one item, its current (maybe stated by the manufacturer) will be your only concern when you supply its own power circuit.


“If you run them in series, the amperage will still be the same as one LED; power will be down.”

No it won’t. First, the LED’s are unlikely to light up because they won’t have enough voltage to trigger them, but if they did, then you have two resistances in series, that doubles the total resistance. Since the circuit voltage does not change, doubling the resistance cuts the current in half.

Each LED would get only half the circuit voltage, and circuit current would be cut in half, each LED would only get a quarter of the power that one LED got by itself. You would have to use LED’s that were designed to operate at 6v for them to work in series.


LEDs by themselves do not act at all like resistors. They are constant voltage drop devices. Most white LEDs have about a 3.6V drop, but it does vary somewhat. Of course, the LED lights that you buy for car use usually include a resistor to drop the voltage and limit the current. Keith, the stuff you say applies to normal incandescent bulbs, but it doesn’t work exactly that way for LEDs, even those with resistors built-in.


Try these 12 vdc led strip lights. There are only 120 milliamp per foot:


I knew that, that is why my first line was “the LED’s are unlikely to light up”. But they also do act like resistors in a way, just not a fixed resistance.