How to test if enough current?
Printed From: the12volt.com
Forum Name: Car Audio
Forum Discription: Car Stereos, Amplifiers, Crossovers, Processors, Speakers, Subwoofers, etc.
URL: https://www.the12volt.com/installbay/forum_posts.asp?tid=67768
Printed Date: July 13, 2025 at 9:38 AM
Topic: How to test if enough current?
Posted By: trobins5
Subject: How to test if enough current?
Date Posted: December 06, 2005 at 12:40 AM
How do you test if a wire will support the load from the component you are trying to add to the circuit.
Example... If I want to tap into the lighter 12v wire for a 12v source for a cellular hands-free kit, how do I test to see if it will give me the extra current? Thanks!!!
Replies:
Posted By: dwarren
Date Posted: December 06, 2005 at 12:45 AM
I don't know of any tests. But there are guidleines for wire size and fuse size according to amp draw. Ohms law https://www.the12volt.com/ohm/ohmslaw.asp and this https://www.the12volt.com/info/recwirsz.asp. I can tell you that what you are suggesting has been many times with no problems. -------------
Posted By: trobins5
Date Posted: December 06, 2005 at 12:49 AM
I know the lighter works. How about when you see 12v on a wire and you tap it and the component doesn't turn on? Not enough current?!
Posted By: dwarren
Date Posted: December 06, 2005 at 12:51 AM
Have you checked this "12v" with a meter? -------------
Posted By: trobins5
Date Posted: December 06, 2005 at 12:53 AM
Yes, it reads 12 volts on the meter. Not a test light!
Posted By: dwarren
Date Posted: December 06, 2005 at 12:56 AM
Interesting, grab constant from the radio then?
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Posted By: stevdart
Date Posted: December 06, 2005 at 8:13 AM
trobins5 wrote:
I know the lighter works. How about when you see 12v on a wire and you tap it and the component doesn't turn on? Not enough current?!
No, because the component in question would try to pull the power it wants. If it pulls too much a fuse in the source will blow; or lacking a fuse, the wire might burn up. In this scenario, given that you have tested the wire to be 12V, either the component is faulty or not sufficently grounded. ------------- Build the box so that it performs well in the worst case scenario and, in return, it will reward you at all times.
Posted By: DYohn
Date Posted: December 06, 2005 at 9:56 AM
Wires don't limit current. As Stevdart said they simply burn up if you try to pull too much through them (or, if the circuit is wired correctly, a fuse blows.) Does whatever you are trying to power have a good ground to the vehicle's chassis? ------------- Support the12volt.com
Posted By: geepherder
Date Posted: December 06, 2005 at 6:04 PM
Tell us you're not trying to get power from a/c controls, please. This will give you problems.
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My ex once told me I have a perfect face for radio.
Posted By: trobins5
Date Posted: December 06, 2005 at 6:15 PM
sometimes A/C, why???
Posted By: geepherder
Date Posted: December 06, 2005 at 6:57 PM
That's usually the case when I hear this of this problem.
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My ex once told me I have a perfect face for radio.
Posted By: DYohn
Date Posted: December 06, 2005 at 7:14 PM
Yes, never try to power any component off anything in the vehicle that is already powered, unless it is designed for it (like a cigarette lighter or power outlet.) Trying to tap a power source off something like an A/C control or dash light is always a bad idea, for many reasons not the least of which is the vehicle's wiring is only designed for the expected loads. Plus you never know for sure when some existing load is and is not actually powered (I've even seen one person try to use their dome light to power a video system and wonder why it only worked when the doors are open. Duh.) ------------- Support the12volt.com
Posted By: trobins5
Date Posted: December 06, 2005 at 9:58 PM
I know about time outs and doom light wires are a no no. I am just looking for about a 2.4 amp draw. What about a 10ga 12v constant blower motor wire?
Posted By: DYohn
Date Posted: December 06, 2005 at 10:05 PM
trobins5 wrote:
I know about time outs and doom light wires are a no no. I am just looking for about a 2.4 amp draw. What about a 10ga 12v constant blower motor wire?
What about it? I'm not sure what you're asking. ------------- Support the12volt.com
Posted By: trobins5
Date Posted: December 06, 2005 at 10:30 PM
Well, you said to only tap into a circuit like a lighter or pwr outlet. I'm asking if I am only adding about 2 more amps to the wire that is already a 15, 20, or 30 amp fuse rating will it hurt? I don't think so because the manufacture over rates the fuse size anyways.
Posted By: DYohn
Date Posted: December 06, 2005 at 11:52 PM
Like I said, in general it is not good practice to tap into an existing load side wire, mainly because car manufacturers use the smallest size wire possible for the expected load, and your additional load may cause problems. If you are tapping into the fuse box, sure go for it you may not cause any issues, but you may cause a fuse to blow. And never change the sizes of OEM fuses. If you need to run a new power cable, do so (with a proper fuse) directly from the battery. ------------- Support the12volt.com
Posted By: trobins5
Date Posted: December 07, 2005 at 8:43 AM
Ok, what if I need a 12v constant and a switched pwr source from the center console area or the passenger kick/ glovebox area?
Posted By: stevdart
Date Posted: December 07, 2005 at 9:21 AM
Note to mods: Please retitle this thread to "Cellular hands-free install". Better than that: trobin5, I suggest you start a new thread with that title, as your original question here has been answered. Post it in the forum "Cellular and communications". Readers searching for answers will find it easier that way. ------------- Build the box so that it performs well in the worst case scenario and, in return, it will reward you at all times.
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