Hey guys,

I have wired some after market switches into my F150 (because I do not have upfitter) Wired to these switches are accessories like CB, Radar and interior lights. I havent had any issues with these switches.

Although, in my work truck, a 2011 F350, there are upfitter switches. one 10A one 15A and two 25A. I know which wires go to what switch, and I know how to ground.

I wired a CB into the 15A switch, (a cobra 29 LTD) and I havent had any problems with it. I recently bought a newer CB (cobra 29lx LCD display) and when I hooked it up, i started to smell a "burning" smell. So I unplugged it right away. I think that CB's draw between 2-3 amps.

So I guess the question that I have is, what does "25A switch" mean? Does it mean that is can handle 25A? or constantly gives 25A? Do I need a resistor before the CB?

What doesnt make sense to me is why the inline fuse on the CB radio didn't blow. Would it possibly be something wrong with the CB?

 

The quick answer is that a 25A switch is rated to carry 25 amps, maximum without burning up.

I tend to be long winded, so please forgive me when it comes to detailed explanations.

An electrical circuit consists of several components: A power supply (as in vehicle battery), the conductors carrying the electrical current, any switching that may be incorporated into the circuit and finally the "load" of the circuit (in your case, the CB, radar and interior lights). Hopefully somewhere along the way, there is over current protection included in the circuit.

The amount of electrical current in a circuit is determined by the load, subject to the ability of the power source to supply the "demanded" current.

You didn't mention the size of the wire that feeds your switch. Depending upon the length of that wire, to have a truly 25A "rated" circuit, the wire should be at least 8 gauge (20 feet maximum length). That means from the battery to the switch you should be using at least 8 gauge wire. Also it is very important to have over current protection (usually fuses, sometimes circuit breakers, rated at 25 amps). This over current protection should be as close as possible to the battery.

"Down stream" from the switch to the actual components, you simply connect the supplied power wires to the switch. Hopefully all of these supplied power wires will have their own fusing of appropriate size to the specific wire used.

The critical factor is to ensure that the total current draw of the components does not exceed the rating of the circuit. For example if the CB draws 3 amps, the radar draws 3 amps and the interior lights draw 20 amps, you are at 26 amps, or 1 amp above in excess of the rating. The result is you'll blow a fuse.

Of course some folks have been known to replace a fuse that blows with a higher capacity fuse (as in using a 30 amp fuse in a 25 amp circuit). That means they are trying to funnel more electrical current through a circuit than it is capable of handling. This is similar to a 250 pound person trying to use a step ladder rated for 200 pounds.

Excessive current in a circuit leads to excess heating of conductors. Excess heating of conductors can lead to excess visits by folks like me (I'm a fire chief).

The following is a link to a table showing recommended wire sizes for circuits based on wire lenght and current draw. Whenever I'm involved with wiring a vehicle for radios, emergency lights and the ton of other electrical stuff in our vehicles, I tend to use the next higher size wire. (I'd rather not have that professional embarassment of a fire vehicle catching fire due to skimping on wire).

http://www.engineeringtoolbox.com/am...uge-d_730.html

As far as why you got the smell and why the fuse didn't blow, it could be a couple of things. One is what you suspected - the radio might have a problem. The other is was the correct size fuse used in the wire for the CB. There's a remote chance that there were some loose connections, which can lead to overheating.

Even more remotely there could have been a nick in the wire which may have gone to ground upstream of the fuse and point of power connection which lead to overheating. (Fusing/over-current protection is effective from the fuse to the load. There isn't any protection between the power supply and the fuse, which is why it is so critical to have fuses as close as possible to point of connection to the power supply).

 

Thanks for the info!

I sent the CB back, they came back to me with "turns out the CB had a faulty/blown final transistor diode" it was replaced under warranty. Just to be on the safe side I am going to re run wire from the switch output to the device and put the fuse right at the power supply like you suggested.

As far as wire length, I am not sure how long the wires from the switch are. The switches came with the F350 stock.


Thanks again for your help. Much appreciated

 

Some times I can be a little dense. Are you talking about a F150 or F350? If you are tapping into the factory installed upfitter switches on the F350, I'm confident that the appropriate sized wire for the length was used.

If you are using the same kind of upfitter switches that are used on the F350 in your F150, a couple of things to remember. First if you indeed want a "good" 25 amp circuit from the power supply to the switch, then you'll need the correct sized wire between the power supply and switch, including a fuse near the power supply. If you tap your CB on the switch, then you'll need a second fuse at that point, since it will probably have a smaller wire going to the CB.

This might be a little confusing, but the purpose of fuses (or circuit breakers) is to protect the wire from overheating due to more current being demanded than it can safely handle. Fuses don't protect components or devices.

With all that said, it is perfectly acceptable to use a 25 amp switch in a circuit that is otherwise designed for less current. Back on the step ladder example - it is perfectly fine for a 200 pound person to be on a 250 pound rated ladder, but not the other way around.