what is a relay?

2. About the same thing. The switching device is light weight, or in a crowded area and it is not desirable nor practical to have heavy current switching and wiring present. So a relay is used in some remote location to do the heavy work while being controlled by a light current control circuit.

Mark

Now I should have also asked about 'do I need a fuse in the line and where...' (I didnt put one in). PLease advise, STU

The fuse is usually rated for 1.5 times the normal current.....if you need 10 amps to run 2 headlights, put in a 15 amp fuse and size the wiring accordingly which is 14 awg (good rule of thumb). IF your headlights are 35watt and you have 2, and a 6 volt system, you are right at (tad over) 10 amperes [if wired in parallel...one side of each to the fused wire (via light switch) and other side of each to chassis ground]. 12 volt system is half that (if you are using 12v lamps). You could still wire up with 14 awg and use the 15 amp fuse however..... .you're protecting the wire (usually), not the thing using the power; however it does get protected also.

Now you could send a 12v wire to 2ea 6 volt 35 watt lights and wire the lights in series across the 12v input (12v hot....from the 15A fuse) to one light then other side of that light to input of the second light (not ground) and the output of that second light to ground). In that case the 14 awg wire would still be correct.

You can get fast blow and slow blow fuses. Slow blow fuses are designed to hold on higher than normal current (overload) for a few seconds.....like when a motor starts.

Even though a lamp usually has about 1/10 the normal resistance when cold, and hence a 10x inrush of normal current, it is very short in duration and in a tractor, a normal blow fuse usually is adequate.

And that's the rest of the story (Paul Harvey)

Ask another question.

This is fun.

Mark

Mark

But the whole amperage thing is about wiring insulation. The current that a wire will safely carry is based on the insulation. Just looked up Fusing current for 14 AWG wire. It's 166 amps....not a misprint, 166 amps. Fusing current is the current that melts the wire.

So you can see 15 is a long way from it. If you have a conductor inside a small pipe with a lot of other wires and have soft plastic insulation it is a lot different deal than having one single wire strung out in air with heavy plastic insulation.

Do this. Put in the fuse at 15 amperes. If it blows, you can either put in a slow blow 15. Or if your wiring is as usual on a tractror, go ahead and put in a 20 ampere. Iffin you get an overload that is that bad, you'll probably smell the insulation getting hot before you melt it to the point where you have a melted wiring harness.

For most plastic insulation that is available at the auto store 85 F rating should easily carry 20 amperes in an overload.

Besides if something shorts and overloads the circuit but isn't quite 20 apmeres, you are going to notice something is wrong and go investigate. So it won't be overloaded all that long anyway.

But if you are really concerned go rewire with either 2 14 amp ckts @15 amps or use 12 AWG wire and the 20 amp fuse.

Mark

Second, you should have an inline fuse, and I'd put it in between the switch and the power source, that way the switch is protected as well, and if for some reason the switch shorts to ground, your source (battery) is protected. Now, the size of the fuse and wiring depends upon the draw of the headlights. Wire needs to be rated at least what the fuse is, but never a bad idea to go with larger guage - but is an awful idea to go with smaller. The point of the fuse is that it is rated equal to the draw and surge of the components, never more than.

Now for a really dumb answer: A relay is where people run around a circle and hand each other a stick because they have nothing better to do. LOL, and have a nice night.