Relays in series?

Long story short, other than being branded lazy, bodgy or anything else, is there a reason I shouldn't use one relay to switch another?

Now for the particulars. I put a set of high powered driving lights on a vehicle, I utilised the existing unused fog light circut to power them. It was a nice neat solution that needed no extra wiring aside from a change of plugs at the point where the new lights connect to the factory harness. Problem is, despite my calculations saying it should be ok, the fuse blows after 5 or so minutes which says to me it's running right on the edge of the fuses capacity.

Now, the easy and particularly shady answer is just uprate the fuse 5 Amps and see if the problem persists. But obviously that's not the brightest idea and may result in some cooked wiring.

So, my thought was maybe I could just throw a relay in near the battery and trigger it via the existing fog light circut. This would result in next to no additional wiring, but its obviously a little redundant triggering one relay with another... Other than that, is the a reason I shouldn't?

The other option would be to unplug the fog light relay and bridge the switch wire to the output wire and switch the new relay that way.

Either way, I don't really want to run a whole bunch of additional wiring just for driving lights. Thoughts?

I don't see anything overtly wrong with your options, however I'd try ohming the wires and/or grabbing a FLIR style camera to look at the wires during that five minute slice of time.  Sounds to me that you've got something just barely unable to handle the load and as it heats up its drawing more amps to power the circuit.

Thats about the only thing that would concern me, you might be masking the problem and letting a little issue grow into a big one.

Maybe throw up your calculations on what the lights pull and what the stock ones were rated for?  See if someone catches something you missed?

In reply to Mr_Asa :

Basically I'm asking to much of the circuit as it stands. The lights are 20a draw and recommended to be on a 25a fuse where as the fog light circut is fused at 20a and would realistically only comfortably run a 15a load.

Now that I've actually asked and typed it out, I feel like I should have just pulled the relay and bridged the switch wire to the relay output and fitted a new fused power supply with heavier gauged wire and relay in the engine bay. Lets me safely use the existing wiring with minimal new wiring etc.

No problem at all.  You'll find that in a lot of vehicles, one relay may control others.

If you want to get philosophical, put enough relays in series controlling each other, and you have yourself a computer.  A very large, loud, slow computer.  And then transistors (solid state relays) were invented, and then integrated circuits, and things began to get smaller quickly.

I don't see any reason why it would be an issue other than redundancy.  Two relays which could potentially fail instead of one.

I think the really UN-lazy version would be to pull the fuse panel and solder a proper wire on the fuse terminal, but depending on the car, that could be easy or impossibly hard.  But that way you could stab in a 30A relay and a 25A fuse and be kosher.

There's nothing wrong with wiring in a relay directly from the battery and using the factory wiring to control it. Just install a fuse as close to the battery as you can. I have a Hella horn that's wired exactly like this. It draws a lot more power than the factory wiring can handle. 

Thanks folks, I couldn't think of a reason why it wouldn't work, but just having a bit of a derp moment.

I think I'll run with the removal of the original relay and make a little jump wire to use in its place so I can control the new relay and power source. Means It can easily be undone and I don't have to pull a bunch of extra wire to replicate a circut that's already there.

May I add, your solution should work, however, the best way to run driving lights is to run a relay with power from the battery, and then ground the relay operation through an arming switch, and power the relay operation from the high beams.  

That way, when armed, they go on and off with the high beam stalk, making it easier and faster to dim them when someone comes over a hill.

It's not complicated, and in many jurisdictions it's the only legal way to run driving lights.

What you are doing is basically what was the impious for the development of microcontrollers and the first computers.  So yes there should be no problem having one relay control a second relay as long as you pay attention to the rated amps of the various components.