Piggy back main 12v 15A supply

PaulBerkshire · Oct 9, 2018

The alarm I have has an inline blue 15A fuse. Is that ok to put a Piggy Back Fuse Holder on my main fuse (15A) constant 12v in fuse box? The Piggy Back Fuse Holder would then have 2 15A fuses, so would my main fuse then be likely to fuse(blow) if alarm had a fault as well as my alarm fuse? Logic tells me the device i am adding (alarm) should have a smaller rating fuse to the one i am Piggy Backing

Andy-1960 · Oct 9, 2018

Might be best if you draw out what you are trying to do, photograph it then post it so we can understand.

If you are trying to ask that if you put two fuses of the same size in series, which will blow first? - that is anybody’s guess.

PaulBerkshire · Oct 9, 2018

Ok. My fuse box has a fuse rated at 15A, this is where i can pick up a constant 12v supply for my alarm. The problem though is if my alarm has an inline fuse rated at 15A, is it ok to piggy back that to the main fuse in the fuse box. My concern is if alarm shorts or has a fault because the main fuse is 15A and the alarm is 15A, i could take out the vans 12v

Using one of these

rsgaz · Oct 9, 2018

It's not a good idea, no.

Are you not fitting the alarm under the bonnet? You just need a ring terminal and an inline fuse holder, directly onto the battery positive. The fuse holder should be as close to the battery as possible.

PaulBerkshire · Oct 9, 2018

I was going to fit the alarm (brains) in the cab. Maybe just extend the cable then and go direct to battery, sounds a better way.

rsgaz · Oct 9, 2018

Yes, seeing as I expect you will be running wires for the alarm sounder, i.e. you're going through the bulkhead anyway.

If you are determined to run from the fusebox, is your cigarette lighter permanently powered? The safe way to use those add-a-circuit taps is, say for example your cigarette socket has a 25Amp fuse, you would then plug in your new 15A fuse and change the fuse supplying the cigarette socket to a 10Amp, thus totalling the original.

PaulBerkshire · Oct 9, 2018

I looked at that, on a 20A fuse but not permanent live.

rsgaz · Oct 9, 2018

Any empty slots?

PaulBerkshire · Oct 9, 2018

rsgaz said:
The safe way to use those add-a-circuit taps is, say for example your cigarette socket has a 25Amp fuse, you would then plug in your new 15A fuse and change the fuse supplying the cigarette socket to a 10Amp, thus totalling the original.

But the 10A would blow before the 15A so it would never have 15A protection? I thought the main (original fuse) went in then the one needed for what you are piggybacking, ideally a lower rating so it would blow first

PaulBerkshire · Oct 9, 2018

rsgaz said:
Any empty slots?

Yes loads, not sure if anything goes to them though. Will get my thin probes out have a prod

rsgaz · Oct 9, 2018

PaulBerkshire said:
But the 10A would blow before the 15A

They're not '2 in a line' like that, they are in parallel, just like all the other fuses.

PaulBerkshire · Oct 9, 2018

rsgaz said:
They're not '2 in a line' like that, they are in parallel, just like all the other fuses.

Oh right!

rsgaz · Oct 9, 2018

There is a right way and a wrong way round to fit those...

If you put it in the wrong way, then yes, the current would sort of be 'back fed' through the 10A fuse first, round to the 15A and then to the new item. But the correct way round is the supply being fed to the left leg in the above diagram.