Discuss Piggy back main 12v 15A supply in the Auto Electrician Forum area at ElectriciansForums.net

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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
 
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.
 
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

Piggy back main 12v 15A supply fuse - EletriciansForums.net

Using one of these

Piggy back main 12v 15A supply 61YE6WCwVfL._SL1077_ - EletriciansForums.net
 
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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.
 
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.
 
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
 
There is a right way and a wrong way round to fit those...

Piggy back main 12v 15A supply addAcircuit - EletriciansForums.net

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.
 
so, learnt about the 12v and load side ( would never have thought of that) and it looks as first thought the new load being added needs to be a lower fuse rating than the one you piggy back. If i do have an empty slot that has power +12v permanent can i assume it will take 15A? If not i am going direct to battery, probably easier and safer Thanks all for your input Also just realized battery is in passenger footwell, so looks like it is out the firewall under the van and in to battery.
 
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If i do have an empty slot that has power +12v permanent can i assume it will take 15A?

Look in the manual to see what fuse would be there if your vehicle had that particular feature. If it's 15A or higher, then you're good to go.

Any items that are permanently powered in a fusebox ought to be fed via some sort of busbar supplied via a nice thick wire from the battery, but I couldn't be sure for all brands/models/etc.
 
Started installing today, where the alarm brains is going is close to the large loom at the steering column, tapped into the 12v that is +12v when ignition is on, would it be ok to use the permanent 12v on there too with the 15A inline fuse or could that damage other things? Be handy as very close to where i need the power.
 

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