Not a stupid question.
I used a multimeter.
Set to “20v”
Connect black probe to a good metal ground (eg a bolt through the bodywork)
Take out the fuse you’re considering replacing with the piggyback fuse, and “probe” the terminals inside the fuse box fuse slot with the red probe.
The side that gives 12v is the live or hot side….. the other shows little or nothing and is the load side.
An unexpected problem I don’t know I had. I believe I’ve put the dash cam. piggyback fuse the correct way round for my MX-5 but opened a full can of wriggles for my Skoda Karoq as I’m sure I could only fit the fuse one way round due to cramped space. I’ve no idea if that was the correct way round. It’s been in for 2years but now need to check. Many thanks for the informative chats and info. Great topic. ( goes off to check both veeekles)
The fuse rating should be below the current carrying capacity of the original wiring or the wiring actually becomes the fuse!
My bad, the fuse will be well above the continuous current draw of the load is where I was meant to be going with that.
(IMHO)the following is important in order to proceed safely.
So the original AUX/PWR circuit I am using, is designed to handle up to 15 A, which is why it has a 15 A fuse.
My piggyback carries the original 15 amp fuse, which protects the original circuit and a 3 amp fuse protecting the accessory circuit. In a parallel circuit, the total current ie that in the accessory circuit PLUS that in the original circuit, should not exceed the original fuse value.
In my case ( as you know) the non- wire blade of the piggyback is in the hot side of the fuse block and the blade with the wire-tail is into the load side. Parallel Circuit.
When you use a piggyback fuse, the two circuits are effectively in parallel after the fuse, but they share the same supply path up to the fuse slot. That means the original fuse still protects the wiring feeding both circuits.
The combined current must not exceed the original fuse rating (15 A in this case).
My accessory phone charger adapter/socket on the dash top speaker grille, is designed to provide either a 1 amp or 2.1 amp current, depending on which socket is used. See photo below. This accessory circuit is protected by the 3 amp fuse in the piggyback.
FOR EXAMPLE-IT WOULD BE UNWISE TO PLUG IN, SAY, A TYRE COMPRESSOR ( drawing 8-12 amps) TO EITHER OF THESE TWO USB SOCKETS, as the 3 amp fuse would blow immediately.
The tyre compressor would need to be inserted into the original AUX/PWR circuit- since this is protected by the 15 amp fuse (cigarette lighter plug).
However, even with a tyre compressor plugged into the cigarette lighter and a phone plugged into the dash top accessory socket, (even the 2.1 socket) - the total current drawn would still be less than the 15 amp limit imposed by the original fuse.
Hope this is helpful.
DC
