Fitting a “piggy-back fuse”

Technical Area Electrics & ICE
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This is intended for information, and hopefully may be helpful to someone else. I would appreciate confirmation.

A while ago I fitted a USB socket into the speaker grille in the centre of the surface of the dash, just in front of the windshield.

I wired it into the fuse box in the cabin, situated just in front of the passenger door and used a piggy-back fuse into the AUX PWR 15amp socket.

I didn’t realise that a piggyback fuse can be inserted the wrong way round, and it does matter.

If the piggy-back fuse is inserted the wrong way round, the accessory you are trying to power may still work, but it will not be fused, and this is obviously a concern.

It’s possible to use a multi-meter to find out which side of the fuse box is the “hot side”, however I found the following very helpful.

Install the piggy-back, with its two fuses.

One of these is the original 15amp fuse, and the other is the fuse for the accessory ( in my case whatever is connected into the new USB socket in the speaker grille as above).

*If you remove only the accessory fuse

• Original circuit should still work

If you remove only the original fuse

• Accessory should stop working

If that doesn’t happen → the piggyback is reversed??

IS THIS CORRECT?

Have now turned it around, and the conditions above (*) now satisfied.

Looking for advice.

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Top photo as fitted originally.

Bottom photo, piggy-back fuse turned round (180 degrees) - this is the correct direction of insertion to ensure your accessory is fused?

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Are you sure?

My understanding is that if installed correctly and you remove the main circuit fuse, the accessory circuit should still work and it would still be protected by its own fuse.

Otherwise you are putting the load of the accessory through both fuses/circuits, which is not ideal.

The first picture looks correct to me.

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3

No, I’m not sure…….. however I was told ;

After installing:

  1. Pull only the accessory fuse
    • Original circuit should still work
  2. Pull only the original fuse
    • Accessory should stop working
    If that doesn’t happen → the piggyback is reversed.

When inserted as in first photo, and original fuse (only) removed….. the accessory still worked.

As per second photo, with original fuse removed the accessory didn’t work.

I will check it again and confirm. I agree the first photo “looks” better- but that’s not the issue really.
Thanks.

Any other thoughts from the group?

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When I said it “looked correct” I didn’t mean aesthetically, hahaha.

Curious to hear what others think.

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6

Thanks, I’ll check my cars, I have two fitted for dash cams.

7

I’m getting conflicting views about the correct way to fit this- depending on which internet site I’m looking at.

I will do more research and try and find the correct answer- I’d be grateful for expert advice.

As you will be aware, I’m no expert!

8

This is an interesting post David, as it was also my understanding that it didn’t matter regards the fuse orientation.

EDIT: due to Nick’s input below :+1:

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It should be the way you describe as the fuse is there to protect the wiring and prevent a fire. So as the original wiring is protected by a 15A fuse, if you spur off before the fuse then the original item that the fuse protects and also the new accessory can be drawing the right current for their application and fuse rating, but the total on the wiring ahead of the spur is going to be the combination of the two and potentially overload the “upstream” wiring. So yes, the accessory should be the other side of the original fuse and should go dead if the main fuse blows.

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10

OK, that does make sense Nick. So in effect the piggyback creates two fully protected circuits, one to protect the original circuit and one to protect the device being added.

So you need to find and confirm the ‘hot’ side and the ‘load’ size, with the help of the multimeter :+1:

11

So to confirm;

After installing:

  1. Pull only the accessory fuse
    • Original circuit should still work

  2. Pull only the original fuse
    • Accessory should stop working
    If that doesn’t happen → the piggyback is reversed.

    are you saying this is a true statement?

Will confirm with multimeter tonight- thanks for replys

1 Like
12

This may help clarify things…

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Think of it like a hosepipe. In the middle of the hose you have a tap (fuse). After the tap, if you have one outlet in simple terms the water out equates to the pressure of the water in the pipe. However you want two outlets. If you put the second outlet after the tap then the flow is still restricted through the tap and so the flow of water is split equally and is effectively what was the output of the single outlet is divided by the two.

If however you put the second hose ahead of the tap, the “fuse” is negated. So then if you increase the water pressure in the hose before the tap (turn up the mains pressure) you could get the flow out of the second outlet to match that of the original first outlet.

However in doing that you put far more stress on the hosepipe before the divergence of the two flows and could burst the feeding pipe.

For wires, the electrical “pressure” will just increase automatically dependant on the demand of the things attached to it. So it is really important to have the additions the other side of the fuse (tap) to protect the wiring.

If you take it to the extreme and you put 20 of these piggyback connectors on top of each other and plugged things into the outlets all at 2A. Then if they are the “hot” side of the fuse (tap) you could pull 40A down a wire designed for 15A and there would be fire. If they are the fused side and you tried to run the 20 outlets, the 15A fuse would blow and no fire.

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CHANGED IT BACK TO ORIGINAL ORIENTATION.

Looks like I may have been correct originally, and changing it around may have been incorrect?

Using a multimeter I established the “live “ side of the fuse box is the terminal nearest the door (LHS)

The fuse blade WITHOUT the wire goes to the live side. ie ……Power should enter the piggyback on the side opposite the wire.

Therefore each circuit is protected by its own fuse.

I’d really appreciate all your comments on the above.

Thanks to all

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I’ll try and check mine out tomorrow, although an ND it’ll be very similar to the NC.

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Thanks very much, and apologies for sowing the seeds of doubt.

As I have it now, in the final photo above, I think it’s installed properly (plain blade → live side):

Power enters the piggyback first (from the live terminal)

  • It then splits internally

  • Each fuse protects its own circuit independently

So if I remove the original fuse now, the accessory will still work. Which suggests to me that both circuits work independently. The other way, when the original fuse was removed the accessory didn’t work- indicating all the power was through the original fuse and none through the accessory fuse- not desirable.

Hopefully this is the correct thinking- if I’m incorrect PLEASE enlighten me.

Thanks

D.

17

I now believe what you said is correct- ie if you remove the original fuse from the piggyback, the accessory should still work.

Thank-you

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18

No worries, I’m not an expert either so it’s good to double check sometimes!

19

Good question. I wasn’t aware of this when I installed a Bluetooth adapter wired the same way. I’m off to check mine. Now where’s my multimeter.:grinning_face_with_smiling_eyes:

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20

No, you need the accessory to go dead if you remove the main fuse, not stay live. You are not protecting the accessory or the main load, fuses protect the wiring. If the item downstream of the fuse draws too much current, the fuse blows to protect the wiring, not to protect the item drawing the load. If you drop your accessory in your coffee and it goes total short circuit, the accessory is already buggered, what the fuse is doing is not allowing your car battery, which is quite capable of sending several hundred amps, straight down the wire and turning it in to a 3 bar electric heater. The fuse blows to open the circuit.

If you have both loads on the supply side of the fuse, the you could plug a cigarette lighter into the 12v socket and draw 15 amps and the new accessory an draw 5A (or what ever). Both fuses are fine because they only see the load being drawn, HOWEVER, the wire from the battery to the fuse block is seeing the total, 20A and it is not designed for it.

You must have the additional load on the fused side of the block.