Wiring in +/- into car fuse (piggyback)

[ChrizK]

Once again I am using the Engineering chat as I think my question is more theoretical rather than 'diy advice'. If I am misusing the forum, please let me know.
This question is back 'in the car' again, but I think it is a fundamental question about 'how electricity works', more so, a circuit (as I understand it, the flow of negative charged electrons to a positive terminal).
My project is to install another car accessory, but I don't want to use the cigarette lighter as I am fed up with wires spewing from the console. As with hardwiring kits, I want to piggyback into the fuse board, thus hiding my 'wiring in'. Given just about everything has a USB-A plug at this end, I am surprised there isn't an off the shelf 5V 'female USB-A to piggyback fuse' product.
There are a few hard wire USB sockets, but they all appear to have two wires, instructed to connect to the positive and negative terminals of a battery.
Most are styled like 'cigarette lighter barrel', but I like the simple version, picture included (my only hesitation being that I can't find a reputable manufacturer).
So, how do I wire in a positive and negative wire into a generic piggyback fuse? Not wishing to insult you guys, but I have included some pictures for clarity (perhaps more so for any others viewing this question).

Can I simply twist the positive and negative together, then crimp directly onto the single wire coming from the generic piggyback fuse?

Bearing in mind my total lack of understanding of electricity, and therefore not knowing if it is a really stupid question, I will attempt to explain my reasoning ...
TBH I haven't given much thought to circuits when using hard wiring before, but it has occurred to me that the fuse has two spades, which I am guessing are inserted into positive and negative 'sides' of the fuse board. My question comes from the fact that the piggyback fuse has a single wire, used to connect the accessory that it is powering. As the externally powered accessory must have a positive and negative side, it makes me think that the accessory (in this case the USB socket) doesn't actually need two wires, and perhaps the electrons flow to the right terminal ... being the reason I am posting here, to ask you clever people 'how it works' (as well as 'how to do it').

 

[DPG]

No, don't connect the positive and negative wires together. The negative 'ground' connection would normally be taken from a chassis connection somewhere - possibly a ring terminal from a bolt.

Only the positive comes from the fusebox. In your case it is from the additional fuse in your piggyback adaptor.

And just to confirm firm, your USB socket does indeed need a positive and negative connection.

 

[ChrizK]

No, don't connect the positive and negative wires together. The negative 'ground' connection would normally be taken from a chassis connection somewhere - possibly a ring terminal from a bolt.

Only the positive comes from the fusebox. In your case it is from the additional fuse in your piggyback adaptor.

And just to confirm firm, your USB socket does indeed need a positive and negative connection.

That completely blows my understanding of +/-.
OK, so negative is ground.
I thought Negative was the path/connection to the negative terminal of a battery ...actually, the provided illustration of the wiring for the USB step-down shows the battery.
If you don't mind asking a real basic question ...does that mean the negative side of a battery is simply a grounding?
An extension of the question being, does that mean the electrons are simply disposed through ground, once they have powered the device/accessory?
I think my understanding is worse than that, being that a battery provides 'power' by passing negatively charged electrons from the negative terminal > powering the device > with the electrons flowing 'back' to the positive terminal. If the fuse is actually providing the 'power' I would guess it should be connected to the negative wire, but perhaps the terminology swaps from the 'consumer side'.

You have answered my question, thanks again, but just to make sure ... I can ground the negative wire by simply bolting it (anywhere) to the chassis of the car. My only doubt being you used the term 'ring terminal', which conjures the thought of a special bolt/terminal located somewhere!

 

[DPG]

Thanks again for your quick answers. That completely blows my understanding of +/-.
OK, so negative is ground.
I thought Negative was the path/connection to the negative terminal of a battery ...actually, the provided illustration of the wiring for the USB step-down shows the battery.
If you don't mind asking a real basic question ...does that mean the negative side of a battery is simply a grounding?
An extension of the question being, does that mean the electrons are simply disposed through ground, once they have powered the device/accessory?
My perception was that the negative terminal 'returned' the electrons back to the battery ... embarrassingly further highlighting my complete ignorance!

You are mostly right yes. The circuit goes from battery, via the accessory, and back to battery. You need a complete circuit for current to flow.

On a car the negative terminal of the battery is connected to the chassis, and so accessories can be connected with their negative to the chassis, rather than having to return each one to the negative terminal of the battery.

It's referred to as ground in automotive systems, but it is not the same idea as a ground/earth connection in a household electrical system. The household earth connection is a protective connection rather than a return one.

Hope this makes sense!

 

[ChrizK]

On a car the negative terminal of the battery is connected to the chassis, and so accessories can be connected with their negative to the chassis, rather than having to return each one to the negative terminal of the battery.

Ahhhhhhhh, that makes sense ...thank you so much for the information.
...however, I now imagine sitting in my car, with electrons buzzing through the framework, charging me as I put my feet on the pedals ...

 

[DPG]

Ahhhhhhhh, that makes sense ...thank you so much for the information.
...however, I now imagine sitting in my car, with electrons buzzing through the framework, charging me as I put my feet on the pedals ...

Remember the brake is negative and the accelerator is positive. Only kidding

 

[ChrizK]

On a car the negative terminal of the battery is connected to the chassis, and so accessories can be connected with their negative to the chassis, rather than having to return each one to the negative terminal of the battery.

Hi, sorry ...additional question(s) if you don't mind.

https://www.amazon.co.uk/dp/B08W21WSTH?psc=1&ref=ppx_yo2ov_dt_b_product_details

The wires connecting this step down are not only incredibly thin (is that referred as a high gauge, or low?), but also short.
I will need to extend the negative/earth.

I have now entered into the world of AWG, which I understand to be complicated by length, ampage, and if the devices are in-line ...

My straight forward question (?) is, would 14AWG be sufficient for an extension of ~1m (tops)?

And, out of interest, a car fuse board (internal) has several fuses with different rating (5,7.5,10, 15, 25 amps). Again, showing my ignorance, does that mean I should only expect 5amps to be 'supplied' where a 5amp fuse is inserted?
(or does the fuse merely protect a 5amp device from pulling too much power from the board which is capable of more?)
Given it is 'standard practice' to use a piggy back fuse, if I want to power a 5amp device, should I select an existing 5amp to piggy back from?
And, given the complication of device configuration when considering AWG, are all devices in-line from a fuse board?

Would really appreciate thoughts on suitable AWG, the rest is just an interest in how things work.

 

[Avo Mk8]

AWG is American Wire Gauge!
You could work with SWG (Standard Wire Gauge) and be more patriotic!
The higher the number, the thinner the wire.
There are lots of tables on the interweb giving metric equivalent cross-section of AWG and SWG, which is what many tend to use these days.

14AWG has a cross section of about 2 sq mm, good for 20Amps+, so rather overkill for the approx. 1Amp that the USB converter will take (from your battery when the USB fully loaded)
I would suggest wire (flex) that is somewhat thinner, say 1.00 sq mm or 0.75 sq mm

In the latter part of your post, you mention a 5A device. I presume you are not talking about this USB adapter, because that won't take much more than 1A (15W apparently)
If you need to take 5A, you need at least a 5A fuse, and preferably one that is not already feeding a device requiring up to 5A!

 

[DPG]

Avo bet me to it! And with a good answer as always.

 

[Avo Mk8]

Avo bet me to it! And with a good answer as always.

You are too kind.

Please note I failed to answer any of the other interesting queries towards the end of ChrizK's post!

I started writing about selecting wire gauge for the load current (or working out the load current from the power consumption) so that there won't be excessive voltage drop, then selecting the fuse to protect the cable, and then revisiting what if any sensible current limitation there should be in case of a fault condition in the electronics of the load, eg considering fire etc.

But my written ramblings were so incomprehensible (as above) I cancelled the post.

Feel free !

 

[ChrizK]

AWG is American Wire Gauge!
You could work with SWG (Standard Wire Gauge) and be more patriotic!
The higher the number, the thinner the wire.
There are lots of tables on the interweb giving metric equivalent cross-section of AWG and SWG, which is what many tend to use these days.

14AWG has a cross section of about 2 sq mm, good for 20Amps+, so rather overkill for the approx. 1Amp that the USB converter will take (from your battery when the USB fully loaded)
I would suggest wire (flex) that is somewhat thinner, say 1.00 sq mm or 0.75 sq mm

In the latter part of your post, you mention a 5A device. I presume you are not talking about this USB adapter, because that won't take much more than 1A (15W apparently)
If you need to take 5A, you need at least a 5A fuse, and preferably one that is not already feeding a device requiring up to 5A!

Haven't spotted SWG (I did wonder why 'everything' seems to quote an American standard).

Apologies again, I should have looked at the step-down again, it actually says 15W DC/DC Converter, IN 12V, OUT 5V3A on the label. I am hoping it is 3Amp out as I think that is reasonable for USB.

Many thanks for the advice. I think your recommendation is close to AWG 18 (which appears to be SWG 19, if I have read a chart properly)

 

[Avo Mk8]

I thought it was normal practice to use flexible cables in vehicles, not solid conductors (cos of vibration etc)
So not a single core copper wire, but flex with the same cross section?

 

[DPG]

Yes, definitely use stranded wire and not solid core.

 

[ChrizK]

I started writing about selecting wire gauge for the load current (or working out the load current from the power consumption) so that there won't be excessive voltage drop, then selecting the fuse to protect the cable, and then revisiting what if any sensible current limitation there should be in case of a fault condition in the electronics of the load, eg considering fire etc.

Actually, would you mind telling me if I can mix different rated fuses within the piggyback/fuse tap? I think the lowest I have installed on the board is 5Amp, but you have pointed out that the device/step down, is only 1Amp (ish). To protect the rather high gauge wire, would it make sense to insert a 2Amp (lowest I have found) alongside the original?
My theory ...hah, so wrong, so many times .. that the original fuse is one circuit, the additional fuse being another.

And, to give an overall understanding, could I mix a 10Amp original with a 2Amp tap .... or even a 5Amp original with a 10Amp tap?
(wondering if the original 'circuit' would influence the tap)

 

[mainline]

Actually, would you mind telling me if I can mix different rated fuses within the piggyback/fuse tap?

You can use various size fuses branching off the supply to the original fuse, that cable must be able to handle the total load.

I think the lowest I have installed on the board is 5Amp, but you have pointed out that the device/step down, is only 1Amp (ish). To protect the rather high gauge wire, would it make sense to insert a 2Amp (lowest I have found) alongside the original?

5 amp will be fine.

 

[ChrizK]

You can use various size fuses branching off the supply to the original fuse, that cable must be able to handle the total load.


5 amp will be fine.

'total load' has thrown me, I believe the cable being the cable from the tap/piggyback (...plus my extension).

If I were to tap into a position containing a 10Amp (original), for a 5Amp device, therefore using a 5Amp 'piggyback fuse', I am guessing the cable must be able to handle the total load/draw of that device, being 5Amp (otherwise the fuse would blow).

Sorry, I am sure your answer makes perfect sense to anybody with knowledge, but I need things spelt out so that I understand the basics.

 

[DPG]

'total load' has thrown me, I believe the cable being the cable from the tap/piggyback (...plus my extension).

If I were to tap into a position containing a 10Amp (original), for a 5Amp device, therefore using a 5Amp 'piggyback fuse', I am guessing the cable must be able to handle the total load/draw of that device, being 5Amp (otherwise the fuse would blow).

Sorry, I am sure your answer makes perfect sense to anybody with knowledge, but I need things spelt out so that I understand the basics.

The new device's load needs to be less than the 5A rating of the new piggyback fuse that is feeding it. The cable used needs to be able to handle 5A.

 

[mainline]

'total load' has thrown me, I believe the cable being the cable from the tap/piggyback (...plus my extension).

If I were to tap into a position containing a 10Amp (original), for a 5Amp device, therefore using a 5Amp 'piggyback fuse', I am guessing the cable must be able to handle the total load/draw of that device, being 5Amp (otherwise the fuse would blow).

Sorry, I am sure your answer makes perfect sense to anybody with knowledge, but I need things spelt out so that I understand the basics.

You have 2 choices when connecting a new circuit to an existing fuse.

1) You join to the cable that supplies the existing fuse and add your own fuse for your extra circuit.

2) you join to the outgoing side of the original fuse with your own fuse.

1) would be the correct way.

2) would be adding an extra load to the existing fuse.

Whichever way you do it, you will be adding an extra load to the cable that feeds the original fuse.