Wiring in +/- into car fuse (piggyback)

[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.