I've purchased a Thlevel Car USB Charger Socket to install in my 12v system campervan and I'm confused regarding the fuse and wire I need to use to wire things together. By my calculations, given that I could be using all the sockets at the same time, I would need around a 30 amp fuse and a 12 gauge wire (4 mm2). But then I wonder, what if I'm using most of the time only the usb chargers (each only 2.1A). Wouldn't it be that the wire and fuse is too big? Could someone point me out in the right direction? Thanks a lot.

  • \$\begingroup\$ most of the time is not all of the time \$\endgroup\$
    – jsotola
    Commented Nov 20, 2020 at 18:49
  • \$\begingroup\$ I missed these kind of answers from my nerd buddies in the university... To make it clear, this is really not my area. I was hoping to understand this since I read that big wires are not a good option for small needs in terms of amperage and also how big amp fuse can actually be of any protecting use for such small amperage needs (again when only the usb are being used) \$\endgroup\$
    – PedroB
    Commented Nov 20, 2020 at 19:03
  • \$\begingroup\$ If you could point me out which wire/fuse I should pick, I'd really appreciate \$\endgroup\$
    – PedroB
    Commented Nov 20, 2020 at 19:04
  • \$\begingroup\$ Doesn't the seller of the thing give good information about what you should use? \$\endgroup\$
    – Andy aka
    Commented Nov 20, 2020 at 19:24
  • \$\begingroup\$ No, that's why I'm asking here :) \$\endgroup\$
    – PedroB
    Commented Nov 20, 2020 at 19:26

1 Answer 1


Since I cannot answer in the comments I'll add some little tips.

About the wiring, if you don't want to bother with calculation (usually better but some quick answer might be sufficient), there is some web calculator that I found well designed: circuitcalculator.

When you define your wire size, take around 20% margin of your wire max current as a max fuse.

About your fuse issue on USB socket, well since it is integrated you cannot add fuse between the internal DCDC converter (12V -> 5V) to add the right fuse. (as @Justme said it have big chances to be a step down converter).

Since you are blind on the efficiency of the converter (it could be LDO or high efficiency converter), You can plus a calibrated resistor that leads you to 1A @5V and check the voltage on 12V line. If it is around 0.5A @12V it means that you have >80% efficiency. So take the max you think it can handle, calculate the approximate current on 12V you get and maybe take a 20% margin on your fuse choice.

Hope that's helpful


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