I am now planning to run an inverter from the battery which says 40Ah on it to a 400w pure sine wave inverter, with peak power 800w. https://www.canadiantire.ca/en/pdp/motomaster-eliminator-400w-pure-sine-power-inverter-0112047p.0112047.html?gclid=CjwKCAjwi9-HBhACEiwAPzUhHCXsctIOh0fzWV6Xm9rxsTSZkAzlWs5CVjhcYpD2P2_j51BLSX6CoxoCOksQAvD_BwE&gclsrc=aw.ds#store=606

I am mainly going to use it for my laptop that draws 65 watts I believe, maybe 90 watts.

The mechanic is telling me to get 8 gauge wire, I am seeing as thick as 4 gauge when doing 800w / 12v = 66.7a and referring to a chart online.
He is also saying put an inline fuse that is slightly above that 66.7 number. What is correct?

My inverters are rated up to 175 watts, I have one in each socket.

The sockets were just replaced because one time I guess I tried to pull too much by using an extension cord/power bar thing with a thick cable, laptop, extra monitor, vape battery charger all plugged into it and it melted some plastic parts in the socket.

I received a firm warning from the dealer saying, "This is an extreme fire hazard to use an extension cord as the 12V is only rated for 12V appliances like specialized mini-fridges, air pumps, etc".

So, I am plugging my laptop now directly into the inverter and then using the other socket's inverter for my extra monitor and vape battery charger.

The head mechanic explained to me, "With such a thick gauge cable in my extension cord trying to pull so much power, that caused the socket to overheat and melt." I am not an electrician but that does not make sense. The appliances like laptop, monitor, battery charger plugged into my extension cord/power bar are what is trying to pull too much power I thought so the way I have it now, should be safe? Also, just plugging the extension cord/power bar into the inverter without any appliances should also be safe regardless of the thickness of the cable and wiring in it.

Can someone clarify?

Note: I can put my car in Utility Mode when parked which then draws power directly from the large Lithium-Ion battery that is used for driving and that is what I normally do.

  • Welcome to Motor Vehicle Maintenance & Repair! Jan 20, 2021 at 14:17

2 Answers 2


You are right, the thickness of your extension cords is not what melted your car's sockets. There was some other issue, such as the inverters exceeded the 180 watt rating of the socket, or the sockets were not designed properly, etc. The root cause was, indeed, the wattage of too many devices connected to the socket, not the voltage. The voltage can vary with the inverter, but it is the wattage that governs the limit of the sockets.

Also, a heavier gauge extension cord is desirable. It will prevent the wires and the power bar from becoming warm with load.

Your final assumption is correct: fewer connected devices will prevent the sockets from melting again. To repeat, the voltages and wire gauge don't matter. Your mechanics simply don't understand electricity.

  • That is good to know, my new setup is slightly noisier from the fans on two inverters but it seems to be running fine with laptop in one, monitor and vape battery charger in the other. I have this way in case the laptop has a sudden surge from higher CPU usage. Jan 20, 2021 at 20:13

What caused your sockets to overheat and then melt the support was drawing too much power and possibly loose / poor connections.

I would not use those sockets for any thing more than charging a smartphone or running a dash cam.

To provide the power to run monitors and laptops I would run a suitably fused direct supply, possibly switched with a correctly rated relay or manual switch, from the battery to a suitable connecting socket. Use a cable rated for 20A or so to avoid any overheating issues.

The mechanic was correct with the point about excess power btw.

  • Can my current setup of having a 175w inverter in each 12v 180w socket, with laptop in one inverter and monitor and small battery charging device for vapes in the other start a fire? Or worst case, will it melt the socket part again or blow the fuse? I remember reading that a laptop should come around 80w, monitor about the same or less and charging device less than either. Also "loose/poor connection" means the way I plugged the inverter or any of my appliances or something related to the manufacturing of the socket? Jan 20, 2021 at 20:03

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