I have a distribution block buried under my center console. Would getting 12v from that line vs directly from the battery present any fire hazard?

Question

My car has an unfortunate lack of always-on 12v power points -- something I'd like to remedy. I want to add a new powerpoint in my center console hooked directly to an always on source, eg: the battery (primarily for charging my phone while camping). I know I should run wire directly to the battery and fuse it within 18 inches of said battery, but that's a pain. I already have a (properly fused) 4 gauge wire running from the battery to underneath the center console for two under-seat subwoofers. The wire is oversized for said subs (young foolish me thought I might replace them), so I don't see any worry adding additional load to said wire.

Would it present a fire hazard if I were to instead of running a dedicated hot line from the battery just stick the wire into the incoming port of the distribution block alongside the incoming hot wire? The distribution block is actually sized to accept anywhere between 0 and 4 gauge, so I suspect I'd have room for an additional wire for the outlet. My only (poorly understood, hence the question) concern is that the ~10 gauge wire for the new 12v outlet is much smaller, so it might burn up before the fuse on the main trunk. If that is the case, could I just get away with secondary fuse just on that wire?

Answer 1

The fuses are there to protect the wiring, the fuse needs to blow before the wiring gets hot. You say that you have a fuse at the battery end of the 4 gauge wire, so that wire is protected against overload.

It is fine to add a small wire as you plan, as long as you add a small fuse at the very start of the wire where it joins the distribution block. This will protect the wiring to the new power point against overload and fire.

This is how all the original wiring in the car is designed. The very thick wires have large fusible links installed. Where the thin wires to individual devices branch off, there will be small fuses chosen to protect the smaller wires.

Any devices within the vehicle that need special protection, for example the stereo, will likely have its own fuse built in.

Answer 2

I've done somethign like this with my 50 year old landrover.

Add a distribution sub-panel in a convenient place. Make sure the link to the supply is secured against chafing and so on, then branch out sub-circuits from there. Try to make the panel easily accessible while still protected from feet or water.

Make sure you use a fused distribution panel which will protect each branch with a standard small automotive blade fuse. Example:

These are available from both car and marine part specialists, the latter being used in wet/damp environments.

The incoming supply to this sub panel should have wire diameters sized and fused for the maximum reasonable load. Main fuse should be as near the battery as possible.

Also, do add a sticker label to your main fuse block saying "To sub panel in glovebox" or whatever. This helps a future fixxer find faults.

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It may also be reasonable to fit the supply leads with a low voltage cut-off in case someone leaves a load on and kills the battery to the point it can't start the main motor. Also known as a Low Voltage Disconnect or LVD, this will kill your always-on loads if the battery gets too low. This limit is adjustable on most cutoffs, and will either auto-reset when the voltage rises, or can be overridden on some models.