130amp alt wire gauge to trunk

The problem I'm having.
The new alt is still a 3 wire. I've already installed it and have yet to bypass the amp meter. I started it just to check my gauges and see what's going on. I have one volt meter from my racepak that gets power from a fuze block and reads 14.5-8 volts. My problem is I have another volt meter gauge on my fitech that gets power direct from battery. Only shows 11.8-12.2 volts. So I did some testing with an external volt meter. Coming into my factory amp meter I'm getting 14.8 volts. Coming out of my amp meter getting 14.8 volts. Out of the amp meter is a 10 gauge wire (for now) to the battery cutoff switch in the trunk. There I check voltage and only showing the 11.8 volts. So by the time it leaves the amp metre and ends up in the trunk it's losing the voltage

My thoughts are that the 10 gauge wire is too little and that's why the voltage is being lost?

I WILL be bypassing the amp meter.

The cut off switch has been installed for more than three years now and works correctly. I recently added fuel injection and never noticed the loss of voltage at battery till now