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I understand a bit about voltage, current, resistance, power, etc., and I know that thicker wires are able to allow more current to flow in them. I also understand the concept of voltage drop across a resistor which is a function of wire length because the longer it gets the greater it’s resistance will be.

I am working on a personal project that entails bringing high current from my car’s 12v battery to a dc-dc charger so that I can charge another 12v battery that is situated in my car towards the end. The wire run without any intermittent connectors is 15 feet and the manufacturer suggests 6 gage wire for the 25A rated current. Dc-dc device itself at 15 feet away has short wires built-in that are 8 gage and we will couple my wires to these.

I am sure the manufacturer is being conservative about the wire gage, and I totally understand. I want to be able to somehow gain confidence that I can run 8 gage wires from my car’s battery to the device instead of recommended 6 gage.

It is awkward that I am connecting a thicker wire to a thinner one, and I can’t understand what I should be expecting in terms of current and voltage across the connection between the two cables. If I measure an unpleasant voltage drop at the end of my cable run, will that drop decrease if I use thicker cables? What should I worry about if I make the wire thinner? Also, if the 15-foot segment has to be in two pieces that are connected together using 75A connectors, would that make any difference one way or another?

I appreciate your time and also if you can guide me to simple reading that can enlighten. I am a civil engineer and have taken some electrical engineering classes and have tinkered with electricity.

Thanks a lot.

Farzad

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Your ideas and concerns are absolutely correct!

In general, the bigger the wire, the less the resistive losses: resistance for the wire is usually stated in ohm/km. A really good approximation can be found from the copper section: as already commented there are several calculators around.

Please note that the energy lost go for heating the wire: if there's to much current it will overheats too with unpleasant consequences. As a rule of thumb you'll be fine up to about 4-5A/mm²

The exact same reasoning holds for the connectors: ohmic losses are stated in the datasheet (usually some microohm) and the stated current is the one for safely dissipating the heat.

Be sure to check the voltage rating of the connectors too (for other reasons, but it's still a safety issue)

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