starter motor grounding

I'm still at the studying phase but a question came upon my mind which bothers me way too much.

It's clear, that the starter motor grounding point is the engine block itself. If i'd like to choose the engine block as my star earthing point, than the backflowing amount of current to the battery will increase, i guess.

The question is, how should i determine the size of the grounding wire between the engine block and the battery? Should I use thicker cable doe to the increased current flow?

Thanks in advance, expect me with more questions in the future!

Hello Bálint,

It depends on how long your ground cable is, the temperature that it will be subject to, the temperature rating of the cable, the expected continuous current requirements and the starter peak current draw.

Have a read of this article, along with this one. They give some good information on the subject. The cable will need the ampacity to support your maximum steady state current requirement for the required distance and temperature, plus a safety margin (How much of a safety margin is up to you; the higher the margin the higher the cost and weight, but more headroom for extreme operating conditions or degraded connections.)

Your starters peak current does not need to be supported for very long, so if it draws a peak of 400A, you don't need a cable that has a rated steady state ampacity of 400A... you could, but it would be overkill and a needless expense... and bulk.

The majority of 12V automotive applications would get away with 2AWG - 1/0AWG (35-50mm²), however you always want to make sure.

Hello Callum!

I'm extremely thankful for your proper answer!

A good rule of thumb is to use the sama gauge of cable, or earth straps, for the ground as is used for the main power cable.

As Callum said, there are several thing to take into account as different vehicles may have very different requirements.

As a guide to the maximum current, you should be able to find the stall current for your starter motor - post it if you can't and we may be able to help - then upsize the nominal requirement a size or two. This is because you want to minimise the voltage drop in the cables/grounds to maximise the cranking speed, and to minimise the voltage drop - this is important as many starter motors need 9-10V to turn over the engine, and the ECU may have a minimum voltage to operate.

The short answer is the ground cable from engine to battery should be the same size as the power cable from the battery to the starter.