Engine to body earth wire specifications

[keith1948]

Hello all

I have been following the thread on the TR4/4a part of the forum and have taken off the earth strap that goes from the front plate of the engine to the left front chassis to clean up the connections because it was looking a bit 'tired'. It is now back on the car.

I have decided, following various comments, that it is prudent to have another earth cable from the engine to body to avoid using the choke cable as a 'hot wire' earth. Next question though is what size earth wire to buy? Looking on the internet there are a variety e.g. 16mm2 = 110A, 20mm2 = 135A, 25mm2 = 170A and 35mm2 = 240A. Some are rated against SWG for cable size. The battery in my 4A is rated 55Ah but 490 En cranking. But you can fit anything up to 80Ah, 660A En cranking. Somewhere I read that the starter draws 250A under load but don't know if this is correct or not. In the TR3 workshop manual it states that at 1000rpm the torque of 8 lb/ft uses 250-270 amps at 9.4-9.0 volts. All this is a bit confusing.

My initial thoughts is to fit one rated 240A but that seems maybe a bit 'heavy duty'. Some suppliers seem to recommend 170A but others seem to think 135A is ok. If the starter does draw 250A under load then I would imagine the cables to the starter and the earth return would need to be similarly rated but since the load isn't constant maybe a lower rating is ok.

So I would be interested to hear from our electronics boffins what they would use and why.

On the plus side I have now been looking at various electrical connections on the car and cleaned up a few already that seemed to have a few more ohms resistance than they should have had.

Keith

 

[RobH]

You are over-thinking this a bit Keith - use the same size as the power cable to the starter.  There is no point in using anything thicker - you already have the braid in parallel anyway. but you don't really want to use thinner since the extra cable will have to take all the current if the braid fails. 

The current drawn by the starter will depend on the work it has to do - it isn't a constant.  On a very cold morning when the oil is very thick it will be high. On a summer's day when the engine is hot it will be much lower.

Edited September 23, 2020 by RobH

[keith1948]

Hi Rob

Just found this Lucas document https://www.mg-cars.org.uk/imgytr/pdf/lucascourse6.pdf 

On page 7 it says earth cable comprising 37 strands 20 SWG tinned copper wire for earth if starter motor current does not exceed 400 amps. That size of wire is also equivalent to 25mm2 and is rated at 170 amps.

So my original feeling that 170 amp rated wire was the one seems to be the one chosen by Lucas.

Lucas also mention a thicker wire but that seem to be overkill.

Not sure what the rating is for the power cable to starter without measuring it but I agree the size of the earth just needs to be same size.

Keith

 

[RobH]

The resistivity of copper means that 25mm^2 copper cable drops 1.75mV/Amp/Metre,  so at 400A the voltage drop is 0.7 volts for a metre length.  I guess you will be using a rather shorter length so the voltage drop will be rather lower and entirely acceptable. 

The 170 amp rating comes from the cable current capacity for power cables in buildings.  The ratings are given for continuous service at full current to limit temperature rise in the cable which can damage insulation. The current capacity is de-rated if the cables are bunched or thermally insulated. 

(For instance, IEE table 4E1A says 25mm^2 - where supply and return conductors are side by side in free air - is rated for a continuous 182Amps for a horizontal run and 161Amps vertical for a maximum operating temperature of 70 degrees C.)  

 

Edited September 23, 2020 by RobH