Cable thickness and amps!

One of the interesting aspects of rebuilding the Golden Arrow loco was the decision we took to replace the 40V DC system with a modern 24V DC electronic controller based one.

We used the highly successful Bosch 1000 watt motor, which gives almost 1 HP on full load, which is pretty good for a single 24 volt unit.

But what thickness of cable to use?

Well, current (ib) is related to voltage and power by the equation:

ib = 1 x watts/volts

So in our case this become..

ib = 1 x 1000/24 or about 43 amps.

That 43 amps is the maximum normal running current but let's play safe and double it to allow for both failure modes and also to reduce the voltage drop over the length of the wire. So we'll allow for about 80-90 amps absolute maximum.

So how thick does the cable need to be?

A good source of practical information is in the published works of marine and canal boat users, as well as the more formal IEEE and similar organisations.

These suggest the following information for a boiler room running at 60'C!

Cable cross section (mm2) Current  Strands
35mm2  87A  127
50mm2 105A 127
70mm2 135A 127

 For more normal temperatures - say 20'C - then we need to look at something like this.

Cable cross section Current  diameter
16mm2 76A  7.2mm
25mm2 101A  8.4mm
35mm2 125A  9.5mm

So the answer to our problem would seem to be that if we use 25mm2 cable, which is about 8.4mm in diameter, we should be safe to draw the maximum stall current of the Bosch motor, that is, about 100 amps.

Out of interest, a test involving a single Bosch motor equipped Golden Arrow hauling a single Pullman coach up a 1 in 20 incline drew a maximum current of 55 amps. So 100 amp cable is probably 'over kill' and so maybe 16mm2 cross section would do for most normal use.