Diesel Enginer Generator-Cranking Motor Cable size

[ksps100]

Hi All,

I have a DEG whose Cranking motor current requirement at firing speed is = 1000A DC.

How would I size the Cable for this?
(Cable to be laid between the Battery terminals to the Cranking motor terminals).

I plan to use XLPE Insulated 1 Core cables.I have sizes of 50mm2,95mm2,120mm2 and 150mm2.

Length of cable from battery terminals to Motor terminal=20 meters.

Please advise.
Thanx in advance

 

[alehman]

Voltage drop will be the determining factor. Consult the DG manufacturer for tolerable voltage drop and size appropriately based on DC resistance of round-trip length.

 

[Warpspeed]

Voltage drop is definitely going to be the thing to watch. A thousand amps through 40m total length of cable will give the following theoretical voltage drops:

150mm^2 = 4.58v
120mm^2 = 5.73v
95mm^2 = 7.24v
50mm^2 = 13.75v

Realise also, that the battery terminal voltage is going to fall as well as the voltage drop along the actual cables. Cable heating may become an issue too, depending on maximum required cranking time.

 

[itsmoked]

If that starter only draws a thousand amps.... They often draw a lot more.

 

[ksps100]

Dear All,
Thanx for all your support.

The manufacturer recommends max 2V drop.
Based on this i chose 2 runs of 120mm2
Thats 2 runs for +ve and 2 runs for -ve.
Resistance of 120mm2 per meter=0.0002Ohms/mtr.

Am I OK?

Thanx

 

[ScottyUK]

Why are the batteries located so far away? The normal solution is to move them as close to the load as possible, certainly within a metre or two. This would be the most sensible solution, unless there are other factors which you haven't mentioned that require the battery to be located so far away.

Based on Warpspeed's figures, you are closer to a 3V drop than 2V using two paralleled 120 mm², and that is assuming that the terminations introduce no additional resistance. If you factor in those drops you are more likely to be looking at paralleled 240 mm² to keep the overall drop including terminations down to 2V.


----------------------------------

One day my ship will come in.
But with my luck, I'll be at the airport!

 

[Warpspeed]

My estimation of cable resistance assumes 0.0172 ohms for a "standard" one metre length of 1mm^2 copper at 20C.

By my reckoning, 120mm^2 works out to 0.0001433 ohms per metre, but that will increase with temperature.

A figure of 0.0002 would be a perfectly reasonable figure to design from.

If you plan on three runs of 120mm^2 in each direction that should certainly be up to the job. Laying three individual cables will be much more mechanically flexible, easier to run, and cable cooling will be better.

I am guessing that the 1,000 amp rating is just a nominal average cranking figure, locked rotor starting amps could go much higher.

It may be wise to make provision for plenty of available cable tray area, ample holes through walls and so on, then do an actual test. If an additional return run of 120mm^2 is needed to meet the two volt specification, then an upgrade is fairly painless.

 

[ksps100]

Dear ALL,
Thanx for all your suggestions.

The Battery is located based on the Building design.
We cant get it closer because that would hamper access for DEG Maintenance.

We plan to use 2 parallel runs of 150mm2 per terminal.
2 Nos of 150mm2 for +ve and 2 Nos of 150mm2 for -ve terminal.

Thank you
With Best Regards

 

[alehman]

You may wish to very maintenance requirements. Mounting of the starting battery directly beside the engine, even in small enclosures, is very common practice.