Starter Cable Size Question

[Marqus]

Good point about the latency of fuses, Rod.

I use Mega fuses and checking Littlefuse's specifications for them indicate the following:
If you use a 100A fuse, and the circuit pushes 200A (double the rating), it will take between 1sec and 15sec for the 100A fuse to blow. So, for this fuse type anyway, sustained current is more important than peak.

Of course, criteria for selecting a fuse is quite divorced from those for selecting a wire size (copper cross section) to counter voltage drop, being the interest of the OP.

It's a potentially subtle issue highlighted by Maine Sail. If you have a lower available voltage exciting a starter motor engaged with a stationary flywheel, there may be consequences for the level of current, and the duration, of the full-length start-up attempt.

For example, if you had "0.5volt" more drop at the starter terminals due to cable selection, are those consequences significant?

With a well-equipped workshop and a couple of diesel engines with 12v starter motors on hand, one could run a few tests using progressively thinner cable and in the space of a morning come up with an informative answer. Don't have those, so can't say.

[mitiempo]

Originally Posted by Marqus

I use Mega fuses......

The other point for fuse selection is AIC rating. The larger the battery bank the larger the AIC rating should be for large loads.

The better choice for main battery bank fusing is:

ANL and MRBF fuses have AIC ratings of 5,000 to 10,000 amps. Class T fuses are much higher @20,000 amps.

Mega fuses rate 2000 amps AIC.

[ramblinrod]

Originally Posted by Maine Sail

I work based on actual field measurements at the "as installed" level...


Here's a 1.4kW starter motor..

This is what the current graph looks like.


Here is average voltage (12.04V) & average cranking current from loaded starter to unloaded (286A). We already know the peak current hit around 640A... It also shows this motor starts, as one should, loaded starter to unloaded, in about .76 seconds.



I have a 1.2kW starter in my shop right now that eats 100A +/- spinning into free air (no load current) and draws about 375A for in-rush, again into free air (no engine load just the in-rush). Average cranking for this starter, which fits a small two cylinder diesel, is in excess of 200A..

If you run the numbers for Yanmar's starter cable requirements, based on their tech manuals, and convert to the closes AWG, you come up with a maximum allowable voltage drop of approx 1.75% or less than a 2% allowable voltage drop, in a Yanmar starting circuit.

This is quite a difference from a 10% allowable voltage drop for a Yanmar/Hitachi starter motor as being acceptable...



20mm Squared is closest to 4 AWG
40mm Squared is closest to 1 AWG

Exactly, Yanmar is not specifying 4/0 cable for their starters.

In comparison, 4AWG = 21.14 mm^2, whereas 4/0 = 107.16 mm^2.

But, But, But...

According to ABYC (Table VI - A), the maximum current allowed in a 105C (most common) jacketed 4 AWG single conductor, non sheathed, non-bundled cable in and engine space is...136 A.

Not sure what that .2V/100A voltage drop rating means.

If the current is 100A, max Vdrop is .2Vdc (1.6%) but if it is 600A, max Vdrop is 1.2Vdc (10%)?????

Crap, now I have to call Yanmar tech support. ;-)

[Marqus]

Originally Posted by mitiempo

The other point for fuse selection is AIC rating. ..........

Mega fuses rate 2000 amps AIC.

Valid observation, although according to Gord May (in another thread):

"The required (ABYC) Ampere Interrupting Capacity (AIC, or Interrupt Rating) of a breaker depends upon the total Cold Cranking Amps (CCA) and Voltage of the connected battery bank, as follows:
< 650 CCA requires 1500 AIC at 12 - 24 Volt.
651 - 1100 CCA = 3000 AIC @ 12/24V"

If that is correct, the Mega's 2000A AIC would pass for my particular starting battery, though admittedly not with as good a margin as the ANL.