Quote:
Originally Posted by gahamby
370 amp potential over a 30 foot run. 4/0 at least. Don't forget the over current protection.
Consult an ampacity chart.
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Not sure if this will help much, but I'm a fan of the engineering expression: "Without data - all you have is an opinion.". So this morning after reading this thread, I went out to the Silver Kitten while it was still 35°F outside (it was 32° overnight) to try and directly measure the starting current of a cold 7.3-liter PowerStroke Diesel engine.
The instrumentation used was my Fluke 337 AC/DC clamp-on ammeter, which is good for 1000 Amps. After zeroing out the drift in the hall-effect sensor, I clamped around the positive cable leading away from the starting battery in the engine compartment. However, due to the zipties, split-loom tubing, and tape on the ends of the cables, I had to also had to clamp around the smaller lead coming from the alternator and battery isolator, which carries current in the opposite direction of the cable to the starter when charging, but the same direction while starting (i.e. minimal RPM = minimal charging current from the alternator). Figuring that this would probably induce minimal error in my measurement during the cranking process, I forged ahead. (Please feel to argue or educate me if you think I've goofed up here.)
With the hood up, and a flashlight shining on the display of the Fluke 337, I could just read its display through the windshield (upside down) by peering through the gap under the opened hood from the driver's seat.
I disconnected my battery maintainer from the vehicle, prior to which the on-board voltmeter and my Fluke 189 DMM both showed the starting battery bank on float at around 13.4 Vdc.
When I first turned the key to the RUN position, the glow plugs drew just over 190 Amperes, which tapered down to about 115 Amperes by the time the "Wait to Start" indicator on the instrument panel shut off. I'm assuming that this reduction is the result of both the increasing glow plug resistance with temperature, as well as diminishing starting battery voltage under load (as opposed to intelligent control).
Unfortunately, using the normal, INRUSH, and MAX modes on my Fluke 337 for several starting cycles all failed to produce consistent results for the actual current drawn while cranking, but on the first try the 337
did register a MAX reading of 337 Amperes. Even if I didn't get all of the data that I wanted, at least the glow-plug currents that were measured suggest possible minimums for the draw from the starting battery during the early part of a starting cycle in colder weather. I'd hate to buy a calibrated 500-Amp or 1000-Amp shunt for my stripchart recorder just to satisfy my curiosity, but I still might do that someday . . .
Question: The numbers displayed by the 337 were too erratic for me to tell if the glow plugs in a PSD are switched off during the actual cranking, but does any more-Diesel-savvy forum member know this answer?
The Bottom Line (opinion): I think that Gahamby's figure of 370 Amperes sounds entirely plausible, and could be used in the relevant calculations for wire sizing on starting battery relocation projects.
Hope this helps!