Good points there Tom. Sounds like Myles has taken a good, conservative approach. I'd like to hear others' take on it as well.
On my latest project (Albin 25 with 8x6V 220AH G.C. batteries and 5KW Mars brush motor) I used 2 gauge for everything. That size was chosen simply because I had a bunch left over from previous projects and was determined by "eyeball estimation" to be adequate. Resistance for AWG2 is given as .1563 ohms per 1000 ft. Figuring about 20 feet total for my wire runs, including all the jumpers between batteries and miscellaneous accessories comes to about .003126 ohms. At my max draw of 110A that works out to about .34V drop. I usually cruise at half that or less.
I haven't done a detailed study of all the various drops in the system, but looking at my charts from some test runs I did last summer I see about a 5V drop when going from 10A to 100A. If I remember right that was measured at or near the controller input terminals, so should include everything upstream of the controller. I'm assuming a good percentage of that 5V drop is in the batteries themselves and would vary based on state of charge, battery age, etc. In other words, actual wire drop is a fairly small percentage of the total in this case. Going bigger would help a little, but not much. There are also 24 terminals, power switch, fuse, and ammeter shunt in line that each contribute a portion. It would be interesting to measure them each individually, but I haven't done that yet. I occasionally do a "touch test" on all the terminals and components after a hard run to check for abnormal heating. That's the easiest and probably the most reliable "health check" you can do on the wiring system and I would highly recommend it.
I haven't included the controller to motor wiring in the above example, but it probably wouldn't hurt to go a size or two larger since there are re-circulating currents and high-frequency "skin effects" that come into play. If the run is short it's not a huge issue, but if the controller is remotely located it warrants some consideration. I used 2 gauge there as well and it does run noticeably warmer than the battery cables, though how much of that is conducted heat from the motor and controller vs. resistance heating I'm not certain.
Jim
--- In electricboats@yahoogroups.com, "Myles Twete" <matwete@...> wrote:
>
> Tom, yours is the practical kind of analysis here. I went thru similar
> estimates with my boat and ended up opting for 1/0 cabling for
> batt-controller-motor. Max current I ever draw is about 130amps and typical
> is in the 20-50amp range.
>
>
>
> -Myles
>
>
>
>
>
> From: electricboats@yahoogroups.com [mailto:electricboats@yahoogroups.com]
> On Behalf Of Tom
> Sent: Saturday, February 19, 2011 10:35 PM
> To: electricboats@yahoogroups.com
> Subject: [Electric Boats] voltage drop
>
>
>
>
>
> This talk about very large cables got me wondering what voltage drop other
> builders were designing for.
>
> I cruise at 10-20A, with a theoretical max of 100A (never done that yet).
> With 1AWG cable running 20' round trip the voltage drop at max current is
> about 1/2 of 1%.
> So at full power (5KW) my cables are losing 25W. But I am usually running
> 20A or less (1KW), so my losses are most often less than 3W.
>
> I could've installed larger cables, of course, but given the additional
> cost, and compared to the other unavoidable (and much larger) losses
> inherent in EP, going with larger cables offered little improvement.
>
> So I'd like to ask others who designed their own system, what voltage drop
> did you design for, and why?
>
> -Tom
>
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