Line Voltage Drop Calculations

Ohms Law applies whatever (at low frequencies) - look up the ohms/m of your cable & plug it it.
1.2kA is a heafty current, so make sure nothing's going to get too hot (as in running several cables near each other). Apply local wiring regulations.

Check the IEE red book there is all the listings for resitances per cable size, and a formula (with explanations) and this calculation is an industry standard

... You didn't mention distances - your cable (3/0 - 1/0) comes out at about 3 ohms/km.

The "old fellers" never worried about voltage drop because back in the all-analog, relay-switched, carbon-mike days, the exact voltage didn't matter. I bet that's not the case now. Voltage drop calculations are the same whether AC or DC; it's just ohm's law.

A.C. voltage drop is commonly calculated with an eye toward overall impedance.
D.C. voltage drop is calculated on straight resistance for the entire circuit length, (both ways).
NFPA 70 will give you figures for both.
You don't say what your installation will tollerate, but you've got an existing setup and a meter, (presumably).
Take some measurements and read the equipment spec.s. By comparing your readings with your calculations,
you'll get a feel for how good your numbers are and a guide in case you need to modify the system.
The "Old boys who never worried about it before" generally put some extra copper into the job just on general
principles just so they won't have to.

As previously stated Ohms law applies to the wlole distribution cable ut you are more likely to get problems in the connectors rather than the cables. Even a 1 Ohm resistance in a connector will result in 1200 watts of heat being generated at the point of contact and this is arc welder sorts of power. I once had a mainfranme computer burn to the ground because a terminal wans't tightened up correctly at only one tenth of the currents you are speaking of.

thanks, I am really at a loss. I am not kidding about the current...if i can figure out how to insert a .bmp I can show ya some of the stuff I have to work with...

This goes all the way back to Edison and the electric chair. AC power distribution is more efficient but more dangerous. DC line losses are higher because there is no ability to optimize for load balance. At least you have 54VDC. At 12 VDC trying to get 12v to the top of a sailboat mast adds a lot of diameter to the copper....

Suggest that you purchasse the latest version of the US NAtional Electrical Code book (NEC) they cover the topic very well regarding correctly sizing conductors for voltage drop in free air & in condute & raceways.

Voltage drops are mostly a concern when any distance is over 150 feet from the origination. If you are feeding several loads from one main feeder, you should actually be concerned with your voltage drop at each station, per say the load factor.

Study OHMs law in parrallel circuits and series circuits. Each cicuit will have a conciderable voltage drop in its usage in comparison to the over all impiedence factor. As stated before, the NEC has the brake down in feeder size, from 14 AWG on up to 1000MCM, for the ohms per foot. Then you should take into concideration the type of feeder cable being used, and if it is an old system, your insulation might actually has outlived its life expectancy, since you mentioned low voltage high current feeds. You might concider replacing the old cable with more efficient cable and improved insulation properties. Mostly you should be concerned on the full load current draw being ran through the cables, this will mandate the size of cable used. Most cables are rated upto 600 VAC, and your circular mil size is rated for the current load as a continuous rating. This is why we have feeders in open cable trays, and the insulation plays an important factor in how this cable is being used. Your conduit size is calculated for the number of feeders and the ability to allow ample cooling principles witht the square area of fill in your conduit. All this plays an important roll in your application. The new code book has planty of information for your needs. I would actually recommend a coarse in power distribution centers from your local community applied science school, they all have great gourney man prep coarses.

Good luck.