Application based on the Western Electric, Bell Laboratories, Bell System DC Power Lead Calculator which is in a circular slide rule format.  
Using allowable voltage drops specifically designed for telephone company central office direct current power plants and distribution wiring. The application references 48 volt battery systems but also works for other direct current voltages such as 24 volt. When using for 24 volts or other, choose by the required loop voltage drop instead of the specific section of the circuit.
Notes:
51. Voltage drop and wire size calculations are based on 2.00 volt LOOP drop from the power plant battery to the load equipment and return. 1.75 Final Volts Per Cell or 40 volts min. operating volts at the equipment for nominal 48V systems.
 (40 + 2 = 42 / 24 = 1.75fvpc) 


52. LOOP length is used in the calculations, often the battery and returns are unequal. The result VD is shown as ONE WAY.
53. The formula used in the lead size calculations is:
 (CM = LoopLength * 11.1 * Fuse or Breaker @ 100% / 2 (Circuits)/ VD)
Where VD is the desired voltage drop for the circuit and 11.1 is the direct current constant resistance of telephone company approved copper wire (KS24194L-3 or equivalent) and the result CM is the circular mils used to determine the wire size required.
54. BDFB is a Battery Distributing Fuse Board. Fuse Panel is rack mounted at or near the load equipment. Load Equipment is the actual device requiring power. 

55. This application is intended for informational use. Based on original Bell System Lead Calculator, Western Electric Co., Inc. Model GS-5420, 4-66.