The protective current required is the total area of bare steel in square meters times the required current per square meter.
Surface exposed = length of pipe (m) x pipe circumference (m) x fraction exposed
Current demand = Surface exposed x current density
For the example of 45.7 m of pipe in 1,000-ohm-centimeter soil:
Surface exposed = 45.7 x 0.314 x 5/100 = 0.718 m2
Current demand = 0.718x 11 = 7.90 mA or 0.0079 A
Number of anodes equal to total current required times the installation life divided by the ampere-hour rate of the magnesium anodes.
Protection capacity required (Ah) = Current demand x life of the system to be protected
Number of anodes = Capacity required / capacity of each anode
The ampere-hour rating varies with different conditions but 0.90 kg of magnesium can be rated at about 1000 ampere-hours (Ah). Thus, a 3.6-kg magnesium anode would be expected to deliver about 4,000 ampere-hours.
Protection capacity = 0.0079 x 24 (hours/day) x 365 (days/year) x 10 years
Protection capacity = 692 Ah
In this case, a single 3.6-kg magnesium anode delivering 4000 Ah could provide nearly six times the required life with an indicated useful life of 50 years.