Total area to be protected = 4 x 9.3 m2 = 37.2 m2
Protective current = 37.2 m2 x 11 mA/m2 = 0.409 A
We know that 0.90 kg of magnesium anodes will provide 1000 Ah of current protection
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 7.7-kg magnesium anode would be expected to deliver about 8,556 ampere-hours.
Protection capacity = 0.409 x 24 (hours/day) x 365 (days/year) x 10 years
Protection capacity = 35,828 Ah
Since a single 7.7-kg magnesium anode can deliver 8,556 Ah, the number of anodes required would be 4.1.
However, in 3,000-ohm-cm soil with a 0.85 V P/S reading a 7.7 kg anode will only deliver approximately 0.030 A (see performance survey)
The corrected number of anodes would therefore be 0.4 ÷ 0.03 = 13.3 or 14 anodes to completely protect the footings