Total area to be protected =

**4 x 9.3 m**^{2}**= 37.2 m**^{2}Protective current =

**37.2 m**^{2}**x 11 mA/m**^{2}**= 0.409 A**

We know that

**0.90 kg**of magnesium anodes will provide**1000****Ah**of current protectionNumber 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