(1) Average soil resistivity is 4500 ohm-centimeters.

(2) Design for 90 percent coating efficiency, based on experience.

(3) Design for 15-year life.

(4) Design for 2 milliamperes per square foot of bare pipe.

(5) Packaged-type magnesium anodes must be used.

(6) Insulating couplings are used on all service taps. Mains are electrically isolated from all other metal

structures in the area.

(7) All pipe has been precoated at the factory and wrapped with asbestos felt. The coating has been

tested over the trench for holidays and defects have been corrected. The coating is considered to be better

than 99.5 percent perfect when installed.

(1) Find the total outside area of piping (table C-3).

(2) Find the area of bare pipe to be protected cathodically based on 90 percent coating efficiency:

A = 4288x0.1

A = 429 sq. ft.

(3) Find the maximum protective current required based on 2 milliamperes per square foot of bare

metal:

I = 2 x 429

I = 858 mA or 0.858 A.

Pipe size

Pipe lingth

Pipe area

Pipe area

(in.)

(ft)

(sq ft/lin ft)

(sq ft)

3

600

0.916

550

2

1500

0.622

933

1

1800

0.497

894

1

2400

0.344

826

3900

0.278

1084

Total area of pipe in square feet

4288

U.S. Air Force.

(4) Find the weight of anode material required based on maximum current requirement and 15-year

life. Use equation C-1:

YSI

W'

,

E

where Y = 15 years, S = 8.8 pounds per ampere-year, I = 0.858 ampere, and E = 0.50 efficiency. Thus,

(15 yr)(8.8 lb/A&yr)(0.858 A)

W'

,

0.50

W = 227 lb.

Note that the 227-pound value is based on an output current of 0.86 ampere for the cathodic protection

system's full design life, 15 years. Strictly speaking, this is not the true condition, because current output after

new installation is much less due to the high coating efficiency. The average current requirement at first may

be as low as 0.03 milliampere per square foot of pipe.