Page 222 - Pipeline Rules of Thumb Handbook

Corrosion/Coatings 209

How to calculate the voltage drop in ground bed cable quickly

Use the following formula for calculating the voltage drop
in a copper ground bed cable:

R 1000
T [
1
V T = ID GR + D A ( [ N - )( )
I T
1000
1
- I A (1 +++234 .... N - )]]
where V T = total voltage drop in volts
R 1000 = cable resistance per 1,000ft
I T = total current in amperes
I A = I T /N current per anode
D GB = distance from rectifier to first anode in ft
D A = distance between anodes in ft
N = number of anodes in ground bed
.2487
[
()(225
Example. What is the voltage drop in a copper wire V T = , 1 000 ( [ 225 )(100 )+ 20 15 )
ground bed cable in a cathodic protection system having a )]]
14
rectifier output of 225 amperes? The distance from rectifier -()(120
to ground bed is 100ft and 16 anodes are spaced on 20ft
centers. The cable used is No. 4 copper having a resistance V T = .2487 ( ,22 500 + 20 695 ])
[ ,1
of 0.2487 ohm per 1,000ft. , 1 000
Then R 1000 = 0.2487 ohm V T = Ê .2487 ˆ ( ,56 400 )
I T = 225 amperes Ë ,1 000 ¯
I A = 225/16 = 14 amperes
D GB = 100ft V T = (.2487 )( .56 4 )
D A = 20ft
N = 16 V T = 14 .00 volts drop

What is the most economical size for a rectifier cable?

Choosing the economical size for a rectifier cable in- cables, and assumes the use of a good grade of direct burial
volves balancing the first cost of the cable against the cable.
power losses. A rule which does not include all of the factors,
but which usually gives a very close approximation, is as Example. Suppose
follows:
I = 60 amps
P = 4 cents
a = 2 280 I P
,
a = (2 280 )( )(60 ) 4
,
where a is the area of the cable in circular mils (refer to a a = 273 600 circular mils
,
wire table for size), I is the current to be carried, in am-
peres, and P is the cost of power in cents per kilowatt-hour. Cable is sized 250,000 and 300,000 circular mils. Use the
This formula is based on the present prices of copper latter.