Page 140 - Pipeline Rules of Thumb Handbook

Electrical Design 127

How to find the resistance and weight of copper wires

It is easy to calculate mentally the approximate resistance Number 10 wire has a resistance of 1ohm per thousand ft.
and weight of standard sizes of copper wires, by remember- (Rule 1)
ing the following rules: Number 20 wire has a resistance of 10ohms per thousand ft.
(Rule 2)
Rule 1: Number 10 wire has a resistance of 1ohm per thou- Number 17 wire has a resistance of 5ohms per thousand
sand ft. Number 2 wire weighs 200lb per thousand ft. ft. (Rule 3)
Rule 2: An increase of ten numbers in the size is an increase Number 18 wire has a resistance of 6.25ohms per thousand
of ten times in the resistance, and a decrease to one-tenth ft. (Rule 4)
of the weight. Number 18 wire has a resistance of 1.25ohms per 200ft.
Rule 3: An increase of three numbers in the size doubles the Answer.
resistance and halves the weight. Number 2 wire weighs 200lb per thousand ft. (Rule 1)
Rule 4: An increase of one number in the size multiplies the Number 12 wire weighs 200lb per thousand ft. (Rule 2)
5
4
resistance by / 4 and the weight by / 5 . Number 22 wire weighs 2lb per thousand ft. (Rule 2)
Number 19 wire weighs 4lb per thousand ft. (Rule 3)
Always proceed from the known value (No. 10 for resis- Number 18 wire weighs 5lb per thousand ft. (Rule 4)
tance, No. 2 for the weight) in the smallest number of jumps; Number 18 wire weighs 1lb per 200ft. (Answer)
that is, go by tens, then by threes, and finally by ones; the
jumps do not all have to be in the same direction. The weight determined by the above rules is that of bare
copper wire. Weight of insulated wire varies widely according
to the type of insulation. Resistance values are correct for bare
Example. What is the weight and resistance of 200ft of or insulated wire, and for solid or stranded. Errors resulting
No. 18? Apply the rules in order, as follows; resistance first: from the use of the rules will rarely exceed 2 percent.

What you should remember about electrical formulas

Ohm’s law Other facts

Easily Remember It, or E = RI The number of watts in a hp is equal to the year Colum-
bus discovered America divided by two, or 1492/2 = 746
watts/hp.
Power
The horsepower of a reciprocating engine necessary to
DC Power Is Easy, or P = IE drive a three phase, 60 cycle electric generator can be deter-
2
It Really Is, or P = IRI = I R mined by multiplying KW by 1.5.

How to calculate microwave hops on level ground

horizon can be found by adding the square root of the tower
height to the square root of the first square root (the latter of
course is the fourth root).

4
x = h + h or h + h
Rule. Excluding obstructions such as hills, etc., the
microwave (line-of-sight) distances between two towers will Example. How far apart can 100-foot-high towers be
be twice the distance from the transmitter to the horizon. The spaced to provide line-of-sight transmission if there are no
line-of-sight distance in miles from the top of the tower to the obstructions between them?