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66 RICHARDSON'S BLUBBOOK OF PROJECTION
area of the larger wire (26250 I mils) since the No. 6 wire would in effect be equal to 26 250 1-mu wir«s
(17) Therefore, to ascertain the resistance offered bv a circuit of commercial copper, multiply its length m Set by two. Multiply the result by 10.5 and divide by he cross section area of the wire in circular mils, ^calculating circuits it is customar) , to take double the mil-foot standard instead of twice the length oi the circuit Either way is correct. Both return identical results. Expressed as a formula the rule is this: Twice the length of circuit in feet 1 '. 10.5
Area of wire in circular mils = Resistance of circuit in ohms
The rule is based upon the supposition that the temperature does not exceed 75 degrees Fahr. (24 degrees Centigrade). If the circuit is not overloaded, temperature may be disregarded. If this temperature is m7e thin 75 degrees Fahr. it will tend to increase
reS(18TAge-several years of use-does not affect the resistance of electrical conductors appreciably unless they are worked above capacity or become overheated An occasional slight rise in temperature above 75 degrees will do no permanent harm but one heavy over-heating will damage a conductor permanently, raising its resistance and weakening the copper. Voltage Drop Calculations
Following are the formulas used by electrical engineers m cafculating voltage drop. If P^dSrSS them intelligently they will save a great deal °f electrical power. L stands for one-way ength of circuit A for cross sectional area of wire in circular mils C for voltage drop in volts. E for voltage in circuit. I for current in amperes. R for resistance in ohms. P for voltage drop, expressed in percentage. . ^n these formulas we have doubled the mil-foot standard instead of doubling the length of the circuit.