280 ELEMENTS OF ELECTRICAL ENGINEERING. 



tables. From this equation it is evident that the poles may be spaced farther and 

 farther apart the greater the value of T' y the greater the permissible sag, and the 

 less the weight of the wire ; but it is to be noted that the ratio T'\w is independent of 

 the size of the wire, inasmuch, as T' and w are both proportional to the sectional area 

 of the vvire. 



The spacing of the poles is usually chosen tentatively as the first step in the design 

 of a pole line. When a great deal depends upon the permanence of a line, as in a 

 transmission line supplying power to many customers, the poles are placed close 

 together in order to make the line substantial and in order that the sag may be small 

 enough to avoid the possibility of the wires swaying into contact. Close spacing is 

 especially necessary in the case of heavy wires so as to distribute the weight of the 

 heavy wire over a large number of insulators, the insulator being one of the weakest 

 elements in the construction. Poles are usually spaced as follows on straight-pole 

 lines : (a] Heavy power transmission lines about 80 feet, which is the spacing on the 

 Niagara-Buffalo transmission line ; (6) Ordinary electric-lighting circuits in city or 

 suburban districts, from loo to 125 feet ; (c] Telegraph and telephone lines 125 to 

 150 feet. In every case the poles should be placed near together where the pole line 

 follows a curve, thus making the line turn a very obtuse corner at each pole, in order 

 to avoid excessive stresses in the supporting structure due to unbalanced tensions 

 of the wire. Furthermore pole spacing is often determined by surrounding local 

 conditions such as the presence of obstacles or the recurrence of cross-streets in cities. 



The amount of sag in a span of line wire should be small in order to prevent the 

 swaying of the wire by the wind. This swaying is objectionable because it tends to 

 break the wire where it is fastened to the insulators and because it is likely to bring 

 adjacent wires into contact. Once the spacing of the poles is chosen, the minimum 

 permissible sag is determined as explained in the next paragraph ; although the 

 amount of sag that may be allowed has a great deal to do with the choice of the pole 

 spacing. Very long spans, * such as spans across rivers, have a sag equal to one 

 twentieth or one thirtieth of the span. In ordinary pole-lines the sag seldom exceeds 

 one one-hundred-and-fiftieth of the length of span, in coldest weather. 



Effects of temperature. Wires are usually strung on poles during warm weather, 

 the wire grows shorter as the temperature falls, and the tension of the wire is there- 

 fore greatly increased during cold winter weather. Hence, it is important to string a 

 wire with sufficient sag (and a correspondingly low tension) so that the coldest weather 

 may not increase the tension of the wire beyond the safe value, T'. Knowing the tem- 

 perature, /, of the wire when it is strung, and the lowest winter temperature, /', the 

 calculation of the necessary sag, h, and tension, T, at temperature, /, is carried out as 

 follows : Take the values of T f (= 7j/) and w from the following tables, and from 

 these, together with the chosen distance, /, between poles, calculate the winter sag, 

 //', using equation (39), and calculate the corresponding length of wire, s' t in a 



* " Long spans for transmission lines," by F. O. Blackwell, Trans. American 

 Institute of Electrical Engineers, June, 1 904. 



" Conductors for long spans, " by F. O. Blackwell, Trans. International Electrical 

 Congress, Vol. 2, St. Louis, 1904. 



