Table A.1 American Wire Gauge (AWG) Versus Wire Diameter and Resistance American Diameter Resistance (S2/km)a Wire Gauge at 20°C 2.305 4.134 2.053 5.210 1.828 6.571 1.628 8.284 1.450 10.45 1.291 13.18 1.150 16.61 1.024 20.95 0.9116 26.39 0.8118 33.30 0.7229 41.99 0.6439 52.95 0.5733 66.80 0.5105 84.22 0.4547 106.20 0.4049 133.9 0.3607 168.9 0.3211 212.9 0.2859 268.6 0.2547 338.6 0.2268 426.8 0.2019 538.4 aThese figures must be doubled for loop/km. Remember it has a "go" and "return" path. We know that this resistor must have a value equal to four of these resistors if there is no change in current. Just sum the resistor values or 250 + 375 + 136 + 741 = 1502 S. The rule here is that when we want to calculate the equivalent resistance of resistors in series, we just sum the resistances of each resistor. A.3.2 Resistance of Conductors The current we can expect through a conductor with a fixed emf source varies directly with (1) the resistivity3 of the type of conductor, (2) the length of the conductor, and Resistivity is a unit constant used to determine the conductive properties of a material. REVIEW OF FUNDAMENTALS OF ELECTRICITY WITH TELECOMMUNICATION APPLICATIONS (3) inversely with its cross-sectional area, a function of its diameter. Unless otherwise specified, all conductors that we will discuss are copper. Outside plant engineers are faced with the design of the subscriber loop (Section 5.4). This is a wire pair extending from the local serving exchange to the telephone instrument (subset) on the subscriber's premises. A major design constraint is the resistance of the subscriber loop. At some point as the loop is extended in length, there will be SO much resistance that its signaling capability is lost. One way we can extend the length and maintain the signaling capability is to increase the wire diameter. In mainland Europe, wire diameter is given in millimeters. In North America a copper wire's diameter is indicated by its gauge. Here we mean American Wire Gauge (AWG), formerly known as Brown & Sharpe (B&