96 Modern Commercial Wiring Copyright Goodheart-Willcox Co., Inc. When working with the correction factor for multiple conductors in a conduit, not all con- ductors are included in the count. Equipment grounding conductors and the neutral conduc- tor in single-phase, three-wire circuits are not counted because they normally carry no cur- rent. Therefore, they generate no heat. Neutral conductors that regularly carry current are counted, however. 6.4 Voltage Drop Most electrical equipment is sensitive to varia- tions in line voltage. In order for the equipment to operate effectively, it is important to supply the correct voltage. The Code suggests, but does not mandate, a maximum voltage drop of 3% for any feeder or branch circuit and a maximum combined voltage drop (from service entrance to fi nal utilization outlet) of 5%. fi Voltage drop is the loss of electrical pressure (voltage) along the length of a conductor. It is often caused by the resistance of the conductor however, many factors affect voltage drop: tem- perature, conductor length, conductor size, and conductor material. However, the greatest cause of voltage drop is conductor resistance, which is proportional to the current. The primary rem- edy for voltage drop is to reduce the resistance of the conductor by increasing its size (diame- ter). Voltage drop can be reduced in other ways, such as reducing ambient temperature, shorten- ing the length of the conductor, and changing the conductor material, but these methods are less practical. 6.4.1 Calculating Voltage Drop Voltage drop is calculated using Ohm’s law, E = I × R. Conductor resistance is listed in Table 8 of Chapter 9 of the Code, which was shown in Figure 6-3. There are two categories of copper in the table: coated and uncoated. Tin-coated copper is often used for conductors having rubber insula- tion to prevent chemical reaction between the copper and the rubber. Most copper conductors are uncoated, however. Single-Phase Voltage Drop Single-phase voltage drop can be calculated using the following formula: VD = ______R_×I×L×2 1000 where VD = Voltage drop L = One-way length of conductor (in feet) I = Current in the circuit (amps) I R = Conductor resistance as shown in Table 8, 8 Chapter 9 of the Code (in Ohms/1000′) In a single-phase system, current travels along the conductor and then returns on the neutral conductor. Therefore, the current travels from the source to the equipment and then back again. The total distance traveled is twice the length of a single conductor, so a factor of two is added to the equation. The entire product must be divided by 1000 because the resistance is based on a length of 1000′ . By dividing the actual length (in feet) by 1000 and multiplying by the resistance per 1000′ , the actual ′ resistance of the conductor is determined. The voltage drop equation can be algebraically rearranged to create an equation to determine the maximum allowable resistance to have less than a 3% voltage drop for a given conductor run. R MAX = X 1000 × 0.03 × (line voltage) ______________ 2 × L × I I NOTE Table 8 8 lists dc c resistanc ce ce rather t th an imped- mp ance, e which includes in resistance es a an d react ta nce. The e voltage d dr op formulas mu assum me no rea ac tance and n d a power e factor of o f f one.