Selecting the correct size conductor is not a difficult task, but there is more to it than just picking a conductor from Table 310.15(B)(16) in the National Electrical Code (NEC). The conductor must be selected and installed in accordance with all applicable provisions pertaining to conductors. For example, the ampacity of a conductor must not exceed the terminal connection temperature limitations in 110.14(C).
Another consideration pertains to a conductor’s conditions of use. The number of current-carrying conductors in a raceway or cable is one condition of use. Ambient temperature is another condition of use. A conductor also shall not be used in a manner that its operating temperature exceeds the limit designated for the type of insulated conductor involved [310.15(A)(3)].
There are other things to consider when selecting a conductor. Is the load, or any part of the load, continuous? Is the small-conductor rule in 240.4(D) applicable? Is the next higher standard overcurrent device rating (above the ampacity of the conductors being protected) below, equal to or above 800 amperes (A)? Will the conductors be feeder taps or transformer secondary conductors? Will the conductors be used as motor feeder or motor branch-circuit conductors? I have previously discussed some of these considerations in this series; I will discuss others in this and upcoming issues.
Last month’s column concluded by covering requirements for counting (or not counting) neutral conductors as current-carrying conductors. This month, the discussion continues with adjustment factors for more than three current-carrying conductors in a raceway, cable or earth (directly buried).
It may or may not be necessary to count neutral conductors as current-carrying conductors. Neutral conductor provisions in 310.15(B)(5) are divided into three sections. The first section, covered last month, states that if the neutral conductor carries only the unbalanced current from other conductors of the same circuit, it is not necessary to count the neutral as a current-carrying conductor. The second section in 310.15(B)(5) pertains to a specific electrical system. The neutral conductor must be counted if it is supplied from a three-phase, 4-wire, wye-connected system, but only if it is in a 3-wire circuit that consists of two-phase conductors and the neutral. For example, three multiwire branch circuits supplying power to incandescent lighting will be installed in a raceway. An equipment grounding conductor will also be installed in the raceway. Each multiwire branch circuit will consist of a neutral conductor and only two ungrounded (hot) conductors. The power system is a three-phase, 4-wire, wye-connected system, and the voltage is 208/120 volts (V). Each ungrounded conductor draws 12A at 120V. Counting the three 3-wire, multiwire branch circuits and the equipment ground, there will be 10 conductors in the raceway. What is the adjustment factor for the conductors in this example? Because each multiwire branch circuit will consist of a neutral conductor and only two ungrounded conductors and they are supplied from a three-phase, 4-wire, wye-connected system, each neutral must be counted. The equipment ground does not count. Because of the six ungrounded conductors and three neutrals, there will be nine current-carrying conductors in this raceway. The Table 310.15(B)(3)(a) adjustment factor for nine current-carrying conductors is 70 percent (see Figure 1).
The reason the neutral conductor must be counted in this type of circuit and system is because the neutral (or common conductor) carries approximately the same current as the line-to-neutral load currents of the other conductors. This is even stated in 310.15(B)(5)(b). This theory can be verified by the electrical formula for finding neutral current when the system is three-phase, 4-wire, wye-connected.
The letter I represents current, and the subscript letters represent phases A, B and C. The superscript 2 means that the current (or number) must be squared. (The square of a number is the product of a number multiplied by itself.) For example, each incandescent lighting circuit in Figure 1 draws 12A at 120V. Each 3-wire multiwire branch circuit will consist of a neutral conductor and two ungrounded conductors. The 3-wire circuits will be supplied from a three-phase, 4-wire wye-connected system. Each 3-wire circuit will use two of the three phases to supply power to the lights. One of the multiwire branch circuits supplying power to the 120V lighting circuits will be terminated on phases B and C. What is the current draw of the neutral in this multiwire branch circuit? Replace the letters in the formula with the known factors and solve for neutral current. Since this multiwire branch circuit does not use phase A, the current on phase A will be 0A. Replace the subscript A with 0. Replace the subscript B with 12. Replace the subscript C with 12. Since there is no current on phase A, the current of A squared is 0. The current of B squared is 144 (12 12 = 144). The current of C squared is also 144 (12 12 = 144). The current of A multiplied by the current of B is 0 (0 12 = 0). The current of B multiplied by the current of C is 144 (12 12 = 144). The current of A multiplied by the current of C is 0 (0 12 = 0). After adding and subtracting, the sum is 144 (0 + 144 + 144 – 0 – 144 – 0 = 144). The square root of 144 is 12. Each 3-wire multiwire branch circuit will have a neutral current of 12A, which is the same current as the line-to-neutral load currents of the other conductors (see Figure 2).
The third neutral conductor provision pertains to specific loads in a specific electrical system. On a 4-wire, three-phase wye circuit where the major portion of the load consists of nonlinear loads, harmonic currents are present in the neutral conductor; the neutral conductor shall, therefore, be considered a current-carrying conductor [310.15(B)(5)(c)]. Electronic equipment, electronic/electric-discharge lighting, adjustable-speed drive systems, and similar equipment may be nonlinear loads. For example, a multiwire branch circuit consisting of five conductors has been installed in a raceway. Three of the conductors are ungrounded (hot) conductors, one conductor is a neutral, and one conductor is an equipment ground. The multiwire branch-circuit supplies power to fluorescent lighting. The power system is a three-phase, 4-wire wye-connected system, and the voltage is 208/120V. Each phase or leg of the multi-wire branch draws 13A at 120V. What is the adjustment factor for the conductors in this example? Because this is a three-phase, 4-wire, wye-connected system and the loads are nonlinear, the neutral must be counted as a current-carrying conductor. Since the equipment ground does not count, there are four current-carrying conductors. The Table 310.15(B)(3)(a) adjustment factor for four current-carrying conductors is 80 percent (see Figure 3).
Next month’s column continues the discussion of sizing conductors.
MILLER, owner of Lighthouse Educational Services, teaches classes and seminars on the electrical industry. He is the author of “Illustrated Guide to the National Electrical Code” and “The Electrician’s Exam Prep Manual.” He can be reached at 615.333.3336, charles@charlesRmiller.com and www.charlesRmiller.com.