CHAPTER 2 EXAMPLES AND TABLES

Transcription

1 CHAPTER 2 EXAMPLES AND TABLES COMMENTARY AT (A) EXCEPTION An overcurrent device that supplies continuous and noncontinuous loads must have a rating that is not less than the sum of 100 percent of the noncontinuous load plus 125 percent of the continuous load. Because grounded/neutral conductors are generally not connected to the terminals of an overcurrent protective device, this requirement for sizing conductors subject to continuous loads does not apply. Grounded/neutral conductors are typically connected to a neutral bus or neutral terminal bar located in distribution equipment. In addition, (A)(1) requires that the circuit conductors, chosen from the ampacity tables, have an ampacity that is: Not less than the sum of 100 percent of the noncontinuous load plus 125 percent of the continuous load Not less than the maximum load to be served after the application of any adjustment or correction factors The rating of the overcurrent device cannot exceed whichever calculation yields the greater result. Calculation Example Determine the minimum-size overcurrent protective device and the minimum conductor size for the following circuit: 30 amperes of continuous load 60 C overcurrent device terminal rating 40 C ambient temperature Type THWN conductors Four current-carrying copper conductors in a raceway Solution STEP 1. Determine the size of the overcurrent protective device (OCPD). 125 percent of 30 amperes = 37.5 amperes. Thus, the minimum standard-size overcurrent device, according to 240.6(A), is 40 amperes. STEP 2. Determine the minimum conductor size based on the noncontinuous load plus 125 percent of the continuous load. The ampacity of the conductor, with no adjustment or correction factors applied.

2 There is only a continuous load, so 125 percent of the 30-ampere continuous load results in 37.5 amperes. Because of the 60 C rating of the overcurrent device terminal, a conductor chosen based on the ampacities in the 60 C column of Table (B)(16) is necessary. The calculated load must not exceed the conductor ampacity. Therefore, an 8 AWG conductor with a 60 C allowable ampacity of 40 amperes is the minimum size permitted. Conductors with a higher allowable ampacity based on their insulation temperature rating may be used but only at a 60 C allowable ampacity. STEP 3. Because four current-carrying conductors are in the raceway, Table (B)(3)(a) applies. The adjustment factor for four conductors is 80 percent. Because the ambient temperature is 40 C, Table (B)(2)(a) applies. The correction factor for the ambient temperature is 82 percent. Calculate the ampacity of the conductor using these adjustment and correction factors: Conductor ampacity = Calculated load [Factors from Table (B)(3)(a)X Table (B)(2)(a)] 30 amperes 0.80 X 0.82 = 45.7 amperes The conductor ampacity determined in Step 2 is not the calculated load that is used in this step. It is a calculation that is used to determine a minimum conductor ampacity and circular mil area based only on the load with a continuous load factor applied. It ensures sufficient dissipation of heat at the terminals of overcurrent protective devices supplying continuous loads. The calculated load is 30 amperes. The phrase before the application of any adjustment or correction factors has the effect of not requiring that the conductor be subjected to a double-derating. This occurs when continuous loads are supplied by a conductor that is also subject to ampacity adjustment because more than three current-carrying conductors are in a cable or raceway or is subject to ampacity correction due to the installation being made where the ambient temperature exceeds 30 C (86 F). In this instance, the application of the adjustment and correction factors indicates that the conductor must have an ampacity of 45.7 amperes. This would require a 6 AWG conductor at 60 C. However, an 8 AWG conductor with a 75 C insulation has an ampacity of 50 amperes. The higher temperature-rated insulation can be used, for derating purposes, as long as the 60 C conductor has an ampacity sufficient to carry the load without overheating the terminations. COMMENTARY AT 215.2(A)(B) INFORMATIONAL NOTE NO. 3 Reasonable operating efficiency is achieved if the voltage drop of a feeder or a branch circuit is limited to 3 percent. However, the total voltage drop of a branch circuit plus a feeder can reach 5 percent and

3 still achieve reasonable operating efficiency. See Article 100 for the definitions of feeder and branch circuit. The 5 percent voltage-drop value is explanatory material and, as such, appears as an informational note. The informational notes covering voltage drop are not mandatory (see 90.5). Where circuit conductors are increased due to voltage drop, (B) requires an increase in circular mil area for the associated equipment grounding conductors. The resistance or impedance of conductors may cause a substantial difference between voltage at service equipment and voltage at the point-of-utilization equipment. Excessive voltage drop impairs the starting and the operation of electrical equipment. Undervoltage can result in inefficient operation of heating, lighting, and motor loads. An applied voltage of 10 percent below rating can result in a decrease in efficiency of substantially more than 10 percent for example, fluorescent light output would be reduced by 15 percent, and incandescent light output would be reduced by 30 percent. Induction motors would run hotter and produce less torque. With an applied voltage of 10 percent below rating, the running current would increase 11 percent, and the operating temperature would increase 12 percent. At the same time, torque would be reduced 19 percent. In addition to resistance or impedance, the type of raceway or cable enclosure, the type of circuit (ac, dc, single-phase, 3-phase), and the power factor should be considered to determine voltage drop. This basic formula can be used to determine the voltage drop in a 2-wire dc circuit, a 2-wire ac circuit, or a 3-wire ac single-phase circuit, all with a balanced load at 100 percent power factor and where reactance can be neglected: where: VD = 2 L R I 1000 VD = voltage drop (based on conductor temperature of 75 C) L = one-way length of circuit (ft) R = conductor resistance in ohms (Ω) per 1000 ft (from Chapter 9, Table 8) I = load current (amperes) For 3-phase circuits (at 100 percent power factor), the voltage drop between any two phase conductors is times the voltage drop calculated by the preceding formula. Calculation Example Determine the voltage drop in a 240-volt, 2-wire heating circuit with a load of 50 amperes. The circuit size is 6 AWG, Type THHN copper, and the one-way circuit length is 100 ft. Solution STEP 1. Find the conductor resistance in Chapter 9, Table 8.

4 STEP 2. Substitute values into the voltage-drop formula: VD = 2 L R I 1000 STEP 3. Determine the percentage of the voltage drop: = %VD = V = 0.02 or 2% 240 V = 4.91 volts A 12-volt drop on a 240-volt circuit is a 5 percent drop. A 4.91-volt drop falls within this percentage. If the total voltage drop exceeds 5 percent, or 12 volts, larger-size conductors should be used, the circuit length should be shortened, or the circuit load should be reduced. See the commentary following Chapter 9, Table 9, for an example of voltage-drop calculation using ac reactance and resistance. Voltage-drop tables and calculations are also available from various manufacturers. COMMENTARY AT 215.2(A)(1) EXCEPTION NO. 2 Example Feeder grounded/neutral conductors that do not connect to the terminals of an overcurrent protective device are not required to be sized based on 125 percent of the continuous load. For example, if the maximum unbalanced load on a feeder neutral is calculated per to be 200 amperes and the load is considered to be continuous, the use of a 3/0 AWG, Type THW conductor is permitted as long as the conductor terminates at a neutral bus or terminal bar within the electrical distribution equipment. COMMENTARY AT INFORMATIONAL NOTE In the example shown in Exhibit CE1 below, each panelboard supplies a calculated load of 80 amperes. The main set of service conductors is sized to carry the total calculated load of 240 amperes (3 80 amperes). The service conductors from the meter enclosure to each panelboard [2 AWG Cu = 95 amperes per 60 C column of Table (B)(16)] are sized to supply a calculated load of 80 amperes and to meet the requirement of relative to overcurrent (overload) protection of service conductors terminating in a single service overcurrent protective device. The main set of service conductors [250 kcmil THWN Cu = 255 amperes per 75 C column of Table (B)(16)] is not required to be sized to carry 300 amperes based on the combined rating of the panelboards. The individual service-entrance conductors to each panelboard (2 AWG THWN) meet the requirement of

5 EXHIBIT CE1 Service conductors sized in accordance with See Exhibit in the handbook for a similar example. In that example, the ungrounded service conductors are not required to be sized for the sum of the main overcurrent device ratings of 350 amperes. Service conductors are required to have sufficient ampacity to carry the loads calculated in accordance with Article 220, with the appropriate demand factors applied. See , , and for specifics on size and rating of service conductors. Part III of Article 220 contains the requirements for calculating feeder and service loads. Part IV provides optional methods for calculating feeder and service loads in dwelling units and multifamily dwellings. Except as permitted in and 240.6, the rating of the overcurrent device cannot exceed the final ampacity of the circuit conductors after all the correction and adjustment factors are applied (such as where the ambient temperature, the number of current-carrying conductors, or both exceed the parameters on which the allowable ampacity table values are based). Calculation Example Determine the minimum-size overcurrent protective device (OCPD) and the minimum conductor size for a feeder circuit with the following characteristics: 30 degrees C ambient 3-phase, 4-wire feeder (full-size neutral) 125-A noncontinuous load 200-A continuous load 75 C overcurrent device terminal rating Type THWN insulated conductors Major portion of the load is nonlinear Four current-carrying conductors (neutral conductor is a current-carrying conductor) in the raceway Solution

6 Based on 215.2(A), there are two application conditions that have to be looked at individually and then compared in determining the minimum size of the feeder conductors. The condition that results in the larger conductor size must be chosen. STEP 1. Determine the minimum feeder conductor ampacity by first totaling the continuous and noncontinuous loads according to 215.2(A)(1)(a). Total load = 125% of continuous load + noncontinuous load = (200 A 1.25) A = 250 A A = 375 A In accordance with 215.3, the same calculation is used to determine the minimum rating for the feeder overcurrent protective device (OCPD) Using 240.4(B) and 240.6(A), adjust the minimum standard-size OCPD to 400 A. STEP1(A). Select the feeder conductor size before ampacity adjustment in accordance with 215.2(A)(1)(a). Using the 75 C column of Table (B)(16) (because of the overcurrent device terminal), the minimum-size Type THWN copper conductor that can be used to meet the minimum conductor ampacity determined in accordance with 215.2(A)(1)(a) (375 amperes) is 500-kcmil copper, which has an ampacity of 380 A. This conductor is permitted to be protected with a standard-size OCPD rated at 400 A. If the OCPD used is listed for operation at 100% of its rating, the minimum conductor ampacity is permitted to be 325 amperes (200A + 125A) and the feeder overcurrent protective device is permitted to be 350 amperes (next standard size above 325 amperes). See 215.2(A)(1) Exception No. 1 and Exception No. 1. STEP 2. Determine the minimum feeder conductor ampacity needed to supply the calculated load and then apply any required ampacity adjustment or correction factors according to 215.2(A)(1)(b). Feeder ampacity (before applying ampacity adjustment or correction factors) = continuous load + noncontinuous load = 200 A A = 325 A STEP2(A). Apply adjustment or correction factors. Section (B)(5)(c) requires that the neutral conductor be counted as a current-carrying conductor because a major portion of the load consists of fluorescent and high-intensity discharge (HID) luminaires. Therefore, this feeder circuit consists of four

7 current-carrying conductors in the same raceway. Section (B)(3) requires an 80-percent adjustment factor for four current-carrying conductors in the same raceway. To determine the minimum ampacity for the conductor use the following formula: Minimum conductor ampacity = Load current/adjustment factor = 325 A / 0.80 = A Because of the requirement to adjust the ampacity of the current-carrying conductors using an 80% factor, the minimum conductor ampacity for this installation is amperes. STEP 2(B). Determine the minimum conductor size. Using Table (B)(16) the conductor can be selected from either the 75 degree C or 90 degree C column. If the conductor insulation temperature rating is 75 degrees C, the minimum size copper conductor is 600 kcmil with an ampacity of 420 amperes. If the conductor insulation temperature rating is 90 degrees C, the minimum size copper conductor is 500 kcmil with an ampacity of 430 amperes. Both conductors, after ampacity adjustment, have an adequate ampacity to supply this load A x 0.80 = 336 amperes x 0.80 = 344 amperes STEP 2(C). Determine OCPD rating. According to 240.4(B) and 240.6(A), a conductor with a calculated ampacity of 336 amperes or 344 amperes is permitted to be protected by a 350-A OCPD. Therefore, either the 600 kcmil THWN (75 degree C insulation rating) copper conductor or the 500-kcmil, Type THHN (90 degree C insulation rating) copper conductor can be used for this installation. The next standard size overcurrent device would be 350 amperes. However, requires that the OCPD be rated for the noncontinuous load plus 125% of the continuous load. Exception No. 1 would allow the OCPD to protect this load if the OCPD is listed for operation at 100% of its rating. If the OCPD is not listed for 100% of it rating, then a 400 ampere rated device would be necessary, as indicated in Step 1. A 400 ampere rated device would be insufficient to protect conductors having an ampacity of 344 amperes because it is larger than the next standard size. If a 400 ampere OCPD is necessary, it is also necessary to use 600 kcmil 90 degree C conductors, which would have an ampacity (after adjustment factors) of 475 x 0.80 = 380 amperes STEP 3. Compare the minimum conductor ampacity determined in accordance with Step 1 and Step 2. The calculation in Step 1 results in the need for four 500-kcmil 75 degree C Type THWN copper conductors in one raceway, each with an ampacity of 380 A, to supply a 375-A load (that consists of continuous and noncontinuous loads) which is protected by a 400-A OCPD. The result in Step 2 will depend on the OCPD available. If the OCPD is listed for operation at 100% of its rating, four 500-kcmil 90 degree C Type THHN copper conductors in one raceway with an ampacity of 344 A can be used with an OCPD rating of 350 amperes (the next standard size in accordance with 240.6). If the OCPD is not listed for operation at 100% of it rating, a 400 ampere rated device is required. Therefore, it is also necessary to use 600 kcmil 90 degree C conductors, with an ampacity (after applying adjustment factor) of 380 amperes (475 x 0.80)

8 Section 215.2(A) requires the minimum conductor to be the larger result determined using Step 1 or Step 2. Step 2 results in either a larger conductor size or a conductor with a higher insulation temperature rating. Therefore a 500 kcmil copper conductor, with a 90 degree C insulation temperature rating and ampacity of 430 amperes can be used to meet the minimum conductor requirement determined in accordance with 215.2(A)(1)(b). Interestingly, the size of the conductor determined by both conditions (215.2(A)(1)(a) and 215.2(A)(1)(b) is 500 kcmil. However the higher ampacity achieved by using a 500 kcmil conductor with a 90 degree C insulation temperature rating is required in order to meet the minimum conductor ampacity determined by 215.2(A)(1)(b). If a conductor with a 75 degree C insulation temperature rating is used, the minimum copper feeder conductor is 600 kcmil with an ampacity of 420 amperes. COMMENTARY AT (A)(6) A common grounding electrode conductor serving several separately derived systems is an alternative to installing individual grounding electrode conductors from each separately derived system to the grounding electrode system. In such an arrangement, a tapped grounding electrode conductor is installed from the common grounding electrode conductor to the point of connection to the individual separately derived system grounded conductor. This tap is sized from Table based on the size of the ungrounded conductors for that individual separately derived system. The minimum size for this conductor is 3/0 AWG copper or 250-kcmil aluminum so that the grounding electrode conductor always is of sufficient size to accommodate the multiple separately derived systems it serves. This minimum size for the common grounding electrode conductor correlates with the maximum size grounding electrode conductor required by Table Therefore, the 3/0 AWG copper or 250-kcmil aluminum becomes the maximum size required for the common grounding electrode conductor. The sizing requirement for the common grounding electrode conductor is specified in (A)(6)(a), and the sizing requirement for the individual taps to the common grounding electrode conductor is specified in (A)(6)(b). The methods of connecting the individual tap conductor(s) to the common grounding electrode conductor are specified in (A)(6)(c). The permitted methods include the use of busbar as a point of connection between the taps and the common grounding electrode conductor. The connections to the busbar must be made using a listed means. Exhibit CE2 shows a copper busbar used as a connection point for the individual taps from multiple separately derived systems to be connected to the common grounding electrode. The following example, together with Exhibit CE3, illustrates the use of a common grounding electrode conductor for grounding multiple separately derived systems (transformers in this case). Calculation Example A building is being renovated for use as an office building. The building is being furnished with four 45- kva, 480 to 120/208-volt, 3-phase, 4-wire, wye-connected transformers. Each transformer secondary supplies an adjacent 150-ampere main circuit breaker panelboard using 1/0 AWG, Type THHN copper conductors. The transformers are strategically placed throughout the building to facilitate efficient distribution. Because there is no accessible effectively grounded structural steel in the same areas as

9 where the transformers are installed and the metal water piping is concealed after it leaves the lower level, each transformer secondary (separately derived system) must be grounded to the water service electrode within the first 5 ft of entry into the building. A common grounding electrode conductor has been selected as the method to connect all the transformers to the grounding electrode system. What is the minimum-size common grounding electrode conductor that must be used to connect the four transformers to the grounding electrode system? What is the minimum-size grounding electrode conductor to connect each of the four transformers to the common grounding electrode conductor? Solution STEP 1. Determine the minimum size for the common grounding electrode conductor. Minimum size required [per (A)(6)(a)]: 3/0 copper or 250-kcmil aluminum The common grounding electrode conductor does not have to be sized larger than specified by this requirement. Additional transformers installed in the building can be connected to this common grounding electrode conductor, and no increase in its size is required. STEP 2. Determine the size of each individual grounding electrode tap conductor for each of the separately derived systems. According to Table , a 1/0 AWG copper derived phase conductor requires a conductor not smaller than 6 AWG copper for each transformer grounding electrode tap conductor. This individual grounding electrode conductor will be used as the permitted tap conductor and will run from where it connects to the grounded conductor in the transformer to a connection point located on the common grounding electrode conductor. This conductor is labeled Conductor B in Exhibit CE3. EXHIBIT CE2 Listed connectors used to connect the common grounding electrode conductor and individual taps to a centrally located copper busbar with a minimum dimension of 1/4 in. thick by 2 in. wide.

10 EXHIBIT CE3 The grounding arrangement for multiple separately derived systems using taps from a common grounding electrode conductor, according to (A)(6)(a) and (A)(6)(b).