EC&M’s Electrical Calculations Handbook - Chapter 14 pps

Electrical Design and Layout Calculations Many design and layout issues arise daily in the work of electrical engineers and designers, such as: What are the minimum dimensions of a straight-through pull box, an angle pull box, or a junction box? How close to a wall can a 480/277-volt (V) panel be placed? How close together can two opposing 13.8-kilovolt (kV) switchgear layouts be placed? How close together can knockouts be punched with- out causing locknuts to physically overlap and interfere with one another? What minimum phase-to-phase and phase-to-ground dimensions must be maintained when con- structing an auxiliary wireway? This chapter provides con- venient answers to these questions by providing the rules for each, along with completed “go by” calculations that engineers and designers can use as templates for their spe- cific calculations simply by changing certain values. Straight-Through Pull Box in a Conduit System When wires are drawn through a conduit, the friction between the conductor insulation and the conduit can become Chapter 14 357 v Copyright 2001 by The McGraw-Hill Companies, Inc. Click here for Terms of Use. too great, damaging the conductors. Therefore, sometimes intermediate pulling points are required within what would otherwise be a continuous conduit run. When wires are pulled in a conduit run that contains pull boxes, the wires are completely drawn through and out of the open cover of the pull box and then are refed into the next part of the conduit run. As most of the wire length is pulled into the next part of the conduit run, the wire loop at the cover of the pull box forms an increasingly smaller radius until the wire is all in the box. The minimum bend- ing radius of the conductor, the size of conduit that each con- ductor circuit will normally fit within, and the possibility of physically damaging the conductor on the sharp edges of the pull box flanges have been considered in specifying the min- imum dimensions of a straight-through pull box, which is eight times the trade size of the largest conduit entering the box. See Fig. 14-1 for an example of this calculation. The other box dimensions are determined by the physical space required to place a locknut on the conduit fitting, as shown in Fig. 14-6 and replicated for convenience in Fig. 14-1. Angle Pull Box in a Conduit System When conduits enter adjacent walls of a pull box, the loop that the wire makes as it is pulled in is less of an issue, and so the box can be smaller (only six times the trade size of the largest conduit, plus other considerations). When more than one conduit enters the wall of a box, then the additional con- duit trade sizes must be added to the “six times the trade size of the largest conduit” to determine the dimension to the opposite wall of the box. In addition, raceways enclosing the same conductors are required to have a minimum sepa- ration between them of six times the trade size of the con- duit to provide adequate space for the conductor to make the bend. Figure 14-2 illustrates these calculations. Working Space Surrounding Electrical Equipment Sufficient working space must be provided for workers to operate safely around electrical equipment, and National 358 Chapter Fourteen Electrical Code Table 110-26(a) defines the minimum dimensions of this working space. The height of the working space must be the greater of (1) 6.5 feet (ft) or (2) the actual height of the electrical equipment, and the width of the working space must be the greater of (1) 30 inches (in), or (2) the width of the electrical equipment. There are three distinct possibilities, or conditions, that determine the depth of the working space: Condition 1. Exposed live parts on one side and no live or grounded parts on the other side of the working space or exposed live parts on both sides effectively guarded by suitable wood or other insulating materials. Insulated wire or insulated busbars are considered to be live parts only if their potential exceeds 300 V to ground. Condition 2. Exposed live parts on one side and grounded parts on the other side. Concrete, brick, and tile walls are all considered to be grounded, just as is sheetrock screwed to metal studs where the metal screws are accessible. Condition 3. Exposed live parts on both sides of the work space with the operator between them. There is one more consideration regarding working clear- ance depth. The doors of the electrical equipment must be able to open a full 90°. Note that practically everywhere in the code and in the electrical industry, voltage is given as phase-to-phase volt- age, but in this case voltage is stated in phase-to-ground units, and with ungrounded systems, the voltage to ground is taken as the phase-to-phase voltage. These requirements apply to equipment that is likely to require examination, adjustment, servicing, or mainte- nance. Some examples of such equipment include circuit breaker panelboards, fused switch panelboards, disconnect switches, individual circuit breaker enclosures, motor starters, and switchboards. Generally, the working clearance requirements for the rear of rear-accessible electrical equipment are the same as for the front of the equipment, except that if the equipment can only be worked on in a deenergized state, then the 36-in Electrical Design and Layout Calculations 359 360 Figure 14-1 Solve for dimensions of a straight-through pull box in a conduit system. 361 362 363 Figure 14-2 Solve for dimensions of an angle pull box in a conduit system. minimum condition 1 clearance for up to 150 V to ground may be reduced to 30 in. No rear clearance space is required for equipment that requires no rear access. The requirements for working space increase as the volt- age to ground increases. Figure 14-3 shows the required space for electrical equipment operating at from 0 to 150 V to ground, Fig. 14-4 on p. 366 shows the required space for electrical equipment operating at from 151 to 600 V to ground, and Fig. 14-5 on p. 367 shows the required space for electrical equipment operating at voltages above 600 V to ground. Minimum Centerline-to-Centerline Dimensions of Knockouts to Provide for Locknut Clearance While trade sizes of conduit are well known, the dimensions of the locknuts that secure conduit connectors are less established. When planning for conduits to enter a wall of a junction box or wireway, it is necessary to provide physical space for the locknuts in addition to providing for the con- duit opening space. When determining the exact centerlines of knockouts, it is essential to provide for the locknut space, or else the connectors will not fit beside one another in the box wall. Figure 14-6 on pp. 368 and 369 provides exact lay- out dimensions that can be used to determine the minimum dimensions from one conduit knockout centerline to the next, regardless of the sizes of the conduits involved. For example, reading directly from Fig. 14-6, a knockout center- line for a 2-in trade size conduit can be no closer than 3.375 in to the sidewall of a box or else the locknut will not fit between the connector and the box wall. Also reading direct- ly from Fig. 14-6, the minimum centerline-to-centerline dimension from the knockout for a 2.5-in conduit to a 1.25- in conduit would be 3 in, but 3.25 in is the recommended minimum distance to allow some clearance between the locknuts. 364 Chapter Fourteen Electrical Design and Layout Calculations 365 Figure 14-3 Solve for working space in front of equipment operating at 0–150 V to ground. 366 Chapter Fourteen Figure 14-4 Solve for working space in front of equipment operating at 151–600 V to ground. [...].. .Electrical Design and Layout Calculations Solve for working space in front of equipment operating at over 600 V to ground Figure 1 4-5 367 368 369 Figure 1 4-6 Solve for minimum centerline-to-centerline dimensions of knockouts to provide for locknut clearance . the locknuts. 364 Chapter Fourteen Electrical Design and Layout Calculations 365 Figure 1 4-3 Solve for working space in front of equipment operating at 0–150 V to ground. 366 Chapter Fourteen Figure 1 4-4 Solve. ground. Electrical Design and Layout Calculations 367 Figure 1 4-5 Solve for working space in front of equipment operating at over 600 V to ground. 368 Figure 1 4-6 Solve for minimum centerline-to-centerline. and the box wall. Also reading direct- ly from Fig. 1 4-6 , the minimum centerline-to-centerline dimension from the knockout for a 2.5-in conduit to a 1.2 5- in conduit would be 3 in, but 3.25 in