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Published byMakenzie Pille Modified over 2 years ago

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The first step in recording information on an electrical panel is to list the circuits according to the plan, realizing that each slot in the panel is good for ONE phase wire. Since all lighting and convenience outlets are single phase, or single pole, then one slot is sufficient for each 20 amp circuit. Special outlets, generally are to remain as single circuits. As you list a special outlet, it is essential to know if the outlet device is single pole, two pole, or three pole, as each pole represents a phase wire. So a two pole outlet requires two numbered slots in the panel, and three pole outlets require three. Remember there are three elements in each of the electrical formulas; amperes, watts, and volts. So the designer must know at least two of the elements in order to design a circuit for the device. It is insufficient to say that a device is rated at so many amps, or so many volts, or so many watts. It takes two elements to define the electrical requirements for a device.

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The next step is to assign wattage to each of the phase wires in the panel. Each phase wire is to be loaded with the single pole wattage of a circuit plus the single pole wattage of the circuit that is on the opposite side of the panel. For instance, phase wire A will be loaded with watts from circuit ONE and TWO; phase wire B will be loaded with watts from circuits THREE and FOUR; phase wire C will be loaded with watts from circuits FIVE and SIX, and so on. Since special outlets are circuits by themselves, the wattage of the device is divided equally between phase wires. For instance in this example the circuit for the AC fan motor is two pole, so the wattage of the fan will be divided between the next two panel phase wires in the progression. Likewise, the AC compressor is three pole, so the wattage of the compressor is divided equally between all three phase wires. The following series of slides shows the progression of circuit wattage as they are assigned to the panel phase wires.

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Your plan shows circuits A-17, A-18, and A-19. You were told at the beginning that would change. From the panel diagram, since the 2-pole and 3-pole outlets must have multiple slots, the proper designation for the above circuits should be: A-17 is properly A-17,19 A-18 is properly A-18,20 A-19 is properly A-21,23,25 After all the circuits are listed and the wattage is shown, add the columns of wattage for each panel phase wire and record the total at the bottom of each column. Then add the three totals to find the total wattage of the panel from the circuits. Then we will divide the wattage into three categories; continuous use wattage; wattage for convenience outlets; and wattage for power equipment.

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The remainder of the calculation for the panel occurs at the bottom of the panel diagram, so for clarity, only the bottom of the chart is shown next...

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The National Electric code requires that CONTINUOUS use wattage be increased by 25%, which in this case continuous use wattage refers to the lighting. So we take the lighting circuits and total the wattage; Lighting: = 7800 watts Lighting load is 7800 x 1.25 = 9750 watts Receptacles are the convenience outlets wattage amounts to circuits 6 through 15, which is 10 circuits: Receptacles : 1920 x 10 = 19,200 watts Receptacle load can be decreased by 50% for all over 10,000 watts, since it is not likely that receptacles are used at the same time. Receptacles load = 10, ( 9200 ) = 14,600 watts Equipment load is what is left: 46,776 – 7800 – 19,200 = 19,776 watts

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The total panel wattage equals 44,126. This is the wattage for which the amperage is calculated, the wire and conduit size is found, and the size of the ground wire. Formula 3, since the panel is 3 pole, 3 phase, 120/240 volt Amperage = watts/1.732 x volts = 44,126 / Total amps = 106 Main Breaker = 110 amps From wire size chart; wire = four # 2 Conduit size = 1 ¼ Ground wire = # 8

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This example problem is a small building. Many buildings such as schools, hospitals, retail stores, offices, and such will be large enough to require numerous panels. Each panel in a large installation will become a circuit from a main switchboard. As an illustration the next 3 slides is of a real project that was built in New Braunfels, Texas.

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END OF DAY 16 And end of Electrical Design

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