Load calculation requirements are in Article 220 of the National Electrical Code (NEC). This article provides requirements for calculating branch-circuit, service and feeder loads.
Article 220 is divided into five parts. Part I provides for general requirements for calculation methods. Part II covers requirements for branch-circuit load calculations. The branch-circuit load calculations can either be used as stand-alone calculations or combined with other load calculations from Article 220. Results from calculations in Part II can be used to size branch-circuit conductors and branch-circuit overcurrent devices. Results from Part II can also be incorporated with results from other sections in Article 220 to size conductors and overcurrent devices for feeders and services. Part III contains feeder and service load-calculation methods. As specified 220.40, the calculated load of a feeder or service must not be less than the total branch-circuit loads that are calculated by Part II of Article 220. The branch-circuit load calculations can include demand factors that are permitted by Parts III, IV or V. Part IV, “Optional Feeder and Service Load Calculations” also contains load-calculation methods for feeders and services. This part contains optional or alternative load-calculation procedures for one-family dwellings, existing dwelling units, multifamily dwellings, two dwelling units supplied by a single feeder, schools, feeders and service loads for existing installations, and new restaurants. Part V provides calculation methods for farms.
Last month’s column covered the calculated load for fastened-in-place appliances, household electric cooking equipment, clothes dryers and water heaters in 220.82(B)(3)(a) through (d). This month, the discussion continues with optional feeder or service load calculations as specified in 220.82.
Permanently connected motors must be added to the general loads when calculating a feeder or service when calculated by the optional method. Although motors are listed separately, some or all of the motors may have already been added to the load calculation as fastened-in-place appliances. For example, a kitchen waste disposer could be added to an optional load calculation as a fastened-in-place appliance or as a permanently connected motor. It is not necessary to add motors that have already been added to the load calculation as fastened-in-place appliances.
In accordance with 220.82(B)(4), add the nameplate ampere (A) or kilovolt ampere (kVA) rating of all permanently connected motors to the general loads covered in 220.82(B). Include all motors that are not already included with the general loads covered in 220.82(B). For example, what is the optional method service load calculation (before applying the demand factor) for a one-family dwelling with the following permanently connected motors: two automatic power attic roof ventilators rated horsepower (hp) at 115 volts (V) each and a swimming pool pump rated 1 hp at 230V? The ampere rating of each motor is not known. These three motors have not been included with the general loads covered in 220.82(B) (see Figure 1).
The first step is to find the full-load current rating in amperes for each motor. Since the ampere ratings for these motors are not known, use the values given in Table 430.248. Table 430.248 provides full-load currents in amperes for single-phase alternating-current motors. The full-load current for each of the automatic power attic roof ventilators rated hp at 115V is 4.4A. The full-load current for the swimming pool pump rated 1 hp at 230V is 10A (see Figure 2).
The next step is to calculate volt-amperes (VA) for each motor. Use Ohm’s Law to find an unknown factor (or quantity) when there are at least two known factors. Ohm’s Law states that the current in a circuit is equal to the circuit’s voltage divided by the resistance of the circuit. Figure 3 shows an Ohm’s Law wheel, containing 12 formulas. Terms that make up the Ohm’s Law wheel include current (I), voltage (E), resistance (R) and power (W).
When choosing a formula from the Ohm’s Law wheel, start with the unknown factor, and select the appropriate letter from the inner circle. Next, out of the three formulas for that particular letter in the inner circle, select the formula containing the two known factors. Finally, insert the known factors into the formula and solve the unknown factor. Since the unknown factor in this example is volt-amperes (or watts), find the symbol “W.” The lower right quadrant of the Ohm’s Law wheel contains three formulas to solve for watts. The two known factors in this example are volts and amperes. Next, select the formula containing “E” for volts and “I” for amperes. To find volt-amperes (or watts), multiply volts by amperes (W = E I).
To calculate the load in volt-amperes for each roof ventilator motor, replace “E” with 115 and “I” with 4.4 in the Ohm’s Law formula. The load for each roof ventilator motor is 506 VA (115 4.4 = 506). To calculate the load in volt-amperes for the swimming pool pump, replace “E” with 230 and “I” with 10 in the Ohm’s Law formula. The load for the swimming pool pump is 2,300 VA (230 10 = 2,300) (see Figure 4).
When calculating the feeder or service load of a dwelling unit by the optional method, do not multiply the full-load current of permanently connected motors by 125 percent. Although there are times when the motor’s full-load current must be increased, this is not one of them. The optional method service load calculation (before applying the demand factor) for a one-family dwelling with two automatic power attic roof ventilators rated hp at 115-volts each and a swimming pool pump rated 1 hp at 230V is 3,312 VA (506 + 506 + 2,300 = 3,312) (see Figure 5).
Do not include heating and air conditioning equipment in the list with permanently connected motors. Heating and air conditioning equipment is covered in 220.82(C). Depending on the type of system, demand factors may or may not apply.
Next month’s column continues the discussion of feeder and service load calculations.
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.