The first edition of the National Electrical Code (NEC) was developed in 1897. Including the 2008 edition, there are 51 editions. A lot has changed, both in the electrical industry and in the Code book, since the first edition. However, the Code’s purpose has always been protecting people and property from dangers that can come from the use of electricity.
Another function of this Code is the practical safeguarding of persons and property from hazards arising from the use of electricity [90.1(A)]. The wording of this first sentence in the NEC has remained exactly the same since the 1975 edition. The revision of the wording 35 years ago did not change the meaning. In accordance with 90-1(A) of the 1971 edition, the purpose of this Code is the practical safeguarding of people and of buildings and their contents from hazards arising from the use of electricity for light, heat, power, radio, signaling and other purposes. The revision changed “of buildings and their contents” to just “property.” Deleting the last part of the sentence was also part of this revision. Protecting people and property from hazards that can come from the use of electricity is the purpose of the Code. How electricity is being used and for what reason is not significant.
Last month’s Code in Focus concluded by covering the calculated load for permanently connected motors in 220.82(B)(4). This month, the discussion continues with optional feeder or service load calculations as specified in 220.82.
There are two components in calculating a dwelling unit feeder or service by the optional method. General loads are the first component; heating and air conditioning loads are the second component. General loads are covered in 220.82(B)(1) through (4). For detailed explanations of each section, see Parts XLV through XLVIII of this series on Article 220—Load Calculations.
After finding the total volt-ampere (VA) load of 220.82(B)(1) through (4), subtract 10,000 VA from the total. Do not derate this 10,000 VA load. In accordance with 220.82(B), the general calculated load shall be not less than 100 percent of the first 10 kilovolt-ampere (kVA) plus 40 percent of the remainder of the loads. The remainder of the general loads can now be reduced or derated. Multiply the remainder of the general loads by 40 percent. For example, a one-family dwelling will have the following general loads: 10,500 VA for general lighting and general-use receptacles; 9,000 VA for small-appliance and laundry branch circuits; 9,156 VA for fastened-in-place appliances; 14,000 VA for a range; 5,500 VA for a clothes dryer; and 6,576 VA for permanently connected motors (see Figure 1).
What is the optional method service load calculation for the general loads in Figure 1? Start by adding all the general loads. The volt-ampere total for the general loads is 54,732 VA (10,500 + 9,000 + 9,156 + 14,000 + 5,500 + 6,576 = 54,732). Next deduct 10,000 from the total (54,732 – 10,000 = 44,732). Since the first 10,000 VA is calculated at 100 percent, add 10,000 to the derated load of 17,893 VA (10,000 + 17,893 = 27,893). The optional method service load calculation for general loads is 27,893 VA (see Figure 2).
To better illustrate 220.82(B), nothing is mentioned in this example about heating and air conditioning loads. Since a service load calculation will usually include heating and/or air conditioning equipment, the calculated load will be the result of adding the general loads from 220.82(B) to the heating and air conditioning loads from 220.82(C).
The optional method dwelling-unit load calculation can be applied to feeders as well as services. As previously mentioned, most service load calculations will include heating and/or air conditioning equipment, but not all feeder load calculations will include these types of loads. If the feeder (or service) will not supply power to heating and air conditioning equipment, calculate just the general loads in 220.82(B). For example, a remote panelboard will be installed in a one--family dwelling. This feeder will supply the following general loads: 7,200 VA for general lighting and general-use receptacles; 6,000 VA for small-appliance and laundry branch circuits; 7,776 VA for fastened-in-place appliances; 18,000 VA for household cooking equipment; 5,000 VA for a clothes dryer; and 3,312 VA for permanently connected motors (see Figure 3).
What is the optional method feeder load calculation for the general loads in Figure 3? No heating or air conditioning load will be supplied from this feeder. Start by finding the volt-ampere total of the general loads. The total for the general load is 47,288 VA (7,200 + 6,000 + 7,776 + 18,000 + 5,000 + 3,312 = 47,288). After deducting the first 10,000 VA, the remainder is 37,288 VA (47,288 – 10,000 = 37,288). The load after applying the 40 percent demand factor is 14,915 (37,288 40% = 14,915.2 = 14,915). After adding 100 percent of the first 10,000 VA, the total general load is 24,915 VA (10,000 + 14,915 = 24,915). In accordance with 220.82(A), the calculated load shall be the result of adding the loads from 220.82(B) and (C). Since there is no heating or air conditioning load, no load will be added for heating and air conditioning. The optional method feeder load calculation for this one-family dwelling unit is 24,915 VA (see Figure 4).
As calculated by the optional method, the calculated load of 24,915 VA is the minimum rating for this feeder. The next step is to find the ampere rating from the calculated load. The Ohm’s Law formula is watts (or volt-amperes) divided by voltage. If the service is supplied by a 120/240-volt, single-phase source, the minimum ampacity for this service would be 104 amperes (24,915 ÷ 240 = 103.8 = 104). The rating of 104A is not a standard ampere rating for a fuse or a circuit breaker. Section 240.6(A) contains a list of standard ampere ratings for fuses and inverse time circuit breakers. The next standard size above 104 is 110A. The minimum size feeder for this example is 110 amperes.
Next month’s Code in Focus continues the discussion of feeder and service load calculations.
Editor’s note: A numbering mistake occurred in this series. Two articles in a row had the same number. We’ve corrected this error and apologize for any confusion.
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.