Branch-Circuit, Feeder and Service Calculations, Part XXVI

by Charles R. Miller
Published: April 2008

Article 220 – Load Calculations

220.55 Electric Ranges and Other Cooking Appliances—Dwelling Unit

Article 220 of the National Electrical Code (NEC) contains requirements for calculating branch-circuit, feeder and service loads. Article 220 contains a lot of these requirements, but calculating branch-circuit, feeder and service loads in accordance with every one of those requirements is not absolutely necessary. However, some requirements in Article 220 are mandatory rules.

Mandatory rules identify actions that are specifically required or prohibited and are characterized by the use of the terms shall or shall not. An example of a mandatory rule can be found in 220.12. A unit load of not less than what Table 220.12 specifies for occupancies therein shall constitute the minimum lighting load. This requirement states that, when calculating the lighting load, the load must not be less than the volt-ampere unit load per square foot (or square meter) that is specified in Table 220.12.

Some requirements in Article 220 are permissive rules. Permissive rules in the NEC are those that identify actions that are allowed but not required, are normally used to describe options or alternative methods, and are identifiable by the use of the terms “shall be permitted” or “shall not be required.” Requirements in 220.55 are permissive rules. For example, the load for household electric ranges, wall-mounted ovens, counter-mounted cooking units and other household cooking appliances individually rated in excess of 1¾ kW shall be permitted to be calculated in accordance with Table 220.55.

We are not required to use Table 220.55. We are permitted to use the table. For example, a 12-kW range will be installed in a dwelling. The range could be added to the branch-circuit, feeder or service load calculation at its total rating of 12 kW. Or, in accordance with Column C in Table 220.55, we are permitted to use 8 kW instead of the total rating of this range (see Figure 1).

There is one instance when using Table 220.55 is required. Calculating in accordance with Table 220.55 is required if you are trying to correctly answer an electrical test question that pertains to this table.

Including the total rating of one range in the service load calculation for a one-family dwelling does not matter much one way or the other. But, including the total rating of multiple ranges in a service load calculation for a multifamily dwelling could make a huge difference in the size of the service. For example, the total rating for 50 12-kW ranges is 600 kW (600,000W). If the voltage at the service is 208-volt, three-phase, the total current rating is 1,667A. By using Table 220.55, the rating is 62.5 kW (62,500W). At 208V, three-phase, the total current rating is 174A. The difference between using the table and not using the table is 1,493A. It is not likely that all surface elements and both elements inside the range will be on simultaneously. If most of the range is on, it will not be on for any length of time because thermostats will cycle elements off and on. As the number of ranges increases, the chances of the ranges being fully loaded decreases. The table that is now Table 220.55 has been in the Code for more than 45 years.

Up to this point in the series, household electric cooking equipment ratings have been within the limits of Column A or Column B, not within the limits of both Columns A and B. Where the rating of cooking appliances falls under both Columns A and B, apply the demand factors for each column separately, and add the results together. This specification is stated in the last sentence of Note 3, under Table 220.55. For example, what is the service demand load for 10 3-kW, wall-mounted ovens and 10 5-kW -counter-mounted cooking units (cooktops)? The ovens have a total rating of 30 kW (10 × 3 = 30). Because the ovens are rated less than 3½ kW, the demand factor percent is within the limits of Column A (10 units = 49 percent). The calculated demand for the ovens is 14.7 kW (30 × 49% = 14.7). The cooktops have a total rating of 50 kW (10 × 5 = 50). Because the cooktops are rated within 3½ through 8¾ kW, the demand factor percent is within the limits of Column B (10 units = 34 percent). The calculated demand for the cooktops is 17 kW (50 × 34% = 17). Now add the results together to find the total demand load (14.7 + 17 = 31.7). The demand for these household cooking appliances is 31.7 kW (31,700W) (see Figure 2).

As discussed at the beginning of this month’s Code in Focus, selecting the lower kilowatt demand load is not required. It is permitted.

Look back at Figure 2 and compare the calculated load from Columns A and B with the maximum demand from Column C. Since there were 10 ovens and 10 cooktops, the total number of units from the example in Figure 2 was 20. The demand load from Column C for 20 units is 35 kW. The calculated load for 10 3-kW, wall-mounted ovens and 10 5-kW, counter-mounted cooking units (cooktops) from Columns A and B was 31.7 kW. After comparing Column C with the combined demand load from Columns A and B, the lower of the two demand loads is from Columns A and B (see Figure 4).

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 NFPA’s “Electrical Reference.” He can be reached at 615.333.3336, charles@charlesRmiller.com or www.charlesRmiller.com.