No More Guesswork: Determining and Calculating Existing Loads

By | Jul 15, 2018
Electrical systems age, usage increases and people upgrade their homes and offices. This combination is pushing many electrical systems to their limits.

Electrical systems age, usage increases and people upgrade their homes and offices. This combination is pushing many electrical systems to their limits. Enhancing a home or office’s electrical service requires an informed decision that should not be taken lightly. Determining the existing load and calculating the new load should be based on the National Electrical Code and not a wild guess. Overloading an existing service can have serious consequences, such as fires and unwanted outages; however, increasing the size of the electrical service can be very expensive.

Part I of Article 220 provides general requirements for calculating branch circuit, feeder and service loads; Part II covers branch circuit load calculations; and Part III covers feeder and service load calculations. These three parts provide methods of standard calculations and are used for new dwelling, commercial building and industrial facility calculations. Using these three parts will provide some degree of demand loads, considering most branch circuits, feeders and services will not have to support all of the loads being used all of the time.

Table 220.12 provides branch circuit unit loads for general lighting by occupancy, such as dwelling units, banks, churches, office buildings, restaurants and many others. The table expresses loads in volt-amperes (VA) per square foot.

Where dealing with feeders and services, Table 220.42 provides lighting load demand factors based on the type of occupancy. For example, dwelling units will be required to have the first 3,000 VA of lighting loads taken at 100 percent, from 3,001–120,000 VA at 35 percent and the reminder over 120,000 VA at 25 percent. For hospitals, the first 50,000 VA are taken at 40 percent and the remainder over 50,000 VA at 20 percent. Hotels, motels and apartment houses without provisions for electric cooking will have the first 20,000 VA taken at 50 percent, from 20,001–100,000 VA at 40 percent and the remainder of any lighting load at 30 percent.

The demand factors in Table 220.42 cannot be used for the lighting portion of feeders and services for hospitals, hotels and motels, since 100 percent of the lighting in those areas will be on all the time. Demand factors for nondwelling unit receptacle loads are located in Table 220.44 with the portion of feeders and service receptacle loads permits the first 10 kilovolt-amperes (kVA) at 100 percent and the remainder over 10 kVA at 50 percent. There are demand factor tables for household electric clothes dryers, household electric ranges, ovens and cooktops, as well as a table for demand factors for kitchen equipment in other than dwelling units. All of these tables and calculations are located in Parts I, II and III of Article 220 and determine the standard load calculations for primarily new installations.

Section 220.87 permits the calculation of a feeder or service load for existing installations to use the actual demand factor to determine the existing load, based on the actual maximum demand data available for a one-year period of time. If the one-year demand data is not available, an exception permits the calculated load to be based on the maximum demand (the highest average kilowatts reached and maintained for a 15-minute interval) continuously recorded over a minimum 30-day period using a recording ammeter or power meter connected to the highest loaded phase of the feeder or service, based on the initial loading at the start of the recording.

The recording should reflect the maximum demand by being taken when the building is occupied and factoring in any seasonal loads, such as air-conditioning or heating. The maximum demand load determined in the recording must be multiplied by 125 percent. Any new load that is added to the system cannot exceed the maximum demand load at 125 percent, and the total cannot exceed the ampacity of the feeder or rating of the service. Using either 220.83 or 220.87 should take the guesswork out of adding loads to existing feeders or services.

Mark C. Ode

ODE is a retired lead engineering instructor at Underwriters Laboratories and is owner of Southwest Electrical Training and Consulting. Contact him at 919.949.2576 and [email protected]

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