Courtesy of www.MikeHolt.com. Based on the 2020 NEC.
Sizing the feeder or service load for a dwelling isn’t simply a matter of totaling the loads or breaker ratings. The Code recognizes there’s load diversity and that not all loads run simultaneously. Therefore, it provides “demand factors” that you apply to calculate the “demand load.” Electrical Submersible Pump
Article 220 allows two methods of calculating the feeder or service load for a dwelling — the standard method [Part III] and the optional method [Part IV].
Using the standard method is almost impossible without software, because of the complexity of the math and the rules. Plus, many judgment calls are needed to decide which demand factors apply to which load. These issues are solved by using the optional method.
How do you use the optional method to determine the demand load on the feeder or service conductors for a dwelling unit?
You can use the optional method only for dwellings served by a single 120/240V or 120/208V, 3-wire set of service or feeder conductors with an ampacity of 100A or larger [Sec. 220.82(A)].
Step 1. Determine the total connected load, less HVAC [Sec. 220.82(B)].
Step 2. Determine the demand load for the Step 1 loads [Sec. 220.82(B)].
Apply a 100% demand load to the first 10kVA of the total connected load, then a 40% demand to the remainder of the total connected load.
Step 3. Determine the air-conditioning versus heating demand load [Sec. 220.82(C)].
Determine the largest of the following loads and add it to the demand load value from Step 2:
Step 4. Determine the service disconnect and conductor sizing.
Determine the total service demand load in amperes for the dwelling by adding the values of Step 2 and Step 3, then divide the sum by the nominal voltage (208V or 240V). Once you know the service demand load, size the service disconnect for a standard size overcurrent protective device per Sec. 240.6(A).
Dwelling service and feeder conductors supplied by single-phase, 120/240V systems can be sized per Sec. 310.12(A) for service conductors and Sec. 310.12(B) for feeder conductors. Dwelling unit feeder conductors supplied by single-phase, 120/208V systems can be sized per Sec. 310.12(B) for feeder conductors.
Even with the optional method, you must consider many factors, as this example shows.
What size feeder/service conductors are required for a 6,000-sq-ft dwelling containing the following loads? (Figure)
Step 1. Determine the total connected load, less air-conditioning, and electric heating [Sec. 220.82(B)].
General Lighting ― 6,000 sq ft × 3VA = 18,000VA
Small-Appliance Circuits ― 1,500VA × 2 circuits = 3,000VA
Step 2. Determine the demand load for Step 1 loads [Sec. 220.82(B)].
First 10kW at 100% ― 10,000VA × 100% = 10,000VA
Demand Load = 10,000VA + 52038VA = 62,038VA
Step 3. Determine the air-conditioning versus electric heating demand load [Sec. 220.82(C)]; air-conditioning at 100% [Sec. 220.82(C)(1)] versus electric heating at 65% [Sec. 220.82(C)(4)].
Zone 1 A/C Condenser = 240V × 50A = 12,000VA A/C Condenser Fan = 240V × 4A = 960VA Zone 1 Air-Conditioning Total = 12,000VA + 960VA = 12,960VA
Zone 2 A/C Condenser = 240V × 50A = 12,000VA A/C Condenser Fan = 240V × 3A = 720VA Zone 2 Air-Conditioning Total = 12,000VA + 720VA = 12,720VA
Total Air-Conditioning Demand Load (Zone 1 + Zone 2) = 12,960VA + 12,720VA = 25,680VA
Electric Heating Zone 1 = 9,600W Electric Heating Zone 2 = 7,500W Total Connected Space Heating (Zone 1 + Zone 2) = 9,600W + 7,500W = 17,100W
Total Space Heating Demand Load = 17,100W × 65% = 11,115W (omit) [Sec. 220.82(C)]
Air Handler, Zone 1 = 240V × 4A = 960VA Air Handler, Zone 2 = 240V × 3A = 720VA Total Air Handler Demand Load = 960VA + 720VA = 1,680VA
Air-Conditioning Demand Load = 25,680W + 1,680VA = 27,360VA
Step 4. Determine the service disconnect and conductor sizing.
Total Demand Load = 62,038VA + 27,360VA = 89,398VA
Service Size in Amperes = VA Demand Load ÷ System Volts
Service Size in Amperes = 89,398VA ÷ 240V = 372A
Using the next size up rule, the service disconnect size should be 400A. Based on Table 310.12, use 400kcmil copper or 600kcmil aluminum wire for this load.
It’s important to be familiar with the requirements of Art. 220 so you correctly apply the demand factors for a given installation. Time spent studying those requirements will reduce your errors and make you more efficient.
Article 220 presents various tables and specific requirements to follow. Review and apply each of them one at a time, and you’ll be well on the road to mastering these calculations.
These materials are provided to us by Mike Holt Enterprises in Leesburg, Fla. To view Code training materials offered by this company, visit www.mikeholt.com/code.
Fish Tank Air Stone And Pump Mike Holt is the owner of Mike Holt Enterprises (www.MikeHolt.com), one of the largest electrical publishers in the United States. He earned a master's degree in the Business Administration Program (MBA) from the University of Miami. He earned his reputation as a National Electrical Code (NEC) expert by working his way up through the electrical trade. Formally a construction editor for two different trade publications, Mike started his career as an apprentice electrician and eventually became a master electrician, an electrical inspector, a contractor, and an educator. Mike has taught more than 1,000 classes on 30 different electrical-related subjects — ranging from alarm installations to exam preparation and voltage drop calculations. He continues to produce seminars, videos, books, and online training for the trade as well as contribute monthly Code content to EC&M magazine.