Mike Dolt Enterprises
Mike Dolt Enterprises
User: erickench, Location: Brooklyn, NY
Asked on: Thursday, March 31, 2011 at 7:52 pm EDT
Answered on: Tuesday, May 14, 2024 at 3:35 am EDT
Question: Hi Guys, just started reading my book titled "Stallcups Electrical Calculations Simplified". I came across a calculation procedure which Stallcup did not complete. Here is the problem:What is the load for 40 duplex receptacles used for continuous and 40 duplex receptacles used for noncontinuous duty in illustration 1-6(a)?The procedure that Stallcup used was this:Continuous duty load: 180VA x 40 x 125% = 9000VANoncontinuous duty load: 180VA x 40 = 7200VAIn applying the demand factors of NEC Table 220.44 Stallcup applies the 100% demand factor to the noncontinuous loads. See the problem? We usually only apply the 125% when the demand factor is 100%. If I were calculating this load I would apply the 100% to the first 9000VA of continuous load and then add on 1000VA to make it 10000VA and to the remaining VA I would apply the 50% demand factor which in this case would be noncontinuous load as it should be. Thus we have9000 + 1000 + (7200 - 1000) x 50% = 13100VAThe only problem now is what would happen if the continuous load went over 10000VA? How do we bring ourselves to apply both a 50% demand factor and 125% to the same quantity of VAs? Opinions gentlemen!
Opinion 1: Apply Demand Factors Sequentially Apply the 125% demand factor to the first 10,000 VA of continuous load, resulting in 12,500 VA. Then, apply the 50% demand factor to the remaining noncontinuous load, which is 7,200 - 1,000 = 6,200 VA, resulting in 3,100 VA.
Total Load
12,500 VA + 3,100 VA =
15,600 VAOpinion 2: Weigh the Demand Factors Average the 125% and 50% demand factors, resulting in an average demand factor of (125 + 50) / 2 = 87.5%. Apply this average demand factor to the entire noncontinuous load of 7,200 VA, resulting in 6,300 VA.
Total Load
9,000 VA (continuous) + 6,300 VA (noncontinuous) =
15,300 VAWhich Opinion is Correct?
Neither opinion is explicitly stated in the NEC. However, it is generally accepted that demand factors should be applied sequentially, as in Opinion 1. This method ensures that the total load will not be underestimated if the continuous load exceeds 10,000 VA.
Conclusion
Based on the precedence of applying demand factors sequentially, the correct answer is Option 1: Apply Demand Factors Sequentially, resulting in a total load of 15,600 VA. Flag for review