Q: My project has single-phase ranges supplied by a three-phase panel. My manual load calculation for the ranges based upon NEC 220.55 doesn’t match Design Master’s calculation. How is the load being calculated?
A: This calculation has several moving parts, including one that is not explicitly mentioned in NEC 220.55 itself but is shown in Annex D, Example D5(a). This article will provide another example with some additional context. Relevant information about the project is as follows:
|# of Ranges, Phase A-B||28|
|# of Ranges, Phase B-C||20|
|# of Ranges, Phase C-A||24|
|Connected Load per Range||9.1 kVA|
To calculate the load:
- Find the maximum number of ranges between any two phases.
In this example, phase A-B has the most ranges at 28.
- Multiply that number by 2.
28 ranges * 2 = 56 ranges
- Using that number, calculate the demand based upon NEC Table 220.55.
This example will use Column C (rating between 8.75 and 12 kVA). For 56 ranges, the demand is 25 kVA plus 0.75 kVA for each range.
25 kVA + (0.75 kVA * 56 ranges) = 67 kVA
- Divide that number by 2 to calculate the single-phase load.
67 kVA / 2 = 33.5 kVA
- Multiply that number by 3 to calculate the three-phase load.
33.5 kVA * 3 = 100.5 kVA
The panel schedule footer will display the connected load and number of ranges for each phase, as well as the calculated load, rounded to the nearest integer (in this case, 101 kVA):