NEC 120.54, 120.55, & 120.84(A)(2): Single-Phase Loads & Three-Phase Service

My project has single-phase dryers, ranges, and/or multifamily dwelling unit panels supplied by a three-phase panel. How are these loads calculated?

Each of these calculations uses the same formula, which has several moving parts, including ones not explicitly mentioned in the code references. A more comprehensive explanation and example is provided in NEC Annex D, Example D5(b), which specifically shows the calculation for multifamily dwelling unit panels. This article provides some additional context, as well as another example.

Underlying Formula & Context

For all three calculations, the steps to calculate the service load are as follows:

1. Find the highest number of ranges, dryers, or panels between any two phases.
2. Multiply that number by 2.
3. Multiply that number by the connected load per unit.
   note

   Because the actual connected loads are known in Design Master Electrical, it will instead use the highest total connected load between two phases, multiplied by 2, for a conservative result.

4. Divide that number by 2 to calculate the single-phase load.
5. Multiply that number by 3 to calculate the three-phase load.
6. Multiply that number by the corresponding demand factor based upon the number of units calculated in step 2.

Example for Ranges

The following example is for electric cooking appliances as defined in NEC 120.55. 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 (kVA)	9.1

note

For this example, Table 120.55 calls for a specific calculated load rather than a demand factor. That load is applied in step 3, in place of the connected load per range.

To calculate the load:

1. Find the maximum number of ranges between any two phases. In this example, phase A-B has the most ranges at 28.

2. Multiply that number by 2.

   $28\>ranges \times 2 = 56\>ranges$

3. Using that number, calculate the demand based upon NEC Table 120.55. 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.

   $25\>kVA + (0.75\>kVA \times 56\>ranges) = 67\>kVA$

4. Divide that number by 2 to calculate the single-phase load.

   ${67\>kVA \over 2} = 33.5\>kVA$

5. Multiply that number by 3 to calculate the three-phase load.

   $33.5\>kVA \times 3 = 100.5\>kVA$

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):