Single-Phase Ranges on Three-Phase Services

Single-Phase Ranges on Three-Phase Services

Where two or more 1-Phase ranges are supplied by a 3-Phase, 4-wire feeder or service, the total load must be calculated based on twice the maximum number of ranges connected between any two phases. This calculation is generally required for multifamily dwellings with 3-Phase electrical service. For example, a multifamily dwelling with twelve ranges rated at 12 KVA would be calculated as follows:

4 ranges from A phase to B phase

4 ranges from B phase to C phase

4 ranges from A phase to C phase

The maximum number of ranges connected on any one phase is eight. The NEC® requires that the calculation be based on twice the maximum number of ranges connected on one phase. For this example, 8 x 2 = 16.  Using Table 220.55, Column C for 16 ranges, the total demand is determined to be 31 kW.

There may be situations where the number of ranges will not distribute equally around the three phases. Again, connect the ranges to phases A to B, B to C, and A to C, Take the phase with the highest number of ranges connected to it and multiply by 2 to determine the number of ranges to use. A simple method to find the total number of ranges is to take the total number of appliances and divide by 3. If there is a remainder, round up to a whole number and multiply the whole number by 2. This is the number of appliances that is used for Table 220.55 to determine the service demand. NEC 220.61(B)(1) allows the neutral load to be based on 70% of the ungrounded conductor load.

For example, what is the range load for 32 ranges rated at 12 kW on a 208/120 wye service? Number of appliances per phase = 22/3 = 10.66 – Round up to 11

Number of appliances per phase = 11 x 2

Number of appliances per phase = 22

See Table 220.55, Column C.

Service demand= 37 kW