If you have more than four separately controlled space heating units, you can use the optional calculation method in NEC 220.83(B) by adding the following two additional calculations:1. Calculate the heating demand for the largest unit.2. Calculate the heating demand for the second largest unit.

Then, add the results of these two calculations to the heating demand for the four largest units. This will give you the total heating demand for your project.

Total heating demand = Heating demand for four largest units + Heating demand for largest unit + Heating demand for second largest unit

Once you have calculated the total heating demand, you can use it to size the electrical service for your project. Flag for review