We have a 13 unit multifamily building going up and I have a question about how the engineer sized the service for the building. He used the optional method for each individual unit in order to determine the feeder size to each. Then he took the actual connected load for each unit, totaled them and then applied the derating factors located in the table for multifamily building services in order to determine the building service. Can he mix calculation methods like this? Thanks
multi family service calculation
- Thread starter patc
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- Lockport, IL
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- Semi-Retired Electrical Engineer
Re: multi family service calculation
Just to make sure I understand your question, what I read is that the engineer used 220.30, ?Optional Calculation ? Dwelling Unit,? for the feeder to each apartment, and then used 220.32, ?Optional Calculation ? Multifamily Dwelling,? for the building service. If that is your question, my answer is ?yes, this is acceptable.? The engineer is not mixing calculation types.
But the engineer could mix calculation types, if that served to the owner?s advantage. For example, it is acceptable to use the standard calculation method for each unit, and the optional method for the building as a whole.
But keep in mind that each calculation must be done separately, and in accordance with the rules for that type of calculation. For example, suppose that the engineer used the optional method, including its demand factors, for each unit. Suppose further that the engineer added up the calculated results, and did not, as you described, add up the connected loads of all units. Suppose finally that the engineer then applied the multi-family demand factors to the total building loads. This method would be invalid, for it would apply the building?s demand factors on top of the individual unit?s demand factors. We don?t get to ?double-dip? in this way.
Does this answer your question?
Just to make sure I understand your question, what I read is that the engineer used 220.30, ?Optional Calculation ? Dwelling Unit,? for the feeder to each apartment, and then used 220.32, ?Optional Calculation ? Multifamily Dwelling,? for the building service. If that is your question, my answer is ?yes, this is acceptable.? The engineer is not mixing calculation types.
But the engineer could mix calculation types, if that served to the owner?s advantage. For example, it is acceptable to use the standard calculation method for each unit, and the optional method for the building as a whole.
But keep in mind that each calculation must be done separately, and in accordance with the rules for that type of calculation. For example, suppose that the engineer used the optional method, including its demand factors, for each unit. Suppose further that the engineer added up the calculated results, and did not, as you described, add up the connected loads of all units. Suppose finally that the engineer then applied the multi-family demand factors to the total building loads. This method would be invalid, for it would apply the building?s demand factors on top of the individual unit?s demand factors. We don?t get to ?double-dip? in this way.
Does this answer your question?
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