You're thinking about this the right way. Here's a breakdown of why your assumption is likely correct: Multi-Family Dwelling Considerations

The 83% demand factor for multi-family dwellings (referring to NEC 220.84 or similar sections)

only applies when the entire building is residential and meets the specific criteria outlined in the code. Mixed-Use Buildings

Since your building has a convenience store (a commercial space), it's considered a mixed-use building. Therefore, you cannot apply the multi-family dwelling demand factors to the entire service.

Load Calculation is Key

You must perform a load calculation according to NEC Article 220 (or your local equivalent). This will involve: Calculating the general lighting and receptacle load for each apartment (NEC 220.14). Calculating the appliance load for each apartment (NEC 220.16). Calculating the convenience store's load, considering its lighting, receptacles, equipment (refrigeration, etc.), and any other connected loads. This will likely be the most significant portion of the calculation. Applying demand factors as permitted for commercial occupancies (NEC 220.42, 220.50 etc.). For example, if the store has a large HVAC system, you might be able to apply a demand factor to its total calculated load. The calculated load will give you the minimum ampacity required for your 300 amp feeders.

Overcurrent Protection

Your 300A overcurrent protection device (OCPD) at the service protects the feeder conductors, not necessarily the individual units. You must still ensure that each 100A breaker is properly sized for the load of the individual unit it serves.

Continuous vs. Non-Continuous Loads

Remember to consider if any loads are "continuous" (operated for 3 hours or more at a time). Continuous loads need to be calculated at 125% of their maximum current.

In Summary

You are correct. Because of the commercial occupancy (the convenience store), you likely can't use the residential demand factors for the entire building. You need to do a full load calculation according to the NEC. Depending on the store's load, your 300A service feeder might be just right, or you might discover that your calculation shows a lower ampacity.

Important



This is for informational purposes only. Always consult the latest edition of the NEC (or your local electrical code) and a qualified electrical engineer or inspector for code compliance and safety. The calculations can get complex, and it's crucial to get them right. Improperly sized feeders can lead to overheating, voltage drop, and potential fire hazards. Your boss should be involved in this calculation process and signing off on the work.Good luck with the project! Flag for review