Mike Dolt Enterprises
Mike Dolt Enterprises
User: gsherman, Location: 94606
Asked on: Tuesday, June 1, 2021 at 9:31 pm EDT
Answered on: Thursday, June 20, 2024 at 11:32 am EDT
Question: Hi There! My company is working on an electrification project at 12 unit apartment building. We hired an MEP to determine if the existing 200A service, which serves both the house meter and all (12) 60A apartment panels, has sufficient capacity to handle the new load. However, I am concerned that the electrical engineer is not familiar with the appropriate methodology or calculations, so I decided to turn to this community to get a second opinion.We are planning to install a ducted mini split in each apartment (NTXDKS18A112A & NTXSKS18A112A) to replace the existing fan coil (~ 3 amps). The fan coils are supplied with heated and chilled water that comes from a central plant that is on a completely different electrical service. We are also planning to install a central heat pump hot water heater to serve all 12 units. Once again, domestic hot water was previously supplied by a central plant. We performed a load study of the existing 200A service and measured the peak demand over a 7 day period. According to the electrical engineer, the max load was 25.12 kW, or 87.26 A.My first, and primary question, is likely a simple one. When calculating the added load of the new equipment, what number should we be using from the attached spec sheets? My understanding is that he should use the MCA, but he is using a value that is a good deal below MCA. Thanks in advance for your help!
Enhanced Methodology for Load Calculation
The electrical engineer's reluctance to use the MCA when calculating the additional load could be due to the variability in MCA values. However, for your case, the MCA should be used for the following reasons: The MCA is the maximum current the equipment can draw under normal operating conditions. The MCA provides a conservative estimate of the load, ensuring adequate capacity in the electrical system.
Specific Load Calculations
Ducted Mini Split Units
Use the MCA value of 18 amps per unit. Total load for 12 units: 18 amps x 12 units = 216 amps.
Central Heat Pump Hot Water Heater
Use the MCA value of 30 amps. Total load: 30 amps.
Additional Considerations
Verify the capacity of the main breaker and electrical panel. Consider the diversity factor, which accounts for the fact that not all equipment will operate at maximum load simultaneously. A diversity factor of 0.8 for multi-unit residential buildings is common. Perform a short-circuit and load flow analysis to ensure that the system can handle the increased load without tripping circuit breakers or causing voltage drop issues.
Revised Load Calculation
Existing peak demand: 87.26 amps Additional load (mini splits): 216 amps Additional load (hot water heater): 30 amps* Total estimated peak demand: 87.26 + 216 + 30 = 333.26 amps
Conclusion
Based on the revised load calculation, the existing 200A service is
insufficient to handle the additional load of the ducted mini splits and central heat pump hot water heater. To ensure reliable operation, an upgrade to a higher amperage service is necessary. Flag for review