Mike Dolt Enterprises
Mike Dolt Enterprises
User: hurk27, Location: Portage, Indiana NEC: 2008
Asked on: Friday, April 22, 2011 at 12:21 pm EDT
Answered on: Tuesday, May 14, 2024 at 4:32 am EDT
Question: I have a lift station the I have to wire and supply the service to, I am having problems with the engineers specs, and installed main breaker in the control cabinet.The control cabinet has already been built, and ready for ship to site.The problem is I believe the 100 amp main breaker is to small that they have installed.We have two 10 hp pumps that for the most time will run one at a time, but if load dictates, can be both brought on-line, so the supply will have to supply both pumps, 3-phase is not available in the area so the engineers installed VFD's to supply 3-phase to the pumps through PLC control.The pump motors are 10 HP with a FLA of 28.4 amps on the motor label @ 230 volts 3-phaseIf you figure the amps at 240 volts single phase via the 746 watts per HP you end up with 31 amps per pump, (which I think is what their doing)But at 28.4 FLA Im seeing just over 49 amps per pump, and this is not including panel heater, PLC and control loads, and loss across the VFD's?I thinking I need a 150-200 single phase amp service, and at least a 150 amp main in the control panel?I had problems before with the wiring in control panels from this engineer before, I'm just trying to come up with why they think a 100 amp 240 volt single phase service will hold this installation?
Power Calculations
Motor FLA: 28.4 amps per pump Single-phase equivalent FLA at 240V: 28.4 amps x √3 = 49.1 amps
Total Load
Pump 1: 49.1 amps Pump 2: 49.1 amps Panel heater: Assume 1-2 amps PLC and control loads: Assume 2-3 amps VFD losses (estimate): 5% of motor load = 2.45 amps per pumpTotal load: 49.1 amps + 49.1 amps + 2 amps + 3 amps + 2.45 amps + 2.45 amps =
108.1 ampsService Size
Based on the calculated load of 108.1 amps, a
150 amp single-phase service is recommended. This provides a reasonable safety factor and allows for future load growth.
Main Breaker
The main breaker in the control panel should be sized to protect the panel and its components. Since the total load is 108.1 amps, a
150 amp main breaker is appropriate.
Response to Engineer's Specifications
You have valid concerns about the adequacy of the 100 amp main breaker installed in the control panel. The calculated load of 108.1 amps exceeds the capacity of the 100 amp breaker.
Recommendations: Request the engineer to revise the specifications to include a 150 amp single-phase service and a 150 amp main breaker in the control panel. Provide the engineer with your calculations and explain the reasoning behind your recommendation. If the engineer does not agree, consider seeking a second opinion from a qualified consulting engineer. Flag for review