Okay, let's break down this VFD service sizing issue in Houston and address your concerns

Understanding Normal Duty vs. Heavy Duty on VFDs:
Normal Duty

This rating typically assumes a lighter load profile with less frequent starts and stops, or a load with lower torque requirements. The VFD can handle higher currents for a shorter period.

Heavy Duty

This rating is for more demanding applications involving frequent starts/stops, high inertia loads, or loads requiring high torque over a sustained period. The VFD's current limit is reduced to accommodate these stresses.

Why the Higher Input Current for Normal Duty?

The VFD input current is determined by the VFD's internal components' capabilities to handle a given load and the required duty cycle. With the reduced load in normal duty, the internal components need to accommodate for less "stress" allowing the VFD to pass more amperage from the service to the motor.

Your Load Calculation and Concerns

You're right to focus on minimizing the service size to avoid utility easement nightmares. Let's examine your approach:1.

Using VFD Input Current



YES, you must use the VFD's input current rating for sizing your service entrance conductors, overcurrent protection, and disconnecting means. The utility sees the VFD as a load that draws current from their system. They don't care what the VFD outputs to the motor.2. Choosing Normal Duty vs. Heavy Duty



Here's the critical point

You can't just arbitrarily choose the "lower" rating. You MUST select the appropriate duty rating based on the

actual load requirements of the pumps. Question for you

What is the actual application of these pumps? Are they handling variable loads (e.g., fluid transfer with fluctuating pressure)? Are they starting and stopping frequently? Are they driving high-inertia equipment?

Justification is Key

You need to be able to justify your choice to an inspector (or the utility, if they ask). If the application clearly fits the "Heavy Duty" definition, you'll need to use that higher current value. If not, you need to provide the calculations or reason behind selecting "Normal Duty."3.

NEC 430.128 and Disconnect Sizing

Your understanding of 430.128 is correct. You can size the disconnecting means at 115% of the VFD input current if the VFD is marked with the minimum supply circuit conductor ampacity and maximum OCPD rating. You are correct in stating that 125% as stated in article 430.110 is no longer needed.
4.

Load Calculation Check

Given the latest information, you need to use the VFD input FLA for both pumps, and size your service based off that.

Pump 1 (400HP)

598.1A (Normal Duty)

Pump 2 (200HP)

315.3A (Normal Duty)

Largest Motor

598.1 x 1.15 = 687.8A

Total Load

687.8 + 315.3 = 1003.1A5.

Impact on Service Size



1003A Exceeds your desired 800A threshold significantly. This is likely going to trigger the larger pad-mount transformer requirement with the utility, and the corresponding easement issues.

Recommendations and Strategies


Re-evaluate Duty Requirements

Carefully document the actual load profile, start/stop frequency, and torque requirements for both pumps. Can you definitively justify using the "Normal Duty" rating based on the actual application? If so, you're still in trouble exceeding 1000A2.

Discuss with VFD Manufacturer

Contact the VFD manufacturer and explain your situation. Ask them for guidance on the appropriate duty rating based on the specific application. They might have insights or suggestions.3.

Demand Factor or Non-Coincident Loads

Can you justify a demand factor? Will both pumps never run at their full rated load simultaneously? This is very difficult to prove.4.

Alternative Service Voltage (if possible)

Check with the utility about the possibility of using a higher service voltage (e.g., 12.47 kV). Higher voltage means lower current for the same power, potentially allowing a smaller transformer. This is likely a long shot but worth asking.
5.

Phase-in the Service (Very Complex)

Could you install the service in phases? Start with a smaller service adequate for one pump. Then, upgrade the service when the second pump is added. This requires careful planning and coordination with the utility and may not be feasible.6.

Consider Motor Efficiency Upgrades

While it won't significantly reduce the service size, using premium-efficiency motors can lower the input current slightly. Every amp counts!

Important Considerations



Utility Approval

Always get your load calculations and service design approved by the utility before you start construction. They have the final say.

NEC Compliance

Ensure your design complies with all applicable sections of the NEC, particularly Article 430 (Motors, Motor Circuits, and Controllers).

Documentation: Keep thorough documentation of your load calculations, justifications for duty ratings, and communications with the VFD manufacturer and utility.Let me know more about the specifics of the application and I will revise my response. Flag for review