You're right to be concerned! You've identified a common point of confusion in applying NEC 670.4. Let's break down what's happening and where the misunderstanding lies.

Understanding 670.4(A) and 670.4(C)



670.4(A) (Supply Conductors)

This section focuses on the

minimum ampacity of the supply conductors feeding the industrial control panel. It ensures the conductors are large enough to handle the continuous load imposed by the equipment inside the panel. This is about preventing overheating of the wires. 670.4(C) (Overcurrent Protection)

This section deals with the

maximum size of the overcurrent protective device (OCPD) protecting the supply conductors. It's there to protect the conductors from overloads and short circuits. It allows for a larger OCPD than might be strictly required by the conductor ampacity because of the nature of motor starting and other inrush currents inherent in industrial equipment. This is about preventing fires caused by insulation failures.

The Discrepancy and How to Resolve It

The core problem is you're ending up with an OCPD (70A or higher) that exceeds the ampacity of your calculated conductor size (6 AWG, rated for 55A, when using the 75C column). This violates the fundamental principle of overcurrent protection:

the OCPD must protect the conductors.Here's the correct approach, combining both 670.4(A) and (C)


Calculate Minimum Conductor Ampacity (670.4(A))

You did this correctly. Your calculation of 54.69A is the minimum ampacity the supply conductors must have.2.

Select Conductor Size based on Ampacity

Here's where you need to adjust. Since your calculated minimum ampacity is 54.69A, a

6 AWG conductor rated for 55A is insufficient. You must use a conductor with an ampacity of at least 54.69A. Using the 75°C column of Table 310.16, you would need to use a 4 AWG rated for 70A.3. Calculate Maximum OCPD Size (670.4(C))

You did this correctly as well. Your calculation of 73.08A is the maximum OCPD size you can use.4.

Select OCPD Size

You must select an OCPD that is:

No larger than the calculated maximum (73.08A in your case). Large enough to handle inrush currents. The VFD's internal OCPD often handles this. No Larger than the overcurrent protection for the cable.5. Re-Eavaluate

Remember, all you're doing is protecting the conductors of the supply cable. If you select a larger breaker and the VFD is going to handle all of the overloads, the supply conductors are being protected at a minimum.

Example



Minimum Conductor Ampacity (670.4(A))

54.69A

Required Conductor Size (Based on 75°C Column of Table 310.16)

4 AWG

Maximum OCPD Size (670.4(C))

73.08A

Select OCPD

You would choose a

70A OCPD (standard size below your maximum).

Why Both Sections Are Necessary



670.4(A) ensures the conductors can continuously carry the load without overheating. 670.4(C) allows for a slightly larger OCPD than you might initially think, to accommodate the inherent inrush currents of motors and other industrial equipment, but only if the conductors are sized accordingly.

Key Takeaways



The conductor ampacity must always be protected by the OCPD. You can't have an OCPD rated higher than the conductor's ampacity (unless specific exceptions apply, which are unlikely in this typical industrial scenario). 670.4(C) allows for a larger OCPD to handle inrush, but only if the conductors are sized appropriately to handle the continuous load and be protected by that larger OCPD.* Always double-check your conductor ampacity ratings based on the insulation type and termination temperature ratings (usually 75°C).In summary: You calculated the correct minimum conductor ampacity and maximum OCPD, but you initially selected conductors that were too small for the continuous load and the OCPD you were considering. Using a larger conductor (4 AWG) solves the problem and ensures code compliance and safety. Flag for review