This is a situation where erring on the side of caution is best. Here's a breakdown of the considerations and a recommendation

Understanding the Situation
40 HP Baler Motor

A 40 HP motor is a significant load. It will have high starting current (locked rotor current).

Your Circuit

100A Panelboard Breaker #4 AWG Wire (upsized for voltage drop – good practice) Baler's Internal 100A Breaker

The Question

Can the baler's internal 100A breaker be considered supplemental, allowing you to keep the existing #4 wire and 100A panelboard breaker?

NEC Code Considerations 430.52 - Motor Branch-Circuit Short-Circuit and Ground-Fault Protection

This is the key NEC article to consider. It dictates the maximum size of the branch circuit protection for a motor. It's not simply about protecting the wire. It's about allowing the motor to start (with its inrush current) without nuisance tripping, while still providing fault protection.

430.22 - Single Motor - Conductors

Determines the minimum ampacity of the conductors to the motor.

430.53 - Several Motors or Motor(s) and Other Load(s)

Deals with situations where multiple motors (or a motor and other loads) are on the same circuit. Likely not applicable here, but good to be aware of.

110.3(B) - Installation and Use

Listed or labeled equipment must be installed and used per its listing. This means following the manufacturer's instructions for the baler. If the manufacturer specifies a maximum OCPD (overcurrent protective device) size, you must adhere to it.

Supplementary Overcurrent Protection (SCOP)

Article 430.53(A)(2) allows using SCOP in addition to required branch-circuit protection but cannot replace the required branch-circuit protection

SCOP cannot replace the branch circuit breaker required by the NEC. Voltage Drop

While you've addressed voltage drop with the #4 wire, it's important to recalculate voltage drop once the actual breaker size is determined.

Steps to Determine the Correct Solution1. Baler Nameplate



This is the single most important step. Find the baler's nameplate and look for: Full Load Amps (FLA)

This is the running current of the motor.

Maximum Overcurrent Protection Device (MOCP)

This specifies the maximum breaker or fuse size allowed. This is critical. The nameplate may refer to Inverse Time Breaker or Dual Element Fuse.

Minimum Circuit Ampacity

The minimum ampacity of the conductors.2.

Motor Code Letter (If Available)

The nameplate may have a motor code letter (A, B, C, etc.). This is used with NEC Table 430.7(B) to determine the locked rotor KVA (LRKVA). The LRKVA is needed to calculate the locked rotor amps (LRA). If the code letter is not on the nameplate, you can assume a code letter of H (NEC 430.7(B)).
3.

Calculate Minimum Wire Ampacity (NEC 430.22)

Ampacity must be at least 125% of the motor's FLA: `Conductor Ampacity = 1.25 FLA`4.

Determine Maximum Overcurrent Protection Device (NEC 430.52)



Use the actual FLA and motor code letter from the nameplate. This article provides the maximum allowed breaker or fuse size. You can't exceed this. Use NEC Table 430.52 to determine the maximum percentage of FLA allowed for different types of OCPD's. For example: Inverse Time Circuit Breaker

Typically, 250% of FLA.

Dual Element (Time Delay) Fuse

Typically, 175% of FLA. If the nameplate says "Maximum Overcurrent Protection Device: 100A", that's your hard limit.5.

Check Nameplate vs. Calculated Values



Nameplate MOCP must be followed. If the calculated maximum breaker size is greater than what's on the nameplate, use the nameplate value.6. Verify Wire Ampacity

Ensure the #4 wire's ampacity (based on its insulation type and installation method in NEC Table 310.16 and adjustment factors in 310.15) is greater than or equal to the calculated minimum wire ampacity (step 3).
7.

Recalculate Voltage Drop

Now that you know the actual breaker size, re-run your voltage drop calculations to ensure they are still within acceptable limits.

Recommendation

Based on what you provided, I would recommend increasing the wire size. Baler's Internal Breaker is Likely Not Just Supplementary

It is more than likely the only way to protect the motor from short circuits and ground faults

It is likely that the MOCP for a 40hp motor is greater than 100A

Refer to NEC Table 430.52 (Inverse time breaker 250%, dual element fuse 175%)

Example (Illustrative - You MUST use the Baler's Nameplate)

Let's assume (and this is a big assumption) the baler's nameplate says: FLA: 50A Motor Code: G Maximum Overcurrent Protection Device: 125A (Inverse Time Breaker)1.

Minimum Wire Ampacity

1.25 50A = 62.5A (Your #4 is probably fine, depending on installation).
2.

Maximum Breaker

250% 50A = 125A. The nameplate also says 125A.3.

In this hypothetical scenario, you would need to increase the breaker to 125AImportant Considerations



Manufacturer's Instructions Prevail

The baler manufacturer's instructions and nameplate information always take precedence.

Local AHJ

Consult your local electrical inspector (Authority Having Jurisdiction - AHJ) for their interpretation of the code and any local requirements.*

Safety First

Motor faults can be dangerous. Don't cut corners.

In summary: Get the baler nameplate information. Perform the calculations based on NEC 430. Follow the manufacturer's instructions. Err on the side of caution and safety. Flag for review