Mike Dolt Enterprises
Mike Dolt Enterprises
User: JovialBulge, Location: New England
Asked on: Friday, May 23, 2025 at 11:42 pm EDT
Answered on: Friday, May 23, 2025 at 11:42 pm EDT
Question: I have a scenario in which I have a VFD which is currently being protected by some 60A Class J fuses on the line side. It has been requested of me to add a motor disconnect between the VFD load side and the motor said VFD is controlling. The motor is 25.0HP, 29.50A @ 480 VAC. The motor disconnect that has been specced to use is rated at 30A however it has some conflicting information on the manufacturers website and I'm hoping someone here may be able to clear up this scenario as a whole for me. On the webpage of the device it states that the "SCCR is 65kA RMS Symmetrical when protected with equally sized Class J Fuse", which to me means that if the disconnect is rated at 30A, it would need to be protected by a 30A Class J fuse and all would be well. On the datasheet for the device it reads that its "Suitable for use as a Motor Disconnect per NEC 430-109 (a)(6)Current Rating @ 65,000 RMS Symmetrical Amps up to 600 VAC Max when protected with 40A Class J Fuses, permitting usage at full amperage rating of the switch."As stated earlier the VFD currently has 60A Class J fuses on its line side, and what I believe we need to do is swap those out for 40A Class J fuses (which will be fine based on the VFD's maximum input current being 33.7A) to adhere to the datasheets specs on the motor disconnect. Is this the correct train of thought? I assume that if we were to keep the 60A Class J fuses protecting the VFD, add the motor disconnect on the load side of the VFD, then the SCCR for that branch and subsequently the entire enclosure would be reduced to only 5kA. Is another option here to keep the 60A Class J fuses on the line side of the VFD but then place another set of 40A Class J fuses on the load side of the VFD prior to the motor disconnect? Which seems not correct to me but was suggested by a colleague and I would like to make sure that I am not the only one whom thinks that's "not ok". Yet another option suggested was to just get a higher amperage rated disconnect whos specs stated that it needed to be protected by 60A Class J fuses, which from this manufacturer would mean a 40A disconnect.In addition to the above, another VFD within the enclosure is protected by 25A Class CC fuses and is required to have a motor disconnect as well. The issue with this branch is that the 20/25A disconnect from the same manufacturer also requires it to be protected by 40A Class J fuses in order to achieve its 65kA SCCR rating. This problem with this branch is that the maximum allowable fuse size to meet the UL 61800-5-1 requirements of the VFD manufacturer have the fuse size capped at 30A, which is less than the aforementioned 40A requirement of the motor disconnect, so what do I do with this branch to achieve the 65kA SCCR?
Okay, this is a multi-faceted problem involving VFDs, motor disconnects, SCCR, and UL standards. Let's break it down systematically.
Understanding the Key Concepts VFD (Variable Frequency Drive)
Controls motor speed and torque.
Motor Disconnect
A safety switch installed near the motor to cut off power for maintenance or emergencies.
SCCR (Short-Circuit Current Rating)
The maximum short-circuit current an electrical component or assembly can withstand without failure. It's crucial for safety.
NEC (National Electrical Code)
The standard for safe electrical installations in the US.
UL 61800-5-1
Safety standard for adjustable speed electrical power drive systems.
Class J Fuse
A fast-acting current-limiting fuse known for its high interrupting capacity.
Class CC Fuse
A compact, current-limiting fuse used for control circuit protection.
VFD #1 (25.0HP, 29.5A @ 480VAC, Currently protected by 60A Class J)
Let's analyze your options and address the concerns:1. Replacing 60A Fuses with 40A
This is the
most likely correct approach. The datasheet is clear: the 30A motor disconnect achieves its 65kA SCCR only when protected by 40A Class J fuses. Your VFD has a maximum input current of 33.7A. Therefore, a 40A fuse is sufficient for its protection, while still allowing you to meet the SCCR of the motor disconnect. NEC Requirements
NEC 430.52(C)(1) says the branch-circuit short-circuit protective device shall not be greater than the values shown in Table 430.
52. Table 430.52 allows for 300% of the motor's full load current for inverse time circuit breakers or 175% for fuses. 29.5A x 1.75 = 51.625A, so the 40A fuse will protect the motor per NEC 430.52 and is less than the VFD maximum input current rating.
2.
Keeping 60A Fuses and Adding 40A Fuses Before the Disconnect
This is not recommended. Adding fuses in series like this can lead to coordination issues. The upstream (60A) fuse might not clear a fault fast enough to allow the downstream (40A) fuse to protect the disconnect. This could damage the disconnect before either fuse trips. This solution is unnecessarily complicated and likely won't improve the overall SCCR.3. Using a Different Disconnect (40A Disconnect Protected by 60A Fuses)
This could work, but it may not be the most cost-effective. If you can find a disconnect rated for 40A that specifically states it achieves a 65kA SCCR with 60A Class J fuses, then this is an acceptable solution. However, it would likely be a more expensive disconnect and may require modifications to your enclosure.4.
SCCR with 60A Fuses, Disconnect on Load Side with no Additional Protection
You are correct that the SCCR for the entire branch would likely be reduced. The weakest link in the circuit determines the overall SCCR. If the disconnect is only rated for 5kA without the specified fuse protection, that becomes the new SCCR for the entire assembly.
Regarding the Suggestion to Get a Higher Amperage Rated Disconnect Whose Specs State 60A Class J Fuse Protection
You stated that to get this disconnect from the same manufacturer would mean getting a 40A disconnect, you'll need to double check this. A higher rated disconnect would allow for the existing 60A Class J fuses to remain intact on the line side of the VFD, however as I stated above this would more than likely require additional enclosure modifications and/or expense.
VFD #2 (Protected by 25A Class CC)
This is a more challenging situation. The Problem
The disconnect needs 40A Class J fuses for its 65kA SCCR, but the VFD manufacturer limits the fuses to 30A Class CC.
Possible Solutions (in order of preference)
Consult the VFD Manufacturer
This is the most important step. Explain the situation and ask if they have any solutions or approved alternative fuse types that would allow you to meet the disconnect's SCCR requirements. They might have a specific exception or a modified installation procedure.2.
Use a Different Disconnect with a Lower SCCR
If the VFD manufacturer can't provide a solution, and your facility's electrical system has a lower available fault current, you could use a disconnect with a lower SCCR rating that is compatible with the 30A Class CC fuses. However, you MUST be absolutely certain that the available fault current at that location is below the disconnect's SCCR. A proper short-circuit study is necessary.3.
Consider a Different VFD
As a last resort (and likely impractical), if the SCCR requirements are critical and no other solution is available, you might need to explore a different VFD model that has a more flexible fuse specification.4.
Exception Based on Engineering Judgment (Use with Caution)
In extremely rare cases, if the available fault current at the disconnect is significantly low and the disconnect manufacturer can provide documentation supporting the safety of using the disconnect with a smaller fuse, you might be able to justify an exception based on documented engineering judgment. This requires VERY careful analysis, and the documentation MUST support the deviation. You would be wise to seek advice from a certified professional electrical engineer before pursuing this route.
Important Considerations & Recommendations Coordination Study
Always perform a coordination study to ensure that the fuses and breakers in your system will trip in the correct sequence during a fault.
Documentation
Keep detailed records of all equipment specifications, calculations, and decisions made regarding SCCR and overcurrent protection.
Labeling
Properly label all equipment with its SCCR rating.
Qualified Personnel
Ensure that all electrical work is performed by qualified and licensed electricians who are familiar with NEC and UL standards.
AHJ (Authority Having Jurisdiction)
Ultimately, the AHJ has the final say on whether an installation meets code. Consult with them if you have any doubts or need clarification on specific requirements.
In Summary
For VFD #1, changing the line side fuses to 40A Class J will allow you to achieve the 65kA SCCR that is intended to be gained from the motor disconnect.For VFD #2, your primary goal should be to consult with the VFD manufacturer to find a solution that allows you to meet the disconnect's SCCR requirements. Only consider alternative approaches if the VFD manufacturer cannot provide an acceptable solution. Flag for review