Okay, let's break this down and see if we can find any potential issues and explore alternative solutions

The Setup (Recap)
Primary

480V Delta, fed from 50A breaker on a distribution panel.

Transformer

480V Delta to 380Y/220V Secondary, 10-15' from the panel.

Secondary Disconnect #1

60A fused disconnect immediately off the transformer secondary.

Secondary Disconnect #2

40A fused disconnect fed from the load side of the 60A disconnect, feeding the equipment.

Equipment OCPD

The equipment itself has 40A overcurrent protection.

Potential Issues and Considerations1. Series Rating

You're essentially creating a series-rated protection scheme. While not explicitly prohibited, ensure both disconnects and their fuse classes are approved for series rating when applied in series. This is crucial to ensure proper clearing of faults. Look for labels on the disconnects and fuses indicating their series rating capability.2.

Selectivity/Coordination

While you're aiming for protection, the coordination between the 60A and 40A fuses is paramount. If a fault occurs downstream (at the equipment), the 40A fuse should clear the fault before the 60A fuse. Otherwise, you lose the benefit of having the 40A fuse there in the first place. Review fuse time-current curves (TCCs) from the manufacturer to confirm coordination under various fault conditions. If the 60A clears first, it defeats the purpose of the 40A fuse. A fuse class that has an inrush current capability may resolve this issue.3.

Redundancy of 40A OCPD

The piece of equipment already has 40A of OCPD. While not wrong to add another 40A OCPD ahead of it, it's arguably redundant unless you need a lockable disconnect immediately at the equipment. A single fused disconnect at the transformer is acceptable if that is the case.4.

Transformer Size & Calculations

I assume the transformer's kVA rating is appropriate for the 40A load. Ensure your calculations are accurate based on the 380V secondary voltage. Calculate Secondary Current: (kVA 1000) / (√3 380V) = Amps Verify Transformer FLA is adequately protected by your 60A OCPD5.

Accessibility

Check your local code regarding the accessibility and location of disconnects. The disconnects need to be readily accessible.

Alternatives to Consider1. Single Fused Disconnect (60A)

If the equipment's 40A OCPD is sufficient and you're mainly looking for a disconnect for servicing the equipment, you can use a single 60A fused disconnect right off the transformer secondary.2.

Single Non-Fused Disconnect

If the equipment already has a 40A OCPD and you ONLY need a physical disconnect, you could use a non-fused disconnect. It simply provides a visible break.3.

Main Lug Load Center (Smaller Size)

You mentioned wanting to avoid this due to cost. However, check prices on smaller 4-6 space main lug load centers. Sometimes the cost difference compared to two disconnects and associated labor may not be significant, and it offers greater flexibility and potential future expansion.4.

Combination Disconnect/Overload Relay

If the equipment requires motor overload protection, consider a combination disconnect/overload relay designed for motor protection. This combines the disconnecting means with proper overload protection tailored to the equipment's motor.5.

Tap Rules (240.21(C)): Depending on your local interpretation, the 10ft or 25ft tap rules may apply if you are feeding the transformer with higher amperage conductors. This may lead to more acceptable scenarios from an inspection standpoint.

Recommendations1. Prioritize Simplicity

Unless there's a compelling reason for redundancy or very specific coordination requirements, a single, appropriately sized fused disconnect is usually the most cost-effective and reliable solution.2.

Check with Local AHJ (Authority Having Jurisdiction)

Before proceeding, discuss your plan with your local electrical inspector or AHJ. They can provide guidance on local code interpretations and requirements that may affect your design.3.

Coordination Study

If you must use the series-rated disconnects, a basic coordination study is highly recommended to verify proper fuse selection.4.

Document Everything

Ensure your calculations, fuse selections, and series rating information are well-documented and readily available for inspection.

In summary

Your proposed approach is potentially viable with careful attention to series ratings and coordination, but it may be unnecessarily complex. A single, appropriately sized fused disconnect or a non-fused disconnect (depending on your needs and the equipment's OCPD) is often a simpler and more cost-effective solution. Always consult with your local AHJ for approval.

Disclaimer: I am an AI chatbot and cannot provide official electrical engineering advice. Consult with a qualified electrical engineer and your local electrical inspector to ensure your design meets all applicable codes and safety standards.* Flag for review