Okay, let's break down your questions about wiring this Kay Industries rotary phase converter (RPC). This situation needs careful consideration, and getting the right information is crucial for safety and code compliance.1. Sizing Conductors and Single-Phase FLA The Blank FLA on the Nameplate

It's highly unusual for a rotary phase converter nameplate not to list the single-phase input FLA. That's the most important piece of information for sizing the input conductors and overcurrent protection. The fact that it's left blank raises a red flag.

Contact Kay Industries

Your first action must be to contact Kay Industries. Explain that the FLA is missing and you need that value for proper installation. There could be a specific reason it's blank, but you need that information directly from the manufacturer.

Using 250% of Motor FLA (Rooftop Units) - Not Appropriate

This is

incorrect and potentially dangerous. The 250% rule (Article 430) applies to sizing conductors for a single motor to allow for starting current. You're dealing with an RPC supplying power to multiple motors. You cannot simply add the motor rated ampacities, and you CERTAINLY can't apply the 250% rule for a single motor to the entire load on the RPC. Using 125% FLA - Maybe, but... Using 125% of the total connected load (sum of the rooftop unit's nameplate ampacities) is a better approach than 250% of a single unit, BUT it's still not the proper way to size the input conductors to the RPC. The RPC has internal losses and must supply reactive power; therefore, the single-phase input current will be higher than the sum of the three-phase load currents. 125% of the total connected load could potentially work on the output side between the RPC and the distribution panel, BUT it may be undersized. 125% of the RPC's input FLA is the only way to properly size the input conductors. Why the Blank FLA Matters

The missing FLA suggests a problem. It could be: A manufacturing error. A specialized model with a highly variable input current depending on the specific load. A potentially mismatched component within the RPC.

Sizing Based on Output Load (Once you have the Input FLA)


Get the Input FLA from Kay Industries. This is non-negotiable.2. Size the Single-Phase Input Conductors

Use NEC Article 310.15 and Table 310.16 to determine the ampacity of the conductors. Factor in derating for ambient temperature and conduit fill, if necessary.

The conductor ampacity must be at least 125% of the RPC's single-phase input FLA.3. Size the Three-Phase Output Conductors (RPC to Panel)

This is where you can consider the connected load of the rooftop units. Calculate the total three-phase load in amps. For each rooftop unit: `Amps = (HP 746) / (Voltage 1.732 Efficiency Power Factor)` If you only have "Max Rated Amps" from the nameplate, use that as a conservative estimate. If you have the MCA, you must use this value to size the conductors. Sum the individual rooftop unit ampacities. Multiply the total ampacity by 1.25 (125%). This is your minimum conductor ampacity for the conductors between the RPC and the MLO panel. Select conductors from Table 310.16 with an ampacity equal to or greater than the calculated value, considering derating factors.

2. Overcurrent Protection Between RPC and MLO Panel Generally Not Required - With Caveats: You are likely correct that a separate overcurrent device between the RPC and the MLO panel is not required, BUT this depends on specific factors: NEC 240.21(C) - Tap Rules

The general principle is that you can tap conductors without overcurrent protection at the tap point if you meet certain conditions, specifically, Article 240.21(B)(1). The length of the tap conductors cannot exceed 10' and the ampacity of the tap conductors must be at least 10% of the rating of the overcurrent device protecting the load side of the feeder.

Overcurrent Protection at RPC Input

The overcurrent protection on the single-phase input to the RPC

must be sized to protect the RPC itself. This overcurrent device (circuit breaker or fuses) is your primary protection. MLO Panel Main Breaker (If Any)

If your "main lug only" panel has a provision for a main breaker, you could install a breaker there. This would provide additional protection for the branch circuits within the panel.

This is highly recommended. MLO Panel Busbar Rating

Ensure the busbar rating of the MLO panel is sufficient for the calculated load.

Important Considerations



Coordination

The overcurrent protection on the input side of the RPC should be coordinated with the potential overcurrent protection in the MLO panel (if you install a main breaker). You don't want nuisance tripping.

Short-Circuit Protection

The OCP on the input side should be sized to protect against short circuits on the output side. This is why getting the correct FLA is so important.

Manufacturer's Instructions

Review the Kay Industries documentation carefully. They may have specific recommendations for overcurrent protection on the output side.

Summary and Recommendations1. Contact Kay Industries Immediately

Get the single-phase input FLA for your specific RPC model. Do not proceed without this.2.

Size Input Conductors Correctly

125% of the RPC's input FLA, considering derating.3.

Size Output Conductors Based on Connected Load

Calculate the total three-phase load of the rooftop units, then multiply by 1.25.
4.

Consider a Main Breaker in the MLO Panel

This adds an extra layer of protection and makes it easier to add/change branch circuits.5.

Review Kay Industries Documentation

Look for any specific recommendations for overcurrent protection on the output side.6.

Coordinate Overcurrent Protection

Ensure the input and output overcurrent devices are coordinated.7.

Double-Check Calculations

Have a qualified electrician review your calculations and wiring plans before installation.8.

Use High-Quality Components

Use appropriately rated conductors, connectors, and overcurrent devices.

Disclaimer: I am an AI chatbot and cannot provide electrical advice. This information is for educational purposes only. A qualified and licensed electrician must perform the installation and ensure it complies with all applicable codes and regulations. Improper wiring can result in fire, electrical shock, and equipment damage. Flag for review