You're right to be hesitant and question the previous engineer's directive. While daisy-chaining might seem like a quick fix, it can lead to several issues and is often not the best practice. Here's a breakdown of why your idea of a dedicated circuit is better, and the considerations involved

Why a Dedicated Circuit is a Better Solution:
Safety and Code Compliance

Relying on existing circuits without verifying capacity can easily lead to overloading. A dedicated circuit ensures the load calculations are accurate and the branch circuit is appropriately sized to supply the VAVs safely.

Voltage Drop

Daisy-chaining from existing circuits, especially if they are already loaded, increases the risk of voltage drop, especially at the end of the chain. This can cause the transformers inside the VAV's to operate inefficiently or even malfunction.

Fault Isolation

If one VAV transformer fails in a daisy-chained system, it can potentially trip the entire existing circuit, affecting other equipment that wasn't intended to be on the same circuit. A dedicated circuit isolates the VAVs.

Maintenance and Troubleshooting

With a dedicated circuit, maintenance personnel can easily isolate the VAV system for troubleshooting and repairs without affecting other critical equipment. It's also much easier to trace the wiring for future modifications.

Documentation and Traceability

A dedicated circuit is clearly documented on the electrical panels and drawings, making it easier to understand and maintain the system over its lifespan. Daisy-chaining often lacks clear documentation and can lead to confusion.

Future Expansion

A dedicated circuit provides headroom for future expansion or modifications to the VAV system without overloading existing circuits.

Load Calculation

Accurately calculate the load of the VAVs. Is it a continuous load? This will impact your breaker size. If you daisy chain and create a circuit overload, it is going to be hard to identify as the issue as a lot of troubleshooting will be required.

Why Daisy Chaining is Problematic



Overloading Existing Circuits

The biggest risk is overloading the existing circuit, potentially causing nuisance tripping or even a fire hazard. You need to verify that the existing circuit has enough capacity to handle the additional load of the VAV transformers before connecting anything. This requires a complete load calculation of the existing circuit, and often that is a headache to conduct.

Unknown Loads

You may not have a complete picture of what else is on the existing circuit. Other equipment may have been added since the original design, further increasing the risk of overloading.

Voltage Drop

As mentioned earlier, adding loads to an existing circuit increases the risk of voltage drop, especially if the VAV's are far from the source.

Potential for Unauthorized Modifications

Relying on existing circuits can make it easier for future technicians to tap into the same circuit for other purposes, further increasing the risk of overloading and creating a system that deviates from the original design.

Code Violations

Daisy-chaining from an existing circuit without proper consideration for load and overcurrent protection can violate electrical codes.

Your Proposed Solution: Dedicated 277V, 20A/1P Circuit

Your suggestion of providing a dedicated 277V, 20A/1P circuit and daisy-chaining the VAVs from that dedicated circuit is a much better approach. Here's why: Control

It gives you control over the load calculations and ensures that the circuit is properly sized.

Safety

It reduces the risk of overloading existing circuits.

Documentation

It allows for clear documentation on the electrical plans.

Maintainability

It makes troubleshooting and maintenance easier.

Important Considerations


Load Calculation

Calculate the total load of all the VAV transformers in amps at 277V. Make sure the total load is less than 80% of the breaker rating (for continuous loads). For non-continuous loads, it can be sized to 100% of the rating.2.

Wiring Size

Select the appropriate wire gauge for the 277V circuit based on the calculated load and the length of the run to minimize voltage drop.3.

Overcurrent Protection

Install a 20A breaker in the panel to protect the circuit.4.

Disconnecting Means

Consider local code requirements for disconnecting means for the VAV equipment. This might require a local disconnect switch near the VAVs or a lockout/tagout procedure for the panel.5.

Grounding

Ensure proper grounding of the VAV equipment and the dedicated circuit.6.

Coordination

Review the specifications for the VAV's to verify voltage and amperage requirements.7.

Permitting

You will likely require a permit for this new circuit, and your local AHJ may have specific requirements.8.

Coordination with other trades: Make sure your design takes into account clearances from mechanical, architectural, and plumbing systems.

Recommendation



Reject the original approach. Document your reasons for rejecting the daisy-chaining approach based on the concerns outlined above. Implement your proposed solution. Provide a dedicated 277V, 20A/1P circuit for the VAV transformers, ensuring proper load calculations, wiring size, and overcurrent protection. Document everything. Update the electrical drawings to show the new circuit and the connections to the VAV transformers. Consult with a senior engineer. If you are still unsure or lack the experience, consult with a more experienced engineer or your supervisor.By taking a proactive approach and addressing the potential issues with the original directive, you can ensure a safer, more reliable, and easier-to-maintain system. Flag for review