Okay, let's break down the situation and address the potential issues with this design, focusing on the NEC and POCO concerns.

Understanding the Setup

You have an existing Main Distribution Panel (MDP) with overhead Current Transformers (CTs) for metering the entire service. You're proposing a new Automatic Transfer Switch (ATS) that will:1. Tap into the existing MDP.
2. Feed power out of the MDP.3. Run that feeder to the ATS.
4. The ATS will then feed back into the MDP.5. This "looped" feeder ultimately lands on the same circuit that it originally tapped from.

Why POCO is Likely Rejecting the Design (230.7)

NEC 230.7 states: 230.7 Other Conductors in Raceway or Cable. Service conductors shall not be installed in the same raceway or cable with conductors that are not service conductors.The POCO's concern is likely rooted in the fact that you are bringing a generator source back into the MDP. Once the generator is running, the generator source will be considered non-service conductor. This will be considered a violation.

Potential Problems and NEC/Safety Concerns


Backfeed and Metering Issues

This is the primary problem. The CTs are designed to measure the net power flow into the building from the utility. By creating a loop around the CTs with the ATS, you introduce a potential backfeed situation. When the generator is running, the CTs might:

Meter the generator output as if it were power being imported from the utility. This could result in the customer being incorrectly billed (credited). Create inaccurate readings and billing disputes. Cause protection coordination issues. The POCO's protective devices might not respond correctly to faults if the power is flowing in an unexpected direction.2. Parallel Operation (Synchronization)

Is the ATS designed to parallel with the utility? If so, this is a very complex situation that requires careful synchronization and protective relaying. Typically, residential and small commercial ATSs are not designed for paralleling. They're designed to switch between the utility and the generator. Paralleling requires a sophisticated control system to ensure voltage, frequency, and phase angle are matched before the generator is connected to the utility.

Accidental paralleling can damage equipment and be dangerous.3. Overcurrent Protection

The overcurrent protection for the feeder running to the ATS must be sized to protect the conductors and the load connected to the ATS. Also, you need to ensure proper coordination with the MDP's existing overcurrent protective devices.4.

Fault Current

Consider the available fault current from both the utility and the generator. The MDP's busbars and connected equipment must be rated to withstand the maximum fault current available from either source.5.

225.30 Number of Supplies. This section limits the number of supplies to a building or structure. If the generator is considered a second supply, you need to comply with this section (and the exceptions). Typically, an emergency system is not counted as a second supply if certain conditions are met.6. Suitability of Equipment

Is the ATS rated for service entrance use? This might be necessary if it's being installed before the main service disconnect.7.

Local Amendments

Always check with your local electrical inspector and utility for any specific amendments or requirements they might have.

Alternatives & Solutions (Discuss with POCO) Relocate CTs (Preferred)

The ideal solution would be to have the POCO relocate the CTs to measure both the utility feed and the generator feed. This is the most accurate and transparent way to handle the situation. The CTs would then be after the ATS.

Standby System Only

Ensure the ATS is a transfer switch only, not a paralleling switch. The generator is not designed to parallel the utility source. It will only supply the load once the utility has failed.*

Dedicated Generator Connection

If paralleling is desired, a completely separate, dedicated generator connection to the MDP with its own overcurrent protection and metering may be required. This would likely involve a much more complex (and expensive) setup with specialized relaying and synchronization equipment.

Recommendation



Do not proceed with the installation without approval from the POCO. Schedule a meeting with them to discuss your design and explore alternative solutions. Provide detailed drawings and specifications for the ATS and generator to help them understand the system. Be prepared to adjust your design based on their feedback and requirements.

In summary, the POCO's rejection is likely valid due to metering and potential paralleling concerns. Relocating the CTs or implementing a non-paralleling transfer switch are the most likely solutions, but require close coordination with the POCO. Flag for review