Let's break down the requirements and address your specific situation. The short answer is: While not explicitly prohibited, it's highly problematic and likely not compliant with code interpretation and best practices. Here's why

1. The "Dedicated Feeder" Requirement and its Intent:
NEC 695.6(A) (2023 Edition, check your specific edition): "...a reliable source of power shall be dedicated to the fire pump. One of the following shall be permitted..." This is then followed by options, including the one you are considering (Utility Source). NEC 695.6(C)(1) - Reliability

This section is crucial. It emphasizes the need for the fire pump circuit to be independent of other loads and arranged to ensure maximum reliability. It also emphasizes that "Other power equipment shall not be connected to, or permitted to be supplied from, fire pump circuit equipment."

Intent

The purpose of the "dedicated feeder" is to ensure the fire pump has an uninterrupted and robust power supply. It's about survivability during a fire event. Having a single point of failure before the individual fire pump controllers undermines this intent. If the transformer fails, both fire pumps are disabled.

2. Why Double-Lugging and Shared Transformer is Problematic



Single Point of Failure

The transformer becomes a single point of failure. A fault within the transformer or its connection to the primary will knock out both fire pumps simultaneously. This defeats the redundancy and reliability principles central to fire protection.

Overload Potential

While each fire pump feeder individually might be sized correctly, the transformer itself needs to be sized to handle the simultaneous inrush current of both fire pumps starting at the same time. This inrush can be substantial and could damage or trip the transformer, again knocking out both pumps. This is even more of a concern if the transformer is already sized relatively close to the existing fire pump's needs. You'd need to demonstrate that the transformer is grossly oversized to handle the worst-case inrush scenario of both pumps. A transformer is a single failure point.

Maintenance/Testing

Maintenance or testing on the transformer would require taking both fire pumps offline, potentially creating a hazard.

AHJ Interpretation

Even if you technically meet the wording of the code, the Authority Having Jurisdiction (AHJ) is likely to reject the design due to the inherent lack of reliability and the single point of failure. They have the final say.

3. NFPA 20 (Installation of Stationary Fire Pumps for Fire Protection)

NFPA 20 works in conjunction with the NEC. It provides more detailed requirements and best practices for fire pump installations. While it doesn't explicitly forbid this exact scenario, it heavily reinforces the need for reliability and independence of the power supply. The intent of NFPA 20 would be violated by a single transformer feeding multiple fire pumps.

4. Alternatives and Solutions to Consider



Separate Transformers

This is the best and most reliable solution. Install a second transformer, dedicated to the new fire pump. This provides true redundancy.

Generator as Second Source

If allowed by the AHJ, consider using an on-site generator as a second power source for one (or both) of the fire pumps. This adds redundancy and addresses utility power failures.

Diesel Fire Pump

While you want to avoid it, a diesel fire pump provides the most reliable independent power source, as it doesn't rely on the utility grid.

Negotiate with the AHJ

Present your design to the AHJ before proceeding. Be prepared to demonstrate, through detailed calculations and analysis, that the transformer can handle the simultaneous starting current of both pumps without any risk of failure or voltage drop. Also be prepared to demonstrate that the transformer would be inspected/maintained to a higher standard to ensure a lower probability of failure. This is unlikely to be accepted, but it's worth exploring.

5. Double-Lugging is a problem of its own

Even if the above were deemed acceptable, double-lugging conductors on a transformer secondary can pose a safety risk if not done correctly. Consult the transformer manufacturer regarding conductor termination requirements. There may be a need for a secondary distribution block to accommodate multiple conductors.

In Conclusion

While you might be able to argue a technical interpretation of the code to attempt* to justify the shared transformer, it's a high-risk approach. The inherent single point of failure and the potential for overload make it unlikely to be approved by the AHJ. The cost savings of sharing a transformer will be dwarfed by the risk of a failed fire pump during an emergency.

I strongly recommend against this approach and suggest pursuing separate, dedicated power sources for each fire pump. Consult with a qualified electrical engineer and the AHJ to determine the best and most compliant solution for your specific situation. Flag for review