Let's break this down and address the likely reason for your contractor's concern.

Understanding the Concern: Dissimilar Metals and Corrosion

The primary reason for using copper bus bars in some electrical applications, especially in outdoor or corrosive environments, is to prevent or minimize galvanic corrosion. When dissimilar metals (like aluminum and copper) are in contact, especially with moisture present, they can form a galvanic cell. This cell accelerates corrosion, potentially leading to: Increased resistance in the connection Overheating Joint failure Reduced lifespan of the electrical systemNFPA 780 and Lightning Protection

NFPA 780 is the Standard for the Installation of Lightning Protection Systems. If your ERCCS (Emergency Responder Communication Coverage System) is part of a lightning protection system, then NFPA 780 absolutely applies.

NFPA 780 (2020 Edition) Considerations (Consult the actual code book for definitive answers)

While I cannot definitively state what the code states without looking at the book, I can make some assumptions.

Bonding

Lightning protection systems require very low resistance connections. Dissimilar metals can compromise this.

Corrosion

Because lightning protection systems are exposed to the elements, corrosion is a major concern.

Material Requirements

NFPA 780 likely has specific requirements for the types of materials used in the lightning protection system, including grounding conductors and bonding connections. It may limit the use of dissimilar metals or specify methods for mitigating corrosion when dissimilar metals are used.

Why Your Contractor is Likely Concerned

Your contractor is likely concerned because:1. NFPA 780 Applicability

The ERCCS system, if it is also part of a lightning protection system, falls under the scope of NFPA 780.2.

Bonding Requirements

NFPA 780 probably requires a continuous low-impedance path to ground, and the Aluminum Bus to Copper Wire connection could be a weak link from a corrosion and resistance standpoint.3.

Inspection

The inspector will check that the system complies with applicable codes (including NFPA 780, if relevant) and will likely flag the aluminum bus bar as a potential issue.

Your Specific Setup: Potential Issues Aluminum to Copper Transition

The transition from aluminum bus bar to copper conductors (#4 to #4) is a critical point. Proper connectors designed for aluminum-to-copper connections are essential. These connectors are filled with oxidation inhibitors and designed to maintain a secure, low-resistance connection over time. If you don't use the correct connectors, you're almost guaranteed to have problems.

NEMA 3R Enclosure

NEMA 3R enclosures are designed to provide some protection from rain, sleet, and snow, but they are not watertight. Moisture can still enter, which exacerbates the corrosion issue.

Recommendations


Get the NFPA 780 Code Book

Purchase a copy of the latest edition of NFPA 780 and review it thoroughly. Pay close attention to sections on grounding, bonding, materials, and corrosion protection.2.

Consult with the Authority Having Jurisdiction (AHJ)

Talk to the electrical inspector or other AHJ before proceeding. Get their interpretation of the code requirements for your specific situation. They have the final say.3.

Use Listed Connectors

Ensure that all connectors used to join the aluminum bus bar to the copper conductors are specifically listed and labeled for aluminum-to-copper connections. Proper preparation of the aluminum surfaces is also crucial (cleaning with a wire brush and applying an antioxidant compound).4.

Consider a Copper Bus Bar

While it may be more expensive, replacing the aluminum bus bar with a copper bus bar would likely eliminate the concern about dissimilar metals and make it easier to comply with NFPA 780.5.

Document Everything

Keep detailed records of all materials used, installation procedures, and any discussions with the AHJ.

In summary, while I cannot say with certainty that the aluminum bus bar will* cause you to fail inspection, it is a significant concern given the potential for corrosion and the requirements of NFPA 780 (if applicable). Your contractor is likely raising a valid point, and it's worth addressing before you get too far into the project. It is best to consult the code book to avoid any future issues and maintain a safe electrical system. Flag for review