The CBC and CFC reference NFPA 2 for H2 specific requirements.  However, I’ve seen reference to the Hydrogen Fuel Gas Piping Manual published by IAPMO.  What are the differences between NFPA 2 and this other standard?  The CBC/CFC currently do not appear to specifically adopt the Hydrogen Fuel Gas Piping Manual, but will future code cycles be adopting this standard?

This is a complex and sometimes contentious topic. The International Code Council (ICC) addresses hydrogen piping in the International Fuel Gas Code (IFGC). NFPA 2 (2023) references both the IFGC and ASME B31.3 for piping, although this will change in the 2026 edition to allow the use of either document. The hydrogen piping provisions in the IFGC are dated and in need of revision, while ASME B31.3 remains the predominant standard for hydrogen piping systems, offering extensive design guidance for a wide range of piping types.

In California, the IFGC does not apply directly; instead, the state uses the California Plumbing Code (CPC), which mirrors much of the IFGC but omits the hydrogen-specific sections. Therefore, hydrogen piping in California must comply with the CPC. 

Regarding the IAMPO document, it appears to focus on residential appliances and is derived largely from natural gas appliance standards. It is not comprehensive (18 pages total), is limited to 5 psig, and allows plastic underground piping similar to natural gas installations. The "Applicability" paragraph is below.   

The content appears more aligned with permitting and approval processes—using terms such as “listed for use” and “authorized”—rather than providing detailed design and operational safety guidance. While NFPA 2 currently contains only basic piping provisions and refers to ASME B31.3 for detailed design requirements, the IAMPO document could be considered for reference in the future if applicable. However, at this stage, it appears to be a narrow, appliance-focused standard rather than a broad design code for hydrogen piping systems.