What is the minimum pipe size connection when piping a steam pressure gauge to the boiler using steel or wrought iron pipe?

The minimum pipe size connection for piping a steam pressure gauge to a boiler using steel or wrought iron pipe is 1/2 inch. This size is standard for ensuring that the gauge can accurately measure the steam pressure within the system without excessive restriction or flow loss associated with smaller sizes.

Using a 1/2 inch connection prevents issues related to pressure drop that can occur in smaller pipes, and it also allows for a more manageable installation, as it's common practice in the industry to use this size for many pressure gauge applications. Moreover, it strikes a balance between providing enough capacity for the pressure transmission while still being practical for installation and maintenance tasks.

For larger options, while they could theoretically work, they might be unnecessary for a gauge application, creating additional costs and complexity without any corresponding benefit. Therefore, the specification of 1/2 inch is widely accepted and used in practice.