What is the permitted maximum size of malleable iron fittings where pressure is at 300 psi?

The permitted maximum size for malleable iron fittings when the pressure is at 300 psi is 6 inches. This is based on the guidelines outlined in NFPA 13, which specifies the appropriate sizes for fittings depending on the service pressure they will experience. Malleable iron fittings are often used in sprinkler systems due to their strength and ability to withstand high pressures, but they also have limits based on the nominal pipe size they can accommodate for any specific pressure rating.

In this case, the specification of 6 inches aligns with the safety and performance expectations for sprinkler systems under typical operational conditions. Larger fittings, such as 8 inches, would exceed the design specifications intended for malleable iron under conditions of 300 psi, leading to potential risks of failure or inefficiency in sprinkler operation.

Thus, understanding the limits placed on fitting sizes helps ensure both compliance with NFPA standards and the safety and reliability of fire protection systems.