Cast iron (spun) water pipes: choose the incorrect nominal internal diameter (mm) from the list of typical standard sizes.

Introduction / Context:
Standardization of nominal diameters simplifies procurement, inventory, and fittings for cast iron (and ductile iron) pipe networks. Non-standard sizes complicate jointing and spares.

Given Data / Assumptions:

• Typical nominal sizes progress in standard steps (e.g., 80, 100, 150, 200, 250, 300, 350, 400, 450, 500, 600 mm, etc.).
• Question asks for the incorrect (non-standard) size.

Concept / Approach:
Identify the odd size that does not belong to the common standardized series used in municipal systems and catalogs for C.I. spun pipes.

Step-by-Step Solution:

Verification / Alternative check:
Checking fittings and valves catalogs also reveals the absence of 550 mm as a usual nominal size.

Why Other Options Are Wrong:
The remaining options correspond to standard sizes commonly available in pipes, valves, and specials.

Common Pitfalls:
Assuming any intermediate number could be ordered; network components must align with prevailing standard sizes.

Final Answer:
550 mm