How is the approximate maximum fixture units for a cold water system calculated, and what guides this calculation?

• A. Use only actual fixture counts, ignoring code values.
• B. Assume all fixtures will operate simultaneously to size the pipe.
• C. Sum fixture unit values per code, apply demand and diversity factors, and select pipe size from code tables (IPC/UPC or local codes).
• D. Base size on building height alone.

Answer: (C)

The main idea is to size a cold-water supply by estimating demand using standardized fixture units, then adjusting that demand with demand and diversity factors before picking a pipe size from code tables. Each fixture type is assigned a fixture unit value in the plumbing codes, which translates different fixtures into a common measure of water demand. You add up these values for all fixtures served by the same supply, but you don’t treat every fixture as if it will run at full capacity at the same moment. You then apply demand factors to reflect how many fixtures are likely to be used simultaneously, and diversity factors to account for the fact usage tends to be staggered over time. The result is an approximate maximum fixture unit load that the system must handle. With that total, you consult the code’s pipe-sizing tables (IPC, UPC, or local codes) to choose a pipe size that provides adequate flow and pressure with acceptable friction losses. This approach is guided by codes to ensure practical, safe, and economical sizing. Using only actual counts ignores standard demand values; assuming all fixtures operate simultaneously is overly conservative; and sizing based on building height alone ignores hydraulic requirements and pressure considerations.