A branch carries 55 water supply fixture units. The demand table published for this project lists 50 units at 24 gallons per minute and 60 units at 27 gallons per minute, with no row in between. The design procedure for this project directs that where a fixture unit total falls between published rows, the next higher row is used. What demand sizes the branch?
- AA design demand of 24 gallons per minute
This takes the lower row, 50 units at 24 gallons per minute, which credits the branch with less demand than the 55 units connected to it actually produce. Every rounding rule in pipe sizing points toward more capacity, not less.
- A design demand of 27 gallons per minute
- CA design demand of 25.5 gallons per minute
This interpolates halfway between the two rows. The demand curve behind the table is not a straight line, so interpolation is not exact, and the procedure for this project directs the next higher row instead.
- DA design demand of 55 gallons per minute
This treats fixture units as though they were gallons per minute. A fixture unit is an index of probable simultaneous use, not a flow rate, and the two are only connected through the conversion table.
Why B is correct
Fifty-five units sits between the two published rows, and the procedure directs that the next higher row be used, so the branch is sized on 27 gallons per minute. Rounding upward keeps the pipe on the safe side of a curve that flattens as fixture counts rise, which is the reason the rule points that way rather than toward the nearer row.
What this question is testing
This checks whether you round in the safe direction when a fixture unit total falls between table rows, and whether you can keep fixture units and gallons per minute apart in your head.
On the job
Every published demand table has gaps, and every jurisdiction has a habit about how to close them. Some accept interpolation, most expect the next higher line, and a plan reviewer who is used to seeing round numbers will stop on a value that could only have come from interpolation. Writing the row you used next to the number on the calculation sheet saves an argument.
Memory technique
Between two lines, step up: a fixture unit is never a gallon.
Exam tip
When a total lands between rows, take the higher one, and never read a fixture unit total as a flow.
Where to look it up
The demand conversion table is in IPC Appendix E; check whether your jurisdiction publishes an interpolation policy. UPC states size water piping from their own fixture unit table and their own sizing appendix, and the numbers are not the same, so confirm against the code your state adopted.