Supply Sizing
WSFU to pipe diameter, and what pressure and length do to it.
7 articles
Water supply pipe size for 30 fixture units over a 150 ft run
30 supply fixture units over 150 ft
1″
30 WSFU is about 18 GPM at once — not 30 GPM
| Building supply sizeType L copper, 1.025″ bore | 1″ |
|---|---|
| Simultaneous demandfrom the Hunter curve — the whole point of fixture units | 18GPM |
| Supply fixture units | 30WSFU |
| Velocity at that flowceiling is 8 ft/s for cold, 5 for hot | 7ft/s |
| Friction over the run150 ft of 1″ at 18 GPM | 13.6psi |
| Pressure left at the fixtureassuming 60 psi at the main, an 8 psi meter, and one storey of rise | 34psi |
Notes
- 30 fixture units is 18 GPM, not 30 GPM, and that gap is the entire reason fixture units exist. Nobody runs every tap at once. The Hunter curve converts a count of fixtures into the flow that actually happens together, and it flattens hard: doubling the fixtures from 25 to 50 raises demand from 16 to 24 GPM, not from 25 to 50.
- Adding up the fixtures' rated flows instead is how houses end up with pipe two sizes too big. Oversized supply is not harmless: hot water takes longer to arrive because there is more cold water sitting in the line ahead of it, more water is wasted waiting, and the extra volume sits still long enough to matter for water quality.
- Velocity is the ceiling, and it is about noise and erosion, not about pressure. 1″ at 18 GPM runs 7 ft/s. Past 8 ft/s a copper line becomes audible and starts eroding the inside of its elbows; hot water is normally held to 5 ft/s because erosion accelerates with temperature.
- 34 psi left at the fixture is comfortable. The assumptions behind that are worth checking against your own house: 60 psi at the meter, 8 psi lost through the meter itself, and one storey of rise at 0.43 psi per foot.
- The service line from the street is a separate question, and it is usually the real bottleneck. A 1″ building supply behind a ¾″ service from the main is limited by the ¾″. Older houses often have a galvanised service that has narrowed to half its bore with corrosion — no amount of repiping inside the house fixes that.
- Fixture unit values are not intuitive, so use the code table. A bathtub is 4 WSFU, a lavatory 1, a flush-tank toilet 2.2, a hose bib 2.5, a dishwasher 1.4. A typical three-bathroom house lands somewhere between 30 and 45 WSFU once the kitchen, laundry and outside taps are counted.
Building supply size
50 calculations · browse all
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- Water supply pipe size for 30 fixture units over a 50 ft run
- Water supply pipe size for 30 fixture units over a 100 ft run
- Water supply pipe size for 30 fixture units over a 150 ft run
- Water supply pipe size for 30 fixture units over a 200 ft run
- Water supply pipe size for 40 fixture units over a 50 ft run
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- Water supply pipe size for 50 fixture units over a 150 ft run
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- Water supply pipe size for 50 fixture units over a 250 ft run
- Water supply pipe size for 60 fixture units over a 50 ft run
- Water supply pipe size for 60 fixture units over a 100 ft run
- Water supply pipe size for 60 fixture units over a 150 ft run
- Water supply pipe size for 60 fixture units over a 200 ft run
- Water supply pipe size for 60 fixture units over a 250 ft run
- Water supply pipe size for 80 fixture units over a 50 ft run
- Water supply pipe size for 80 fixture units over a 100 ft run
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- Water supply pipe size for 80 fixture units over a 200 ft run
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- Water supply pipe size for 100 fixture units over a 50 ft run
- Water supply pipe size for 100 fixture units over a 100 ft run
- Water supply pipe size for 100 fixture units over a 150 ft run
- Water supply pipe size for 100 fixture units over a 200 ft run
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