Minimum Supply Size to Each Residential Fixture

Why this matters

A tub that takes twenty minutes to fill, a one-piece toilet that refills so slowly the customer thinks it is broken, a pot filler that dribbles: the cause is often a supply that was smaller than the fixture needed for the last few feet, put in because a fitting was handy. The minimum fixture supply sizes are a short table and an inspector reads them at rough-in. This card gives the IPC table for residential fixtures, says exactly which piece of pipe it governs, and runs it against a kitchen and laundry remodel. Sizing the branches and the main that feed these fixtures is a different job, owned by 'Pipe Sizing for Water Supply Reference'.

Which pipe the table governs

The fixture supply is the pipe from the branch to one fixture: the stub out of the wall or floor that the stop valve goes on. It is not the branch line serving several fixtures and not the flexible connector from the stop to the faucet.

IPC 2021 Section 604.5, as adopted by your jurisdiction, sets the minimum size of the fixture supply by Table 604.5, and says in substance that the fixture supply pipe terminates not more than 30 in from the point of connection to the fixture, with any reduced-size flexible connector between the supply pipe and the fixture being of an approved type. So the chain is: branch sized for its load, then a fixture supply at or above the table size ending within 30 in of the fixture, then a listed connector that may be smaller.

Why the connector may be smaller. A short connector costs little pressure because it is short. The table protects the run that can be long, which is the fixture supply in the wall. A 3/8 in compression outlet on a stop feeding a faucet through a braided connector is normal; a 3/8 in tube run 12 ft through a wall to a tub is not.

The table

Minimum fixture supply sizes from IPC 2021 Table 604.5, as adopted by your jurisdiction, for residential fixtures. Sizes are nominal.

The one-piece toilet row. One-piece and many high-efficiency toilets use a fill valve that wants more flow than a two-piece tank, and the table reflects it. A one-piece bowl set on an old 3/8 in stub is the slow-refill callback.

The manifold footnote. The adopted table carries a footnote allowing an individual line from a manifold in a parallel (home-run) system to be one nominal size smaller than the table, under stated conditions of developed length and available pressure. Read the footnote's conditions in your edition before running 3/8 in home runs to a tub or shower.

Nominal size is not bore

Two pipes with the same nominal size do not carry the same water. Type L copper at 1/2 in nominal has an inside diameter of about 0.545 in. SDR 9 PEX at 1/2 in nominal has an inside diameter of about 0.485 in, since its wall is one ninth of its 0.625 in outside diameter. Bore area goes with the square of the diameter: (0.485 / 0.545) squared is about 0.79, so the PEX has roughly four fifths of the copper's flow area, before the insert fittings narrow it further. Cold-expansion fittings narrow it less than crimp or clamp inserts do.

The table is written in nominal sizes, so either pipe meets it. But on a long fixture supply to a fixture that wants flow (a soaking tub, a body-spray shower), a PEX run at the table minimum delivers noticeably less than copper at the same size. That is a sizing decision on top of the table, not a code failure. See 'PEX Installation Procedure Reference' for fitting types.

Telling an undersized supply from everything else

On a slow-fill or weak-flow call, the supply size is one suspect among several, and it is the expensive one to fix because it is inside a wall. Rule out the cheap ones first, in this order:

1. The stop. A multi-turn stop that was never fully opened, or a quarter-turn stop with a reduced port, restricts more than any length of pipe behind it. Open it fully and re-test.
2. The fixture's own restriction. An aerator, a flow restrictor in a showerhead, a fill valve screen on a toilet. Pull it and test the open flow.
3. The house pressure. Read static and flowing pressure at a nearby outlet (the two numbers are defined on 'The Pressure and Temperature Numbers on Every Residential Call'). A house at 45 psi static that falls to 20 flowing has a branch or main problem, and the fixture supply is not where it lives.
4. The fixture supply. Only when the first three are clean does the stub size become the answer. Time a bucket fill at the fixture with the restrictor out, compare it with the rated flow on the fixture's spec sheet, then look at what was stubbed.

What an undersized supply looks like. Flow is weak at that fixture only, it falls further when a neighboring fixture on the same branch opens, and the stub out of the wall is smaller than the table row. A tub filler with a 3/8 in tube stub behind the escutcheon is a typical find in an old remodel. A fixture that is weak while every other fixture is fine, on a stub that meets the table, points back to the fixture or to a blockage, not the size.

UPC: same idea, different table

The UPC does not use the IPC's Table 604.5. It sizes water supply from water supply fixture units, and its fixture unit table in Chapter 6 carries a minimum fixture branch size column of its own. Do not assume the IPC's 3/8 in entries carry over; read the column in the adopted UPC edition for each fixture. The same distinction applies: that column governs the branch to the fixture, and the building's distribution is sized from the total fixture units.

Worked example: a kitchen and laundry remodel

The house is on the IPC, 2021 edition as adopted. Static pressure 62 psi, copper main, PEX for the new work. The new layout:

• Kitchen sink with a pull-down faucet, at the island.
• Dishwasher beside the sink.
• Pot filler on the range wall, cold only.
• Refrigerator ice maker.
• Laundry moved to a closet: washer outlet box and a laundry tub.
• New hose bib on the back wall, frost-free.

 FIXTURE SUPPLY CHECK - IPC 2021 Table 604.5
 ----------------------------------------------
 FIXTURE         TABLE   DRAWN    RESULT
 kitchen sink    1/2     1/2      pass
 dishwasher      1/2     3/8      FAIL  stub size
 pot filler      n/a     1/2      see note
 ice maker       n/a     1/4      see note
 washer box      1/2     1/2      pass
 laundry tub     1/2     1/2      pass
 hose bib        1/2     1/2      pass

The dishwasher. The draft drew a 3/8 in PEX stub to the dishwasher, the old habit of tapping it off a 3/8 in outlet. The table says 1/2 in for the fixture supply. The fix is a 1/2 in stub for the dishwasher, or a 1/2 in hot stub at the sink with a dual-outlet stop whose second outlet feeds the dishwasher connector. Either way the fixture supply is 1/2 in.

The 30 in termination on the dishwasher. A dishwasher connector routed from the sink base under the cabinets commonly runs well over 30 in. Whether your authority reads an appliance's own listed connector under the 30 in rule, or wants the fixture supply brought closer to the appliance, is a question to settle with the inspector before the cabinets go in, not after.

The pot filler and ice maker. Neither is a row in the table. A pot filler is commonly supplied at 1/2 in to match its inlet, and an ice maker on its own tubing per the refrigerator maker's instructions. Where a fixture has no row, the maker's installation instructions and the authority decide the size, and the instructions are part of the listing.

The count. Seven fixtures. Five have a table row; four of those five pass as drawn and one, the dishwasher, fails. The other two have no row and go to the maker's instructions.

Re-check from the branch side. The kitchen sink hot, dishwasher and laundry tub hot all hang off one 1/2 in PEX hot branch in the draft. The table does not rule on that; whether one 1/2 in branch carries those three is a branch-sizing question on 'Pipe Sizing for Water Supply Reference'. A plumber who reads Table 604.5 as the branch size would conclude a 1/2 in branch is fine because every fixture on it is 1/2 in, which is the exact misreading this card exists to prevent.

What would change the answer. If the house were fed from a manifold with home runs, the footnote might allow 3/8 in individual lines to some of these fixtures, but only if the run lengths and the available pressure meet its conditions. At 62 psi static with short runs that may hold; on a long two-story run it may not.

How to verify you got this right

• Each stub on the rough-in carries its size and the table row it was checked against, and any fixture with no row has the maker's instruction sheet in the job file.
• Every fixture supply ends within 30 in of its fixture, or the authority's position on an appliance connector is written down.
• The branch feeding a group of fixtures was sized separately, not read off this table.
• Any reduction under the manifold footnote has its run length and pressure recorded beside it.

References

• IPC 2021 Section 604.5 and Table 604.5, as adopted by your jurisdiction.
• UPC 2021 Chapter 6 water supply fixture unit table, as adopted by your jurisdiction.
• The fixture or appliance maker's installation instructions, which are part of the listing.
• See related: Pipe Sizing for Water Supply Reference; PEX Installation Procedure Reference.
• See related: How to Plumb a Kitchen Sink with Disposer and Dishwasher; The Pressure and Temperature Numbers on Every Residential Call.