How to Size a Branch From Fixture Units Without Guessing

Why this matters

Three separate checks govern a horizontal branch, they are governed by different rules, and the branch size is the largest answer the three return. Sizing by the first check that passes is the most common way a remodel branch gets built one size light, and it is invisible on the day of the install because a new branch drains beautifully at half its design load. It shows up eighteen months later as a fixture that works alone and backs up when the house is busy, on a pipe that is buried in a slab. The worksheet below exists to force all three answers onto paper so the one that governed is written down rather than assumed.

The worksheet, and what each field is for

Fill this in per branch. Every field has a rule attached, and a blank field is a guess.

Step 1: list the fixtures, and pick the use column before any arithmetic

The use column is a statement about frequency, not about the fixture, and a mixed-use building can legitimately carry two columns in two parts of one system. Pick it first and write it down, because changing it later silently changes every number below it. The companion article on what a fixture unit is actually counting owns the reasoning here.

Step 2: convert continuous flow, do not count it as a fixture

Condensate pumps, ice machines, commercial dishwashers on a continuous drain, water treatment backwash. These discharge with a probability of one, so the diversity built into a fixture unit does not apply to them. Model codes convert them at a stated rate, commonly on the order of two fixture units per gallon per minute; take the rate from the plumbing code your jurisdiction has adopted and amended. Converted units go into the same subtotal as the fixtures.

Step 3: check A, the fixture unit capacity of a HORIZONTAL branch

This is the check people think is the whole job, and it has one trap in it: a horizontal branch and a vertical stack of the same nominal size do not carry the same fixture unit load. Water in a stack falls in an annulus around a core of air and reaches terminal velocity in a short drop, so a stack moves a great deal per unit of bore. A horizontal branch is an open channel running partly full, and its capacity is set by depth and velocity at its slope. In the IPC as published, a 2 inch horizontal branch is tabulated at 6 fixture units and a 3 inch horizontal branch at 20, while a 3 inch stack over one branch interval carries several times the horizontal figure; your adopted table's numbers govern, and the building drain has its own table that varies with slope. The number to hold is not the integer, it is the fact that the horizontal figure is much the smaller of the two.

Reading a stack figure against a horizontal run is the single highest-yield sizing error in a remodel, because it always errs toward a smaller pipe.

Step 4: check B, the minimum size the largest fixture forces

Independent of every total. A water closet requires a minimum drain size in every adopted plumbing code this trade meets, commonly 3 inches, and no fixture unit arithmetic reduces it. A fixture's trap size sets a floor on the pipe leaving it. On short branches this check is usually what governs, because the totals there are far below any table's capacity.

Do this check even when check A has already returned a large answer. It costs one line and it catches the case where a total-driven size happens to land below a fixture minimum.

Step 5: check C, the geometry

Two parts, and both are pass or fail rather than a size.

Fall over the trap arm against one internal diameter. If the arm falls more than about one internal pipe diameter between the trap weir and the vent connection, the water surface can reach the crown of the pipe at the connection and seal the vent off, and the fixture self-siphons no matter how good the vent above it is. Print both numbers, not the verdict.

Slope maintained over the developed length. A branch is only at its tabulated capacity if it holds its slope the whole way. A run that has to duck a beam and come back up has a local reversal in it, and the capacity table no longer describes it.

Step 6: take the largest of the three, then write down which one governed

The governing check is the field with the most value to the next person, because it tells them what changes the answer. A branch governed by check B does not get bigger when fixtures are added. A branch governed by check A does.

Before you cut into the existing branch

Opening a live DWV line releases sewer gas at the opening and may release standing water: cut with the fixture group upstream out of use, keep your face out of the plane of the cut, and ventilate the space rather than relying on the room's air. If the customer has used a chemical drain product on that line, the standing liquid above a partial blockage is a caustic or acid splash exposure and needs sealed goggles, a face shield and chemical-resistant gloves rated for the product class from its safety data sheet.

Solvent cement and primer for plastic DWV are an inhalation route, not a skin route: use them with the space actively ventilated and your head out of the vapor path, take the exposure controls from the product's safety data sheet, and where a respirator is the control it belongs under a written program meeting 29 CFR 1910.134 rather than a dust mask off the truck. Cutting cast iron with a chain cutter releases stored energy in the chain and the pipe drops when it parts: support both sides before the cut.

Worked example: a basement half bath added to an existing branch

Existing. A 2 inch horizontal branch serving a kitchen sink and a laundry standpipe. Private use column throughout.

Proposed. Add a lavatory and a water closet on the same branch, plus route the air handler's condensate pump discharge to the laundry standpipe.

Filled worksheet:

Read what each line did. The fixture values came from the adopted table and are shown as illustrative integers, the same way your own table's integers would enter. The continuous flow was converted rather than counted, and it added 2 units, which is 20 percent of the 10 unit subtotal from a single small pump. The largest-fixture minimum was checked even though check A had already reached 3 inch, and it is the answer that survives: adding two more small fixtures later raises the subtotal but does not move the size, while removing the water closet would drop check B out and leave check A at 10 units, still calling for 3 inch. Both roads land on 3 inch here, and naming which one governs is what tells the next tech that this branch has 10 fixture units of headroom (20 minus 10) before check A takes over.

The counterfactual that shows why the horizontal column matters. Suppose this branch also picked up a second-floor bathroom group and the subtotal reached 24 units. A 3 inch stack carrying those same 24 units passes comfortably. The 3 inch horizontal branch does not: 24 is greater than 20, and the horizontal run goes to 4 inch while the stack above it stays 3 inch. A tech who read the stack figure across to the horizontal run would have built a 3 inch branch, and it would drain acceptably until the day both floors are busy at once.

The failure mode in the field. An undersized horizontal branch does not back up. It runs deeper than it should at peak, which means the free air area above the flow shrinks, which means the branch starts acting as a pressure source for the traps connected to it. The complaint that arrives is a gurgle at the lavatory when the washer empties, and it gets diagnosed as a vent fault, because it is one: the vent is fine and the branch has stolen its air space. That is why the branch size and the venting behaviour are the same subject.

How to verify you got this right

• Confirm three answers exist on the sheet. A worksheet with check C blank has not been done, it has been started.
• Re-add the subtotal with the continuous flow line covered up, then uncover it. If the size did not change, say so; if it did, the pump is a load-bearing part of the design and belongs in the record.
• Check that check A was read off the horizontal column. Say the word horizontal out loud while your finger is on the table.
• State the headroom as a count, not as a feeling: this branch has 10 fixture units left before check A governs. That number is what a future remodel needs.

References

• The plumbing code as adopted and amended by the local jurisdiction, for the horizontal branch, stack and building drain capacity tables, minimum fixture drain sizes and the continuous flow conversion rate; the model documents are the IPC (ICC) and the UPC (IAPMO), neither of which binds on its own
• 29 CFR 1910.134, where a respirator is the selected control for solvent cement or primer vapor
• Safety data sheets for solvent cement, primer, and any drain-cleaning product present on the line being opened
• See related: What a Drainage Fixture Unit Is Actually Counting; Why Drainage Pipe Is Sized for Partial Flow and Not Full