2.2 Discharge Unit & Fixture Unit Sizing under G13/AS2 & AS/NZS 3500.2

Discharge-unit sizing

A discharge unit is a design value representing the probable hydraulic effect of an appliance. It is not a litre-per-second measurement and should not be converted directly to flow without a method that authorises that conversion. The design process is: choose a compliance path, list connected appliances, total the correct units, select diameter and gradient from that path’s table, and verify all accompanying limits.

G13/AS2 Table 2

For DN100 foul drains, Table 2 lists these maximum loads: 255 DU at 1:40, 205 DU at 1:60, 149 DU at 1:80, 122 DU at 1:100, and 104 DU at 1:120. For DN150 drains, it lists 1040 DU at 1:80, 855 DU at 1:100, 760 DU at 1:120, 677 DU at 1:140, 611 DU at 1:160, 558 DU at 1:180, and 515 DU at 1:200.

Those rows disprove two common shortcuts. First, DN100 does not have one universal 1:60 minimum: a flatter listed gradient may be available for a smaller connected load. Second, capacity changes with gradient; it is incorrect to attach one capacity to a diameter without stating the grade.

G13/AS2 instructs the designer to use the maximum practicable gradient. A flatter row is not a convenience for avoiding level coordination. Establish the outfall and upstream levels, then use the steepest practicable compliant grade while preserving cover, access, and building interfaces.

Do not mix methods

G13/AS2 uses discharge units. AS/NZS 3500.2, cited through G13/AS3, has its own terminology, tables, conditions, and edition. A design cannot take appliance values from one method, capacity from another, and access rules from a third, then call the result an Acceptable Solution. State the path on the drawing and apply it consistently.

Similarly, do not use a branch-length or venting rule without its conditions. A branch serving a gully is treated differently from a general main drain, and fixture-discharge-pipe rules are not automatically drain rules. Read table headings, notes, and definitions.

Worked example

Suppose a DN100 drain serves 118 DU and the level survey makes several grades possible. At 1:120, the table capacity is 104 DU, so that row is insufficient. At 1:100, capacity is 122 DU, so the load fits, subject to all other G13/AS2 requirements. At 1:80, capacity is 149 DU and also fits. Because G13/AS2 calls for the maximum practicable gradient, the designer should not automatically select 1:100 if 1:80 is reasonably achievable.

For a 700 DU commercial load, DN150 at 1:140 carries only 677 DU and is insufficient. DN150 at 1:120 carries 760 DU and fits. The calculation sheet should show the total load, selected row, available levels, and why the grade is practicable.

Verification

Before installation, cross-check every appliance against the correct source table and account for future or unshown connections only where the design brief requires them. On site, measure actual horizontal run and fall. After installation, survey the inverts and compare the actual grade with the selected row. A pipe laser setting is an instruction, not proof of the finished grade; bedding movement, socket lift, and disturbed pegs can change the result.

When a load exceeds a row, valid responses include increasing grade, increasing diameter, revising the route, or using a specifically designed Alternative Solution. Inventing extra capacity or averaging compliant and non-compliant segments is not valid.

Independent source-check exercise

For 2.2 Discharge Unit & Fixture Unit Sizing under G13/AS2 & AS/NZS 3500.2, practise answering from evidence rather than recall. Write the controlling source, its edition or effective date, its scope, and the exact paragraph, table, figure, or manufacturer instruction that supports the decision. Then state the site inputs that can change the result. In this section those inputs include Pipe size and gradient are selected together from the connected discharge-unit load and the chosen compliance path.; G13/AS2 Table 2 gives DN100 capacities from 255 DU at 1:40 to 104 DU at 1:120.; G13/AS2 Table 2 gives DN150 capacities from 1040 DU at 1:80 to 515 DU at 1:200..

Build a four-column check: proposed condition, source requirement, evidence observed, and action. Test the answer with one changed assumption. A different pipe size, discharge-unit load, ground condition, licence status, authority, compliance path, or consent condition may produce a different result. If it does, explain why; if it does not, show which requirement remains controlling.

Finish with a field-verification plan. Name what must be inspected before concealment, what instrument or record demonstrates it, who has authority to accept a change, and what appears on the as-built or completion file. This exercise turns an open-book fact into certifying-level judgement. It also exposes accidental mixing of standards: if the source for a dimension differs from the source for the test or approval, either return to one complete compliance path or document the proposal as an Alternative Solution. Do not promote an example value into a national rule.