7.1 Main Drain Testing Procedures & The 10% Comparison Rule

7.1 Main-Drain Testing and the 10 Percent Comparison

A main-drain test provides a repeatable indication of water-supply condition at a riser. It is not a full fire-flow test and does not by itself prove hydraulic adequacy. The useful result comes from a controlled procedure and comparison with a valid earlier test under similar conditions.

Current reference and frequency

In NFPA 25 (2023), main-drain provisions are in Section 13.2.2, not the 13.2.5 numbering found in older editions. Table 13.1.1.2 lists the main-drain test at annual or quarterly intervals as applicable. Use the current table, its notes, and Section 13.2.2 to determine the installation's frequency. Do not attach an old subsection number to a remembered backflow or pressure-reducing arrangement.

Preparation

Identify the correct riser, main-drain valve, supply and system gauges, and discharge route. Compare the setup with the prior record. Confirm that the drain can carry full flow without flooding, erosion, ice, exposure to traffic, or contact with electrical equipment. Establish required alarm and monitoring notifications because water movement can operate devices.

Record initial system status and static pressure. Use the same gauge as the prior test when possible and note its identifier, range, and units. Municipal demand, pump operation, tank level, and other water use can affect results, so record unusual conditions that could impair comparison.

Procedure under direct supervision

1. Record the static supply pressure with the main drain closed and no test flow.
2. Open the main-drain valve fully in a controlled manner.
3. Allow the pressure to stabilize while observing the discharge.
4. Record the full-flow pressure and any required time or discharge observation.
5. Close the drain slowly to limit water hammer.
6. Verify that the valve is fully closed, leakage stops, pressure returns toward normal, and alarms or monitoring status restore.

Stop if the drain cannot handle the flow, piping moves dangerously, the gauge fails, uncontrolled leakage develops, or another unexpected condition occurs. Do not throttle the valve to produce a preferred pressure. A partially open drain invalidates comparison with a full-open prior test.

Correct calculation

NFPA 25 Section 13.2.2.3 addresses a 10 percent reduction in full-flow pressure compared with the original acceptance test or a previously performed test. The percentage reduction is:

(previous full-flow pressure − current full-flow pressure) ÷ previous full-flow pressure × 100

If the previous comparable full-flow pressure was 60 psi and the current full-flow pressure is 48 psi:

(60 − 48) ÷ 60 × 100 = 20%

This exceeds 10 percent, so the cause must be identified and corrected if necessary. Do not calculate (static − current flow) ÷ static and call that the historical ten-percent comparison. The static-to-flow drop within one test describes that test condition but is not the comparison specified by the rule.

Interpretation

A reduction can be associated with water-supply change, a valve not fully open, obstruction, backflow-device condition, gauge error, noncomparable flow, or another cause. The Level I technician records the facts and immediately informs the supervisor. Investigation may include confirming valve positions, gauges, test setup, and supply conditions under an approved plan.

Do not state that every result above 10 percent is automatically a critical deficiency or automatically an impairment. NFPA 25 classification depends on the actual condition and its effect on system performance. The threshold means investigation is required.

A result below 10 percent is not a blanket declaration that every supply component is satisfactory. Abnormal static pressure, poor recovery, leakage, valve problems, or other evidence still must be reported.

Documentation

Record property, riser, drain and gauge identifiers, date and time, initial static pressure, stabilized full-flow pressure, final static or recovery observation, prior comparison value and date, calculated percentage, discharge condition, notifications, and restoration. State whether the drain was fully opened and whether alarms were restored.

If the prior test used a different gauge, drain position, or supply condition, flag the comparison limitation. A precise record lets the supervisor distinguish a water-supply trend from a test-method difference and decide what investigation is required.

Exam focus

Keep the denominator straight: the prior comparable full-flow result is the baseline. A current static-to-flow difference is not the historical percentage. Also separate the threshold from the diagnosis—ten percent calls for investigation, while the discovered cause determines classification and correction.