Water Supply & Distribution

Every building intended for human occupancy shall be provided with an adequate supply of pure, potable water. The water supply shall:

Potable water supply system — requirements & installation. Every occupied building gets an adequate supply of pure, potable water through approved materials, protected from cross-connection by air gaps and backflow devices, sized by water-supply fixture units (WSFU), and maintained at adequate residual pressure (boosted by pump/pressure-tank or elevated tank where utility pressure is insufficient). (NPC §500)

Where an existing building's water service pipe is to be extended or new fixtures added, the plumber shall evaluate whether the existing service pipe is adequate. A new or enlarged service pipe is required when:

• The calculated fixture-unit demand exceeds the capacity of the existing service pipe size (per Table 5-2)
• The available pressure at the meter drops below the minimum required pressure at the critical (highest and farthest) fixture
• The existing pipe shows evidence of scale, corrosion, or flow restriction reducing effective diameter
• The building use changes to a higher-demand occupancy

Water piping pressure test: test pressure = 125% of working pressure or 344.5 kPa (50 psi) air — whichever is greater. All ends capped, all joints visible, hold for minimum 15 minutes. No visible leakage or pressure drop permitted.

Required Air Gap Dimensions

Backflow Prevention Devices

Where an air gap is not practicable, an approved backflow preventer shall be installed:

Air gap — the fundamental backflow prevention concept: supply pipe outlet must be above the flood level rim. Minimum air gap = 2× pipe diameter, never less than 25mm. A submerged inlet with no air gap allows contaminated water to back-siphon into the supply.

Air gap minimum distances for three outlet conditions: unobstructed (2× pipe dia., min 25mm), near a single wall within 300mm (increased), and near two intersecting walls (further increased). Always measure vertically from the lowest point of the outlet to the flood level rim.

Atmospheric Vacuum Breaker (AVB): must be vertically oriented and installed minimum 152mm above downstream piping and flood level rim. No downstream shutoff valve allowed — a downstream valve would defeat the protection by allowing back-pressure to build.

Height comparison: AVB requires minimum 152mm (6 in.) above downstream piping and flood level rim; PVB requires minimum 305mm (12 in.). Both must be vertically oriented. PVB offers higher protection level suitable for irrigation systems.

Reduced Pressure Zone (RPZ) assembly clearances: 305mm above, below, and from side walls; minimum 914mm clearance in front for testing and maintenance. Relief valve discharge must drain to an open, air-gapped drain — never directly to a sewer.

Double Check Valve Assembly (DCVA): same 305mm side/above/below clearances as RPZ, 914mm minimum front clearance. Used for medium-hazard non-toxic applications such as fire sprinkler systems and non-drinking water loops.

Reduced Pressure Backflow Preventer (RPBP): highest mechanical protection for high-hazard toxic applications (chemical feed, boiler makeup, irrigation with fertilizers). Install with isolation valves on both sides, strainer upstream, and test cocks accessible. Relief valve discharge must not connect to sewer.

* Flushometer valves & unlisted outlets: water-supply outlets for fixtures not listed above shall be computed at their maximum demand, but in no case less than the values below (Private / Public):
  a. 9.5 mm outlet — 1 / 2    b. 13 mm outlet — 2 / 4    c. 19 mm outlet — 3 / 6    d. 25 mm outlet — 6 / 10
See §609.9 for the method of sizing flushometer-valve installations using Table 6-6.

For quick planning purposes, the simplified reference below summarizes typical minimum branch sizes against approximate WSFU ranges (mid-range 46–60 psi assumptions, short runs). It is a planning aid only — final sizing must use Tables 6-6.1–6-6.3 with your project's actual supply pressure and pipe-run lengths.

Approximate WSFU bands derived from the lower-load (longer permissible run) columns of Tables 6-6.1–6-6.3 for orientation only — actual minimum size for any specific run shrinks as developed length increases. Always verify against the governing table for your supply-pressure range.

Thermal Expansion Tank (TET): required downstream of a backflow preventer or PRV where a closed system is created. Connect downstream of the backflow preventer, with isolation valve for maintenance. Minimum 305mm clearance on all sides; 914mm in front.

Water heaters — installation requirements. Provide a temperature-and-pressure (T&P) relief valve sized to the heater input, with the discharge piped full-size to within ~150 mm of the floor (no cap or reduction); a drain pan with indirect drain where leakage could cause damage; seismic strapping; full service clearances; and adequate combustion-air supply for gas units. Storage temperature shall not exceed 60 °C. (NPC §500)

• Potable drinking water shall be accessible within 60 meters of travel distance from any occupied work area.
• Drinking fountains are required per §411 minimum fixture table (see Chapter 4). For offices: 1 per 75 persons; for schools: 1 per 100 students.
• Drinking fountain supply shall be connected only to potable water — no grey-water or reclaimed water connections permitted.
• Fountains shall have an angled, non-removable nozzle that delivers water in an arc — the mouth shall not contact the nozzle during drinking (anti-contamination design).
• Minimum pressure at fountain: 138 kPa (20 psi); maximum: 415 kPa (60 psi).

Non-potable water sources may be used for toilet flushing, irrigation, and cooling tower makeup — never for drinking, cooking, or personal hygiene. Requirements:

Pressure limits: static pressure shall not exceed 552 kPa (80 psi) at any point — install PRV where exceeded. Temperature limits: lavatories/sinks/bidets max 49°C (120°F); bathtubs/showers max 49°C; dishwashers max 60°C (140°F); clothes washers max 60°C. Temperature control mixing valves (TCMV) required where scalding risk exists.

Water hammer (hydraulic shock) occurs when a fast-closing valve suddenly stops water flow, creating a pressure spike that can reach 10× the normal working pressure. This can crack pipes, loosen joints, and damage equipment.