Stormwater Pit Float Switches Australia: Duty/Standby, High-Level Alarms & AS/NZS 3500.3
A specifier's guide for hydraulic designers, pump-station builders and plumbing wholesalers: how many floats a wet well really needs, where each one sits, and what the standard and council guidance actually ask for.
Basement car parks, below-ground amenities and stormwater pump-out pits are the highest-consequence level-control jobs in Australian buildings. A pump that fails during a storm does not merely inconvenience somebody — it floods a habitable or storage area. That is why the wet-well chapter of the stormwater drainage standard and many council water-management manuals expect duplicate pumps, alternate operation, a visible and audible alarm, and a limit on starts per hour. All of those functions begin with float switches. This guide maps the requirements to an actual float bill of materials, so the RFQ is right the first time.
1. Why one float and one pump is usually not enough
Guidance commonly cited from AS/NZS 3500.3 (Stormwater Drainage) for pumped wet wells calls for high- and low-level alarms in each wet well, pumps in duplicate, controls set up to alternate the duty pump at each start, a visible alarm if one pump fails, an audible alarm if both fail, and a control arrangement that limits motor starts per hour. NSW council water-management manuals echo this with a minimum of one duty and one standby pump, an audible alarm at a predetermined level, and a requirement to consider power failure when sizing buffer storage.
That is a system requirement, not a float requirement — a float switch is only one component, and ELEHPD does not claim that its floats certify a complete installation to AS/NZS 3500.3 or any WaterMark scheme. The applicable edition, the class of building and the local authority conditions must be confirmed by the hydraulic designer or licensed plumber. But the practical consequence for procurement is simple: plan for several floats at different heights, plus a control panel, not a single tethered float.
For the broader domestic and rural context, see our rainwater tank float switch guide and the stock trough float valve vs float switch guide.
2. Map each switching level to a float
Australian dual-pump controllers are commonly sold with three floats and derive the stop point from the duty float; installer guides describe four distinct switching levels. Either way, decide the heights before ordering cable lengths:
- Stop / pumps off — lowest. Both pumps stop while still fully submerged, so no pump runs dry.
- Duty pump start — the normal start level. The panel runs one pump.
- Standby / assist start — higher again. If inflow outruns the duty pump, or the duty pump has failed, the second pump joins and an alarm is raised.
- High-level alarm — highest, on an independent circuit. It must still work if the pump control chain has failed.
Space the floats so cables cannot tangle, and keep them clear of the inlet discharge so turbulence does not cause false trips. Guidance cited from AS/NZS 3500.3 places the high-level alarm no higher than 100 mm above the inlet pipe invert, and lower again where flooding of habitable rooms, storage areas or garages could occur — confirm the current edition and local conditions with the designer. A common commissioning check is to lift each float by hand and verify the panel and alarm respond at the intended level.
3. Keep the alarm circuit independent
The whole point of a high-level alarm is that it survives a pump failure. Australian pump-pit alarm panels are typically 240 V / 50 Hz devices with a low-voltage float input: a sealed float on a 10 m lead can often be extended to 50 m because the float side is low voltage, which also reduces voltage-drop problems on long cable runs. Good installation practice puts the alarm on a separate power circuit from the pumps, and provides a voltage-free changeover contact (often up to 10 A at 240 V) for a BMS, dialler or remote indication. Some panels offer high/low mode switching, a mute button and a repeating audible alert.
We supply the float switches, not the alarm panel. On an RFQ, state the alarm panel's input voltage and whether the float must be a simple volt-free contact — that decides the wiring and the float variant, not the pit depth alone.
4. 240 V wiring, contactor inputs and AS/NZS 3000
On anything beyond a very small single-phase pump, the floats are inputs to a control panel or contactor, never direct motor switches. Typical Australian tethered floats are rated 6 A or 10 A at 240 V AC; motor inrush on a 2.2 kW pump exceeds what a float contact should carry, and two pumps plus alternation logic cannot be run from one switch at all. All fixed 240 V wiring is done by a licensed electrician under AS/NZS 3000 (the Wiring Rules), with RCD protection as required. Council manuals also ask for electrical fittings and supply to sit with at least 300 mm freeboard above maximum water level or overland flow paths — a detail that affects where the float cable enters the pit.
Where a solar, telemetry or BMS loop runs at 12/24 V DC, keep that low-voltage circuit electrically separate from the 230/240 V pump circuit. A float rated for 250 V AC does not make the whole installation compliant by itself; it is one component the installer signs off. For the NO/NC and pump-down vs pump-up detail, see how to wire a cable float switch to a pump and the float vs electrode comparison.
5. What kills floats in a stormwater pit
A stormwater wet well is a sediment trap. Car-park grit, silt, leaves and — in mixed-use buildings — fats, oils and grease form a crust that can trap a float and make the pit look empty when it is full. Debris wrapping around the cable ("ragging") stops the float tipping. Turbulent inflow makes a float chatter on and off, driving the short-cycling that the standard asks designers to limit. Annual service checklists in Australia specifically call out float water ingress, cable damage, ragging and manual alarm testing. Specify for that environment:
- Sealing and temperature: IP68CS for permanent immersion, rated 0–70 °C (working range to 80 °C), suitable for clean water, wastewater and mild acid/alkali.
- Endurance: over 100,000 mechanical cycles and over 50,000 electrical cycles — relevant when a panel is limiting starts per hour but the site still sees storm surges.
- Body and cable: moulded polypropylene float with PVC or HO7RN-F flexible cable; 3-core black / blue / brown lead gives both pump-down and pump-up wiring from the same float, with NO/NC stated on the order.
- Tether length and spacing: free cable between tie-off and float sets the on/off differential. Stock lengths 2 / 3 / 5 / 10 / 15 m; a sliding polypropylene counterweight sets the switching level without tools, which makes commissioning several heights in one pit much easier.
- Physical layout: floats clear of the inlet stream, cables strain-relieved and separated so they cannot tangle, and enough room for the float to swing without catching on the pit wall, pipework or guide rails.
All figures above come from our published UKY series specifications. See the float switch selection guide for the full spec checklist, and our septic & AWTS guide for dirtier effluent duty.
6. RFQ checklist for a wet-well job
- Number of pumps (single / duty+standby) and the control panel make — how many float inputs does it expect?
- Switching levels: stop, duty start, standby/assist start, high alarm. Give pit depth and inlet invert level.
- Alarm arrangement: separate circuit? low-voltage float input? volt-free contact for BMS?
- Pump voltage/current, single- or three-phase, and whether floats feed a panel or contactor coil only.
- Cable length per float, NO/NC per float, and whether a counterweight is needed for field adjustment.
- Medium: clean stormwater, silt-laden runoff, grease, or sewage — plus IP/temperature requirements.
Send those six lines and we will confirm the closest models from the ELEHPD catalogue, the datasheets available for them and the lead time. For the full electrical and standards context of the Australian market, see our Australia market page.
FAQ
How many float switches does a duty/standby stormwater pit need?
Typically three or four, depending on the control panel. A common arrangement uses a duty-start float, a higher standby/assist float, a high-level alarm float and a low-level stop (some panels derive the stop from the duty float). Australian dual-pump controllers are commonly sold with three floats, while specifier guides describe four distinct switching levels: pumps off, duty on, standby on and high alarm. Confirm the panel's input count before ordering floats.
Can one float switch run two pumps directly?
No, not as a reliable or compliant arrangement. A float switch is a signal device with a limited contact rating (for example 10 A at 250 V AC); two pumps, their starting current and duty/standby alternation logic need a control panel or contactors. The floats feed the panel inputs, and the panel switches the motors. All 240 V fixed wiring must be done by a licensed electrician under AS/NZS 3000.
Where should the high-level alarm float be set in a stormwater wet well?
Guidance commonly cited from the stormwater drainage standard AS/NZS 3500.3 places the high-level alarm no higher than 100 mm above the inlet pipe invert, and lower again where flooding of habitable or storage areas or garages could occur; alarms should also sit clear of the inlet discharge to avoid false trips. The applicable edition and the local authority's requirements must be confirmed by the hydraulic designer or plumber, not by the float supplier.
Related ELEHPD products
- UKY cable float switch, 2 m cable
- UKY cable float switch, 3 m cable
- UKY cable float switch, 5 m cable
- UKY cable float switch, 10 m cable
- UKY cable float switch, 15 m cable
- Water level controller for pump
Related guides
- Rainwater tank float switch Australia
- Septic & AWTS float switch Australia
- Float valve vs float switch for stock troughs
- How to wire a cable float switch to a pump
- Float switch selection guide
- Cable float vs electrode level switch
Supplying the Australian market
ELEHPD manufactures and wholesales float switches and level controllers to distributors, pump-station builders, panel builders and OEMs. For the Australian market we supply factory-direct with English datasheets and labelling, OEM/private-label options, and short lead times — spares and documentation are supplied from the factory, not from a local warehouse. See our Australia market page for voltage, standard and application notes.
Talk to us about your specification
Send the pit depth, the number of pumps and the switching levels you need, and we will confirm the closest match from the ELEHPD catalogue, the documentation available for it and the lead time. OEM branding and buyer-specific cartons are available on qualified volumes.