Number of wires from fan coil to the thermostat / three-speed switch: depends on the control scheme - thermostat + normally-closed motorized two-way valve most commonly 6 wires, plain three-speed switch 4-5 wires (wire composition, take-off essentials and code basis included)

Topic Positioning

Answers the common HVAC/electrical take-off and construction question "how many wires from the fan coil to the thermostat (or three-speed switch)": the wire count is not fixed, it varies with the control scheme. This card gives a quick wire-count reference by common control scheme, the wire composition, the quantity-calculation essentials and the current code basis (original texts).

Core Conclusions

1. Wire count varies with the control scheme; the construction drawings and the manufacturer's wiring diagram govern. Common bases:

Control scheme	Common count	Wire composition	Common wire size
Plain three-speed switch (no motorized valve, constant flow)	4-5	Live L + high/medium/low speed control lines; N and PE normally bypass the switch and feed the coil from the power circuit	BV/BYJ 4x1.5 common
Thermostat + normally-closed two-wire motorized two-way valve (most common)	6	Live L, neutral N, 3 speed lines, 1 valve-control line	6x1.0 or 6x1.5 (sheathed or in conduit)
Thermostat + three-wire valve	7	The above 6 + valve "close" control line	>= 0.75 mm2 (mostly 0.75-1.5)
Four-pipe (cooling/heating dual valves) or networked thermostat	8-10 or more	Cooling and heating valves controlled separately, plus communication lines etc.	Per the manufacturer's wiring diagram

2. Take-off essentials: distribution box -> fan coil is the power circuit (commonly BV-3x2.5: L, N, PE), fan coil -> switch is the control circuit; the two conduit runs are measured separately; drawings usually annotate the count directly (e.g. "fan coil to thermostat WDZD-BYJ-6x1.5 / MT25") - calculate the wire-pulling quantities per the annotation; the PE protective wire bonds the coil casing and normally does not pass through the switch; the practical lower limit for control-wire cross-section is 0.75 mm2.

3. Code basis (original texts):

• GB 51348-2019 "Standard for Electrical Design of Civil Buildings", Clause 18.9.3 "The monitoring of fan coils shall comply with the following": 1 fan coils should be automatically controlled by on/off thermostats switching the motorized water valves, with a manual three-speed switch selecting high/medium/low fan speeds; 2 fan start/stop shall interlock with the motorized water valve, and two-pipe fan coils operating in both winter and summer should have a manual summer/winter changeover switch; 3 for places with higher control requirements, dedicated fan-coil micro-controllers should be used; the micro-controller shall provide continuous modulation of the hot-water and chilled-water valves of four-pipe systems and three-speed fan control, with automatic summer/winter changeover for two-pipe systems; 4 the micro-controller shall provide Ethernet or fieldbus communication interfaces forming an open field network layer; 5 networked fan-coil micro-controllers shall allow the building-equipment monitoring system to control fan-coil start/stop and temperature regulation, or may operate as a self-contained system. This clause is the design basis for the wire counts of the various schemes: item 1 corresponds to the most common "thermostat + motorized water valve + manual three-speed switch" scheme; the four-pipe/networked requirements of items 3-5 correspond to the higher-count schemes.
• GB 50738-2011 "Code for Construction of Ventilation and Air-conditioning Works", Clause 15.11.6: the actions of the fan-coil three-speed and thermostat switches shall be correct and correspond one-to-one with the unit's operating states.
• GB 50303-2015 "Code for Acceptance of Construction Quality of Building Electrical Works", Clause 20.1.5: thermostat wiring shall be correct, the display indications normal, and the installation level per the design. Inspection: 10% of each inspection batch, not fewer than 1 set; method: observation.
• T/CBDA 51-2021 "Technical Specification for Residential Decoration Works", Clause 22.3.13: the routing of air-conditioning unit pipes and wires shall comply with: 1 power and control wires of concealed works shall not be bundled with water pipes and shall be routed separately in electrical conduits; 2 wires of different voltages shall not share one conduit; 3 electrical switches and wiring are selected per the design; 4 air-conditioning equipment shall be fully earthed.

Applicable Boundaries

• This card states the engineering-convention bases for conventional civil and public-building fan-coil control wiring and does not replace construction-drawing design; the count and sizes of a specific project must follow the drawing annotations and the manufacturer's wiring diagram.
• The counts per scheme are common bases: valve type (two-wire normally closed / three-wire), presence of a summer/winter changeover switch, networking, four-pipe operation etc. all change the count; different manufacturers define thermostat/valve terminals differently (presence of "open/close" terminals, valve type) - verify the manufacturer's wiring diagram before construction.
• This card is not specific to any region, project or brand; multi-split (VRV/VRF) indoor units and other non-fan-coil systems are not covered.

Further Reading

• Design selection: GB 51348-2019 "Standard for Electrical Design of Civil Buildings", Chapter 18 "building equipment monitoring system", Clause 18.9.3 (three-speed switch, valve interlock, micro-controller and networking requirements).
• Construction and acceptance: GB 50738-2011 "Code for Construction of Ventilation and Air-conditioning Works", Clause 15.11.6 (testing); GB 50303-2015 "Code for Acceptance of Construction Quality of Building Electrical Works", Clause 20.1.5 (thermostat wiring, display, level checks).
• Wiring routing: T/CBDA 51-2021 "Technical Specification for Residential Decoration Works", Clause 22.3.13 (power and control wires in separate conduits, different voltages not sharing a conduit, equipment earthing).
• Practical network references (for comparison): fwxgx Q&A "fan coil and fan-speed switch wiring" https://www.fwxgx.com/questions/572145 ; "wiring of the fan-coil switch" https://www.fwxgx.com/questions/1191593 .