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Underfloor, Trace & Space Heating Specification
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Specifying Electric Heating to BS 7671
Electric underfloor heating (UFH), trace heating, and fixed space heaters are classed as continuous, purely resistive loads ($cos \phi = 1$). Unlike general power circuits which benefit from diversity, heating circuits will draw their maximum calculated amperage for hours at a time. It is critical to specify the correct circuit protection and switching hardware to comply with BS 7671:2018+A4:2026 and prevent thermal damage to switchgear.
The 13A Fused Connection Unit (FCU) Limit
A standard 13A Fused Spur is designed to handle a maximum load of approximately 3,000 Watts (3 kW) at 230V. If your total calculated heating load falls comfortably below 13 Amps, you may feed the thermostat via an FCU spurred off an appropriately rated ring final or radial circuit. However, if the load is exactly 13A, continuous running may cause the fuse and faceplate to degrade over time due to heat generation.
When to Install a Dedicated Radial Circuit
If your heating load exceeds 13A, it cannot be safely supplied by an FCU. You must install a dedicated radial circuit straight from the consumer unit, protected by an appropriately sized MCB/RCBO (e.g., 16A or 20A), using correctly sized cable (e.g., 2.5 mm² or 4.0 mm²) calculated for voltage drop and installation method.
The Thermostat "Burnout" Risk: When to Use a Contactor
The vast majority of domestic underfloor heating thermostats have internal relays rated to a maximum of 16 Amps. If your load exceeds this, wiring the heating mat directly into the thermostat will cause the internal relay to weld shut or burn out, creating a severe fire hazard.
- Under 16A: The thermostat can directly switch the heating load (though running it at exactly 15.9A continuously will reduce its lifespan).
- Over 16A: You must install a suitably rated Contactor (e.g., a 20A, 40A, or 63A modular contactor in an enclosure). The thermostat is used purely to send a low-current control signal to the contactor's coil (A1/A2). The heavy contacts of the contactor (L1/T1) then bear the burden of switching the heavy heating load.