I am hoping to have under floor heating fitted, can I just add this to my existing electrical circuit?
Before adding any additional fixed load to an existing electrical circuit, there are some checks that need to be carried out.
- What is the total load already on the circuit?
- What is the total additional load to be added?
- Is there enough spare capacity?
- Is the current earthing to the property correct?
- Will the circuit comply with the current British Standards BS7671 IEEE wiring regulations (17th edition)?
Working out the extra load of the under floor heating mat
For example, if you have an area of 42metre and you use 150W/M matting, it will use 4 x 150W = 600 Watts. Divide the 600 Watts by the standard 230 Volts = 2.61 Amps.
This would be acceptable to run from an existing 32 Amp ring main via a 13 amp fused spur unit.
The circuit would have to be fed via a 30mA residual current device (RCD), plus using the manufacturers safety thermostats and timers etc.
Some under floor heating systems can use up to 242metres with 3.6KW on each thermostat. Loads such as this would require separate circuits and indeed the capacity of the main supply may have to be recalculated if used throughout the property.
Is the current earthing to the property correct?
When any electrical work is carried out, the current system needs to be checked to ensure that it is earthed correctly. The earth is often supplied by the main electricity supplier. It can be supplied either as protective multiple earthing (P.M.E.) or via the main cable sheath and is usually made of lead. Sometimes the property owner may have to supply their own earth via an earth rod, stake or metal plates burried in the ground, known as a TT system.
As a contractor, we sometimes find that there is no main connection to earth at all, this is highly dangerous and needs to be rectified.
To ensure that the system is setup correctly, we temporarily disconnect the main cable going to the earth, then use test equipment to make sure the the earth is functioning correctly. Problems usually arise when the the system uses a stake or rod, as these can get damaged or even removed completely. On some occasions new protected multiple earth have been installed but not connected. This again means that the system lacks a main earth, making it highly dangerous, and is another reason why you should get your electrical system tested at least once, then repeat testing every ten years.
Original questions from Simply Spalding and answers provided by Kelvin Goulden