fused spur for underfloor heating

hi everyone,

I would like to put a fused spur from the ring main that runs through my kitchen and conservatory for some underfloor heating.
The underfloor heating is rated at 2200watts.
I was hoping that if I described what sockets and appliances are on the ring I might get some guidnce as to whether or not this sounds feasable.
The ring main has 2 double sockets and 1 single socket in the kitchen and a fused switch for the boiler controls. There are 2 double sockets in the conservatory. The sockets in the kitchen take a 1.36 kwh washing machine, a toaster, a kettle and a fridge. The ones in the conservatory take a 2200watt heater, a hi-fi and some outdoor fairy lights for the garden.
There is a fused 20a spur from the same ring that feeds a single socket for a tumble dryer.
The kitchen and conservatory are about 5m from the consumer unit and the cooker and hob run from their own seperate fuses from the consumer unit.
It would be ideal if I could spur from an existing socket in the kitchen or using a junction box off the ring.

Any advice appreciated.

Thanks,

Graham.

Sure the WM value is 1.36kW?

Sure the UFH is 2.2kW?

Ring Final btw, not Ring Main.

EDIT: My mistake, you said kWh for the WM.

On the face of it (from what you say) you could place the UFH on a fused spur connected to the ring-final-circuit. If it were me then I would run a new radial circuit in 2.5mm with appropriate MCB based upon the actual load (As SECURESPARK said, are you sure the UFH is 2.2KW?) if so then a 10amp or 16amp MCB would be fine (actually an RCBO would be a better option if your CU allows)

Why do I think you should do this?
1) Because you would not want to lose your UFH, Kitchen Sockets and Boiler in the event of a fault.
2) Because in your case you may abnormally load the ring-final-circuit with the fixed/variable equipment. A ring-final-circuit is designed to have a reasonably balanced load across it (taking into account diversity etc)
Unless you can actually see or map out exactly where the loads occur on the circuit then you will not know if it is well balanced or not.
3) With your fixed equipment and sockets you are likely to have in access of 8KW loading on the ring during certain times (cold day, washing machine/kettle/toaster and UFH on) that’s more than 34 amps. Not a problem for short periods on a well balanced ring but may overload certain lengths of cables within the ring if not balanced.
4) each time you add new junctions to a ring then you increase its resistance slightly (even the tightest connections have more resistance than a continuous piece of wire)
5) If you have gone to all the trouble of installing a UFH system then it surely deserves its own supply..

Whatever you decide to do then you should absolutely do the following:
1) Ensure that you have one or more RCD(s) protecting all circuits (and the UFH if you do not decide on an RCBO)
2) Test the circuit! Continuity, Insulation resistance, loop impedance, RCD/RCB trip time...

Thanks for the quick responses guys.
Unfortuntely I couldn't find the wattage of my particular washing machine on the back or on- line, only the kwh.
The underfloor heating is definitely 2200 watts. (ecoflex underfloor heating) As there is an electric heater in the conservatory I was intending on using it to compliment it rather than be the primary source of heat.
I do appreciate that the UFH having its own circuit would be the ideal way to go but would much prefer to use a fused spur as there are no spare ways on my CU and in addition it would prove very difficult access wise for me..... I could go for the lower wattage version of UFH at 1900watts if you think it would be more prudent????

Please don't think I'm dismissing your advice as it is greatly appreciated but the UFH is just a thought before I tile the floor in the next week or so and if not a feasable and cost- effective undertaking as well as considered safe by the professionals on this forum I will just do without and wear socks when in there in winter!

Understood.. I don't think you need to switch to the 1.9KW rating. The 2.2KW UFH will be fine and I assume you will have a local roomstat/timer? It may be best to set the timer for on-times that avoid peak usage such as breakfast/dinner times etc.

From a regulations point of view what you plan is acceptable provided you make sure the ring is in good condition (that is really important)

To get a feel for the ring condition do the following:
(I am assuming you are reasonably competent with electrics and practice safe procedures)

1) at the CU identify the kitchen ring final circuit.
2) remove the two reds/browns from the mcb
3) remove the two blacks/blues from the neutral block
4) remove the two green-yellows from the earth block
5) With an accurate ohmmeter measure the continuity resistance across
each pair (live-live, neutral-neutral, earth-earth) Expect to see readings under 1ohm on the live and neutral pairs and expect to see something like 1.6 times higher on the earth to earth reading.
If you see anything above 1ohm on live and neutral
Or
If you see a significant difference between the live/neutral pairs
Or
If you see an earth-earth reading much higher than 1.6 times the Live/Neutral readings: Then you will need to check all existing socket/spur connections (loose connections etc) then repeat the test.

This is not the official way to check (test) a ring-final-circuit but it will provide you with a general view of its electrical state. If you have an insulation tester then you should also check all conductors to ensure they have an insulation resistance above 100Mohms (you need to make sure everything (all loads) are unpluged/disconnected

Again, what you plan is OK but not testing it before and after installation is not OK! You must test before and after.

Also I feel that I should mention the following:
Please remember that "officially" you are supposed to inform your LABC prior to installation and then have them inspect the work. Before they inspect the work they will want to see an electrical certificate which you can download from several sites but do you have the test equipment
to be able to provide the data for the cert? The cert will require much more testing then I have proposed above.

sorry for the delay in responding, I was away for a couple of days.

I have carried out the tests as suggested.
live live gave me 0.001 ohms, neutral neutral 0.002 ohms and earth earth 0.001 ohms.
Although all below 1ohm are the readings too low??
Do I need to satisfy the 1.6 ratio or maybe my meter isn't accurate enough to see this??

btw I will be using a local room stat and timer.

I don't think your meter is anywhere near accurate enough to do anything with.

Unless, of course, your ring circuit is only 13.5cm long from end to end, in which case it's fine.

It is 13.5cm...... according to the tape measure that came with the meter

I'll buy a new one and report back with some revised results...

Yea, I thought that might happen. Electricians use ohmmeters that are very accurate from about 0.1 - 200ohms (usually calibrated regularly too)
Most (but not all) "electrical" systems offer resistance values down in the sub 200ohm range. Most (but not all) "Electronic systems" offer resistance readings much higher. Most general multimeters are tailored towards electronics applications rather than electrical though I accept that I'm making that sound very philosophical



Its not only accuracy but also resolution that you need. You might place a 10ohm resister in series with the circuit during measurement which may take your meter into a more linear part of its scale.

1) measure just the resister first and note the reading.
2) now place the resister in series with the circuit using good, tight croc clips.
3) Subtract the resister reading from results.
4) if that does not improve the results then try a 100ohm resister.

If that fails then its an investment in an accurate, hi-res, low range ohmmeter I'm afraid.

The good news is that in general you appear to have good ring continuity for all three conductors which is always a good starting point.

ok, I've tried again as you suggested using 3 different resisters and croc clips.
resister measurement: 10ohms alone. With circuit L-L 11ohms, N-N 11ohms, E-E 12ohms.

resister measurement: 23ohms alone. With circuit L-L 23ohms, N-N 23 ohms, E-E 24ohms.

resister measurement: 47 ohms alone. With circuit L-L 48ohms, N-N 48 ohms, E-E 49ohms.

Is it reasonable to infer from these results that the continuity resistence on the L-L aand N-N is less than 1ohm and that the E-E measurement doesn't appear to be significantly more than these so I can continue with my UFH plan?

Your meter does lack resolution but I would say the results are OK.

The higher earth reading is typical and related to the ratio of cable cross sectional area. With 2.5mm cable the live/neutral conductors are 2.5mm2 and the earth(CPC) is 1.5mm2. Resulting is a ratio of approximately 1.6 which means you would expect the resistance of the CPC to be roughgly 1.6 times higher.

Strongly encourage you to repeat this test after you install the fused spur.

Other safety points:
1) Make sure the ring circuit is protected by an RCD.
2) Make sure you have GAS and WATER main bonding in place
That is: 10mm" earth cable from main earth terminal (maybe inside consumer unit) to within 600mm of gas meter and 600mm of water main as it enters the house (often under sink in kitchen)
3) Assuming you do not have lots of plastic water pipes in the kitchen and bathroom (assuming they are copper) then make sure they are equipotential bonded (sometimes called cross bonding) That is: 6mm2 earth cable clamped between all pipework under sink/boiler/bath/basin etc....

graham1 said:

I'll buy a new one and report back with some revised results...

Click to expand...

Plan B:

http://cgi.ebay.co.uk/BEHA-Unitest-...891?pt=LH_DefaultDomain_3&hash=item2a0bc5f033

http://cgi.ebay.co.uk/Unitest-Beha-..._Measurement_Equipment_ET&hash=item3a60440da5

One other point: When you have the ring circuit open (to add the fused spur) take your readings again at that point. You should find that the N-N/L-L/E-E pairs are open circuit. That will confirm that your spur is really "within" the ring final circuit and not off on some spur.

The test equipment that Ban-All-Sheds lists would be a great investment and allow you to do a range of tests inlcuding insulation resistance and accurate low resitance measurements.

Thanks guys.

Just to recap then, I can feed the UFH from a socket on the ring with a 13amp fused spur. Would I still be ok if I upgraded from 2200 to the 2500watt system?
I'd like to protect the ring with an RCD at the consumer unit 32A 32mA RCBO?

The water main does come into the house under the sink and has 10mm earth bonding. The water pipes are equipotential bonded in 6mm as are the pipes around the HW tank upstairs.

The only thing I'm stumped with is the gas bonding.
The gas meter is on the outside wall and goes underground. The CU is on the inside of the same wall. Should I be able to see the 10mm earth cable between the 2 in this area because I can't, well not easily anyway? Would the gas normally run under the house to the boiler in the kitchen? is there somewhere obvious I should be looking?

sorry to be a pain.

Yes, you can place a 2.5KW UFH system on a a fused spur fused at 13amps.

Connect the FCU (the new fused spur box) to the existing socket with 2.5mm2 twin+earth. before doing that, take the existing socket off, separate the wire pairs then retake your measurements at the CU. Expect to see open circuits between L-L/N-N/E-E pairs. This confirms that your existing socket is really part of the ring. When you have added the FCU you will have three 2.5mm2 cables connected to the existing socket. Go back to the CU and confirm that you have good continuity again between wire pairs.


regarding the gas bonding, you are looking for a metal clamp with 10mm2 green/yellow cable secured within 0.6meters of the gas meter on the consumer side. It is quite typical for a gas pipe to be run from the meter under the floor to gas appliances.
If you can't see a 10mm2 earth bond from your CU or Main Earth Terminal (MET) to within 0.6 meters of the consumer side of your gas meter then you must assume that you do not have gas bonding. You must add it.
If you have brick/block walls then a nice long masonry drill will be needed to drill through the wall then link the CU/MET to the gas meter (within 0.6meters of the consumer side of the meter) with 10mm2 earth cable. A decent hammer drill and good masonry bit will take less then 5 minutes to drill through.