Single or double pole fused spur

[kjacko]

Hi all,
I’m installing electric UFH.
I shall be installing a fused spur off of an existing ring socket.
My calcs for the heating mat will be 4.15a total, so was thinking of putting a 5a fuse in the spur or maybe a 10a(?)
Not decided whether to go for a switched or non-switched tbh.
Also, this is where my knowledge falls down so will have a sparky on-site to help me, but I’d like to know in advance please…….should I get a single or double pole spur socket?
What would dictate my decision? And why?

Thanks for any advice.

 

[bernardgreen]

should I get a single or double pole spur socket?

If the mat develops a fault to Earth then full isolation from the supply would be needed to prevent the RCD tripping. Hence a double pole FCU should be fitted.

 

[winston1]

The fuse is to protect the wiring not the mat. So size your fuse according to the wiring.

 

[kjacko]

The fuse is to protect the wiring not the mat. So size your fuse according to the wiring.

So standard 13a then? Using 2.5mm t&e for the spur.

 

[kjacko]

Also, what’s your thoughts on switched or non-switched? The spur will go inline with the socket it’s being spurred off. So in the hallway, low level with the potential for some nugget to turn it off and kill the heat to the floor.
That said, hopefully it won’t be needed for half of the year, so we’d need to either turn it off or take the fuse out.

 

[winston1]

Once you discover how much it costs to run you may be pleased that someone turns it off.

4.15a at 240v=996 watts or 24kWh per day. At 20p per kWh that is £4.80 a day. £4.80 times 365 days comes to £1752. OK let’s say off for 3 months ‘summer’ that is still £1314. Then there is the 54% energy cost increase coming in April so up to £2023 per year just to warm your toes. Hm how about a pair of furry slippers. Electricity is a very expensive way to provide heat.

 

[kjacko]

Once you discover how much it costs to run you may be pleased that someone turns it off.

4.15a at 240v=996 watts or 24kWh per day. At 20p per kWh that is £4.80 a day. £4.80 times 365 days comes to £1752. OK let’s say off for 3 months ‘summer’ that is still £1314. Then there is the 54% energy cost increase coming in April so up to £2023 per year just to warm your toes. Hm how about a pair of furry slippers. Electricity is a very expensive way to provide heat.

Appreciate your input winston1 but’s that’s quite a bit of jungle maths there surely?!
It wouldn’t be on 24 hours a day, more like 5-8 when needed as it’s really to take the chill out of the floor and provide some rising warmth. It would almost certainly only be on from November to March (as we’ve lived without any form of heat in that area for about 3 years now. So with that in mind it would dent your figures quite a bit I think. The UFH represents about 30% of the cost of the flooring project which we’re ok with. I’d rather have it and not need it, than need it and not have it. And yes, there is the 3rd statement of need it but can’t afford it, but hey ho.
Now, regarding the ampage, would that change depending on the temp we set the UFH at?

Unfortunately wet UFH is not an option due to floor depths and re-instating a rad is out too due to space that is now used as built-in storage.

 

[boringoldcodger]

Also, what’s your thoughts on switched or non-switched?

Previously:

If the mat develops a fault to Earth then full isolation from the supply would be needed to prevent the RCD tripping.

I reckon Bernard's recommending Switched, as with Unswitched you'd only be able to remove the fuse which leaves the Neutral connected.

 

[Taylortwocities]

Back to the original question. I dont think that you can buy a single pole fused connection unit.

 

[bernardgreen]

I reckon Bernard's recommending Switched,

Yes I am.

 

[JohnW2]

Back to the original question. I dont think that you can buy a single pole fused connection unit.

Indeed - and I'm not sure that one has been able to buy a SP one for decades. To be honest, I'm not sure that I have personally ever seen a SP FCU, although I imagine/suspect that they did exist 'once upon a time'!

Kind Regards, John

 

[sparkwright]

To clarify, you need a SWITCHED fused connection unit really, so it can be isolated completely should a fault develop.

I've never seen a single pole switched FCU, they have always been double pole for many decades, though don't know for certain about anything over 60 years or so ago.

The comment someone made about the fuse size depending on the cable size and all that is all very well if you can do the maths, but your theory about the fuse being just over the current rating of the mat is absolutely fine.

 

[SUNRAY]

The fuse is to protect the wiring not the mat. So size your fuse according to the wiring.

Despite the fact that the calculations for maximum load are placed hand in hand with cable size, there is nothing anywhere to indicate it is in anyway incorrect to use a smaller fuse rating than the cable can handle.


Personally I'd use a fuse that is suitably rated for the load rather than the larger capacity of the cable.

 

[JohnW2]

Despite the fact that the calculations for maximum load are placed hand in hand with cable size, there is nothing anywhere to indicate it is in anyway incorrect to use a smaller fuse rating than the cable can handle.

Quite so - and not just fuses. There is certainly nothing 'incorrect' (particularly given that it is essentially universal practice!) to have a 6A MCB/RCBO protecting a (lighting circuit) cable which will commonly have a CCC of 16A or 10A!

Personally I'd use a fuse that is suitably rated for the load rather than the larger capacity of the cable.

Much as I sometimes get criticised or ridiculed for it, that is also usually my practice. As I've often said, I am perhaps one of the world's greatest users of 1A BS1362 fuses Why? ... although I doubt that it will hardly ever make any difference, there is just a chance that a 1A fuse might occasionally minimise the damage to (or prevent destruction of) a very small load on the end of the cable, in the event of an over-current situation.

Kind Regards, John

 

[bernardgreen]

he fuse is to protect the wiring not the mat. So size your fuse according to the wiring.

As others have said the size of a fuse does NOT have to be decided by the amount of current the cable ( wiring ) can safely carry. That only sets the maximum fuse rating for the circuit the fuse is protecting.