FCU not permitted for immersion heaters, really?

[ericmark]

It seems the IET has pointed out due to the time an immersion heater runs for, we should not use FCU to supply them.

OK, easy enough to fit a cooker unit, but that means 16 amps, not a 13 amps supply. I am happier to have my iboost+ supplied with a 13 amp supply. It is an expensive lump, I want it to have a 13 amp fuse, even if there is a 16 amp RCBO feeding the FCU.

I would not think my immersion heater runs that long? 35 kWh over 28 days is 1.25 kWh per day average. It will start to use power on a sunny day around 11 am, and it will be using way under 3 kW to start with, so highly unlikely to draw 3 kW in daylight hours for more than ½ an hour, at night it can draw 3 kW for 5 hours when on off-peak, but unlikely again to draw it for more than ½ an hour as only topping up.

I have seen these

used with immersion heaters for years. I suppose one could set it hour on, hour off, but the whole idea of not having a FCU at the immersion introduces safety lock off questions. For an electrician or plumber to work changing an immersion heater with a local switch is great, easy to lock off, and no worries about if the correct circuit has been isolated. With a cooker connection unit, to replace the FCU one would need to isolate at the consumer unit, and then we have the problem of MCB locks not fitting all makes.

It just seems daft to me, we have used FCU for years, what has changed?

 

[Harry Bloomfield]

It just seems daft to me, we have used FCU for years, what has changed?

I have never liked running 13amp plugs, or FCU's, close to the limit, for long periods, due to the heating up of the 13amp fuses. 20amp DP switches/ flex outlets, are much less troublesome.

 

[Gasguru]

Everyone ok with using 1.5mm² to the immersion heater (that's what the cable clamps designed for) on a 16A breaker?

 

[AndyPRK]

Who the hell uses a 13A fcu for an immersion. Madness

 

[Ragnar_AT]

Everyone ok with using 1.5mm² to the immersion heater (that's what the cable clamps designed for) on a 16A breaker?

The load limits the current and 1.5 mm2 flex is good for 16 A anyway.

Also keep in mind that a 16 A MCB provides better overload protection than a 13 A fuse and that the FCU is designed to protect the flex rather than the heater.

 

[Harry Bloomfield]

Everyone ok with using 1.5mm² to the immersion heater (that's what the cable clamps designed for) on a 16A breaker?

Rated for 16 to 20amps, so yes, that is OK.

 

[EFLImpudence]

Everyone ok with using 1.5mm² to the immersion heater (that's what the cable clamps designed for) on a 16A breaker?

 

[SUNRAY]

Who the hell uses a 13A fcu for an immersion. Madness

In my experience it was the main way immersion heaters were installed until our BS1363 products were derated by the penny pinching manufacturers.
However in my opinion and I stress it is my opinion which is not shared by many others; BS1363 products containing a BS1362 fuse never have been suitable for continous heavy power draw such as 3KW portable heaters.

 

[Gasguru]

The cable clamps inside immersion heaters are often designed for a maximum cable size of 1.5mm² so is it compliant to use that sized cable with a 16A MCB alone (and no FCU).
I'm after answers from qualified electricians...

 

[Harry Bloomfield]

so is it compliant to use that sized cable with a 16A MCB alone (and no FCU).

Cable (T&E), or 3core flex?

 

[EFLImpudence]

The cable clamps inside immersion heaters are often designed for a maximum cable size of 1.5mm²

Are they? I can't see there would be much difference.

Anyway, in my experience, nearly everyone uses 2.5mm² heat resistant flex for immersion heaters.

so is it compliant to use that sized cable with a 16A MCB alone (and no FCU).

Yes.

An FCU would not be necessary.
Although they should be adequate but the fuses seem to become loose with the constant full load which leads to more overheating.
The same with plugs - yet I have known one that lasted 'for ever'.

 

[Gasguru]

T&E has never been acceptable to feed an immersion heater. I'm only talking round cable here.

On a recent job the cable clamps supplied with the immersion heaters were not suitable for the existing 2.5mm² round cable....so the contractor binned off the cable clamps and left the elements with no proper cable restraint.

So the regs would deem it's fully compliant to use a 1.5mm² round cable (to suit the clamp) with a 16A breaker despite the cable having a typical rating of 16A.

 

[JohnW2]

Who the hell uses a 13A fcu for an immersion. Madness

It may be 'madness', and certainly usually unnecessary but, in my (limited) experience, is pretty common.

As often discussed, one of the reasons may be that (presumably due to relative sizes of markets) FCUs tend to be appreciably cheaper than 20A switches?

 

[EFLImpudence]

T&E has never been acceptable to feed an immersion heater. I'm only talking round cable here.

Ok. I don't think anyone would disagree.

On a recent job the cable clamps supplied with the immersion heaters were not suitable for the existing 2.5mm² round cable

I find that hard to believe. How much bigger can it be? I'll have a look.

....so the contractor binned off the cable clamps and left the elements with no proper cable restraint.

Ok.

So the regs would deem it's fully compliant to use a 1.5mm² round cable (to suit the clamp) with a 16A breaker despite the cable having a typical rating of 16A.

Yes. Why wouldn't it be?

Anyway, in my experience, nearly everyone uses 2.5mm² heat resistant flex for immersion heaters.

Perhaps it would be useful to say that this is a remnant from the use of BS3036 15A fuses which, because of their derating factor, BS3036 fuses, in effect, require the next size up flex - like people still use 6mm² cable for 32A cooker circuits.

 

[JohnW2]

So the regs would deem it's fully compliant to use a 1.5mm² round cable (to suit the clamp) with a 16A breaker despite the cable having a typical rating of 16A.

I don't really understand your "despite" - there's surely nothing wrong with a cable rated at 16A being protected by a 16A breaker, is there?