Fused spur question for 3Kw heater

[Mottie]

I have a 3Kw heater that has a plug on it. Works fine but after a few hours on maximum power (it has three settings), the plug is very hot. It’s been like that for a year, I’m just concerned about it as it doesn’t seem right. Would it be better to wire it into a 13a switched fused spur or a 20a double pole switch or is it okay to be left as it is?

 

[winston1]

The bit that gets hot is the fuse in the plug. If you put it on a FCU you will have the same problem in the FCU.

A double pole switch will be OK as long as the feed is from a 16 or 20 amp MCB. Don’t put it on a 32 amp ring.

 

[ericmark]

It does seem the 13 amp plug has been down graded, the necking of the live pins to put plastic sleeves on them has reduced the ability to transfer heat away from the fuse, black plugs are better than white, and clearly need to be in free air, the whole design of the ring final and 13 amp plug during the war was with the idea of powering heaters, so they should be OK, and with a thermostatic controlled oil filled radiator I have not had a problem, but 3 kW does seem today to be two much as a continuous rating. So as to if 20A switch or 13A fused spur depends what feeds it, from a ring final would need to be fused spur and with the down grading of cable from 7/0.029 (2.9 mm²) to 2.5 mm² one needs to be careful drawing over 2 kW from a ring final, OK near the centre but near the consumer unit can cause one leg to be over loaded.

The IET BS 7671 now warns of the problem with over 2 kW fixed appliances and suggests over 2 kW should have a dedicated supply, which we have done for years with the immersion heater, but do not tend to have done with tumble drier or washer drier, there has been a move to 2 kW for cloths drying, but it means the answer to your question in not cut and dried due to the down grading of the ring final.

Least we forget the ring final was designed during the war to assist with the rebuilding they realised would be required post war, and the shortage of copper. It does seem a FCU can dissipate the heat from the fuse better than a plug.

 

[winston1]

The 13 amp plug has always been poor at 13 amps even before the sleeved pins. So much so that the Post Office down rated them to 8 amps on their property.

As for overloading one leg of the ring that will never happen with a single 13 amp load and is extremely difficult with two 13 amp loads considering 2.5mm cable is rated at 25 amps.

 

[SUNRAY]

I was in BT 1972 to 1994 and never heard of the downrating to 8A, we had many 2.5-3KW heaters on 13A plugs AFAIA this was without dispute.

I personally have always reckoned 13A plug/socket arrangements are not fit for long term 13A loads like heaters & lighting. and also agree overloading one leg of a ring is unusual. I have only known it twice and both were very heavy loads almost beside the CU.

 

[endecotp]

One possibility is that there is a bad connection somewhere, either in the plug or in the socket; it’s not unusual to find socket terminals and cable insulation that has discoloured due to overheating due to a bad connection. You might like to check that all the screws are tight.

Some heating is to be expected and is not a problem, up to a point. After all the heat is contributing to warming the room! Is the plug too hot to touch?

 

[Mottie]

Is the plug too hot to touch?

Not with my hand but I wouldn’t like it on my cheek, for example. Is there such a thing as a 'heavy duty' plug?

 

[SUNRAY]

It does seem the 13 amp plug has been down graded, the necking of the live pins to put plastic sleeves on them has reduced the ability to transfer heat away from the fuse, black plugs are better than white, and clearly need to be in free air, the whole design of the ring final and 13 amp plug during the war was with the idea of powering heaters, so they should be OK,

Ah yes but that was in the days when manufacturers built quality to a standard rather than the rubbish products we find today.

Years ago I managed to get hold of a load of 'Bill' DSOs in brown and cream and have been using them for my portable kit since 1970's without failure other than physical damage. In 1974 I constructed 6 units with 4 metal clad DSSO's and one by one replaced with the Bill sockets as needed. By 1992 I had only 3 of the boards [due to theft] with only one of the original sockets remaining, due to site temporaries at a place of work I changed one of the boards to 4 white DSSO's [I suspect MK as that was what they usualy installed] but that has been back to Bill for a long time.
I doubt I've changed more than 3 of the Bills for failure or damage in all of that time.

 

[bernardgreen]

I was in BT 1972 to 1994 and never heard of the downrating to 8A, we had many 2.5-3KW heaters on 13A plugs AFAIA this was without dispute.

I came across this when dealing with the GPO for approvals and installation of interfaces between PABXs and radio paging systems in those years.

The limit was 9 Amps. Exemptions were granted when necessary for more than 2 kW loads but then the plug had to be an MK plug

 

[EFLImpudence]

Not with my hand but I wouldn’t like it on my cheek, for example. Is there such a thing as a 'heavy duty' plug?

Fit a brand new plug and see if it is any better.

 

[SUNRAY]

I came across this when dealing with the GPO for approvals and installation of interfaces between PABXs and radio paging systems in those years.

The limit was 9 Amps. Exemptions were granted when necessary for more than 2 kW loads but then the plug had to be an MK plug

I wonder if this may have been a regional thing, or a PO and not BT, as I've spoken to a number of local BT colleagues [some going back to POT / PO times] with experience in a selection of different sections and none have heard of it.
It's only recently on here [forums] that I've become aware of it.

 

[ericmark]

Yes there are heavy duty plugs,

however in the main good quality black not rubber, and a good quality socket, and no problems. I noted an old fuse the other day slightly longer than the 13 amp fuse but same diameter and it is marked 12 amp continuous 25 amp blow unlike the 13 amp fuse the fuse wire inside is in air, but we are looking at the wire inside the fuse running near melting point, and a little thought that heat could be concentrated at centre

or over the whole fuse, I have never really considered the quality of a 13 amp fuse, we see BS 1362 written on them and think they are all the same, but are they?

 

[JohnW2]

I have a 3Kw heater that has a plug on it. Works fine but after a few hours on maximum power (it has three settings), the plug is very hot. It’s been like that for a year, I’m just concerned about it as it doesn’t seem right. Would it be better to wire it into a 13a switched fused spur or a 20a double pole switch or is it okay to be left as it is?

I would be inclined to suggest that, before you consider anything else, you should try replacing the plug with a new (good quality) one. That's certainly what I would do myself.

Kind Regards, John

 

[Mottie]

I would be inclined to suggest that, before you consider anything else, you should try replacing the plug with a new (good quality) one. That's certainly what I would do myself.

Kind Regards, John

It was a new one - it came without a plug. It’s been like it from new.

 

[JohnW2]

It was a new one - it came without a plug. It’s been like it from new.

Fair enough - but, if it were me, I might well nevertheless try 'another new one' (by the way, if you bought it in UK, it would probably not have been legal to sell it without a plug).

As others have said, despite what one might think (and despite the fact that the Standard requires them to pass a 'temperature rise test' at 14A for 'hours'), they quite often do get hot (sometimes heat damaged) at anything approaching 13A / 3kW. However, as has been said, if you want to avoid a plug, then options are limited. As has been said, the fuse in an FCU might get just as hot, so that's not a very good answer, but, as also has been said, if you wanted it to be hard-wired just with a switch, it would preferably be connected to circuit rated (i.e. with an MCB/fuse) less than 32A - probably only 16A.

Kind Regards, John