Diversity Factor

[hard-work]

Electric UFH heating being considered in a 63A supply. Using diversity it still is over the limit, so more inventive means have to be used. The two socket rings are being dropped to 16 amp, not the usual 32A. The main ring will have mainly a TV and a vacuum cleaner plugged in occasionally. The kitchen ring will have a fridge/freezer, m/wave, washer/dryer, extractor fan and a few double sockets over the worktop. This kitchen setup is currently on a 13A fused spur. Not once has the fuse blown. The 16A rings is to cap the supply to keep it within limits.

What do those here usually do in these situations?

 

[hard-work]

I noticed the basement supply to the flats is in a big MEM fused distribution box. Three phase supply with fuses off bus bars, with 25mm cable running 30 foot to the flat CU. It could take a 100A main fuse without any problems, but the fuse box states only 66A. All the flats have gas heating, except this one, as on conversion they could not gas to it, so no big loads in the building.

 

[chivers67]

A 16A fuse will not cap it's supply. It can still be overloaded without tripping over a period of time. Just because the 13A fuse has not blown doesn't mean it hasn't been overloaded either.

What is the rating of the UFH?

 

[hard-work]

approx. 48A for UFH.

 

[JohnW2]

approx. 48A for UFH.

Goodness - I'm glad that I'm not paying the lecky bill!

Kind Regards, John

 

[hard-work]

A 16A fuse will not cap it's supply. It can still be overloaded without tripping over a period of time. Just because the 13A fuse has not blown doesn't mean it hasn't been overloaded either.

The cables are safe for sure, before the main fuse (a bus bar), after the main and inside the premises - no danger of fire. If a 13A fuse and a 63A main fuse are exceeding their ratings the system is still very safe.

I just want to gain the experience of other who have had to cap the supply inventively.

 

[JohnW2]

The cables are safe for sure, before the main fuse (a bus bar), after the main and inside the premises - no danger of fire. If a 13A fuse and a 63A main fuse are exceeding their ratings the system is still very safe.

It's not too clear (at least, not to me) what your concerns relate to, given that it's really the safety of the cables that is the primary consideration.

I just want to gain the experience of other who have had to cap the supply inventively.

If by 'cap', you mean 'limit', then there really isn't any way you can do that effectively by circuit design, particularly when the OPDs are fuses (hence likely to have fusing factors at, or approaching, 2). Even a 13A BS1362 fuse will allow something like 22A to flow indefinitely and a lot more than that for appreciable periods of time.

Kind Regard, John

 

[hard-work]

Sorry 36A for UFH. The safety of cables is fine, cables are size & fused properly. It is exceeding 64A with the main fuse blowing, that is a concern. As I wrote the cables before the main fuse and all after are good enough for 100A.

Other appliances wanted, tumble dryer, w/machine, dishwasher, microwave, electric kettle, two towel warmers in two bathrooms, 3kW immersion (there are two 3kW immersions in cylinder with only one used), ring for skts, ring for kitchen, lights (LEDs).

 

[JohnW2]

Sorry 36A for UFH. The safety of cables is fine, cables are size & fused properly. It is exceeding 64A with the main fuse blowing, that is a concern. As I wrote the cables before the main fuse and all after are good enough for 100A.

As I wrote before, and despite your attempt at clarification above, I'm still not sure of the nature of your concern.

If it's a BS3036 fuse, then you could draw at least 126A through it for at least an hour without it blowing. Furthermore if, as you have said, the cable is such that it would be adequately protected by a 100A fuse (which should carry at least 200A for at least an hour before blowing) what is it that is stopping you having the fuse 'upgraded' to 100A, if you are concerned about the rating of the current one?

Kind Regards, John

 

[hard-work]

The MEM fuse box in the basement states 63A fuses only. Do not want to fall foul of makers. Electrically, and as you have noted, there is not a safety issue. It is keeping within regs that is the issue. The diversity rules are the guide. So a work around to ensure a cap is not exceeded is needed.

 

[JohnW2]

The MEM fuse box in the basement states 63A fuses only. Do not want to fall foul of makers. Electrically, and as you have noted, there is not a safety issue. It is keeping within regs that is the issue.

Current regs require only that Manufacturer's Instructions be "taken into account" - there is no complulsion to 'comply' with them.

The diversity rules are the guide. So a work around to ensure a cap is not exceeded is needed.

As I've said, there is no practical way to 'limit' possible total demand in the manner you seem to want.

Many domestic CUs contain circuits which, if all simultaneously 'fully loaded', would represent a total load of well over 150A, yet both the Main Switch and CU as a whole will be 'rated' at 100A or less, and the whole installation usually protected by a 60A or 80A DNO fuse.

Even one small sockets circuit cannot be 'prevented' from imposing a load vastly in excess of the rating of the circuits OPD (just 6 double sockets, each with 2 x 13A lods = 156A!).

Kind Regards, John

 

[hard-work]

Even one small sockets circuit cannot be 'prevented' from imposing a load vastly in excess of the rating of the circuits OPD (just 6 double sockets, each with 2 x 13A lods = 156A!).

that is why I propose 16A rings. If rings then are within diversity calcs then fine, which is different to being electrically sound.

 

[JohnW2]

that is why I propose 16A rings. If rings then are within diversity calcs then fine, which is different to being electrically sound.

Reducing the rating of the OPDs protecting the two sockets circuits will not, in itself, change what people may try to plug in to those circuits.

Even two "16A" sockets circuits would allow at least 46.4A (16A x 2 x 1.45) to flow for at least an hour. Add to that your 36A of UFH, and you are presumably already (at over 82A) into an area that you feel you should be concerned about?

Furthermore, given that you say that this flat has no gas, does it not also have an electric cooker and electric water heating - factors which don't seem to have come into your calculations so far?

Kind Regards, John

 

[hard-work]

It is 36A max, may be 30A when all assessed accurately, ballpark for now. Induction hob with small electric oven under. Trying to get the oven and microwave all in one unit (combination oven) so both are not on at the same time.

Two 16A rings as opposed to two 32A rings, will create a cap for sure.

 

[hard-work]

Hoping to have a combination oven (oven and microwave) and washer/dryer to reduce peak loadings.

3kW immersion.