two ring mains joined

PBC_1966 said:

It's interesting that 30A radials (using appropriate 7/.036, later 4 sq. mm, cable) were restricted to six sockets while a 30A ring could serve unlimited sockets within a floor area of 1000 sq. ft. Did that ever really make much sense?

Click to expand...

Not to me. Quite the contrary, really - since, unlike the situation with a radial (whose cable has an appropriate CCC in relation to the OPD), the more loads one has plugged into a ring, the more likely that some part of the cable might become overloaded.

PBC_1966 said:

And while the radial specifications changed multiple times over the years, the ring final has remained pretty much the same for decades, other than 1000 sq. ft. being rounded up to 100 sq. m with the change to metric in 1970. ... Come to think of it, does the 100 sq. m ring final limitation still stand, or has that now been reduced to a "historical note" in an appendix too?

Click to expand...

As I said, all we have for rings is the 100m² "historical note" in an Appendix.

If one moves even further from "the regulations" to the OSG, this implies that the floor-area figures are rules/regulations - but, far from imposing any limit on the number of sockets, also includes a table of the minimum number of sockets for different types of rooms of various sizes.

Kind Regards, John

JohnW2 said:

The restrictions on unfused spurs clearly should be part of the regs

Click to expand...

Why?

since, otherwise, the regs might as well also allow 2.5mm² 32A radials.

Click to expand...

They do.

I must say that the limit on number of sockets and floor area were likely to have been removed because it was realised how pointless they were.

What relevance could/can floor area have on anything?

ban-all-sheds said:

JohnW2 said:

The restrictions on unfused spurs clearly should be part of the regs

Click to expand...

Why?

Click to expand...

Why not? Are you perhaps just trying to be 'clever' by thinking of a situation other than that (which is obviously what I was talking about) of a 2.5mm² unfused spur (from a 32A ring final) that is supplying multiple sockets? If not, please clarify.

ban-all-sheds said:

JohnW2 said:

- since, otherwise, the regs might as well also allow 2.5mm² 32A radials.

Click to expand...

They do.

Click to expand...

Again, unless you are thinking of something other than what I was obviously talking about (a 2.5mm² 32A radial sockets circuit), please clarify.

Kind Regards, John

EFLImpudence said:

I must say that the limit on number of sockets and floor area were likely to have been removed because it was realised how pointless they were.

Click to expand...

I agree that, since situations vary so much, it was, in practice, pretty pointless - one could have a massive potential load plugged into a small number of sockets in a room of small floor area, or very little load plugged into a circuit of hundreds of sockets spread over hundreds of m² of floor area.

EFLImpudence said:

What relevance could/can floor area have on anything?

Click to expand...

One can understand what they were trying to achieve. Properly designing a sockets circuit is obviously impossible, since the designer has no control over what gets plugged in; the theoretically possible maximum load (N x 13A) is usually extremely high. Given that, all one can do is to make intelligent guesses as to what the design load should be. In general, increasing the number of sockets and/or the 'amount of building' (floor area) served by those sockets is likely to increase the 'probable' load, so they are not unreasonable 'handles' to use. However, as above, there is going to be so much variation between buildings/situations that any attempt at a "one size fits all" regulation, or even guideline, is really pretty futile.

Kind Regards, John

JohnW2 said:

As I said, all we have for rings is the 100m² "historical note" in an Appendix.

Click to expand...

Ah, sorry, I missed that you already said that the area limit for rings had been relegated to a "historical note" along with radials.

Obviously in practice there would be nothing to stop somebody plugging, say, two or three appliances of 3kW each into three sockets all on the same 20A radial circuit, but looking at the 13th & 14th edition rules I think they were written with the limits regarding number of sockets, floor area, and the stipulation of feeding only a single room in such a way as to try and make such a thing unlikely (with due acknowledgment about the probability of kitchen sockets carrying multiple higher loads).

The later change to just floor areas without room restrictions seems to make the possibility more likely, although perhaps the committee making that change had in mind that by then central heating was much more widespread and so larger portable heaters were not nearly so common.

The BS546 radial rules were the ultimate way of ensuring no overload no matter what devices were plugged in, since they permitted only a single 15A socket or up to three 5A sockets on a 15A circuit (although in practice, that three-socket limit for 5A sockets was widely ignored, and without any real problems given that many were just feeding radios, TV sets, table lamps, etc.).

For comparison, the American NEC sets out where specific dedicated circuits are required, then provides that fixed lighting and general-purpose convenience outlets should be provided on the basis of so many watts per square foot, the precise value varying depending upon the use of the building (dwelling, bank, office, school etc.).

PBC_1966 said:

Obviously in practice there would be nothing to stop somebody plugging, say, two or three appliances of 3kW each into three sockets all on the same 20A radial circuit, but looking at the 13th & 14th edition rules I think they were written with the limits regarding number of sockets, floor area, and the stipulation of feeding only a single room in such a way as to try and make such a thing unlikely (with due acknowledgment about the probability of kitchen sockets carrying multiple higher loads). ... The later change to just floor areas without room restrictions seems to make the possibility more likely, although perhaps the committee making that change had in mind that by then central heating was much more widespread and so larger portable heaters were not nearly so common.

Click to expand...

Indeed. Given that (with one exception - see below), proper design of a sockets circuit is 'impossible', it seems that the regs "did their best" in relation to "what was likely".

PBC_1966 said:

The BS546 radial rules were the ultimate way of ensuring no overload no matter what devices were plugged in, since they permitted only a single 15A socket or up to three 5A sockets on a 15A circuit ...

Click to expand...

That is, of course, the one exception to the above - it is 'almost' possible to properly design a sockets circuit if it feeds only one socket (or equivalent). I say 'almost' since, in the absence of fused plugs, there was no guarantee that the load plugged in would not exceed the 'rating' of the plug/socket. A requirement for a dedicated circuit for each BS1363 socket would make 'proper design' possible!

PBC_1966 said:

For comparison, the American NEC sets out where specific dedicated circuits are required, then provides that fixed lighting and general-purpose convenience outlets should be provided on the basis of so many watts per square foot, the precise value varying depending upon the use of the building (dwelling, bank, office, school etc.).

Click to expand...

Interesting. Do I take it that we're talking about a minimum watts/ft² figure? What sort of figures are we talking about for, say, a dwelling?

Kind Regards, John

JohnW2 said:

I say 'almost' since, in the absence of fused plugs, there was no guarantee that the load plugged in would not exceed the 'rating' of the plug/socket.

Click to expand...

I suppose we have to draw the line somewhere, and assume that for these purposes nobody is going to be wiring a 6kW load to a 15A plug! Of course, there were all those BS546 adapters which would allow up to three 5A plugs to be connected to a 5A socket, or two 5A plus one 15A plug on a 15A socket etc., so still not foolproof.

Interesting. Do I take it that we're talking about a minimum watts/ft² figure? What sort of figures are we talking about for, say, a dwelling?

Click to expand...

Yes, it specifies the minimum, but there's nothing to stop you allowing for much more if you wish. The figure for a dwelling is 3 watts per square foot, which has been in place since at least the 1950's. That's excluding dedicated circuits which are required for the kitchen & dining areas where higher-powered devices are likely to be concentrated.

To give a general overview of how it's done here, major appliances will each have a dedicated circuit. In the typical home that means a 40 or 50A 120/240V circuit where an electric range is used, and a 30A 120/240V circuit for a dryer. Central air-conditioning will have a 240V circuit sized to the unit (e.g. ours is 50A), and if there is any fixed electrical heating (baseboard heaters throughout etc.) then there will be one or more dedicated circuits for that.

The rest which follows is straight 120V. It's normal to provide a dedicated 15 or 20A circuit for a washer (20A is required in new homes now), and also dedicated 15 or 20A circuits for garbage disposal and dishwasher, if present. The NEC then requires that general-purpose outlets in the kitchen & dining areas be fed by a minimum of two 20A circuits, with the outlets distributed reasonably evenly between them, and with nothing else on those circuits (the requirement for two 20A kitchen/dining circuits goes back to 1959).

An amendment a few years ago now also specifies a separate 20A circuit for the bathroom (due to the increasing power of hair dryers, apparently); previously bathrooms were just grouped in with everywhere else.

The remainder of the home - general-purpose outlets in living rooms, bedrooms, and all other areas, plus fixed lighting in all areas - is then calculated on the 3W per sq. ft. basis using 15 and/or 20A circuits. So, for example, a 1500 sq. ft. house would need a minimum of two 20A circuits or three 15A circuits. It's normal to mix fixed lighting and wall receptacles on the same circuit.

PBC_1966 said:

I suppose we have to draw the line somewhere, and assume that for these purposes nobody is going to be wiring a 6kW load to a 15A plug!

Click to expand...

Sure, but single 6kW loads are rare - I was thinking more of 2-3kW loads being wired into each of three 5A plugs (the sockets all being on the same 15A circuit)! I was brought up in a house in which the only upstairs sockets were 5A ones supplied by the lighting circuit - and I hesitate to tell you about some of the things that got plugged into them!

PBC_1966 said:

Yes, it specifies the minimum, but there's nothing to stop you allowing for much more if you wish. The figure for a dwelling is 3 watts per square foot, which has been in place since at least the 1950's. That's excluding dedicated circuits which are required for the kitchen & dining areas where higher-powered devices are likely to be concentrated.

Click to expand...

Fair enough. Even given that most major 'fixed' appliances have dedicated circuits, that's not an awful lot - you'd need 1000 ft² before you were required to have enough for even one 3kW load. Do designers commonly go appreciably beyond that minimum?

Kind Regards, John

JohnW2 said:

Sure, but single 6kW loads are rare - I was thinking more of 2-3kW loads being wired into each of three 5A plugs (the sockets all being on the same 15A circuit)!

Click to expand...

Well, in theory nobody should be connecting 2kW or more loads to a 5A plug, since of course, even at the highest 250V nominal used in the past, 5A is only 1250W. (Although that still wouldn't prevent 3kW or more being drawn from a single 5A socket if somebody used one of those unfused 3-way adapters.) How far do we want to go in safeguarding against a blown fuse or a tripped breaker under such abuse? So long as the wiring is adequately protected, surely that's the main concern?

Even given that most major 'fixed' appliances have dedicated circuits, that's not an awful lot - you'd need 1000 ft² before you were required to have enough for even one 3kW load.

Click to expand...

Don't forget that as well as dedicated circuits for the major appliances, there are also the two small-appliance branch circuits of 20A each for the kitchen & dining area, which is where a significant proportion of power is consumed in the average home. Once the toasters, waffle irons, coffee makers, slow cookers and other kitchen appliances are taken out of the picture, just how much power does the average house consume elsewhere in the main living and bedroom areas? Unless somebody is "making do" with a lot of portable heaters, it's mostly just lighting, clocks, radios and similar electronics, and other low-power devices, plus the occasional use of something higher powered such as a hair dryer, vacuum cleaner, or power tool.

Do designers commonly go appreciably beyond that minimum?

Click to expand...

It seems to vary. Some places are wired to bare NEC minimum, while others pretty much have a dedicated circuit per room. Our place we just moved into is about 1680 sq. ft. and has five 15A general-purpose circuits, although one is dedicated to the attached garage, the bathroom receptacles, and the outdoor receptacle (built before the requirement for a separate 20A bathroom circuit, but clearly done that way to cut costs since those are the locations which required GFCI protection at the time). There will certainly be more by the time I've finished!

PBC_1966 said:

Once the toasters, waffle irons, coffee makers, slow cookers and other kitchen appliances are taken out of the picture, just how much power does the average house consume elsewhere in the main living and bedroom areas? Unless somebody is "making do" with a lot of portable heaters, it's mostly just lighting, clocks, radios and similar electronics, and other low-power devices, plus the occasional use of something higher powered such as a hair dryer, vacuum cleaner, or power tool.

Click to expand...

That's all true. I suppose what I was mainly thinking of was portable heaters, and one would not have to have "lots" of those for there to be a potential problem. With a low floor area, just one or two might become an issue with a 'minimum' power provision.

Kind Regards, John

Here is Section A of the 14th, 1966:

Socket regs are 30-40 and some in the fifties, from memory.

A37 is interesting....

uncut ring conductors at every socket!

Why is it repeated verbatim in A53?

And is A58 asking for DP switches on lighting circuits?

It's also interesting that it appears that A.55 banned unfused spurs from ring finals.

Kind Regards, John

That's only for BS196.