Why 13 Amps?

[twocvbloke]

Here's a question that just popped into my head while looking at some 15 Amp sockets on ebay...

Why did the British Standards people choose 13 Amps as the maximum current allowed to flow through a plug? The 15 Amp BS546 is basically the same design, just lacking a fuse, but can deliver 3.6 Kilowatts, whereas a properly fused 13A plug can only deliver about 3.1 Kilowatts!! That's a loss of 500 watts that could be put to good use, so, why the reduction in power?

Just think, if we retained the 15A plug for high-power appliances, the washing machine & tumble dryer could have their own feeds, and use an "incompatible" plug, and being on such a setup would reduce the risk of people using extension strips to power everything from one socket, usually ending up with charred electrics, or at the very least, many blown fuses...

Not to mention that the plugs could probably handle a lot more than 15 Amps, so could have led to more efficient high-powered appliances...

A fella I know over in the US says his dryer uses about 5 or 6 Kilowatts, whereas ours use maybe 2, so his clothes are dry in 20 minutes, but over here, we have to wait for about an hour or more (not to mention our dryers are Positive-pressure, very inefficient, rather than negative pressure like over in the states)....

Any thoughts?

[dave walsh]

Unfortunately, to some extent, you are advocating the very system that the 13amp plug was designed to replace on a "one size fits all" basis for convenience!

I was given to understand [in the sixties] that this relatively new ring wiring [then] had a built in over-rating ie a thirty amp ring socket used cables that could actually carry 20 amps to each side of the socket, although ostensibly 15. This was downsized again to 13 in the plug giving a further margin. No doubt this seems rubbish in these Part P days but it made sense at the time. I also thought that the 13 amp plug was originally some sort of supposed international standard-like the irritating length of a CD! What's the real story?

It interesting that ring mains seemed common in factory enviroments long before they reached the domestic situation.
Dave W

[Coastwatch]

Unfortunately the Wiki on BS 1363 reveals nothing about the choice of 13 amps for plug fuses (unless I missed it) but it is interesting reading never the less.

Seems the ring main was introduced to reduce the amount of copper wire used in houses following WWII.

Cheers n Beers,

David........

[Sean Williams]

There is certainly some milage in the claim to reduce copper requirements.

Technically (and for the life of me I cannot remember who's law it is) it makes good sense to alleviate voltage drop on long runs of cable - long time since I was at college and did this stuff though!

All I remember about it now was pages of calculations based around ring mains, and the applications as seen on the Railways. (Thanks Privatisation - might still have been on the railways!)

Mind numbing stuff!

[murphyv310]

I sometimes don't think electrical wiring and the regulations make a lot of sense. OK I will be shot down in flames and I am prepared for that.
I would say though our 13a plugs are actually good (British made ones only) and they do have greater safety than foreign ones less the fuse, although I do admit we do need a higher current rating. Our Whirlpool oven came with a 13amp plug fitted and it is the worst oven we have ever had, it takes an age to get up to temperature and often cools slightly when a cold item is placed inside. what happened to the good old 30amp feed?

[Tractorfan]

Hi,
I've often wondered that too. Not so much from the technical side, but why 13? A supposedly superstitious number. How do tryskaidekophobics manage?
I remember reading in the early 70s that a new system was mooted to replace the 13amp system. This publication showed a wall socket and a plug that looked just like the EIC connectors we use today on computers & kettles, etc. Anyone remember this? Then there was the Elektrak system that "Tomorrow's World" said would be the plug of the future. Now reduced to specialised supplies like EPOS tills.
Cheers, Pete

[Andrewausfa]

There's a pdf file from the IETs 'Wiring Matters' magazine telling some of the background here
http://www.google.co.uk/url?sa=t&sou...NT_Zqg&cad=rja

Andrew

[mark pirate]

I have had several high power appliances through my hands with overheated moulded plugs fitted, the latest was a tumble dryer fitted with a 13A fuse that had severe signs of overheating round the live pin, is this down to c**p plugs or faulty sockets?
I remember as a child, we had 15A round plugs fitted to 3KW fires etc with no signs of overheating, i think the contact area on the round pin plugs was far better than the modern 13A plugs.
Another thing is very interesting, all the overheated plugs had silver prongs, rather than brass, does this make a difference?

Mark

[Tractorfan]

Hi,
The hospital my wife used to work at once had a policy of cutting off factory fitted plugs and fitting red MK "Hospital Property" ones however small the appliance. They probably don't do it now for cost reasons.
Cheers, Pete

[Lucien Nunes]

I don't think there is an issue with the plated pins as such. In fact they can be beneficial in preventing tarnishing and the resulting high-resistance. Contact area itself is not an easy factor to assess, as the effective area is much less than it appears. What matters more is the force applied to the miniscule area of metal-metal contact that exists, and in the case of 13A plugs this depends on the contact design in the socket. A cheap socket such as one in a four-way extension often has no proper spring to clench the contacts around the plug pins, relying instead on the contact material itself which weakens with use and heating.

One of the greatest sources of heat in a 13A plug-and-socket pair is the fuse element, and potentially its contact with the clips if they are poor. Also the plug generally has less metalwork to conduct away and dissipate heat than a socket. Therefore a weak socket contact will often burn out the plug, without at first appearing to be damaged itself. Of course 15A plugs did not have the fuse as a heat source and were capable of withstanding higher heat dissipation from the contacts.

Lucien

[Peter.N.]

The first 13a plugs I saw were fitted in an estate in Bromley, near where I used to live, built just after the war, these were not the familiar ones though, they had round pins and the live one was the fuse which unscrewed. Our house had two pin 5a and 10a plugs!

Peter

[dave walsh]

I can see the copper wire arguement [never heard it before] as there should be less single runs but overall, surely the ring principle must also be electrically more elegant ie power coming from two directions reduces cable size as well as lengh. Also being able to easily run a certain number of spurs to meet extra need would be intrinsically be safer than daisy chains! Dave W

[Skywave]

Could it be that the original, prototype 13A plug and socket mechanical connection arrangements were such that 13 Amp. was calculated as simply being the maximum safe current permissable - with a safety margin, of course?

Al.

[Ray Cooper]

Originally Posted by Peter.N.

The first 13a plugs I saw ...had round pins and the live one was the fuse which unscrewed....

These sound like the early Dorman and Smith (D&S) connectors. Apart from domestic use, were quite widely used on BBC premises too. Had a design problem: if the fuse became unscrewed, or the ceramic fuse tube became fractured, whilst the plug was inserted, then when you removed the plug you were left with a protruding live fuse, or the end of the fuse wire. Not nice...

[threeseven]

Wouldn't it be a case of the rating being selected is 3KW rather than 13 amps ? The current then being a result of that power rating. Some committee must have convened and postulated the question "what is the largest practical single appliance power demand likely to be needed in the average home ?"

[dave walsh]

That makes sense. Our 3kw fire was a bit deadly with all the bars on-never saw a 4kw version. Dave W

[Coastwatch]

Originally Posted by threeseven

Wouldn't it be a case of the rating being selected is 3KW rather than 13 amps ? The current then being a result of that power rating. Some committee must have convened and postulated the question "what is the largest practical single appliance power demand likely to be needed in the average home ?"

Correct, the link in post seven gives that precise explanation.

The final unanimous decision was for a 3 kW (230V; 13 A)
socket-outlet with fused plugs.

This seems to answer the original question.

Cheers n Beers,

David.......

[BGmidsUK]

At 230V, the rating at 13A is LESS than 3KW, albeit slightly, but certainly no margin for headroom.

At 240V, 3120W gives a bit of headroom

Our mains is supposed to be 240V although they have fiddled the figures for its specification in the name of harmonization with Europe. This just means in practice that appliances are manufactured with a wider tolerance rather than having voltage switches. My Technics amp from around 1990 has a 230/240V switch .. but strangely enough my BRC1400 telly doesn't appear to have any voltage selecton but claims a 200-250V range!

My average mains voltage IS a little on the low side, usually around 235-236V but is regularly below 228V on peak and around 244V during the night. I'd certainly be interested to hear what the average voltage is in the homes of our members who live in Europe.


Brian

[twocvbloke]

Interesting thoughts on the 13 Amp spec, it does still seem a bit on the weak side in my view though, and as mentioned, using an oven on a 13A supply is just useless, the proper 30A supply to a high-powered cooker would be better (but personally, I prefer using Gas!!!), but still, they're telling us what to do cos it's all "safer", we're not allowed to make our own choice as to our electrical preferences...

I remember reading about the saving copper thing, but if you add up the amount of thick copper wire used to ring a house and it's two or more floors in the modern age, is it really a saving?

Just so long as we don't end up using Aluminium wire, like they tried (and failed) in the US, and produced the results below...

[bluepilot]

Originally Posted by BGmidsUK

I'd certainly be interested to hear what the average voltage is in the homes of our members who live in Europe.


Brian

The standard German mains voltage was 220V. The standard French one 230V. Then came the Euronorm of 230V +/- 10V. When I measure the voltage in Germany it's still 220V and in France 230V but they all conform to the norm.