Cable widths and Socket Heights

[ericmark]

It is considered that 20A is drawn from centre of ring and the remaining 12A is even drawn throughout the ring. So Amps are considered as being 26A. This is not my assumption but is the official guide lines. After going to a 17th Edition lecture I was surprised at the 106M length so did some enquiries to find out why this was the official maximum length.
Also taken into consideration is voltage 230, tp max temp 70, Ca rating factor for ambient temp 1, Cg rating factor for grouping 1, Ib design current 26, It tabulated current carrying capacity of cable 40, Giving a Ct rating factor for operating temperature of conductor of 0.923, this gives maximum length of 26.62259 meters but since ring main times 4 giving 106.4904 meters.

So sorry fact it is ring main already taken into account. 106 meters is absolute max. Same calculation with 4mm cable gives 179 meters.

 

[ban-all-sheds]

And that 106m makes perfect sense given those assumptions.

I wonder why the assumptions are different for a ring on a 30A BS 3036, for which the OSG says 111m?

 

[securespark]

Ban, don't look now but you've gone and broken one of your own golden rules.

Well, I'm not going to say anything if you don't, so I'll just keep mum.

 

[ban-all-sheds]

No I haven't, I've just been a bit slow to reply, that's all - been meaning to do it for ages....

 

[JohnW2]

Ban, don't look now but you've gone and broken one of your own golden rules. Well, I'm not going to say anything if you don't, so I'll just keep mum.

That's the second time in recent days - maybe he's changed the rule?

Kind Regards, John

 

[ban-all-sheds]

No - honestly - I did mean to reply to it.

It came about via this topic: //www.diynot.com/forums/electrics/voltage-drop-calculation.345233/

I couldn't remember the figure in the OSG, so I looked it up, and therefore noticed that the value for a ring on a BS 3036 was higher, and since it's limited by voltage drop I wondered why. And remembered wondering why before.

I know that Eric had posted on the subject of assumed loading on ring finals, so I searched for '106m' in posts by him.

And this was the only one found.

I knew there must be one more recent, but really it makes no difference which I reply to.

 

[JohnW2]

No - honestly - I did mean to reply to it. It came about via this topic: //www.diynot.com/forums/electrics/voltage-drop-calculation.345233/p[/QUOTE]
Yes, I realised that, but ....

I know that Eric had posted on the subject of assumed loading on ring finals, so I searched for '106m' in posts by him. And this was the only one found. I knew there must be one more recent, but really it makes no difference which I reply to.

... why reply to one of the old threads about this (which, as has been pointed out, appears to be a contravention of your 'golden rule'), rather than talk about it in the current thread on the topic (as above)? Just curious!

Kind Regards, John

 

[ban-all-sheds]

Well - just possibly because that post from Eric isn't in that topic?

 

[JohnW2]

Well - just possibly because that post from Eric isn't in that topic?

Hmmm - fair enough. In any event, I guiess that it is the prerogative of he who has a golden rule to interpret and/or modify said rule however he sees fit

Kind Regards, John

 

[ericmark]

I got the info from attending lecturers given by the IET number of speakers including scheme runners.

I have questioned the whole idea of maximum lenght as to me cables need to be short enough to ensure with a short circuit the magnetic part of the MCB will operate which would answer question as why using MCB and fuse gives different results.

To me it's the prospective short circuit current which with use of RCD is now all important. So if the MCB is a B32 then 160A must be able to flow. That's 1.4375 and clearly one should round down not to nearest so that's 1.43 not 1.44 as shown in the big book.

Since measured line - neutral not line - earth and the earth wire is often smaller than neutral then likely 1.44 will be OK but question must be are electricians measuring the line - neutral reading as with many meters this would need one to make your own patch lead or stick probes in the holes on the socket.

Personally I don't care about volt drop. It does not really matter if the volts drop to 130 vac on a lighting circuit using a HF florescent which can run 100 - 300 vac. However with a induction florescent marked 240 vac then even at 225 vac it may fail to start. So the volt drop needs to relate to what is being supplied and a blanket 3% or 5% is really not required.

I personally think the idea of designing a circuit to comply with BS7671 rather than to supply the item in question is flawed. Back in 1960 I remember going into a farm house with flickering 110 vdc lighting and my father commenting on the poor lighting. The farmers reply was it was far better than the oil lamps he used before electric lamps were installed.

And it would seem the number of house fires dropped when electric lamps replaced the oil and gas lamps so on a risk assessment electric lighting even with no earth on the lights was far better than what when before. Candles were very bad as to safety and on the Falklands farm managers would keep generators running all night when an event was staged to reduce fire risk.

I would say any builder who goes to the 106 meter limit is being rather silly. One 50 meter role per circuit should be normal. And when extending one has not a clue how much cable is already installed one can only go by the loop impedance meter readings as to if more can be added. The 91.4 meter of cable used on a first fix seemed OK to my mind. But then we went metric reduced the cable thickness and increased the lenght.

Flounders and Swan put it very simply. "The English, The English and best and to hell with the rest." pity we ever went metric.

 

[JohnW2]

Given that BAS has issued an edict that this old thread can be re-opened ...

IPersonally I don't care about volt drop. It does not really matter if the volts drop to 130 vac on a lighting circuit using a HF florescent which can run 100 - 300 vac. However with a induction florescent marked 240 vac then even at 225 vac it may fail to start. So the volt drop needs to relate to what is being supplied and a blanket 3% or 5% is really not required.

To be fair, that's more-or-less what 525.1 and 525.100 of the regs say ("the voltage at the terminals of any fixed current-using equipment shall be greater than the lower limit corresponding to the product standard relevant to the equipment" and "... Where fixed current-using equipment is not the subject of a product standard the voltage at the terminals shall be such as to not impair the safe funtioning of that equipment").

However, 525.101 then goes on to 'spoil it', by saying that the above requirements are deemed to be satisfied by the standard/magic (essentially arbitrary) 3%/5% limits!

Kind Regards, John

 

[ban-all-sheds]

I have questioned the whole idea of maximum lenght as to me cables need to be short enough to ensure with a short circuit the magnetic part of the MCB will operate which would answer question as why using MCB and fuse gives different results.

Mmm. The figures in the OSG indicate which are limited by voltage drop and which are not.

A ring on a B32 is limited by VD to 106m.

A ring on a 30A rewirable is limited by VD to 111m.