Maximum length of cables limited to 3m

[unphased]

A debate on another forum has prompted me to ask why there is common reference in the regulations to limiting the length of a cable to 3m to satisfy certain conditions. There are some clever people on here so hopefully I will learn. I will give some examples to illustrate the point of my question.

433.2.2 (ii) Its length does not exceed 3 m,........

434.2.1 (i) not exceed 3 m in length, and

Limiting the length of a cable to 3 m does what exacly? Why 3 m? Why not 5 m?

I am a logical kind of person. I look at rules and like to understand the reason for them. In this case I just cannot. It comes over as a notional length picked by the IEE at random when writing the regs. Don't get me wrong, I can see that limiting a cable length will fix voltage drop, fix resistance/impedance over that length, but I just do not understand why 3 m?

Some background to the question. The debate that was discussed on the the part of one poster decreed that this 'rule' applied to unfused spurs off a ring final circuit. Several posters, including me, said that the 3 m rule did not apply to unfused spurs off a ring.

Thank you.

 

[kerching]

Bearing in mind that I know nothing...puts on NOMEX Skids and boots and awaits the wrath of the T'interWebNettum

AFAIUI 3m rule is to do with max length of 'unprotected' meter tails. It is derived as a function of network impedance, Ze, along with associated max Ip. Given all the maximum theoretical parameters of the supply characteristics AND 3m of tails ALLOWS the designer to use the DNOs protection as THEIR protection....otherwise bang a switched fuse in along with an appropriately sized submain....remembering all the while that I know nothing

........................ :coat

 

[johninlondon1]

One is for appliances used close to zones in bathrooms, ie sockets must be 3m away from zone 1, to avoid the hairdryer or other appliance falling into the bath.

 

[kerching]

One is for appliances used close to zones in bathrooms, ie sockets must be 3m away from zone 1, to avoid the hairdryer or other appliance falling into the bath.

Whoops, hangover head on! Sorry. I really should read the posts properly

 

[samson337]

433.2.2 a reduction in current carrying capacity has been brought into a circuit then you must use an over current device, a fuse, and this has to be within 3 meters of the change. So when installing a spur following the regs unless a FCU is used then there is a limit of 3 meters. But could be wrong

 

[unphased]

I am sorry I have not explained myself corrrectly. I am trying to understand why 3 m is used as opposed to any other length. What is so 'magical' about 3 m. I am not specifically referring to meter tails, here, it is any circuit requiring the rule to be applied. Why 3 m?

 

[Evans Electric]

Your post is not very clear , Unphased , what , exactly are you talking about.? I guessed it was meter tails but no so a little more concise please.

 

[unphased]

Its just why is 3 m chosen in the regs, Deke. The regs I have given as an example. Why is 3 m specified as opposed to any other length.

 

[Evans Electric]

I,ve just read 433.2.2 .....and the reason is ....erm....well why its 3m is ...um.... ......er.... when is Specs back from holiday ?? :C

I see the point about inserting a smaller fuse to protect the smaller cable but I don't get the 3mtr bit.

Don't remember it on the course either . Must admit some Regs seem to contradict themselves or it may be me losing it .

I found some of the exam questions somewhat ambigious too .

Something like :- Which of the following is NOT the wrong method ?

 

[Andy™]

no idea where the 3mtr came from. probably because its a resonable length in which to fit a smaller fuse, without having too much

 

[springcrocus]

Lets put all the numbers in a hat and see which comes out...

Makes no sense to me, either. Meter tails, OK, we all know that one one; but generally???