Install method 101 or 103 for a ring main

[Paulr]

hi
I would like clarification on the following: 101 shows a cable clipped to a ceiling joist and then covered with 100mm of insulation ( from memory). 103 shows a cable fully surrounded by insulation in a stud wall. If I have a cable clipped to a stud in a wall which is insulated with sound insulation ( not thermal) and the cable is not quite in contact with the inner skin of the wall is this 103 or possibly 101 ?. If the cct is a ring and run in 2.5mm twin and Earth and it's considered 103 then the cable rating reduces to 13.5 amp, with it being a ring this is then a total of 27 amps I believe , should this be protected by a 25 amp CB type B or would it be considered safe at 32 amp ?

 

[Ian1981]

Each leg of the ring final circuit must be able to carry a minimum of 20amps in each part to comply with section 433 on overload.
If whatever reference method is adopted and in the tables the ccc of 2.5mm is 20amps or greater then 2.5mm can be used.
It isn't just added together as the load is supposed to be equally shared around the circuit if your using ref method 103 then you need to go larger than 2.5mm in fact larger than 4mm as well so I'd try to avoid this method.
If the cable is touching the outta skin (inner wall surface)of the stud wall then reference method 102 can be used giving a ccc of 21 amps and then 2.5mm can be used.

 

[Paulr]

Appreciated ian. I have read many views on this yesterday with all sorts of solutions from downgrading the CB, to simply dismissing the rule out of hand as being over cautious. The remedy I think in this case is to simply adjust the wire ( one run down a stud) to pull it forward of the insulation so that it touches the inner surface of the wall, as you say method 102 then applies. I think probably 50% of my wiring completed by a registered niceic spark back in 1993 was concealed in insulation and I have never had a problem. Guess we need to simply move with the times.

 

[Paulr]

Sorry ian, can I ask a question of my own house wiring. If I suspect my ring ccts are subject to a 103 situation and are currently on. 32 amp CB would it be prudent to downgrade the protection to a 25 amp breaker ?. I know it's been 20 plus years now of no issues but will this offer any better protection in the event of a problem ?.
Regards
Paul

 

[Andy78]

If you cannot guarantee contact with the outer skin for the entire length of run then method 102 becomes 103. The most onerous install method applies to the whole circuit regardless of how much of the circuit is subject to the install method.

Method A would be my favourite option in an insulated stud wall.

 

[Ian1981]

Sorry ian, can I ask a question of my own house wiring. If I suspect my ring ccts are subject to a 103 situation and are currently on. 32 amp CB would it be prudent to downgrade the protection to a 25 amp breaker ?. I know it's been 20 plus years now of no issues but will this offer any better protection in the event of a problem ?.
Regards
Paul

I would say realistically speaking if you have reason to suspect thermal damage to the fixed wiring then you may want to downgrade the circuit breaker however this maybe problematic for a kitchen ring circuit as a 20-25 amp mcb maybe subject to overload with the loads connected.

 

[Wilko]

If it's been working for 20yrs and if there is no damage and no extension of it planned, then why worry now ?
Edit : Sorry, I didn't mean to sound harsh

 

[Ian1981]

Carry out an insulation resistance test if worried about any suspected damage.

 

[Paulr]

Carry out an insulation resistance test if worried about any suspected damage.

Thanks Ian, noted. I have a megger multifunction meter so duly noted.
Cheers
Paul

 

[Paulr]

Chaps, I know I'm flogging this a little but the more I research this issue the more confused I get. I have tried to get more info from the official bodies but as yet no reply so wondered if you have a view on the following:
If the wire is clipped to the timber stud in a wall and is not actually touching the inner wall, and the insulation is only rated at 0.038 w/m2 k ( not a min of 0.1w/m2k ) in the osg - how do you deal with this ?
It still does not meet a 102 requirement but is it still a no no ?. The actual insulation I refer to is rock wool sound bats and are I believe fire resistant ?
Would be interested in your view on this ?

 

[Wilko]

If I recall correctly, the lower the U value the better the thermal insulating effect. So your U of 0.038 W/m2K is more insulating than the installation method assumes, so heat rise in the cable would be worse and current carrying capacity would have to be reduced. But I'm not sure you have actually got this U value, as it depends on both the insulating material and it's thickness. Check this little guide
How to Calculate U Value | CCF Ltd - http://www.ccfltd.co.uk/Trade-Support/Calculating_U_Values

 

[Andy78]

Is this for the design of a new installation or are you worried about a previously installed circuit ?

 

[Paulr]

If I recall correctly, the lower the U value the better the thermal insulating effect. So your U of 0.038 W/m2K is more insulating than the installation method assumes, so heat rise in the cable would be worse and current carrying capacity would have to be reduced. But I'm not sure you have actually got this U value, as it depends on both the insulating material and it's thickness. Check this little guide
How to Calculate U Value | CCF Ltd - http://www.ccfltd.co.uk/Trade-Support/Calculating_U_Values

I've just read a thread on the iet site and they say cables at this stage with conductivity ratings of less than 0.04 are not currently derated ???????

 

[Paulr]

Is this for the design of a new installation or are you worried about a previously installed circuit ?

Hi Andy it's a previously installed ( recently) not about to start run - it's just that its thrown up a query and I like to bottom these out once I get my teeth into them. I find the rules not that clear or specific by way of guidance.

 

[Ian1981]

Regulations state derating factors values given in tables are to be used unless more precise data can be obtained

 

[Andy78]

Hi Andy it's a previously installed ( recently) not about to start run - it's just that its thrown up a query and I like to bottom these out once I get my teeth into them. I find the rules not that clear or specific by way of guidance.

I've just read back a bit and seen that this arrangement has been in place for 24 years ?
I'd not be worrying too much unless you suspect a recent increase in the circuit load prompting cause for concern.

 

[Wilko]

Just for fun and discussion ...
A typical insulation roll quotes Lambda (thermal conductivity) as 0.044 W/mK. With 100mm of this (as assumed in method 100 say) then R (thermal resistance) is 0.1/0.044 = 2.273 m2K/W. And the U (thermal transmittance) is 1/R or 0.44 W/m2K. This is bigger than the 0.1W/m2K quoted as minimum for method 100 so it's ok.
Perhaps reconfirm your insulation values, as I think you may have used Lambda as U ? If any of that makes sense

 

[Paulr]

Just for fun and discussion ...
A typical insulation roll quotes Lambda (thermal conductivity) as 0.044 W/mK. With 100mm of this (as assumed in method 100 say) then R (thermal resistance) is 0.1/0.044 = 2.273 m2K/W. And the U (thermal transmittance) is 1/R or 0.44 W/m2K. This is bigger than the 0.1W/m2K quoted as minimum for method 100 so it's ok.
Perhaps reconfirm your insulation values, as I think you may have used Lambda as U ? If any of that makes sense

I think I need to revisit my building science books !. Thanks for the clarification.

 

[Wilko]

I think I need to revisit my building science books !. Thanks for the clarification.

When / if you do, you'll probably find I'm talking rubbish anyway