17th Edition - Cable Capacity - Which Method ?

[self builder 123]

Hi

In my house we have the PVC Twin + Earth 6 sq.mm cable clipped to the side of a timber ceiling joist which is 200mm deep. The cable is in the centre of the joist.

The same cable after crossing the ceiling drops down in a stud partition which is 150mm deep. It is not clipped to the vertical timber and it also does not touch the rear of the plasterboard.

Both the ceiling and stud partition are fully insulated so the insulation touches the side of the cable which is not clipped to the timber.

In order to confirm the current carrying capability of this cable which method from the 17th Edition is most applicable ?

Cheers in advance.

 

[scarypants]

The cable should be clipped on it's vertical run 300/400mm,
Ref meth c then calculate for thermal insulation this will depend distance the cable runs through it.

 

[self builder 123]

Scarypants

The cable runs pretty much trough out the run touching the side of the insulation. The cable run is approx. 15m.

 

[ericmark]

Sounds like Reference Method 103# (in a stud wall with thermal insulation with cable not touching the inner wall surface) so 6mm = 23.5A.

 

[scarypants]

the thermal insulation will prevent heat dissipating from the cable, this is why factor Ci is used.
The regulation state whenever possible cable should not be covered by thermal insulation, but due to heat loss through lack of cavity wall,ceiling and loft insulation is near impossible. So cables are going to come across this.
Appendix 6 of OSG covers this factor
Appendix 4 of the big red book covers the ref meths by your description I'd assume ref meth 102 or103 depending if cable touching the inner wall worst case your current carrying capacity will half.

 

[self builder 123]

Hi

Looking at Table 4D5 Method A ( cable in conduit in an insulated wall ) a 6 sq.mm cable is derated to 32A.

Where would this method be applicable ?

 

[scarypants]

If the cable was routed using conduit as containment and this containment was not wall mounted but routed through a stud wall that was thermal insulated.

 

[scarypants]

If you clip your cable so it is touching inner wall and not covered by thermal insulation more heat can be disipated.

 

[self builder 123]

If a cable was run inside a conduit which was run inside thermal insulation ( so as to completely encapsulate ) the conduit and thus the cable it would not be possible for the cable to dissipate heat ?

So is the 32A rating for Method A wrong ?

 

[scarypants]

No it's right, when running cables through conduit/trunking again factors and calculation are made so there is space in the conduit so the cable can dissipate heat. You can't go cramming 45 lengths of cable into 16mm conduit. You should leave 45% of it empty. If you have osg Appendix 5 covers this.

 

[self builder 123]

Under the 16th Edition in Table 4D5A Installation Method 15 ( installed directly in an insulated wall ) a 6 sq.mm cable was rated at 35A.

Do you happen to know was this based on the cable touching the inner wall ?

 

[scarypants]

same as ref meth A of 17th

 

[self builder 123]

cheers

 

[ColJack]

if a cable runs in a variety of installation methods over it's course, you should always use the lowest value for it's CCC.

 

[self builder 123]

Hi

One other question - the ccc of a cable, are the values listed in the Installation Methods in the 16th and 17th Edition based on a continuous use i.e. 24 hrs a day ?