Wiring & floor insulation

[duncb]

Hello everybody,
Sorry if this seems to be another of hose repeated questions, but i have looked through other post with similar situations but found mixed answers,

My question is, i am planning to istall floor insulation to my suspended ground floor, and having read about how the insulation affects the wiring i am after some advice, the wiring is clipped to the bottom of the joist so that is not a problem, however where the wiring then comes upwards to the sockets through the insulation (about 6") what is the best way to do this.
I was thinking about putting some 50mm plastic trunking around the wire so creating a sleeve with airspace around the wire, would this be ok.

PLEASE dont get to technical with your replies if you can help me, i've read about diversity but dont totally understand it (Think its to do with reducing cable capacity??)

P.S. the insulation will probably be celotex or rockwool if this helps.
Thanks Duncan

 

[GaryMo]

Don't worry, just insulate away. There won't be any problems nor any need to derate any cables that have a covering of insulation over a mere 150mm of their length.

 

[ban-all-sheds]

That's not what Table 52.2 says....

 

[riveralt]

My question is, i am planning to istall floor insulation to my suspended ground floor, and having read about how the insulation affects the wiring i am after some advice, the wiring is clipped to the bottom of the joist so that is not a problem, however where the wiring then comes upwards to the sockets through the insulation (about 6") what is the best way to do this.

Just for your information - Table 52.2 refers to cable totally surrounded by thermal insulation - this only has to be 50mm in length for derating to apply.

This table will obviously apply if indeed your cable is totally surrounded but not if it is clipped to the joists or wall when passing through the insulation.

 

[GaryMo]

The OP mentioned 6 inches of cable passing through insulation, no more.

 

[ban-all-sheds]

The OP mentioned 6 inches of cable passing through insulation, no more.

this only has to be 50mm in length for derating to apply.

Does anybody have access to a proper design tool which will calculate the de-rating of a cable installed as per the OP's suggestion?

 

[riveralt]

The OP mentioned 6 inches of cable passing through insulation, no more.

this only has to be 50mm in length for derating to apply.

Does anybody have access to a proper design tool which will calculate the de-rating of a cable installed as per the OP's suggestion?

Not aware of one.

However, the Neher-McGrath Calculations provide a method for calculating underground cable temperatures or ampacity ratings - perhaps it has been derived from this formula.

http://www.electrician2.com/articles/ampacity.htm

This article discusses heat transfer and thermal insulation issues - it also uses the Neher-McGrath formula -

 

[GaryMo]

I have to admit that I honestly thought that it was 500mm before a factor was applied to cables passing though insulation.

How they come to derate cables passing through 50mm of insulation is beyond me.
Bear in mind that it's unlikely for the cable to be running at 100% of its rated capacity so surely and extra heat incurred along the 50mm within insulation will sink either way to escape.

 

[ColJack]

500mm of insulation derates a cable by 50%..

 

[GaryMo]

500mm of insulation derates a cable by 50%..

I know. It's just that figure I had in mind at the time of my reply.

 

[ban-all-sheds]

How they come to derate cables passing through 50mm of insulation is beyond me.

For one of two reasons.

A) As part of a British Standard they just made the figures up.

B) As a result of actual testing.

Which do you think is the more likely?

Bear in mind that it's unlikely for the cable to be running at 100% of its rated capacity so surely and extra heat incurred along the 50mm within insulation will sink either way to escape.

Oh well that's OK then.

Just before I go, could you tell me just how you know that at no time in the several decades ahead these circuits will be loaded to capacity, and also if your clairvoyance extends to knowing what the lottery numbers will be on Wednesday?

Oh - sorry - one more thing - I'd find it useful to know where in the regulations it says it's OK to ignore 433.1 if you think it won't matter if you ignore it, because that provision may be worded in a general way which means that we can ignore any regulations we choose if we can tell by looking into the future that nothing bad will happen if we do.

 

[duncb]

Thanks everybody for your replies, But basically i'm still confused, looking at the replies i guess the majority of you say the answer is a no no.
Just to clarify the wiring is clipped to the wall where it will rise through the insulation and then trunking over it to keep it seperate from the insulation.

Thanks again for your replies, any other ideas.

Dunc.

 

[riveralt]

Thanks everybody for your replies, But basically i'm still confused, looking at the replies i guess the majority of you say the answer is a no no.
Just to clarify the wiring is clipped to the wall where it will rise through the insulation and then trunking over it to keep it seperate from the insulation.

Thanks again for your replies, any other ideas.

Dunc.

You still haven't been clear regarding how the cable goes through the insulation.

Clipped direct (no insulation) 2.5m2 rated at 27Amps.

If it is completely surrounded by insulation then table 52.2 applies and the cable has to be derated after 50mm in length through the insulation.

50mm derating factor 0.88 = 23.76 Amps
100mm derating factor 0.78 = 21.09 Amps
200mm derating factor 0.63 = 17.01 Amps
400mm derating factor 0.51 = 13.77 Amps

At 500mm the derating factor is 50% - which for 2.5m2 is 13.5Amps

If it is clipped direct to the wall, joint or plasterboard as it passes through the insulation (insulation less than 100mm) then reference method 100 applies and a 2.5m2 cable is derated to 21Amps.

If is placed in conduit and that is completely surrounded by insulation then that is reference method A and the 2.5m2 cable is derated to 20Amps.

Alternatives - re-route cable away from insulation.
Do not insulate the area whether the cable runs.

 

[ban-all-sheds]

Well - firstly apologies for the sarcasm earlier, but honestly - in a domestic environment where you have no control over what might happen in the future, and where there isn't going to be supervision by someone skilled, you can't say that it's OK to have a circuit where the cable can't carry the maximum current it might be called on to do because you don't think it will be run like that very often.

You ought to be able, for example, to turn on your shower during a record breaking heatwave, go away on holiday for a fortnight and return to nothing more unpleasant then a huge electricity bill.

The problem we have is that the installation method you described didn't (but now might) match the ones in the Wiring Regulations for which derating factors are given.

The following won't mean much unless you have a copy of the regs, but hopefully those that do can comment on the reasoning...

The basic situation of a cable running through 150mm of insulation would derate it to about 70% (interpolating from Table 52.2), which would drop the capacity of your 2.5mm² cables to 18.9A, which is too low to be used in a 32A ring final.

Methods 1, 2 & 3 in 4A2 don't show that putting the cable in conduit makes any difference when the cable or conduit is in contact with a surface with a thermal conductance of 10W/m²K, I don't know what the conductance of wood is, and the specific methods for flat twin & earth cable in 4A2 don't use conduit and assume the cable running through >500mm of insulation. Reference Method 103 matches the 0.5 factor from Table 52.2

However, originally I thought you were describing the cables or conduit clipped to wooden joists as they came up - if they are going to be touching a brick/stone/plastered etc wall then we're looking at Method 2, which is Reference Method A, or Reference Methods 101 or 102.

2.5mm² twin & earth is rated at:

Ref Method A = 20A = OK
Ref Method 101 = 17A = not OK
Ref Method 102 = 21A = OK

BUT - your cables are not going to be running through insulation for >500mm, so the figures for all of those methods can be uplifted. Table 52.2 gives a generic derating factor of 70% for 150mm and 50% for 500mm or more, so it would seem reasonable to uplift those ratings above by 0.7/0.5, i.e. 1.4, which would meant that if we take it to be Reference Method 101 the capacity would be 23.8A, i.e. OK.

In the absence of any design tools which could be applied to your particular situation that's the best I can come up with - hopefully others will comment on whether my analysis makes sense or not.

 

[riveralt]

The basic situation of a cable running through 150mm of insulation would derate it to about 70% (interpolating from Table 52.2), which would drop the capacity of your 2.5mm² cables to 18.9A, which is too low to be used in a 32A ring final.

BAS, help me out here, isn't there a requirement for the current carrying capacity of any cable to exceed the overcurrent protection device for the circuit.

The derating factors apply to individual cable conduct csa's. So the maximum rating (Clipped direct) for a 2.5mm2 conductor is 27Amp - which does not exceed the overcurrent protection capacity.

Therefore, for a 32A protected ring final circuit do we not double the current carrying capacity of the circuit - 2 * 27Amps = 54 Amps.

If this is the case then your calculations of 18.9Amps outlined above are still within the criteria of the 32A circuit (18.9*2) = 37.8Amps.

Of course there is nothing stopping the OP derating the overcurrent protection device to 20A.