Installation Reference Method for T&E cable running through joists with no thermal insulation? (Lacking from table 4D5 in BS7671)

[eveares]

Say you had a cable running horizontally through floor joists without any thermal insulation (assuming horizontal cable support requirements were met), how is one supposed to know how much to de-rate the cable by and what the de-rated current carrying capacity is for a given conductor size.

Based on Table 4D5 in BS7671 does not mention anything about 70c thermoplastic insulated sheathed flat cables with CPC running through joists in free air, does that mean it's not a permitted installation method?

Maybe I'm missing something here, but I'm sure I have seen cables run for several meters through the middle of joists. Assuming no thermal insulation and a single circuit (i.e. no cable grouping), what is the current carrying capacity of T&E cable installed in such a way? Same as method C/clipped direct, or something else?

Any reg numbers to help answer my question above would also be helpful.

Regards: Elliott.

 

[ericmark]

I would say Reference Method C* (clipped direct) C* - For full installation method refer to Table 4A2 Installation Method 20 but for flat twin and earth cable. but rare for some lower method to not also be used, and
70°C thermoplastic insulated and sheathed flat cable, but one can get 90°C thermal setting still flat twin and earth cable.

In the main, used for ring finals, and no option to use less than 2.5 mm² unless using mineral insulated. So does it matter?

 

[EFLImpudence]

I would say Reference Method C* (clipped direct) C* - For full installation method refer to Table 4A2 Installation Method 20 but for flat twin and earth cable. but rare for some lower method to not also be used, and

I would agree it is Method C but all indications in the Regulations refer to it as Method B.
Whilst not in Table 4D5 for T&E other tables can be used to compare the ratings.

Surely (rather than 20) Installation Method Number 47 (too many Methods) in Table 4A2 with the associated notes is more like it.

Does going through a joist hole have any effect?

70°C thermoplastic insulated and sheathed flat cable, but one can get 90°C thermal setting still flat twin and earth cable.

That doesn't make any difference to the answers to Elliott's question.

In the main, used for ring finals, and no option to use less than 2.5 mm² unless using mineral insulated. So does it matter?

I agree. In domestic settings only shower circuits could possibly need any consideration.

 

[JohnW2]

Does going through a joist hole have any effect?

For what it's worth, I would personally suggest that it would have so little effect as to not be worth considering.

In fact, considering it as Method C (which is what I would certainly do, and do do) is probably a little on the 'conservative' side, given that most of the cable run would usually be 'in free air' (Method E) , hence a somewhat higher CCC than with Method C (30A for 2.5mm² 70° T+E, as compared with 27A for Method C).

 

[eveares]

I guess what I'm getting at here, does a cable have less effective cooling when in free air such as when run through joists than when clipped directly to a wood joist in open air for example?

i.e. Would the direct contact with the wood have greater thermal conductivity/desperation into the wood than if the cable was in free air?

 

[pcaouolte]

Presuming that your hole through the wood is bigger than the cable, which it should be, then the cable is only resting on the bottom of the hole with free air above the cable. I cant see that this is significantly different to a cable clipped direct to a wooden joist. In both cases one side of the cable is touching the wood and the other sides are in free air.

Things might be different if you drill a small hole in the wood and wedge the cable tightly into the hole, but who does that?

In any event, if we are talking about 75mm joists with 300mm gaps between then any heat in that 75mm section will dissipate along the cable into the 300mm section which has free air on all sides.

 

[JohnW2]

Presuming that your hole through the wood is bigger than the cable, which it should be, then the cable is only resting on the bottom of the hole with free air above the cable. I cant see that this is significantly different to a cable clipped direct to a wooden joist. In both cases one side of the cable is touching the wood and the other sides are in free air.

Quite so - and don't forget that, as I said, a cable in 'free air' actually has slightly more current-carrying- capacity that does one which is 'clipped direct' (or buried in plaster/masonry).

Things might be different if you drill a small hole in the wood and wedge the cable tightly into the hole, but who does that?

Well, I've certainly seen it done - but, as you go on to say ...

In any event, if we are talking about 75mm joists with 300mm gaps between then any heat in that 75mm section will dissipate along the cable into the 300mm section which has free air on all sides.

Again, quite so - but usually 'better' than that, since joists these gays are often only 50 mm wide with 350-400 mm gaps between them.

 

[eveares]

Presuming that your hole through the wood is bigger than the cable, which it should be, then the cable is only resting on the bottom of the hole with free air above the cable. I cant see that this is significantly different to a cable clipped direct to a wooden joist. In both cases one side of the cable is touching the wood and the other sides are in free air.

Things might be different if you drill a small hole in the wood and wedge the cable tightly into the hole, but who does that?

In any event, if we are talking about 75mm joists with 300mm gaps between then any heat in that 75mm section will dissipate along the cable into the 300mm section which has free air on all sides.

I was more thinking of the space in between the joists where the cable was in free air than the bit through joists.

My flawed, stupid and bizarre logic was that like how water conducts heat 5 x greater than air, if one side of a T&E cable was always in contact with the wood joist, that would dissipate the heat away from the cable into the joist (A bit like a body in cold water) quicker than if much of the cable was floating in free air with no thermal conductivity to dissipate the head away from the cable.

But that theory of mine does not make much sense as when a cable is clipped direct to a joist but also has thick insulation over it, it still gets de-rated despite one side of the cable being in "thermal" contact with the a wood joist to dissipate the heat.

Think I was just over thinking things.

Regards: Elliott.

 

[pcaouolte]

At least you are thinking about it and not just blindly following the rules. Timber won't act as a very efficient heatsink to take heat away from a cable.

 

[JohnW2]

My flawed, stupid and bizarre logic was that like how water conducts heat 5 x greater than air, if one side of a T&E cable was always in contact with the wood joist, that would dissipate the heat away from the cable into the joist (A bit like a body in cold water) quicker than if much of the cable was floating in free air with no thermal conductivity to dissipate the head away from the cable.

You're thinking only of conduction, and you're probably right that many/most 'solid' materials would conduct heat away better than air. However, we're not thinking about 'still air', and what you are overlooking is convection - which causes air adjacent to the cable to take heat away ('rising') constantly being replaced by cooler air.

As I said, this is reflected in the tabulated current-carrying capacities, which are a little (about 10%) higher 'in free air' than when clipped to a surface or buried in plaster/masonry.

But that theory of mine does not make much sense as when a cable is clipped direct to a joist but also has thick insulation over it, it still gets de-rated despite one side of the cable being in "thermal" contact with the a wood joist to dissipate the heat.

It doesn't get de-rated as much when it is in contact with s 'conductive surface on one side but covered with thermal insulation on the other as when it is completely surrounded by thermal insulation.

Think I was just over thinking things.

Over-thinking is usually far more desirable than under-thinking

 

[EFLImpudence]

...and is a ceiling/floor void free air?

 

[pcaouolte]

...and is a ceiling/floor void free air?

Yes. Provided that there is a clearance between a cable and any adjacent surface of at least 0.3 times the cable external diameter for multicore cables, natural air convection is not impeded, and any heat sources are taken into account.

 

[JohnW2]

...and is a ceiling/floor void free air?

It's obviously all 'relative', with essentially arbitrary definitions - but, yes, for what it's worth, given the depths of floor joists, I think it would be fairly reasonable to consider it as 'in free air'.