Can I avoid applying correction factor (Ci) when cable tray passes through a concrete wall?

[s harris]

Hi

This is about a design problem in an assignment.

A cable basket containing several circuits passes horizontally through an internal concrete wall (100mm) and into a ceiling void where it provides lighting throughout the building.

I'm trying to size the cables for the circuits and I'd really like to avoid having to use the correction factor for thermal insulation Ci for where the tray passes through the wall.

No practical experience here, so I don't even know if the gap over the cable tray has to be filled, or how I should 'make good' the cavity. It's an intake room which is the only place in the building without a false ceiling.

I've had a try at it but got in a muddle trying to imagine using fire batt or intumescent foam etc. without the practical know-how and failed to find contenders with thermal properties I could somehow 'match' to the concrete wall.

Does anyone have a solution please?

Thanks, Suki

 

[Strima]

Try running the cable in conduit through the wall as this could help with selection.

Take a look at wall gland kits such as Roxtec as these could be an option.

But if the design requires fire stopping you may have to go up a cable size.

Has your tutor given any direction or hints?

 

[s harris]

Try running the cable in conduit through the wall as this could help with selection.

Take a look at wall gland kits such as Roxtec as these could be an option.

But if the design requires fire stopping you may have to go up a cable size.

Has your tutor given any direction or hints?

Hi Strima,

Thank you very much your comments. I don't think firestopping is needed here as it's just an intake room and not an escape route. It's good to know about wall gland kits, but I really like your idea of using conduit instead. Brilliant - I'll go with that. I'd not even considered it, and it makes me wish I'd posted on this forum sooner.

Thank you very much for taking the time and effort to advise,
Suki

 

[westward10]

Fire compartments are not restricted to escape routes. Fire compartments help to reduce the risk of fire and smoke spread. Being an electrical switch room it is almost certainly going to be a fire compartment and hence any breaches in its structure should be fire stopped.

 

[Andy78]

Well worth looking at 527.2 here to establish the requirements for sealing the cable route. You may need help from your tutor to determine the required fire resistance properties of the building elements.

 

[s harris]

Fire compartments are not restricted to escape routes. Fire compartments help to reduce the risk of fire and smoke spread. Being an electrical switch room it is almost certainly going to be a fire compartment and hence any breaches in its structure should be fire stopped.

Thank you for this information Westward10. You've prompted me to look at guidance documents on fire safety and I see what you mean, it would be a fire compartment. For the assignment I've decided to use separate runs of steel conduit, patched up with mortar.

Thanks again for your comment!
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Well worth looking at 527.2 here to establish the requirements for sealing the cable route. You may need help from your tutor to determine the required fire resistance properties of the building elements.

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Thank you too for your contribution Andy78.

As I'm going with conduit I guess it's a moot point now but it would be nice to know I if I've managed to follow the drift.

If it had to be cable basket passing through an interior concrete wall of a building, is this correct?

If it was a fire compartment:
I'd have to seal the cavity completely using firestop and just use the 0.5 Ci factor;
or I might be able to use a wall-gland kit as long as it didn't impair the cables' ability to dissipate heat (ie. it would need a total thermal resistance equivalent to or better than that of the wall, U value = 2w/m2k)

For a non-fire compartment:
I'd not need to seal the cavity

Thanks, Suki

 

[westward10]

I would speak to your tutor. On a large new build fire stopping is often carried out by specialist contractors so for your project it may be acceptable just to state this.

 

[DefyG]

A 100mm concrete wall would offer minimal thermal impact on cables passing through and would not warrant a Ci of 0.5. Concrete is not classed as thermal insulation!
The fire barrier medium 'might' but again over only 100mm again would be minimal!

 

[s harris]

A 100mm concrete wall would offer minimal thermal impact on cables passing through and would not warrant a Ci of 0.5. Concrete is not classed as thermal insulation!
The fire barrier medium 'might' but again over only 100mm again would be minimal!

Thank you for your reply DefyG.

Sorry, and I should have clarified this - In the first part of my question in my last post I meant to ask if it was ok to pass basket through a hole in a wall without using any foam etc (assuming there was no need to firestop). It might seem surprising I would ask such a basic question but I really don't know the answer.

In the second part of my question, where I wanted to know about the thermal properties of what would immediately surround the cables, I now realise I mistook Ci 0.5 for applying to 100mm length in thermal insulation, instead of referring to Ci 0.78 as I should have. Got a bit confused there.

Where any solutions I've seen that might seal such a cavity claim mention thermal properties at all, it's always that they offer good thermal insulation, and I'd like to avoid that 0.78 factor if possible.

I'm getting the feeling I've misunderstood something big, but I'm not sure what.

Also I'm not sure what lambda/R/U values etc. are considered representative of 'thermal insulation' in the eyes of the Regs, but it seems odd to my mind that the factors in Table 52.2 describe a minimum thermal conductivity value of 0.04W/mk for thermal insulation (523.3). Surely even a wall with a thermal resistivity of 2Km/W has a thermal conductivity of 0.5W/mk.

Thanks, Suki

 

[bbudding]

Also I'm not sure what lambda/R/U values etc. are considered representative of 'thermal insulation' in the eyes of the Regs, but it seems odd to my mind that the factors in Table 52.2 describe a minimum thermal conductivity value of 0.04W/mk for thermal insulation (523.3). Surely even a wall with a thermal resistivity of 2Km/W has a thermal conductivity of 0.5W/mk.
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Yes indeed! Why do they ask for a MINUMUM of 0.04 when the higher that value the more thermally conductive and LESS insulating the material is. I think that must be a typo in the regs!!