Foam and cabling

[Unknownstuntman01]

 

[Taylortwocities]

Around the cables?
That could change the current carrying capacity of the cables by up to 50% so you may have to install larger cables to compensate.

 

[mfarrow]

What TTC meant is the length of foam coverage has a significant bearing on the ability of the cable to work OK surrounded by foam.

The strangest application for this was holding CTs round cores in a cable box. Looked hideous but did the job on cables 200mm+.

 

[skotl]

I'm not sure the OP will completely understand why there may be an issue - with the deepest of respect; I certainly don't understand the issue from the two well-meaning responses so far.

Why does it change the current carrying capacity? I'm guessing that the foam compresses the cable when it expands?
If that is the case, why would length of foam coverage make a difference as I would have thought the cable was as weak as its weakest point, so even putting foam into where it travels through a wall would have an effect?

And is there a specific way of measuring the effect?

Sorry to butt in; I can see that the answer to the OP's question is "yes" but it would be interesting to understand why.

Cheers

 

[Monkeh]

Why does it change the current carrying capacity? I'm guessing that the foam compresses the cable when it expands?

Expanding foam is potent stuff, but it can't compress copper.

The foam is an insulator. It makes the cable run hotter.

 

[Taylortwocities]

The amount of current that a cable can carry safely (i.e. before it melts) is determined by testing the cable in "ideal" conditions.
Ideal would be something like: in free air, clipped direct to backgrounds or on a perforated metal tray. Using these methods the cable is cooled by the circulation of air, or being next to surface that dissipates the heat.

For this reason, the maximum current carrying capacity of a cable will change depending on the installation method. There are tables in the BRB that determine the derating factor for each installation method. Enclosing the cable in insulating material (including expanding foam) has a big impact on a cable, as there is nowhere for the heat to go.

It has nothing to do with the copper being squeezed.

 

[skotl]

Cool - I get it now - thanks

 

[Taylortwocities]

Great, so how are you planning to go about resolving it?

 

[skotl]

Great, so how are you planning to go about resolving it?

You may be mistaking me for the original poster (which isn't surprising, as I do appear to have hijacked his thread somewhat!)

 

[Taylortwocities]

Aah yes.



I wonder what the Stuntman is planning…….

 

[JohnW2]

Aah yes. I wonder what the Stuntman is planning…….

We currently can but guess! It's quite probable that the only question he thought he was asking was whether expanding foam damages PVC cable (and the answer is that polyurethane expanding foam is fine in contact with PVC cables) - but, as all the answers so far have indicated, he will then have discovered that there are other questions that he should have been asking!

We are most often asked this question in relation to just 'sealing around' a cable where it goes through some hole, and I am personally inclined to say that one should not worry about possible CCC issues in that situation.

Kind Regards, John

 

[ban-all-sheds]

You may be personally so inclined.

But that is not an inclination which is informed by Table 52.2

 

[JohnW2]

You may be personally so inclined. ... But that is not an inclination which is informed by Table 52.2

It essentially is, in the context I was referring to. As you might imagine, I am aware of that Table. I was thinking/talking cable about going though a hole in, say, a panel or the casing of an enclosure (e.g. a back box), such that the 'length of the insulation' would/could be appreciably, perhaps considerably, less than the smallest length (50mm) mentioned in that Table.

Both Table and associated text are silent about what happens when the length of insulation is less than 50mm, but one interpretation is that no de-rating is then necessary. The 'worst interpretation' would be that there has to be a de-rating pro-rata to 12% for 50mm, which could be 'negligible' in the sort of situation I'm talking about. Common sense also suggests that if the length of the insulation gets appreciably less than 50mm, then sideways conduction of heat should make the need for de-rating pretty negligible.

Kind Regards, John

 

[ban-all-sheds]

It essentially is, in the context I was referring to. As you might imagine, I am aware of that Table. I was thinking/talking cable about going though a hole in, say, a panel or the casing of an enclosure (e.g. a back box), such that the 'length of the insulation' would/could be appreciably, perhaps considerably, less than the smallest length (50mm) mentioned in that Table.

But that isn't what you said - you did not qualify your advice with anything related to length:

We are most often asked this question in relation to just 'sealing around' a cable where it goes through some hole, and I am personally inclined to say that one should not worry about possible CCC issues in that situation.

You may have been thinking about a cable about going though a hole in, say, a panel or the casing of an enclosure (e.g. a back box), but IMO if someone is using expanding foam to seal around a cable the most likely reason is that it's going through a wall.

 

[JohnW2]

I was thinking/talking cable about going though a hole in, say, a panel or the casing of an enclosure (e.g. a back box), such that the 'length of the insulation' would/could be appreciably, perhaps considerably, less than the smallest length (50mm) mentioned in that Table.

But that isn't what you said - you did not qualify your advice with anything related to length:

We are most often asked this question in relation to just 'sealing around' a cable where it goes through some hole...

Yes, it's true that I did not explicitly talk about panels and cases of enclosures - I suppose I should have made it clearer what I was thinking about.

You may have been thinking about a cable about going though a hole in, say, a panel or the casing of an enclosure (e.g. a back box), but IMO if someone is using expanding foam to seal around a cable the most likely reason is that it's going through a wall.

Yes, that's also quite possibly true.

For those who wish to play strictly by these rules, there must be a strong temptation to add unnecessary (probably concealed) joints. If I wanted strict compliance and had a circuit of, say, 30, 40 or 50 metres total length, and there was somewhere a 100mm length of cable going through insulation which reduced the CCC to the extent that a larger size of cable (larger than would be fine for the rest of the circuit) would be needed, I would be very tempted to 'splice in' a very short length of that larger cable, rather than run the whole circuit in that larger cable.

Kind Regards, John