Impact of temperature on cable rating?

[Tipper]

I have noticed that some of my house wiring cables run adjacent to underfloor heating plastic pipes in service ducts that take both cables and pipes around the house. The ufh runs at a max of 55C so the cables will be close to that temperature.

Is this an issue with respect to cable rating and should the circuits be downrated or is 55C OK for normal pvc t&e cables (2.5 and 1.5 mm2) based on there being no electrical heating load on the power rings?

 

[EFLImpudence]

There is a derating factor but it is unlikely to be an issue in a domestic situation.

 

[ban-all-sheds]

Not so.

For 70° thermoplastic cable, the Ca for 55° ambient is 0.61, so the free-air CCC of 2.5mm² cable drops to 18.3A, which is not enough for it to be used for a ring final - that requires a minimum of 20A.

And that's if you consider that the cables in the service ducts are Method E in the first place. If you rate them as clipped direct then their capacity drops to 16.5A, and with the (arguably valid) Method B it's 14A.

 

[EFLImpudence]

That assumes the cables would rise to the 70° in normal usage - fully loaded for long period.

Do you think that a pipe at a maximum 55° means that that is the ambient temperature?

 

[JohnW2]

Do you think that a pipe at a maximum 55° means that that is the ambient temperature?

Indeed. If the maximum temperature within the pipes is 55°, I rather doubt that the ambient temperature in the vicinity will be anything like that high, even if both are in the same duct.

Kind Regards, John

 

[ban-all-sheds]

That assumes the cables would rise to the 70° in normal usage - fully loaded for long period.

So do ALL cable ratings.

If the ambient temperature factor reduces the capacity of the cable to 18, or 16, or 14A, then that's what it does and it then does not comply with the requirements for use in a ring final. No matter how much you say that doesn't matter, the fact remains that it no longer complies, and the only way you could certify it would be to identify its use as a departure and present a sound engineering based argument showing that its use is just as safe as if it did have a capacity of at least 20A.

The figures involved are pretty much the same as you'd encounter for 1.5mm² T/E, so would you use the argument that it would be OK to use that in a ring final because the cable won't be fully loaded for a long period?

Do you think that a pipe at a maximum 55° means that that is the ambient temperature?

Don't know. Depends on the thermal conductivity of the pipes and any lagging, so it would be good to know what the temperature in the duct would be, as I suspect it will be less than that, and that is an important material fact. But unless they are lagged, then the temperature is likely to be close to 55° as the whole point of the pipes is to allow heat to escape from them.

In the absence of that, 55° has to be what you use. You may not stick a finger in the air and say "I reckon it will only be X° in there" - that's not how engineering works.

 

[EFLImpudence]

That assumes the cables would rise to the 70° in normal usage - fully loaded for long period.

So do ALL cable ratings.

The ratings do but the cable may not. It is, after all, 27/20 over spec.

If the ambient temperature factor reduces the capacity of the cable to 18, or 16, or 14A, then that's what it does and it then does not comply with the requirements for use in a ring final.

We need to determine the actual ambient temperature.

No matter how much you say that doesn't matter,

I didn't say it doesn't matter; I said it was unlikely to be an issue.
It is already there.

the fact remains that it no longer complies, and the only way you could certify it would be to identify its use as a departure and present a sound engineering based argument showing that its use is just as safe as if it did have a capacity of at least 20A.

...or determine that the cable did not get heated to 55°.

The figures involved are pretty much the same as you'd encounter for 1.5mm² T/E, so would you use the argument that it would be OK to use that in a ring final because the cable won't be fully loaded for a long period?

Not really the same ; 1.5mm² would be fully loaded at 20A.

Don't know. Depends on the thermal conductivity of the pipes and any lagging, so it would be good to know what the temperature in the duct would be, as I suspect it will be less than that, and that is an important material fact.

So do I.

But unless they are lagged, then the temperature is likely to be close to 55° as the whole point of the pipes is to allow heat to escape from them.

Really?
It seems very inefficient if it heats things 'adjacent' (OP's word) to it to the same temperature as the water inside.

In the absence of that, 55° has to be what you use. You may not stick a finger in the air and say "I reckon it will only be X° in there" - that's not how engineering works.

You could (well, OP could) stick a finger on the cable.

 

[ban-all-sheds]

The ratings do but the cable may not. It is, after all, 27/20 over spec.

The cable must have a CCC, after all derating factors are applied, of at least 20A.

I didn't say it doesn't matter; I said it was unlikely to be an issue.

Same thing.

It is already there.

Maybe it should not be.

...or determine that the cable did not get heated to 55°.

Indeed.

Not really the same ; 1.5mm² would be fully loaded at 20A.

So would 2.5mm², Method E, ambient temperature 40°.

Really?
It seems very inefficient if it heats things 'adjacent' (OP's word) to it to the same temperature as the water inside.

The pipes are meant to heat things. If those things get to the same temperature as the water that would be very efficient, not inefficient.

You could (well, OP could) stick a finger on the cable.

Nowhere near enough engineering rigour.

 

[ericmark]

Interesting and for once I have to agree with BAS guessing is not good enough. The poster state PVC however both thermal plastic and thermal setting cables look the same, you need to actually read the writing on the cables to know which is which, yes thermal setting is normally white but not always. To my mind with good quality cables there is unlikely to be a problem but that does not mean they are not non complainant.

Big question is what was fitted first? If under floor heating was fitted before or at same time as cables likely the electrician would have allowed for the heat. But if laid after the cables then the electrician would be likely unaware. I have seen where central heating pipes have been very close to cables and seen no damage but that does not mean that will always be the case.

 

[EFLImpudence]

Original post:

I have noticed that some of my house wiring cables run adjacent to underfloor heating plastic pipes in service ducts that take both cables and pipes around the house. The ufh runs at a max of 55C so the cables will be close to that temperature.

Is this an issue with respect to cable rating and should the circuits be downrated or is 55C OK for normal pvc t&e cables (2.5 and 1.5 mm2) based on there being no electrical heating load on the power rings?

I do not read that as the cables being in contact with the UFH; merely in service ducts with the plastic pipes supplying it - as is written.


I now realise what Bas meant by "the whole point of the pipes is to allow heat to escape from them" which I thought odd.

 

[ban-all-sheds]

I didn't think they were necessarily in contact, but they might be. Even so, they shouldn't be damaged by 55°. The issue is solely what is the ambient temperature in the duct?

 

[JohnW2]

I didn't think they were necessarily in contact, but they might be.

I suspect that EFLI's point was that, although the "whole point" of the actual UFH heating pipes is to "allow heat to escape from them", the same is not (or should not be) really true of UFH distribution pipes (which is what we seem to be talking about).

Kind Regards, John

 

[ban-all-sheds]

I don't know enough about UFH systems to say. Maybe the distribution pipes perform a heating role in passageways. If they are lagged would the OP be concerned about temperature in the ducts?

 

[JohnW2]

I don't know enough about UFH systems to say. Maybe the distribution pipes perform a heating role in passageways.

Not impossible, I suppose, but it wouldn't then really be just "UF" heating!

If they are lagged would the OP be concerned about temperature in the ducts?

In general, lagging just affects the rate of heat transfer. Given enough time (with the pipe hot) the temperature within the duct would probably be much the same with or without lagging. That's why, for example, lagging does not give protection (in terms of freezing) against long-term sub-zero ambient temperatures - eventually the water temperature will itself become sub-zero.

Kind Regards, John

 

[ban-all-sheds]

Not impossible, I suppose, but it wouldn't then really be just "UF" heating!

I was assuming his passageways have floors.

In general, lagging just affects the rate of heat transfer. Given enough time (with the pipe hot) the temperature within the duct would probably be much the same with or without lagging.

And that the heating does go on an off.