Cable ratings - constant load?

[danieleither]

For argument sake, lets say a cable is rated at 100A clipped direct, ambient temp 30 deg. Does this mean it can take a constant load of 100A in those conditions or is the rating based on cyclic current?

The reason I ask is on a recent install, following current cable guidelines we fitted a 35mm 4-core SWA XLPE cable to provide 150A per phase over a distance of 15 metres. At 150A constant load the cable has over-heated after 12 hours of operation (insulation melting).

I can't seem to find anything online about whether cable ratings are based on constant load or cyclic. Have we under-specced? If not, perhaps it's a damaged cable.

 

[Julie.]

They all look a bit overheated to me though.

I suspect the current curve would look like a very flattened -almost square rather than sine!

 

[danieleither]

They linked to the CEF site which says nothing about the manufacturer.

However, I do happen to have a short bit of the non-LSF version of that cable from CEF and it says on the side "EL SEWEDY CABLES" which I think are an Egyptian manufacturer.

It is interesting that the neutral is one showing the melting action after what Julie just posted!

The melted section on the Netrual is where Brown / P1 was originally pressed up against it. The mark is where I pulled the two apart from being stuck / melted together (they were originally touching). Therefore there is a similar mark on the brown cable, but you can't see it because of the camera angle.
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They all look a bit overheated to me though.

Agree - you can see how they have turned from a matt to shiny appearance, also very soft. You could scrape the insulation with a soft implement.

 

[pc1966]

Given the marginal rating anyway (154A clipped direct from table 4E4A) and the fact this is possibly a non-linear load, you should really look at replacing it with something heavier.

50mm is the next obvious thing but depending on the results of checking for harmonic currents it might be a case of looking at 70mm, and check before that the various connectors are able to take that size (they ought to, but just in case...)

It might also be worth going to a supplier like Yesss that do a brand like Doncaster Cables with a known reputation, as the small price difference over CEF's best-price choice is nothing compared to the time and effort being wasted on problems!
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Check though as cable-wrangling with 70mm is going to be a lot harder and typically they have sector-shaped conductors.

 

[danieleither]

Given the marginal rating anyway (154A clipped direct from table 4E4A) and the fact this is possibly a non-linear load, you should really look at replacing it with something heavier.

50mm is the next obvious thing but depending on the results of checking for harmonic currents it might be a case of looking at 70mm, and check before that the various connectors are able to take that size (they ought to, but just in case...)

It might also be worth going to a supplier like Yesss that do a brand like Doncaster Cables with a known reputation, as the small price difference over CEF's best-price choice is nothing compared to the time and effort being wasted on problems!
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Check though as cable-wrangling with 70mm is going to be a lot harder and typically they have sector-shaped conductors.

We have since run a 2nd supply and split the load - so all working fine now. But I was still curious to get to the bottom of this issue for future reference...

 

[Alex-SL]

Hi,
When choosing a cable, you should focus on the installation conditions. Derating factors can significantly change the base value of current carrying capacity. Are there any loaded neighboring cables or communications?

In any case, initially the cable was selected at the limit, without current margin. Next is your case without correction factors:

and the report ….

Can you clarify the installation conditions and the effect of the loaded neighboring cables?

 

[Ian1981]

Hi,
When choosing a cable, you should focus on the installation conditions. Derating factors can significantly change the base value of current carrying capacity. Are there any loaded neighboring cables or communications?

In any case, initially the cable was selected at the limit, without current margin. Next is your case without correction factors:
View attachment 59020
and the report ….
View attachment 59021
Can you clarify the installation conditions and the effect of the loaded neighboring cables?

You should be limiting your maximum cable temperature to 70 degrees thus the ccc will decrease, unless it is ascertained that the Switchgear that the cable it is terminating to is suitable for 90 degrees, which in most cases is no.

 

[Alex-SL]

If we limit the maximum cable temperature to 70C degrees, then the current carrying capacity of the cable Cu 35 mm2 is 119 A only (with the C-installation method).

To supply a load of 150A per phase, then you need to use a cable Cu 70mm2 (this will be with a small margin).