SWA cable rating - 2 core or 3 core?

[teacherspark]

Hi guys and girls,

When calculating current carrying capacity of SWA; if you have 3 core swa but one core is being used as the cpc, is the SWA classed as 2 core?


Many Thanks

 

[multimick]

Hi guys and girls,

When calculating current carrying capacity of SWA; if you have 3 core swa but one core is being used as the cpc, is the SWA classed as 2 core?


Many Thanks

no

 

[Pete999]

No

 

[teacherspark]

no

Thanks for your reply multimick.

So 3 core: phase L1, phase L2 and neutral with separate cpc can carry same current as 3 core: Phase, neutral and cpc?

They both get as warm as each other?

I appreciate the time you took to write your answer.

Many thanks

 

[multimick]

Thanks for your reply multimick.

So 3 core: phase L1, phase L2 and neutral with separate cpc can carry same current as 3 core: Phase, neutral and cpc?

They both get as warm as each other?

I appreciate the time you took to write your answer.

Many thanks

Yes, that took a long time as well

 

[HandySparks]

When calculating current carrying capacity of SWA; if you have 3 core swa but one core is being used as the cpc, is the SWA classed as 2 core?

If you use 3 core SWA for a single phase circuit, it won't dissipate as much heat as the same cable used for a 3 phase circuit running at the same current. I use the 'single phase' column values in the current carrying capacity tables.

 

[westward10]

I tend to judge it on the number of cores and not their function as more cores are likely to prevent heat dissipation. Some feel the third core used as a cpc is negligible.

 

[westward10]

I just love helpful responses.

 

[multimick]

i agree but he original question was about CCC.

 

[westward10]

i agree but he original question was about CCC.

.??

 

[HandySparks]

I tend to judge it on the number of cores and not their function as more cores are likely to prevent heat dissipation. Some feel the third core used as a cpc is negligible.

If an extra core carrying no/negligible current made a significant difference to the heat dissipation, then there would be separate columns for 3 phase 3 core and 3 phase 4 core.

 

[multimick]

sorry west i wrote it before your post

 

[teacherspark]

More warm cores (more phases) would slow heat dissipation. A cold core would increase the rate of heat loss.

Heat would be transferred from phase to colder cpc and out 'the other side', dissipating it quicker. If all the cores were warm to begin in with you wouldn't get heat transfer as they are all warm.

I guess this is why I'm here discussing it.

 

[westward10]

If an extra core carrying no/negligible current made a significant difference to the heat dissipation, then there would be separate columns for 3 phase 3 core and 3 phase 4 core.

Possibly but it is interesting that YY, CY and SY cables give a ccc for the conductor csa and then you apply correction factors for core quantity with no relevance to core function.

 

[Ian1981]

Possibly but it is interesting that YY, CY and SY cables give a ccc for the conductor csa and then you apply correction factors for core quantity with no relevance to core function.

Yes but they are devil cables designed to confuse numpties like myself

 

[Bobster]

Possibly but it is interesting that YY, CY and SY cables give a ccc for the conductor csa and then you apply correction factors for core quantity with no relevance to core function.

YY, CY, and SY wont be found in BS7671 as they aren't recognised.

 

[westward10]

YY, CY, and SY wont be found in BS7671 as they aren't recognised.

I know, I just find it interesting. I think the Tables in BS7671 are bound in the past with regards to conductor function above two core.

 

[David M]

Possibly but it is interesting that YY, CY and SY cables give a ccc for the conductor csa and then you apply correction factors for core quantity with no relevance to core function.

I guess the values are calculated on worst case scenarios, i.e on the assumption that all cores have to be considered as load current carrying.

 

[Lucien Nunes]

Comparing 2-core and 3-core cable with two cores loaded, there are two thermal factors at work. The greater outside diameter of the 3-core increases the surface area in contact with air, which is usually the main barrier to heat dissipation, so the 3-core can dissipate more heat in total. On the other hand the internal construction determines how well heat can flow from the two loaded cores to the outside - I would expect this effect to be much less significant, although if it is a rope-wormed cable rather than extruded solid or sectored, the air pockets might have an effect. Overall, I would expect a 3-core cable to be as good or bettter than 2-core so I use the 2-core figures.

In contrast, a 3-core cable for a 3-phase circuit could have all three cores loaded so the dissipation could be 50% greater. Since the surface area cannot increase by more than the square root of the CSA, and in practice increases less, the CCC must be lower than for a 2-core. This is why I would not apply the 3-core 3-wire 3-phase tabulated rating to a single-phase circuit. The difference between a 3-phase 3-wire and 3-phase 4-wire is less significant. The difference between the cable O/D's is less, and although the heat dissipation is likely to be lower (without harmonics, sharing some current with the neutral decreases the sum of I²R values of the separate cores) there is the possibility of harmonics increasing the dissipation above the 3-core value for the same RMS total load.

This difference is why I would use different figures for 2- and 3-core cables used for a single-phase circuit, but the same figures for a 3- or 4-core cable used for a 3-phase circuit.

The unusual case mentioned above, of a 3-core cable used for L1, L2 and N would seem to fall into the 3-phase scenario.