SWA cable current carrying capacity

Table 4D2A refers to cables "with or without a protective conductor" (asterisk).

Perhaps Table 4D4A assumes that the armour is used as the CPC because there is no similar note.

I interpret the table as stating that the capacity of a two-core (SP) cable is 99A, but with three or four cores, that is 82 A per phase, so a lot more than 99A in total. The use of a third core as CPC can hardly change the CCC, because most of the time no current is flowing.

Adding a further load of 40 A could bring voltage drop into play. From Table 4D4B (using just the z value), VD = 1.75 x 85 x 75 mV = 11.2 V.

Hi Chris

Thanks for your reply.

Yes, would agree the volt drop is right on the cusp. Furthermore, I was going to change the 60 amp switch fuse in the main building to a 100 amp, however “In” (100amp) would be greater than “It” (99 amps).   

The only options I can think of are -

1. Renew the cable to a 35mm (very costly).
2. Install some kind of load depended switch, so as when the new load is on it doesn’t allow certain other loads to be used (loads that aren’t important). Would you know of any kind of switch that would carry this function out?

The armouring on the 25mm²  swa should be adequate for the cpc according to a table I’m looking at. The main waterpipe coming into the building is plastic and no other extraneous conductive parts present, so the yellow conductor isn’t really required as an earth.

This is maybe a silly question, however it’s how you learn “by asking”. Could the swa be reterminated into a 3 pole isolator (only using two of the poles) then split using henley blocks, using the existing red conductor to supply the existing loads & the yellow conductor to supply the new load.  Not sure how this would affect the current in the neutral conductor though? as the neutral would now be shared between the red and yellow. Would this be wrong to do this kind of set up? Am I barking up the wrong tree.   

Thank you for your help.

Regards,

CPC

you may parallel cores to reduce the dissipation in the cable - as now two of the three cores are running at half current. But, you should not share a single neutral to 2 live circuits. Personally if I had to I'd prefer to see 2 parallel neutrals and one live - the thermal situation is the same, but the risk of loss of a single shared neutral between 2 ccts is more serious than loss of half of a parallel one.

25mm and 100A should be fine if the cable is well cooled, but is it ?
Mike.

I would apply engineering judgment and consider the existing cable sufficient to for 100 amps, on the grounds that the heaviest load is the shower, which is only used intermittently.

Or apply engineering judgment and use the "2 core rating" for the existing cable, justified on the grounds that only 2 cores are loaded.

Or fit an 80 amp fuse in the new 100 amp switch fuse..

Hi Mike, Thanks for your input. 

I didn't think it would be right to share the neutral. Can you explain how its ok to do this on a 3 phase distribution board, where there are single phase loads across the different 3 phases but only one neutral conductor which is normally the same size of conductor as the line conductors? 

Could you also explain a bit more regarding "if the cable is well cooled". The cable inside the main building is clipped direct to the wall for about a 4 meters, then go's under ground, then clipped direct again to the wall in small building for about 2 meters before being terminated into a single phase distribution board.   

Regards,

CPC

CPC said:

I didn't think it would be right to share the neutral. Can you explain how its ok to do this on a 3 phase distribution board, where there are single phase loads across the different 3 phases but only one neutral conductor which is normally the same size of conductor as the line conductors?

That's a very good question.

In the three phase circuit, the neutral is not shared because there is just one circuit.

The neutral conductor is the same size as the line conductors because the sum of the currents in the lines is never more than the highest current in any one of them because of the phase differences: at any one time, two might be on a positive part of the cycle, but the third is negative.

In theory, I see no reason why you could not run two-phase plus neutral: after all, it would be the same as a distribution circuit to a TP board which happens to have nothing connected to phase 3. (ETA: you would, of course, have to have a TP board at the load end.)

What you cannot do is run two lines of the same phase plus neutral because the neutral current would be the sum of the lines, so potentially twice as much.

I was trying to insert a wee spreadsheet to illustrate the point, but failed miserably. Is this possible please?

ETA: in the mean time, here are some graphs.

A perfectly balanced load has no current in the neutral, which can be omitted if the load will always be perfectly balanced, e.g. a motor.

If one of the phases is removed, the neutral current rises to the line value, but no more.

If the loads have different values, the current in the neutral is lower than the highest line.

If two of the loads are small, the situation becomes similar to an SP circuit with its equal line and neutral.

HTH.

Would you know of any kind of switch that would carry this function out?

Often complete units are sold as "shower priority switch" or suchlike - e.g. https://www.meteorelectrical.com/distribution-control/consumer-units-accessories-1/garo-priority-shower-board-choose-priority.html but you can often get the bits separately and put them together as you need.

   - Andy.

If you can parallel  up 2 cores to give a double neutral, you can exceed the 2core rating, as now 2 of the 3 cores are running cool - because the total dissipation for a given load current, which is after all the heat the cable has to sweat off, is lower.

Noddy example, consider a very long length of cable such that each core is 1 ohm, utterly impractical due to the VD, but it makes for some easy sums,

At 10 amps, any one core dissipates 100 watts (I squared R...) spread out over the length.

So 3 core cable, designed for 3 phase with each of 3  cores carrying 82 amps. 3 lots of 82 squared. == 20182 watts. This is the maximum total power for that length of that cable construction.

Now, change that to 2 cores each carrying 100A, so twice 100 squared --  20000 watts, so a touch less power, and would run very slightly cooler than the 3* 82A of the 3core max rating.

Now consider  the parallel neutral case, so 1 core carrying 100A, plus 2 cores each carrying 50A each.

10000 watts plus 2 lots of 2500 watts = 15000 - so that is actually quite a bit  cooler than the 2 cores at 100A.

The risk as that the chosen cable rating is not the right one, because it runs through thick insulation or something,.. which in this case,  no it does not, so you are OK.

Of course, you wont be dropping 82V, or 100V, and with a length of ~  75m,  presumably expect a resistance per core of 50 to 70 milliohms, and corresponding voltage drop, but the situation  all scales - unless the cable is coiled back on itself,  it is the linear heating of the cable (in watts per unit of length) that sets the maximum safe current.

Mike.

CPC said:

Existing load + new load = 85amps.

Is there no diversity between the loads?

   - Andy.