Cables In Parallel, Max Load?

[bogney]

Hi

Just wondering if someone could help me out with this one

Basically I'd like to know what's the maximum load (single phase) a 4 core 6mm SWA could deliver if cores were connected in parallel

SWA in buried under ground and the length is three hundred meters

The cable used to supply a 3 phase pump on a farm many years ago. The farmer would like to renovate pump house, and yep you guess it, want power down there.

Used a cat scanner to find and determine length.

Any help would on this would be much appreciated

Best regards

Dave

 

[Richard Burns]

You need to ensure the safety of the two cables, in general the regs advise adding the CCC of each core.
In practise the CCC may be slightly greater but this is not accounted for in the wiring regulations.
Have a look at 433.4 and 434.4.

 

[bogney]

Thanks Richard, I'll dig out my regs book.

 

[Taylortwocities]

Were you thinking of just using the armour as the earth? Or is it TT at the far end?
Don't know what your calcs say but the steel isn't as good as copper, especially over 300metres.

 

[westward10]

Were you thinking of just using the armour as the earth? Or is it TT at the far end?
Don't know what your calcs say but the steel isn't as good as copper, especially over 300metres.

It's a farm it probably doesn't have any earthing system.

 

[bogney]

Definitely Earth Rod at the pump house end

 

[Leesparkykent]

What sort of load are you looking at?

 

[bogney]

Doesn't need to be huge, Pump house very small. 4 LED down lights, 1x PIR light, 2 Double sockets and a panel heater.

 

[bogney]

Only problem is I know he wants to extend off the supply again at a later date to provide power for 4 caravans!!

Main issue is going to be volt drop

 

[Leesparkykent]

good luck with that lol.

 

[Taylortwocities]

Your average caravan hook up is 16A, but you could make a practical view on max demand being a lot less than 4x16A.
Could limit the hookup supply to 10A, maybe?

 

[bogney]

good luck with that lol.

Thanks! do you want the job?

 

[bogney]

I have told him my concerns, when its cold the heaters go on and the kettle goes on and so on...
I'll tell him the pump house is the limit.

He'll tell you white is black, when you tell him it's white he doesn't really give a .........

 

[westward10]

Shoot me down if I am barking up the wrong tree, I get conductors in parallel but what columns in Table 4D4A or 4E4A are you using to determine the current carrying capacity of the cable. Column 6 refers to a single phase load in a 2 core cable, however you have a single phase load in a 4 core cable so does this equate to Column 7 which is a 3 or 4 core cable on a three phase load. My view is Column 7 due to the quantity of conductor and not the fact it is single or three phase.

 

[Richard Burns]

Your CCC would be 92A and your pump house perhaps a maximum of 50A so that would leave you 40A to divide among the caravans.
However your volt drop will be well beyond limits at that loading, especially as you want to extend further for the caravans.
I have just checked and I think you would need to keep your current below about 6A to keep within 6.9V (3%) drop over 300m with a 12mm² cable!.

 

[bogney]

Unconventional column 6...?

 

[bogney]

This is another issue I've told him about and my main concern

 

[westward10]

Unconventional column 6...?

Because of the use of 2 core? The OP is suggesting this with the 4 core in parallel.

 

[Richard Burns]

Have a read of appendix 10 that provides fun calculations for cables with unequal current sharing, say where two cable on the same phase are bunched together.

 

[westward10]

Your CCC would be 92A and your pump house perhaps a maximum of 50A so that would leave you 40A to divide among the caravans.
However your volt drop will be well beyond limits at that loading, especially as you want to extend further for the caravans.
I have just checked and I think you would need to keep your current below about 6A to keep within 6.9V (3%) drop over 300m with a 12mm² cable!.

I assume you are doubling Column 6 from Table 4D4A to get 92A, problem is it ain't a 2 core cable?

 

[Richard Burns]

I assume you are doubling Column 6 from Table 4D4A to get 92A, problem is it ain't a 2 core cable?

Yes I was doubling the tabulated CCC as recommended when using cables in parallel, however the regulations are thinking of two separate two core cables suitably arranged to ensure equal current sharing, which may well not be the case here, I agree.
However this is moot since the VD is something in the region of 50V!

 

[westward10]

Yes I was doubling the tabulated CCC as recommended when using cables in parallel, however the regulations are thinking of two separate two core cables suitably arranged to ensure equal current sharing, which may well not be the case here, I agree.
However this is moot since the VD is something in the region of 50V!

Not calculated the volt drop but under maximum loading I don't doubt it will exceed requirements.

 

[westward10]

My view that cable if PVC insulated will be rated at 76A, if XLPE then 88A.

 

[Spoon]

OP: Email the concerns you have to the customer. That was you have proof that you have warned him about all the things. Just to CYA.

 

[Bobster]

My view that cable if PVC insulated will be rated at 76A, if XLPE then 88A.

This is such a common mistake I see, you may well know this so it could just be for the benefit of others.

XLPE only has a higher current rating if everything it's connected to is also rated to operate at 90 deg C. If it isn't it's CCC is reduced to that of a PVC cable.

 

[Spoon]

This is such a common mistake I see, you may well know this so it could just be for the benefit of others.

XLPE only has a higher current rating if everything it's connected to is also rated to operate at 90 deg C. If it isn't it's CCC is reduced to that of a PVC cable.

Reg 512.1.5 (Page 117) for reference to whoever wants to look it up.

 

[westward10]

This is such a common mistake I see, you may well know this so it could just be for the benefit of others.

XLPE only has a higher current rating if everything it's connected to is also rated to operate at 90 deg C. If it isn't it's CCC is reduced to that of a PVC cable.

Yeah know this, that however is the maximum rating of the cable where conditions satisfy.

 

[Lucien Nunes]

In this case it clearly doesn't matter one jot what the CCC is, because at 300m the voltage drop limits you to about 6-7A as per RB above and the OP's observations.

If it were on my own turf, I would look at some kind of fun scheme like launching two phases into the two paralleled cores and transforming down 400 to 230 at the far end, which would give you three times the current for the same drop (before correcting for transformer regulation and loss which would undo some of the benefit) With 600/1000V cable and a tranny each end one could theoretically get about 29A through it at 3% drop and deliver over 100A at 230V at the far end, however the cost of buying and installing the transformers and the lifetime cost of their loss would probably exceed the cost of relaying in suitably sized cable.

 

[marconi]

I did some searching on-line and found this UK supplier of transformer pairs for the compensation of cable volt-drop along long cable runs. http://carroll-meynell.com/volt-drop-compensation-transformers

 

[Knobhead]

A matched pair of buck/boost transformers would be cheaper.