Underground cable size needed?

[mfarrow]

Still thinking John, but out of interest BS 5467 gives a maximum resistance value of 0.727Ω/km at 20°C, per conductor, for two- to five-core cables.

 

[JohnW2]

Still thinking John, but out of interest BS 5467 gives a maximum resistance value of 0.727Ω/km at 20°C, per conductor, for two- to five-core cables.

That sounds right. BS7671 gives 0.875Ω/km at a 70°C conductor temperature. If one assumes a temp coefficient of 0.04/°K, your 0.727Ω/km at 20°C would correspond to 0.872 Ω/km at 70°C. Close enough for me!

Whether it's 2-core, 5-core or 50-core obviously makes no difference to the resistance per conductor at a particular conductor temperature.

Kind Regards, John

 

[ban-all-sheds]

if one assumes a temp coefficient of 0.004/°K

FYI, it's 0.003862 for copper, and 0.00393 for annealed copper.

 

[JohnW2]

if one assumes a temp coefficient of 0.004/°K

FYI, it's 0.003862 for copper, and 0.00393 for annealed copper.

Sorry - simply a typo . As I presume you realise, my calculation correctly used 0.004 (which I presume you'll agree isn't far from the actual figures you quote).

Do you have any thoughts about my uncertainties/confusions about the 3 x single-phase loads situation?

Kind Regards, John

 

[Lectrician]

There is no real rule for sub-mains, and you would think it would be covered in a regulation somewhere.

You can look at it from both ways - Should you assume the possibility of one phase being heavily loaded compared to the other two, and so calculate using the single phase figures, or should you assume the phases will remain fairly balanced and use the three phase figures?

The regs do state "balanced phases have been assumed" for the volt drop tables.

Interestingly, my software has the option to choose "Single Phase Submain", "Three Phase Submain" and "Three Phase And Neutral Submain". Both the three phase options (neutral and not) give exactly the same results, basing on three phase.

 

[JohnW2]

There is no real rule for sub-mains, and you would think it would be covered in a regulation somewhere. You can look at it from both ways - Should you assume the possibility of one phase being heavily loaded compared to the other two, and so calculate using the single phase figures, or should you assume the phases will remain fairly balanced and use the three phase figures? The regs do state "balanced phases have been assumed" for the volt drop tables.

Indeed. I suppose the Tables in the regs have no choice - once one doesn't have balanced phases, there is an infinite number of possible scenarios, so there's no way they could be tabulated. As you say, we do know the 'worst case' - 'full' loading of one phase (and neutral) with no load at all on the other two phases. However, it would be a bit OTT to design on the basis of that extreme case.

Interestingly, my software has the option to choose "Single Phase Submain", "Three Phase Submain" and "Three Phase And Neutral Submain". Both the three phase options (neutral and not) give exactly the same results, basing on three phase.

As with the Tables, if the software is not provided with information about phase balance, it really has no choice but to assume that a 3-phase load is balanced (i.e. no neutral current), so the results are bound to be the same. I suppose the question to ask is why they have bothered to give the two options, if both are always going to give the same answers!

Kind Regards, John