sub main cpc size?

[chrisreid186]

how do you work out the size of cpc supplying a sub main? and is it the same method for working out the main earthing conductor size?

cheers in advance

 

[telectrix]

that's how i would do it.

 

[chrisreid186]

so a 16mm twin and earth supply to a sub main on a tncs and tns system requires a 16mm cpc but if its on a tt system the cpc would only need to be 4mm if unprotected and not buried?

 

[BertUK]

main earth should be at least half the size of the main conductors

 

[Deleted member 26818]

so a 16mm twin and earth supply to a sub main on a tncs and tns system requires a 16mm cpc but if its on a tt system the cpc would only need to be 4mm if unprotected and not buried?

Not necessarily, you could size it by using the adiabatic equation.

 

[pushrod]

how do you work out the size of cpc supplying a sub main? and is it the same method for working out the main earthing conductor size?

cheers in advance

All protective conductors inc the earth are worked out the same way - the only exception is main protective bonds. So it is with either table 54.7 or calculated with the adiabatic. Cables with a cpc already incorporated will comply with the adiabatic if they have been selected properly.

main earth should be at least half the size of the main conductors

No, at 16mm or less the main earth should be equal to csa of line conductor (if not calculated) and at 35mm for a line conductor the protective conductor can be 16mm from table 54.7 .

 

[BertUK]

543-01-01 says that CSA of every protective conductor other than equipotential bonding conductors shall be

(i) calculated in accordance with 543-01-1 (adiabatic equation: S=sqrt(I²t)/k

or

(ii) selected in accordance with 543-01-04:

54G says that if the phase conductor is less than or equal to 16mm2 then the protective conductor should be the same size as the phase conductor.


​

 

[chrisreid186]

thanks for the replys, i'll have a go at the adiabatic later

in ref to pg 29 OSG how can the csa (TT system ) be 4mm unprotected? this seems very small or am i reading it wrong?

 

[telectrix]

that's because the Ze of aTT system is usually a lot higher and so the pefc is a lot less than with a TN system.

 

[Electro-tech]

TT 21 ohms.

 

[telectrix]

with a Ze of 21ohms the pefc would be 230/21 = 11A. ( to nearest whole no.) so 4mm is adedquate.

 

[pushrod]

thanks for the replys, i'll have a go at the adiabatic later

in ref to pg 29 OSG how can the csa (TT system ) be 4mm unprotected? this seems very small or am i reading it wrong?

That is a minimum size, if you used the table 54.7 it would be very much larger. However that would be complete overkill because, as tel said, pefc will be very small. When you put these small fault current values into the adiabatic you will usually get csa for the cpc in the order of <1mm hence the need for minimum sizes

 

[telectrix]

have to allow a safety margin with TT systems as the Ra will go down when your dog p*sses on the rod.

 

[chrisreid186]

yeah definatly add FUDGE factor for rainy days as lower risistance = higher fault current

would the cable from the met to the earth electrode be worked out as per table 4.2 (osg pg 29) ?

if so would green and yellow earth cable be classes as "Protected against corrosion" ?

 

[pushrod]

would the cable from the met to the earth electrode be worked out as per table 4.2 (osg pg 29) ?

if so would green and yellow earth cable be classes as "Protected against corrosion" ?

For a TT system - yes and yes.