ring main end to end resistance.

[john h owen]

working out by calculation the end to end resistance of a ring main given L = 65m cables = 2.5 mm and 1.5 mm and mohms/m = 7.41 and 12.10 respectively at 20 degress. and cables are 70 degress thermoplastic bunched in trunking and conduit.Cpc in the same trunking and conduit

Would you have to use the bunched 1.2 factor when calculatring the end to end resistance of the cables?

 

[poheZ]

your grouping factor would apply to how many different circuits are in the trunking, which you have not told us

 

[xraytek]

working out by calculation the end to end resistance of a ring main given L = 65m cables = 2.5 mm and 1.5 mm and mohms/m = 7.41 and 12.10 respectively at 20 degress. and cables are 70 degress thermoplastic bunched in trunking and conduit.Cpc in the same trunking and conduit

Would you have to use the bunched 1.2 factor when calculatring the end to end resistance of the cables?

You apply factors in order to determine the CSA of cable to be used.

The reading you will get from measuring the end to end resistance of 75m of 2.5mm T&E will be the same whether it was bunched or not.

 

[poheZ]

Then what if his resistance readings + ze were to high for the given circuit breakers max zs, would he not have to change cable size?

 

[ezzzekiel]

if he sticks to the rules for a ring main size then he shouldnt really go over , unless there was a high Ze, in which case
larger cpc could be run

an rcd gives a zs of 1667 ohms (although brb values should be met)