16mm t+e distribution cables

[Mike Watters]

a 5 story house has 10 sub mains 16mm t+e feeding wylex consumer units in each of 10 rooms.each room has 1x 6amp lighting circuit, 1 x 32 amp radial and 1 x 32 amp cooker circuit. the cables were fitted about 10 years ago. the wylex boards 2 years ago. a periodic report says the 16mm dis circuits are inadequate and i was also told by a spark mate that the 6mm cpc in the 16mm dis cable is too small a csa. the dis cables are protected by 60 amp fuses. to test fully do i do the ze at the dis boards in the rooms or at the origin. do i zs test the dis cables 1st and then the final circuits after and do i add the results. i know 6 + 32 + 32 is greater than 60amps but i can put 20 amp breakers on the sockets circuit to stop overload. any help is great. thanks.

 

[telectrix]

16mm protected by a 60A fuse is perfectly adequate . i would agree with the cpc being undersized though. measure your Zdb's and PEFC's then use adiabatic. personally, i would run a separate 16mm cpc to each flat.

 

[Mike Watters]

i think16mm is far too big. 10mm maybe,thanks

 

[Heisenberg]

What's the CSA of the cable before it splits into the 10 16mm SMs?

 

[johnboy6083]

you need to do an adiabatic check on the CPC. i doubt if it would be too low. As for the loading problem, diversity is king. Also strictly speaking the MCB's are pversized for the circuits. if you only have one light in the centre of the room and its going to pull 100W max, then the Ib of that circuit is 100/230 = 0.4A. So you could run that circuit on a 1A MCB (and yes they do exist). The socket outlets are never going to pull 20A, never mind 32A. Have you carried out the diversity calc on the cooker?

In the electricans design guide, it mentions a rule of thumb calculation for acsertaining maximum demand. It is: total installed load x demand factor. the demand factor is 0.4 for domestic. So add up your total load, and multiply by 0.4 and it will tell you a figure to bhe used as IB for the submain. I would be very very suprised if a 16MM submain is too small.

 

[telectrix]

table 54.7. p130 red book. not sure if it's a different page in the green book. mine's on order.

 

[Heisenberg]

Just another thought.. Does the property have any kind of central heating?

 

[Mike Watters]

i think the lad who done the periodic report was fishing for work.the sub main fuse never blew up b4 due to overload so i guess the tenants don't use much but i would rather be 100?% that they can't over load. tanx for the 1amp breaker info for the lights.

 

[Mike Watters]

no central heating which puts the amps up a bit but the cooker mcb can be reduced to 16 amp as most cookers are only plug in type

 

[Mike Watters]

25mm 3 phase

 

[mcauto]

i think16mm is far too big. 10mm maybe,thanks

i agree 10mm is enough 16mm would be over-sizing it but no harm in over-sizing either

 

[Marvo]

It can have a negative effect with PSCC.