Reduction in cable size

russmossy · Mar 27, 2015

I have come across a situation where 6mm tails have been installed on the live side of the main switch of a distribution board. The supply to the DB is 35mm and I would guess on either 63 or 100amp fuses (unable to isolate at main panel to check). It appears that no spare ways were available so someone has tagged the cables in to feed a 4 pole, 32 amp breaker in an enclosure mounted on top of the DB. As usual the regs relating to this are clear as mud and our NIC guy will pick up his messages in about six months!

I think reading reg 434.2.1 in isolation, the situation conforms being that the tails are under the 3mtrs (only 1 mtr) and being run inside the DB, has reduced the fault risk to a minimum? However, I am not sure and would appreciate any advice.

On a similar note, I also have the situation where a 3 phase DB has been made into single phase by linking the incoming side of the main switch with 10mm cable from a 95mm incomer. Is that acceptable based on the criteria above?

D Skelton · Mar 27, 2015

First scenario is fine. The 6mm is protected by the 32A MCB. The regs are clear as day, not mud. Section 433 is where you need to be looking for information. The one thing to look out for is how well the 6mm is terminated. Can't see it sitting well in that termination next to a 35mm tail

Second scenario I cant say without looking at the loading and/or the potential loading of the board. Sounds rough as a dogs dinner but may comply.

Knobhead · Mar 27, 2015

The only thing I’d criticise on the 6mm[SUP]2[/SUP] is them running through the existing board. Had the 6mm[SUP]2[/SUP]’s been on the load side of the isolator I’d have no problem with it.

3Ph+N board used as Ph+N, done it a few times especially for 110V control supplies.
The jumpers used were equal to the feed. Mainly because I’d use cores from the supply cable. You can use smaller for the jumpers, to be honest it’s not something I’ve given any thought to.

Just to add, the 110V supply to one of our central control rooms was 2x15KVA 433/110V (136A) transformers feeding two RedSpot 20A 24 way boards.

russmossy · Mar 30, 2015

Thanks both for your help and advice, I have researched further and think I have it resolved.
The 6mm tails are OK because they feed a device protecting the tails against overload on the outgoing side and they conform to the criteria of short runs on the incoming side (less than 3 mtrs and minimum risk of fire, fault or danger to people as they are within a steel enclosure, being the DB board).
The 10mm tails however, are none compliant. Whilst the latter of the the above is true, the first part about a protective device to limit the load on the cable is not. As the tails feed multiple ways in a DB, anything could be installed and could overload the 10mm in normal use.

Hope this helps others.

shanky887614 · Mar 30, 2015

russ feed the 10mm through a downstream protective device, this is how they power bms type systems.

bassically they have it doubled up with the incoming tails into a fcu then to bms (so people can isolate pumps etc from outsite but still keep controls online

Engineer54 · Mar 30, 2015

russmossy said:
Thanks both for your help and advice, I have researched further and think I have it resolved.
The 6mm tails are OK because they feed a device protecting the tails against overload on the outgoing side and they conform to the criteria of short runs on the incoming side (less than 3 mtrs and minimum risk of fire, fault or danger to people as they are within a steel enclosure, being the DB board).
The 10mm tails however, are none compliant. Whilst the latter of the the above is true, the first part about a protective device to limit the load on the cable is not. As the tails feed multiple ways in a DB, anything could be installed and could overload the 10mm in normal use.

Hope this helps others.

Most if not all manufacturers can provide a single phase conversion kit for their 3 phase DB's, and these conversion kits should be the only acceptable means for converting 3 Phase DB's to single phase. ....End Of!!