School Sub-mains Install

Festerfly · Mar 10, 2011

Ive been tasked with providing the electrical supply for a new Home Economics room in a local private school.

I have the plans for the installation which comprises of

8x Freestanding Electric Cookers (32A Max each)
12 x 2g Switched Sockets (light load only)

Now this isnt normally an issue, however! The main incoming board is around 20 meters away. It is currently full to capacity and is also an Old Federal Electric Board

so didn't relish the idea of finding a used breaker to fit to this ageing discontinued board. Replacing the main board isnt an option. My plan was to take a feed from the tails into an isolator and then run the supply to a smaller 3PH board located in the Home Econ store cupboard and then run the feeds out from there.

My concern is that if i give each cooker its own supply from the local board they would end up on different phases and with some of the cookers standing back to back, if there was ever to be a fault then there is the potential for a shock across two phases which i am keen to avoid tbh. I was planning on running 4mm radials to each cooker and a 2.5mm ring to the sockets. the rest of the wiring (lighting/extractor etc) is straightforward. I would probably go with RCBO's on all circuits to be on the safe side.

Any thoughts on this as i want it to be as safe as possible given the location of the install.

Thanks

bugsy · Mar 10, 2011

Whats at the incommer (capacity wise )? is the existing board at the origin, ? whats the load on the existing DB

Festerfly · Mar 10, 2011

Existing Federal Board is located at incomer. incomer is 3ph 100a ea.

the current load on the DB isnt high from memory (sorry i havent got the exacts to hand) There was one 63a TP and a couple of 32a tp. the rest were all 6a and 20a singles i think. 250v iso in the main db.

bugsy · Mar 10, 2011

Calculating the prospective load and allowing for diversity your existing supply will need upgrading. allowing 10a + 30% for the cookers 316a thats without socket outlets and existing connected load,

Festerfly · Mar 10, 2011

that was my fear tbh. i think i was kidding myself on the loading front. back to the drawing board...

thanks for the help!

bugsy · Mar 10, 2011

Get the DNO to upgrade the supply

Festerfly · Mar 10, 2011

actually....

just been informed that the cookers will be max of only 1kWh each. meaning i could run them on 1x circuit of 10mm with a 40A MCB. brining the whole loading down substantially.

bugsy · Mar 10, 2011

You could but if one develops a fault they all fall over, and would be a 40a rcbo

Festerfly · Mar 10, 2011

my bad, yep RCBO.

I have informed them that if they try to increase the loading any more then they will need to speak to the DNO to increase supply etc. with that they were very happy to keep the loading down!

I might spread the ovens across a couple of phases (4x each say) that are well out of arms reach between each other so that way if one does trip then only 4 would go out etc.

bugsy · Mar 10, 2011

thats a better way, I'd still go with your suggestion of 4mm radials though, you will also have to consider local means of isolation ( 2 x 4 gang 20a DP gridswitch )