New Build School Wiring.

[green-hornet]

A builder mate of mine asked me to quote for a new school he was building, I gave him a detailed quotation to present to the owners and the requirments for the supply.

The school is quite small, single storey and consisted of main hall six classrooms and store with male, female toilets and disabled toilet.

The supply is 45 meters from the main hall and the customer requested a total of 47 strip lights, and 47 sockets distributed throughout the building.

I asked for a three phase supply because of the lighting load and harmonics.

I told him it should be wired in lsf cable with cable management to ease future maintenance etc etc. Sockets where to be wired in 4mm and lighting in 2.5mm because of the length of runs voltage drop and load.

The owners decided the quote was too expensive and got an electrician to wire it.

The builder who knows I do thing right was a bit concerned and asked me to look at what had been done.

The electricians had used 2.5 for the sockets even though the out run and in run was 35mts, he had clipped them to the ceiling joists under 300mm of insulation. The lighting was in 1.5 again clipped to the ceiling joists, and told the owners 100 amp single phase would be ok because the maximum demand would only be 40 amp.

I just shrugged my shoulders and told him if it was a house I would say he was a neat and tidy installer, if he is registered I would eat hay with a donkey.

He has 70 meters of 2.5 flat pvc twin and earth running to the sockets without the link from socket to socket!

47 cat two double strip lights on 1.5 cable on a single phase!

I never saw the emergency lighting or the fire alarm god help those kids is all I can say.

 

[Gforce]

Sorry are u saying he has done sockets as a radial and not a ring? Obviously the clipping to rafters under insulation is wrong. With regard to number of sockets isnt it down to prospective load?

 

[green-hornet]

Sockets are wired as a ring, I am going to go back when the owner has taken charge of the building at the request of the building inspector and the local fire officer due to the issues raised.

The building inspector relies on the work to be carried out in accordance with the requirements of the contractors governing body.

By the way cables buried in insulation whilst being bad practice is not detrimental providing there is a "cooling" distance of 1mtr (I think) not looking any regs up at this late hour!

 

[Apache]

Thought cables could be buried in insulation if suitable de-rating factors applied?

Also what is likely to be plugged in at school? Few computers? Not going to get a bandsaw! Thought number of sockets related to floor area served and expected load?

 

[Gforce]

Just out of interest how would 3 phase supply work for running standard sockets-do you use a special 3 phase socket or does in convert down to a single phase circuit before terminating into sockets?

 

[Oracle]

Hi,

If they are 13A 230V sockets, then they are just the same as normal domestic ones.

You just simply use one of the phases and the neutral from the 3 phase supply (actually you may want to use all 3 phases and neutral to balance the load).

O.

 

[a1]

Thought cables could be buried in insulation if suitable de-rating factors applied?

There is, If I remember correctly it is something like... If over 500mm of the cable is covered with insulation then a correction factor applies.

There are two, I believe, half covered and completely surrounded by insulation. Our resident BS7671:2008 Expert can confirm or kick out that one. (And Supply Reg Numbers).

Or just wire them all in MICC (Pyro) - no need for de-rating factors then. In fact, they can be wired in 1.5mm

 

[nemesis]

BS7671 page 104 reg 523.7 table 52.2 states the derating factor of cable in insulation:

for a length of 50mm is 0.88,

for a length of 400mm is 0.51.

 

[Andy™]

another case of money over quality. just wait until they have to get it re-wired becuase it was done so badly

 

[Gforce]

Surely another way would be to use a sub main to make the run length and number of sockets on one system lesser.

 

[green-hornet]

The main reason for requesting a 3 phase supply is because of the lighting arrangements.

The lighting load could then be shared amongst the 3 phases, to stop harmonic flicker etc.

From what the builder has said it looks like normal ballast lighting has been used rather than cat 2 HF lighting.

So computer interference, lighting flicker, and a big industrial floor polisher= very bad design and materials.

The sockets are mounted in dado trunking, so easy to overload a circuit when they need more sockets, plus the length of runs especially to the main hall.

Sub mains is not an option because of the layout and space required.

The derating factors are cables in or covered by insulation, grouping, conduit, and trunking so there are a lot of factors effecting cable choice.Overall its just a shoddy peice of work. To give you an idea I was told it took two men 2 days to first fix???

 

[Andy™]

Did they work 2x 24 hour days?! even then, would still be tight getting all work done in that time

 

[a1]

Surely another way would be to use a sub main to make the run length and number of sockets on one system lesser.

I was thinking that when I was in me Workshop/shed, earlier.

 

[Andy™]

I was thinking that when I was in me Workshop/shed, earlier.

You need more than a submain in your shed!

 

[a1]

Completed my challenge Last Night.

 

[Zeespark]

If something really bugs Me when it comes to construction, its people playing it cheap with kids lives.

X(X( X(

AndyGuiness Drink

 

[Andy™]

Completed my challenge Last Night.

pics?

 

[Badger]

what challeneg was that then admin ?

 

[Andy™]

what challeneg was that then admin ?

his motorised shed

 

[Evans Electric]

Green Hornet you don't think you may have been over engineering your design do you.? Schools don't usually draw a great deal of current on the sockets. I don't understand the reference to " a big industrial floor poloisher" ! Old fashioned ballast fluoros have been used for years in offices, still is, along with Cat 2 louvres . Don't see any problem with computers. I would have no problem with dado trunking either TBH. The lighting in 1.5mm is , surely, ok , presumably divided into , say, 4 circuits.

If you quoted for emgy. ltg and fire alarm and they have not then they are going to beat your quote every time !!