Saturday, 10 September 2011

First Fix Electrics

All the rooms were defined in block work and shortly we would be ready for plastering, however just a small matter of first fix for plumbing and electrics. 

Now I am one for having a go at most things DIY but for the novice a word of caution. Electricity can kill!  I have a background in electronics, a good understanding of the difference between Volts, Amps and Watts, I have studied Ohms law and my training in the telecommunications industry encourages me to use best practices in all things electrical. If I sound like I am talking gibberish then its best for you to leave this part of a build to the professionals, however if like me you have the knowledge to undertake some of this work then it is not beyond the self builder armed with a good guide book to do so, with the proviso that a qualified electrician carries out the final inspection and tests and connects to the mains supply. 

So armed with my Collins complete wiring and lighting guide in hand I set about installing the mains and lighting circuits. Unfortunately my wife did not find taking photos of bits of wire very interesting so I am a little short of pics for this post.

Anyway, in simple terms a ring main starts at the consumer unit and runs around the house passing in and out at each socket outlet and finally returning to the the consumer unit. In theory the number of sockets is unlimited but in practice a limit is placed on the number by the floor area that one ring main can supply and that is 100 sq mtrs also generally each floor needs it's own ring. That's how I came to provide four rings as there are three floors and the ground floor is greater than 100 sq mtrs. Lighting circuits are a different matter and in their simplest form start out from the consumer unit and run in a radial circuit out to the last light with the limit being the maximum wattage of 1200watts ie:- 12 x 100watt bulbs. With fancy lighting this limit is soon used up hence I ended up with six radial ccts excluding the garage. Cookers, storage heaters and immersion heaters which draw rather more current than either lighting or ring mains all require a dedicated radial feed from the consumer unit and much heavier cable. 

Running cables is a case of finding the best route through the house usually in the loft, in the floor space between floors, in studded walls or chasing them into solid walls. Because I am anal, I had to have a more sophisticated solution and that involved providing cable runways and risers, which were born out of my communications background. So all my cables rise (on a vertical runway)from the garage, through two floors to the loft and then across the loft on a cable runway to a riser where they would fall to the floor voids and start running around the house, dropping down walls to the various sockets. Large numbers of cables running alongside each other tend to develop some heat and and this was another of the reasons that I provided the runway to take the cables above the insulation rather than on or under it. Similarly where lighting cables drop to lights in the loft the cables where attached to battens fixed to the rafters above the insulation then dropping directly to the light fittings, in this way any heat generated by the cables easily escapes reducing any potential fire risk.

Typical Riser, this one in the Utility Room mainly for pipe work

A drawback to having solid internal walls is that all cable drops need to be chased in to the wall and protected and this turned out to be a very long and messy job. Best practice for cable drops dictate that they should be vertical ie:- no diagonal runs to save cable, however where sockets are reasonably adjacent to each other short horizontal runs are permissible. This  practice ensures reasonable confidence as to where cables are located after plastering and hopes to avoid fireworks with a wayward drill. Wall boxes of varying types were provided at the end of each drop and the punch out holes in the metal types protected with a rubber grommet. Cable tails (ready for second fix) at the wall box were left 150mm to 200mm long to help with termination later.

There are several fittings which require a spur off  the ring mains and these included extractor fans, auxiliary lights over bathroom mirrors and fixed appliances all of which require a fused outlet. Smoke detectors however must have a separate feed from the consumer unit and be linked to each other. The regulations with regard to bathrooms are very strict and need to be observed closely but in any case any electrician worth his salt will not pass any installation not up to the regulations, so I say again the system needs checking and testing by a qualified person before connecting to the mains. 

A loop of the ring main rises under the island unit. Kitchen ring shown running around at floor level no more than six inches from the floor is permissible.

Regulations and practices change so it is with some concern that I have written this post. I have not given specific details about providing mains electricity in a self build as it is not intended as a definitive guide and should not be taken as one, moreover reference should be made to a good guide and IEE regulations if necessary.

In the event fortunately for me, my installation passed testing and was duly connected to the supply by my friendly electrician without any problems or rework. There are many electricians out there quiet willing to undertake the commissioning of a domestic system like this as it is the more intellectual part of the job and removes much of the physical effort whilst still being profitable. It is also worth noting that a completion certificate issued by the electrician is required by the BCO before they will issue a completion certificate for any build.

That was all the cables in for the first fix, fitting of all the electrical goodies was to come after plastering. I now needed to get on with doing first fix for plumbing but first to design the central heating system. That's for next time on the 'realselfbuildblog'

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