Electrical Installation Design Documentation

[Fash]

After much discussion with my local building control department I have finally reached an agreement on how to work with Part P as a (competent) DIYer. If I provide some evidence of my competency (degree certificate in electrical engineering) and system design information, they will use the local authority in-house electricians to test and sort out the required certification.

Generally this is good news!

However since I'm not an electrical contractor and installation design in my degree was mostly around power stations, national grid and large three phase stuff I don't know what they are expecting in terms of design information and calculations. At the very least I will be preparing layout drawings based on floorplans I already have, and a circuit list, so the question is what else should I provide? Is it sufficient to produce something that contains the information that would be found on an NICEIC/NAPIT... installation certificate.

I'm not asking anyone to do the calculations for me, just which ones I need to do to be in the clear. Installation is for a single dwelling albeit large (270sqm over 4 floors) and I'm planning to split the installation up into areas (i.e. lots of circuits - big MK CU) these being basically 1. 2nd floor (bedrooms x 3), 2. 1st floor (bedrooms x3), 3. Ground floor right (reception rooms x2), 4. Ground floor left (kitchen dining room +utility +pantry) 5. Basement. The reason for som many areas is that these are separate refurbishment areas so wiring them in this way minimizes the disruption to the rest of the house. The only oddity will be a radial socket circuit running to the kitchen island unit which will be disconnectable under the floor (but access requiring the use of tools), oh and my Wife's kiln (basically a 16A radial so not very odd)!

Incidentally the 100A supply WILL be sufficient as there will be no electric showers, immersion heater will be backup only and cooker only part electric.

Any advice from installers (or anyone else who knows) as to best practice very welcome.

Best regards,

Fash

[Andy Wade]

The April 2006 revision to Part P removed the specific requirement to "provide information" (P2), so so far as the building inspector is concerned, nothing in particular is required, provided you meet P1, which only requires "reasonable provision in the design and installation [...] to protect persons operating, maintaining or altering the installations from fire or injury."

P1 is usually met by complying with BS 7671, which requires the completion of an Electrical Installation Certificate (EIC), complete with its associated schedules of inspections and test results. To see what is required you can download the blank EIC form (& the other certificates) from the IET web site. A comprehensive and accurate schedule of circuits is the main thing to provide, with clear identification of what is fed from where. The test schedule from the EIC provides the necessary framework. Also you need to provide information about how to isolate the installation completely if there is more than one main switch or source of supply, and to identify the location of any earth electrodes (e.g. if a TT system).

In general, little or no design calculations are required for domestic installations, provided you use the standard circuit arrangements throughout, and you follow the relevant maximum circuit lengths given in Table 7.1 of the On-Site Guide (OSG). Get a copy of the OSG (brown cover edition) if you don't already have one, it's a must-have that will answer most of your questions. Some examples of where you may need calculations:

- cable grouping factors where several circuits are run in proximity; - derating for cables completely surrounded by thermal insulation or non-standard installation methods; - anywhere the cable rating is smaller than the fuse/MCB rating (e.g. spurs with downstream fusing), calculate adiabatic compliance; - sub-mains to outbuildings (voltage drop, Zs, etc.

Best practice would include providing detailed drawings showing locations of all equipment and cable runs, and a file containing manufacturer's details and instructions for all equipment installed.

[<me9]

Would a grouping factor be required when both ends of a ring circuit pass through a piece of trunking? Intuition says yes, but they /are/ the same circuit.

My existing ring circuits use some T&E with 1mm^2 earth conductor. Would this require changing if the main work was replacing a CU? I seem to recall that

1mm^2 was large enough in most circumstances but not all, the details escape me.

[Fash]

Spoke to the IET and they said I should use their installation forms + drawings. Since I have a qualification in electrical engineering they consider me to be competent but not registered and say it's not a problem, provided I'm happy to declare myself as competent!

Fash

[ironer]

/snip/

yes, but...if you use the OSG standard circuits these - with qualifications - allow for up to 5 touching single layer circuits - see OSG sect7.2.1

The 'yes' is because the reason for derating is the need to allow space around a cable to disperse I^2R heat generation in the cables & both ends are generating heat - not, as might at first be assumed, electromagnetic coupling.

Heat rots FTE cable. It makes the plasticiser in the plastic sheath and insulation leech and then it goes hard. Life of the cable depends on how fast it leeches, which depends on how hot it has been allowed to become during its life. Can be 1year or less to 20 years or more. Which is also why it pays to have separate circuits for high wattage appliances & not to run cables near maximum current for long. Rating tables assume a maximum cable temperature of 70deg C - very high if the cable is to last a long time. More info in P Cook's book mentioned in my response to your other post.

If you are putting the cable thru trunking this is regarded as the same as conduit & extra derating is needed (conduit = decreased ability to loose the heat to ambient air) - allowed for by using different columns (installation method) in the tables of the 16th Ed IEE wiring regs. It maybe however that there is enough margin in the cable rating for this not to matter. Eg a ring circuit requires a 20A cable, but (depending on installation method) 2.5mmsq cable is often rated 27A or more, so derating factors may still keep you within the 20A.

On a slightly different aspect, how you avoid derating in proximity to the CU is a mystery to me as IMHE all the cables come together in that area & it isn't practical to keep the cables as far apart as the regs require.. Up to now I've just connected them up neatly & forgotten about grouping factors. It sounds as if you will have this problem in Spades. Anyone able to throw light on this?

HTH