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electrical design specifications / calculations

Postby Sparkydelux » Thu May 13, 2010 9:49 pm


On new installations / additions to installations (and as a one man band set up) is it a requirement to submit design calculations for the installation to building control? are they REALLY interested in that sort of thing? I thought they had enough on their plate without looking for more work to sift through.

I met a person recently who said they had to submit calculations of a new domestic installation he was doing.

He submitted it, but as you can imagine without all the expensive calculation programs engineers are priviledged with, he had to do it manually.

This is the first time I had heard of this happening, I was just curious if this is the way things are going or was he just unfortunate of having someone difficult (which does happen).

I know for large scale industrial /commercial loads; calculations are submitted but this was a first.

What are the requirements? And should there be some form of calculation documentation around or on file on behalf of the installer?

Does make sense if you think about it, but also is a massive time consumer.


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Postby ericmark » Fri May 14, 2010 1:11 pm

I used Excel to both produce the form and to check if anything was outside the limits and I had considered it was likely anyone checking other peoples work would do the same once the sheet is made it is so easy.

However my son was working doing all the inspection and testing for a number of electricians gangs and during this they issued a new form. As a result he missed the changes in the form and entered figures in wrong section and did so for a week before he noticed.

On realising his mistake he corrected on new issues but was about to go on holiday and forgot to tell his boss. It was not until much latter when he realised nothing had come back to him which must mean both his firm and the LABC had failed to check any of his readings.

We use to have rule of thumb now referred to historicity and it works quite well. I seem to remember for example with a ring main using 2.5mm cable by time you temperature correct and use diversity you can use around 106 meters of cable and still be within the 5% volt drop. Since the cable normally comes on a 100 meter drum quite easy you use no more than one role.

The calculations are more to help us than the LABC as of course if we were to over step the mark we would need to correct it and that can be expensive.

Did have a group of houses where the electrical engineer had looked at them and they were being fitted with the beams with pre-made holes for wires and pipes which mean the 1/3 rule did not apply and like with old houses the cables just went up and down the wall and went from socket to socket very near to the wall.

During the build the builder decided to save money by returning to old wood beam construction and as a result the 1/3 rule applied and much more cable was required. 5 houses were wired before the first one with wood beams was tested and then it was found the volt drop was exceeded. The 32A MCB's were replaced with 20A MCB's as a result but I was not convinced this really complied as although we have an agreed 20 amp load at furthest point and 12 amp even distributed so we calculate a 32A ring main as having a 26A load when you reduce the MCB size to me any MCB below 26A you will then consider it as having full load at mid point. So a 20A MCB instead of 32A will give you just 20 meters extra length.

I would guess most times no one checks. But those records will be around for a long time and if I was making mistakes I would worry as once they find one how far back will they check?

My son also when into business as a sole trader then after a couple of years gave it up. He is suppose to keep the records for 5 years. Not sure why? He sold house and is now of no fixed abode. He bought a narrow boat. If anyone wanted to contact him they would have a hard time finding him. Not that he wouldn't correct any faults just he would not even know they were looking for him.

But he like me used Excel so would have been unlikely to have made mistakes over length of cable. He like me used a PDA phone which had Excel built into phone so very easy to check.

After 2008 I never bothered to update program but really no need if passes BS7671:2001 for power then it will pass BS7671:2008 and with lighting very rare we get anywhere near the limits.
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