Granny Annexe (again)


Postby jackhawk » Wed Mar 24, 2010 10:45 pm

Hi all,

Sorry to bother you again but I have an inquiry to make and confirm that I am correct in my advise I have given.

This is what I wrote last night but lost it in the sending.

I have just completed a full installation to a new build, in the form of a granny annexe. The main house is a 5 bed large house and has a 15 way C/U and 100a supply cut out. Two showers, 32a and 40a, cooker 32a, various 16a radials for power and various 6a circuits for lights to main house and out buildings (stables, sheds and caravan pitch)

The new build annexe electrics that I designed and installed has a 40a shower, 32a cooker radial, 32a kitchen ring, 20a radial up, 20a radial down, 6a lights up and down and 6a smoke/heat non RCD. This is a one bed two story with on suit. Front room, utility and kitchen. I designed this with the understanding that more power was being applied for through supplier.

Basically it is two houses working independently as you would expect. The mother-in-law is able bodied and works for a living.

By applying diversity, I worked out the main house to be working at its max so have not touched that side. Diversity for the annexe, I worked out to be 120a. 40 for the shower, 25 for the cooker, 32 for the first largest power circuit washing machine, tumble dryer, dishwasher, 40% of the rating for the two radials each and 66% of all the lights @100w each.

The supplier has confirmed (E-on), 17kva to the main house. A = 17x 1000/Uo. A = 70.53. Overhead cables rated at 100a, meter at 100a meter tails 25mm2 at 114a max.

I told the builder and the customer that there would be too much demand if the tails were split to supply both house and annexe and could overload the tails, meter, and supply cables. The only option is to apply for either an extra phase to supply the annexe or a separate meter and supply to the annexe. The mains supply is 25m away from the new build annexe C/U and the volt drop over this distance alone would require swa above 25mm2. 120ax25mx1.80=5.4V. This is just below the 6.9V, 3% for lighting before it gets to annexe CU which complies with 7671 (would have gone 35mm2). I am happy with my calculations and advise. I have been waiting for the extra phase to be supplied so I can live test and cert. The builder and customer have asked a 3rd party for a second opinion (as my advice was expensive) (who can't self-cert)). They have split the tails and run a cable to the annexe. This has been done by splitting the tails to a two way CU, 100a/30mA DP main switch and 63a type C MCB with 16mm2 T&E over a 25m cable run in trunking and clipped direct through the roof. (Tidy job) I have refused to complete the live tests now as I can only cert my own work and told them to go through LABC as full plans were deposited.

Calculations are:

16mm2 Ref Method B 69a
Design current of installation 120a (max demand with diversity)
Volt drop = 120aX25mX2.8 = 8.4V

This does not comply with 6.9V @ 3% of 230V for lighting before it gets to CU.

As human life is more important than cost, my thoughts are as follows:

1. Do you think I am scare mongering?
2. Am I just not getting it?
3. Only the cpc of the 16mm2 T&E has been used for the conductor.
4. Overhead cables (100a), splitter box (100a) meter (100a) tails (114a) overload with both installations running at the same time.

Yes it is long winded but needed to get it all down for opinion.

Kind regards

Jackhawk
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Postby ericmark » Fri Mar 26, 2010 12:35 am

My son lives on a narrow boat and can do everything with 16A supply. In fact he is about to reduce that to a 4A supply and will use an inverter with storage batteries so that items with over 4A can still be used.
As the premises grow the base needs stay the same and using a change over switch like used with the baby belling cookers between ring and oven we could run nearly every house, which does not use electricity for space heating, from a 16A supply.
With mobile homes the supply of 32A has long been considered as a minimum in order that an electric cooker can be used and with electric showers a 60A supply is required to allow the short time high load.
So really speaking there is no need to have any more than a 63A MCB from main house except for the safety aspect of lighting. The only real concern if that if the main incoming fuse was to rupture the house could be left in darkness. However with the use of RCD protection on lighting circuits most houses would require some emergency lighting anyway or an electric storm would likely cause danger as the house is plunged into darkness.
So as to supply I see no real danger. May be a problem, if main fuse blows too often, but not a danger.
So that brings us to volt drop. Forgetting about regulations the main reason that 3% is considered as limit for lighting is induction controlled florescent fitting will fail with voltage less than 220 volt so if HF units are used instead then they would continue to work so yet again down to design and most emergency lights are also HF so easy to remove both problems.
So finally we get to real problem. Under fault conditions will the automatic disconnection system work.
First I will consider the RCD. In most cases the RCD needs voltage to power the electronics so with a short circuit if the voltage falls below 100 volts many RCD’s will fail to operate. Cure is to use active rather than passive RCD’s but this would mean losing power with every brown out. So consider worse case scenario socket at mid point on ring has short circuit to earth. Consider the loop resistance of ring main is 1.2 ohms and 25 meters of 16mm will give around 0.005 ohms then the voltage at the RCD will be high enough for it to trip well over 100 volts.
Second is short circuit line to neutral will the 630A required to trip a C63 MCB flow? Easy method is to measure with meter most ELI meters have a PSC scale as well. Rough calculated consider the incoming resistance is 0.35 plus 0.01 ohms for 16mm cable that’s right on the edge and here I do see a problem that the C63 MCB would likely not trip on the magnet release and it would need a B63 MCB to be safe.

So to summarise to be safe it will need all discharge lighting to be HF type plus emergency lighting and the MCB reduced to a B63.

Regulations are a different kettle of fish and you must consider if it is worth taking a chance with your Part P status and in real terms this means you would not want to sign off the whole job. However there is nothing to stop anyone filling out the schedule of test results under special notes write no supply dead tests only. Maybe use the three signature installation certificate and only sign designer 2 and put comment that all live tests to be carried out by designer of supply. Or phone your provider and ask advice that’s what you pay them for.

But other than size of MCB and lighting failure I see no real danger. I hope Sparx will answer this too as he has more experience on Part P requirement but be careful not to mix up safe with complainant. If there is an overload and the fuse/MCB will open in the required time then it's safe.

I would expect you will find it will not open in required time but you give no test results.
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Postby jackhawk » Fri Mar 26, 2010 1:50 pm

Hi ericmark,

Thanks for the time and effort you put into your reply. I really appreciate it. I fully get what you are saying and I feel a bit of a numpty now. Think I may have over reacted to it all. I had completed all dead tests some time ago and that was all good. I went back to the house this morning and carried out some live tests and they all comply too (red faced) ZE 0.38, pfc 1.4KA. Furthest socket on radial 20a, 0.80 pfc 210a and so on.

Sorry to quiz people like this but I would rather look a fool for a question asked than worry about a question I didn't ask. If I understand and learn some thing new each day, then that day was worth it.

All I want to do is ensure things are safe and you have put my mind at rest. Will live test fully and note what is mine and 3rd party.

Thanks again. Will sleep well tonight.

constantly learning,

jackhawk
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Postby ericmark » Sat Mar 27, 2010 3:16 pm

We have all done the same and better to air on side of safety.

One of the things which often come to mind is where with less experience I have made mistakes and later when working for different firm or even in different country one realises ones errors.
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Postby jackhawk » Sun Mar 28, 2010 11:51 am

Fair play,

We all live and learn. On a refresher course I saw hardened REAL sparks struggle with two way and intermediate switching. We all keep learning.

Thanks again ericmark.
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Postby TheDoctor4 » Tue Jun 29, 2010 7:57 pm

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