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Postby tte » Thu Apr 12, 2007 9:02 pm

Hello again thanks for the last reply, I'v purchase edition 8 of the regs and part P building regs.

Question for a trainee. Theory.

Before i ask this question i know this is a part P area only but this is about understanding the reason behind how you determined what size incomer to use and what type of MCB and when to use a split type CU and when to use RCD types.

The CU where I live has a 100a incomer,
The MCBs
32a Shower, about 8000w 6mm cable
32a cooker, 7000w 6mm cable
32a sockets kitchen side
32a sockets lounge side
16a emersion
6a lights.
Please shed some light to understand how it works. for example can more MCBs be add if yes what sizes can be used before it would be to high for that 100a incomer.

Thank you for any help.
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Simply Build It

Postby Mill Spark » Fri Apr 13, 2007 7:32 pm

You should also buy a copy of the On Site Guide, as this book describes the wiring regs in easier to understand terms.
Once you have a copy, Appendix 1 on page 84 explains diversity and Table 1B gives allowances.

From what I remember

Lighting is 66% assuming 100w per lampholder
1st Ring circuit is 100%
Additional Ring circuits are 40% each
Fixed heating ( immersion heaters, showers etc) are 100% each circuit
Cooker is 1st 10A + 30% of remainder & add 5A if a socket outlet is incorporated into the cooker control unit.

So for your installation

Shower = 35A
Cooker = 21A (26A with 13A socket)
Kitchen sockets = 32A
Lounge sockets = 12.8A
Immersion (normally 3Kw) = 13A
Lights (assuming 10 @ 100W) = 2.3A

So adding them together you get the total of 116A or 121A.
Although this is still above the main CU switch and possibly the cut out fuse, under normal conditions you will never require that much current at any one point.

As a side note I believe the supply companies use a figure of 10A per dwelling on new build estates.

Hope this info was helpful

Mill Spark
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Postby sparx » Fri Apr 13, 2007 10:18 pm

for a domestic supply will have a max. size cut out fuse in the service head of 100A, so you will not overload your main incoming isolator.
you can have as many circuits as you have spaces for in the C.U. 14 way C.U. not uncommon
you quote for instance cooker total load but you can allow a diversity factor which reduces this by abt 40 %, this is the principle of the ring circuit . EG, if you add up the total load on the circuit it would probably come to much more than 32A but you don't often use everything at once.
the only item on your list that uses full power for any lenght of time is immersion heater @ abt 12A
worst circuit you have is shower because :
8000watts /230volts = 34.78Amps so cable size ok if run not too long. Will assume 15 meters from CU to shower...
From table 4E2A in regs enclosed in wall, or clipped direct volt drop would be 7.9 mv/A/mtr, so:
34.78 X 7.9 X 15/1000 = 4.12 volts dropped , well within 6.9v limit.
As for split load etc it very much depends on your supply type, Eg if T.T. system needs RCD main switch, for most other types of supply then it's best to have lights, immersion & cooker (if not including a socket on panel) not on RCD other circuits best on RCD end of C.U.
hope this helps
Regards SPARX
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