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doing a course in domestic electrical installation and stuck

Postby omm4r » Sat May 26, 2007 11:58 pm


I am currently undertaking an electrical installers course and have got an EAL practical exam coming up where I am given a house like pod and have to build shower, cooker, ring main, lighting, garden, and a smoke detector circuit. I have to work out my own cable selections out for each of the circuits, volt drop and thermal constraints etc.

For the cooker, Shower, and lighting circuit I have done all the calculations and come up with what circuit protective device and cable I can use because I either know the power rating or the current rating of the Cooker shower and lighting.

I am a bit stuck in working out the circuit protective device and the cable ( and how to work this out) for the remaining circuits. (Ring Main, Smoke detector and garden). As these don't have a specific power or currrent rating. I don'r rely know where to start with these.

I know that for the ring mains the BS7671 recommends that a circuit protective device of 30 - 32A is used and a minimum of 2.5mm2 conductor with a 1.5mm2 protective conductor.
Because I have to put all my calculations on paper, as far as I know (I may be wrong) for the ring main there is no cable calculations and protective device calculation. obviously I will have to calculate the voltage drop and so on. Is this correct? :?:
What do I have to do for my smoke detector circuit and my garden circuit?

I would be very grateful for any reply that can help as this is now stressing me out :(

Thanks in advance

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Simply Build It

Postby Jeffmo » Sun May 27, 2007 2:43 pm

check out section 7 final circuits in the onsite guide
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Postby sparx » Sun May 27, 2007 4:14 pm

Hi Ommar!
You seem to be doing very well so far;
For your ring circuit, you are quite correct to say not possible to calculate V.D. as load varies and REG433-02-04 says provided the criteria for an A1 ring are met ie, no more than 100m2 floor area per ring; and allowing for any derating of cable current carrying due to say running in insulation not dropping the 2.5mm2 cable rating lower than 20A, then "no futher calcs. reqd."

Your smoke detector circuit is interesting. My attitude would be that as smoke detectors draw only a few milliAmps each & with the max number of them you could fit even in a large multi level house you could not draw enough current to cause a volt drop problem even on a 1mm2, 5/6A circuit. Don't forget you need tw/e to the first one and three core/e to link them together!

As for the 'garden circuit' is this a trick question, as not being part p registered you can't wire anything out of doors?
You can't begin to design a circuit without knowing loads/lengths of run, installation method reqd. etc so you can show you have thought it out and say, "not enough Info."
Don't let it get to you, THEY may have forgotten to give you the details!
I get the feeling you will do OK, just stay focused, try to see what they are looking for in a question, it may not be as 'deep' as you think!

Very best wishes, hope this was some help,
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