Installation design

Postby tte » Sun Apr 15, 2007 12:51 am

hello sparx and mill spark thank you for replying

I'm starting to understand some but not all areas finding out the amps or watts is ok,
As an electrician do you have to design the installations you do
Example outside workshop
CU in the house needs another MCB added so would you use a 50a
there is 40m of cable needed 15m inside 25m out would you use 15m 10mm T&E to get outside then a trip of some sort to join it to the 10mm armoured which would go straight into the CU in the workshop then this CU has to run an air compressor min 30a supply, would you use a 32a MCB and 6mm T&E just the same as a cooker. Also min 30 supply welder/large battery charger would you use a 32a MCB and 6mm T&E plus another 32a MCB and 2.5mm T&E for about 10 double sockets in a ring. 6a MCB 1.5mm standard lighting. For out side lighting would it be advisable to use a 10a MCB with 2.5mm T&E for halogen 500w lights

Could you tell me how you get work out what is the best type of thing to use and why. Not difficult please


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Simply Build It

Postby Mill Spark » Sun Apr 15, 2007 8:13 pm

When designing an installation three current values need to be taken into consideration thes are Ib, In & Iz.

Ib = design current and may include diversity
In = nominal current of the fuse or circuit breaker
Iz = tabulated current of the cable for method of installation (after any extra factors have been applied)

In must be greater than or equal to Ib
Iz must be greater than or equal to In

Iz is the value that must be calculated using tables from BS7671 or the On Site Guide. This ensures the cable can carry the load and that volt drop will be kept below 4% of the nominal voltage, which for domestic applications of 230V is 9.2V.

IMO I would't put such a large load onto an existing CU, I would install another CU alongside to provide a separate supply to the workshop, not forgetting RCD protection as well. You can then run one piece of 3 core SWA, and sleeve the Black with G/Y sleeving for an extra Earth. Concentric cable could be another option to use for the workshop supply as well, although I don't know how readily available it is for short runs.

Don't forget Part P.

Hope this explains a bit more for you

Regards Andy
Mill Spark
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Postby tte » Mon Apr 16, 2007 9:49 pm

thanks for your reply

For me to understand these terms you are using i need the books to decode the symbols. I'v never had anything to do with electrics only the wiring i'v done, i would be happy to rewire a house from the CU on what knowledge i'v found out and asking my brother who was doing a HND in electroinics, he told me some things about wires and sizes but he took his own life last year, because of a few factors in his life, and he found the hnd very hard.
So i want to learn this trade because i did enjoy doing it. There is alot to know but i'v got to start at the beginning and work my way up to the Part P standard

I'l get the books this week,

I'm thankful for any guidence and help with it.

Thanks tom
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