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Just a little info on RCDs needed. I know these are to protect circuits, but can you please help on the ratings of them. I know they are usually 30mA on TNS and TNCS systems but there is another figure like 30A, 60A….. Is this the rating for the supply or have I got things wrong? How do...

If a house has 2 meters, one is for normal and one is for economy 7 and a rewire is needed, does the electricity board have to come out and change it to a normal supply before work is started by an electrician? Will an electrician do a new installation and just use the current supply and...

If there is an old meter in a house with 16mm tails from the supply and also from the meter, can a rewire be done as it is, or does the electricity board have to come out first and change both the supply and meter tails to 25mm before work can be done? If the earth from the supply is...

Routing cables, can you route cables right up from the floor to the ceiling where a light switch is. Do the same rules apply to a light switch as they do to a plug socket? From a plug socket you can run cables vertically and horizontally, can you do the same from a light switch or is it just...

Does this cable have to be run in steel conduit in a notched floor joist? If I cable Is running from one side of the room to the other and it is run through the centre of each joist all the way along, but when it comes to the end one, the joist is against the wall. To get the cable up the...

In Appendix 4 of the OSG on page 103 it tells you where to clip the cable for vertical and horizontal runs. It gives the value of the cable diameter in mm. The first one is not exceeding 9mm, do I just measure the width of the cable and use that figure? Is this for if the cables are running...

Is the conduit diameter the width of the conduit gap? Im a little confused on Appendix 5 of the OSG which states what conduit can be used for cables. I don’t really understand this section, can somebody explain? Does this include the plastic capping that you put over the cables in walls...

Can anybody help me out with a detailed guide to all the fuse numbers, rating and types and also how to identify them? Fuse types and numbers keep cropping up in the OSG but im not too sure what they all mean. There is BS EN 60898 and 3 types. I sort of know what these are, these are the new...

Current carrying capacity of selected cable and correction factors. Im looking at page 115 of the OSG Appendix 6 and im a little confused. It says that the overcurrent device will be providing both fault and overcurrent protection. Is this referring to the MCB, the RCD or both? I assume...

Volt drop is calculated using ohms law U = I x R U is the cable volt drop I is the circuit current (A) R is the circuit resistance W(ohms) Volt drop is allowed as 4% of 230V which is 9.2V For the everyday standard standard multicore pvc insulated cables we can use this table...

Im a little confused about working out design current. If for example I wanted a lighting circuit and I wanted 6 lights on it. I would count up the watts for each light (assuming the minimum as 100) and then divide it by 230V to get Amps. If I had 6 lights all at 100W each is my design current...