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When the regulations say for design persons responsible it does not mean the person saying I want a socket there, there, and there. We have to work out the volt drop and the loop impedance although I will admit after one has done many installations one gets to know when one is on the edge, and so where one knows one will be well within the limits one tends to install and measure the loop impedance rather than calculate the loop impedance first.
The problem with C type is the regulations require a lower loop impedance. Today with the use of RCD protection the earth loop impedance is not so much of a problem, but the line - neutral impedance still needs considering as does volt drop.
One of my concerns when the 16th edition came out was with the push to follow regulations together with exams to show one can read the book, that some time in the future some one can come along and when doing there electrical installation condition report highlight errors made some 10 years back and with a signature on the original form it could come back to me.
With a new installation I was reasonably sure I was not making an error, but with additions one did not know how much cable had already been used so measurements where more important to ensure the installation was within limits. As a result I at first wrote a excel spread sheet to work it all out, and latter a java script using a web page to work it out.
Once done I realised that because of the tolerance of measuring equipment it needs to be quite a large amount over the limit before one could be certain this was a installation error rather than just a meter slightly out. But using C and D type MCB's means it can easy exceed the limits. And yes I have made mistakes and I have swapped the B32 MCB for a B25 MCB because I have miscalculated how much cable was going to be required for the route selected. But it was because I made the mistake once I was far more careful when doing the next job not to make the same mistake.
With electricians meters it is quite easy, we plug in the loop impedance meter note the 0.88Ω reading and know exactly how much cable can be added. But the cheap DIY plug in meters like the EZ150 by Martindale only has a 1.50Ω pass light so you have really no idea how close you are to pass or fail and with a C type it's half that figure so even with a pass light it may still be failing. With a 20A MCB you have around 0.8Ω to keep within volt drop add the incoming reading likely around 0.35Ω and likely you need somewhere around the 1.15Ω mark, and the first light on the cheap £50 plug in tester is 1.5Ω so yes using one likely means the installation is safe, but not necessary compliant.
You ask within the regs, and in real terms DIY installations are rarely within the regs. DIY people simply don't have the test gear.
Measuring the length of cable required however you can work out somewhere near what the figures will be. A 20A radial has a limit of around 32 meters to be within the 5% volt drop for sockets. 32 meters is not that much, which is why we use the ring as with the ring we can use 106 meters, although today not sure the ring really helps, by time you use 1/3 rule for drilling beams. Using RCBO's rather than MCB + RCD each circuit costs an extra £20 for the RCBO rather than MCB so using 4 radials rather then one ring gets expensive. Plus the consumer unit is far bigger to take all the RCBO's. So when wiring a house the ring saves money, but for a single supply the radial is likely the better option.
What you don't want is when selling the house some electrician doing his electrical installation condition report and highlighting loads of errors, all houses will have some, but when a fault is found one starts to look for more. Read these pages and when there is something found which is wrong, we always question the rest of installation, what you don't want is for your work to cause the tester to look deeper and find even more faults.
The problem with C type is the regulations require a lower loop impedance. Today with the use of RCD protection the earth loop impedance is not so much of a problem, but the line - neutral impedance still needs considering as does volt drop.
One of my concerns when the 16th edition came out was with the push to follow regulations together with exams to show one can read the book, that some time in the future some one can come along and when doing there electrical installation condition report highlight errors made some 10 years back and with a signature on the original form it could come back to me.
With a new installation I was reasonably sure I was not making an error, but with additions one did not know how much cable had already been used so measurements where more important to ensure the installation was within limits. As a result I at first wrote a excel spread sheet to work it all out, and latter a java script using a web page to work it out.
Once done I realised that because of the tolerance of measuring equipment it needs to be quite a large amount over the limit before one could be certain this was a installation error rather than just a meter slightly out. But using C and D type MCB's means it can easy exceed the limits. And yes I have made mistakes and I have swapped the B32 MCB for a B25 MCB because I have miscalculated how much cable was going to be required for the route selected. But it was because I made the mistake once I was far more careful when doing the next job not to make the same mistake.
With electricians meters it is quite easy, we plug in the loop impedance meter note the 0.88Ω reading and know exactly how much cable can be added. But the cheap DIY plug in meters like the EZ150 by Martindale only has a 1.50Ω pass light so you have really no idea how close you are to pass or fail and with a C type it's half that figure so even with a pass light it may still be failing. With a 20A MCB you have around 0.8Ω to keep within volt drop add the incoming reading likely around 0.35Ω and likely you need somewhere around the 1.15Ω mark, and the first light on the cheap £50 plug in tester is 1.5Ω so yes using one likely means the installation is safe, but not necessary compliant.
You ask within the regs, and in real terms DIY installations are rarely within the regs. DIY people simply don't have the test gear.
Measuring the length of cable required however you can work out somewhere near what the figures will be. A 20A radial has a limit of around 32 meters to be within the 5% volt drop for sockets. 32 meters is not that much, which is why we use the ring as with the ring we can use 106 meters, although today not sure the ring really helps, by time you use 1/3 rule for drilling beams. Using RCBO's rather than MCB + RCD each circuit costs an extra £20 for the RCBO rather than MCB so using 4 radials rather then one ring gets expensive. Plus the consumer unit is far bigger to take all the RCBO's. So when wiring a house the ring saves money, but for a single supply the radial is likely the better option.
What you don't want is when selling the house some electrician doing his electrical installation condition report and highlighting loads of errors, all houses will have some, but when a fault is found one starts to look for more. Read these pages and when there is something found which is wrong, we always question the rest of installation, what you don't want is for your work to cause the tester to look deeper and find even more faults.
