The start point is the loop impedance readings, from that you can work out what extra cable if any can be added to the ring final. It is considered today that 106 meters is the limit, but that is for volt drop, the earth loop impedance is what is important to ensure the magnetic part of the MCB will operate in the event of a fault.
The
Wiki on this forum gives the safe zones etc. And on completion some one will be inspecting and testing, be it a scheme member electrician, the LABC or some one appointed by LABC they will need to test and inspect, so exceeding the limits is not really an option.
106 meters seems a lot, but by time one has gone up and down the walls a few times, one can soon exceed it, so it does need some design, which could mean spurs and fused spurs to comply.
It is all well and good when LABC is not involved to exceed the limits, and cross your fingers, but converting rooms from and to bathrooms and kitchens is subject to planning permission so LABC is involved, I know as I got caught out.
I also found the LABC inspector was not keen on me doing the inspection and testing, he did in the end permit it, when we pointed out if some one was to say what I have done was wrong, he would need to hold same or above qualifications and I was level 5.
It seems is LABC or scheme member is doing electrics it is one tick box on the form, do ensure the LABC knows if you are using them to inspect and test, you don't want to have to expose cables again. Also they wanted to use a third party to inspect mine, which I would need to pay for, although the LABC selects who does it, so that can easy add another £200 to the bill.
I took over a job, and had assumed the LABC already informed, so wanted to get my name not the sacked builders name on the forms, it seems the builder had not informed the LABC and also it was not them who had to tell them, it is down to the owner. Only exception is if the electrician has some where indicated he is a scheme member.
I think the Part P is a waste of time and money, when I tried to get copies of the certificates on selling the house I was told it would take 4 months, but I don't have a clue how much cross reference there is, if a scheme provider tells the LABC a new kitchen has been wired up, do they look to see if planning permission granted? I suspect not. But not to tell you what should be done, is clearly wrong. Also it depends on who owns the building there are exceptions, I know at work very little requires planning permission as it is a railway, and some things like trespass is a criminal offence, where else where it would not be. Also rules England and Wales vary.