Discuss Zs on ring main high in the UK Electrical Forum area at ElectriciansForums.net
Lounge area general Power Points… ref method CWe may be able to make this circuit compliant with the minimum of fuss.
What is this circuit likely to power? Is it kitchen, utility, heaters? Or just general purpose sockets?
Also: What is the reference method for the
Would a design current of 16A be adequate for this space?Lounge area general Power Points… ref method C
I’m pretty sure it’s not the actual 32 amp circuit…more volt drop…Would a design current of 16A be adequate for this space?
Yes, the volt drop is why this circuit does not comply. And the volt drop depends on two things: 1) The circuit resistance, and 2) the design current.I’m pretty sure it’s not the actual 32 amp circuit…more volt drop…
I agree that would be perfectly safe.This circuit is now compliant. Just drop the breaker to 16A, and everybody's happy.
Thanks we’ve got 28 days to adjust this circuit & yes he was more interested in the resistance on the RM …& the volt drop…I agree that would be perfectly safe.
The problem I see is that it only works if the 16A is shared between 2 legs, i.e. it needs to be a ring.
If the NICEIC guy was being a pedantic pr**k. he could say that 433.1.204 specifies a 30 or 32A protective device.
I still reckon it's worth a try though.
I thought it was 32A as for the MCB?The standard 2.5mm2 ring final from the OSG uses a design current of 26A, and is 106m long, the limiting factor being voltage drop.
Based on the measured r1 & rN of 1.12 & 1.13 ohms the worst case R1+RN resistance is 0.563 ohms at the mid-point, so for 5% = 11.5V drop the max current = 11.5 / 0.563 = 20.4A and so that makes 20A MCB OK. While 16A is perfectly OK, I would prefer the extra margin for diversity that 20A offers.This circuit is now compliant. Just drop the breaker to 16A, and everybody's happy. This will also mean the circuit Zs meets the maximum for the OCPD, just in case your assessor was grumbling about that too (which he shouldn't have been, but there you go.)
You'd think so, wouldn't you? But no, 26 amps: see table 7.1(i) on p74 of the OSG.I thought it was 32A as for the MCB?
Those figures are assuming 20 deg, at 70 deg you'd only be allowed 17A.Based on the measured r1 & rN of 1.12 & 1.13 ohms the worst case R1+RN resistance is 0.563 ohms at the mid-point, so for 5% = 11.5V drop the max current = 11.5 / 0.563 = 20.4A and so that makes 20A MCB OK. While 16A is perfectly OK, I would prefer the extra margin for diversity that 20A offers.
Good point.You'd think so, wouldn't you? But no, 26 amps: see table 7.1(i) on p74 of the OSG.
Interestingly most of the Table 7.1(i) are showing about 80% working load compared to OCPD, much the same as the 80% factor for cable resistance over nominal temperature range.Those figures are assuming 20 deg, at 70 deg you'd only be allowed 17A.
Actually, the ring final conductors won't run at 70 deg with the worse case of the full load at the mid-point, even if the cables are installed with a CCC of 20A. There is a correction factor to allow for this (P429 of the brown book), but even with that applied, this circuit just about misses the 20A mark.
I think that's at the end... 1.96ohmsWhat is your r2?
I come across ring finals of this sort of length too from time to time, they don't seem to be a problem. I'll be honest, I don't tend to take VD for this sort of thing too seriously, at least when working on existing circuits. The theoretical volt drop calculation is based on a nominal voltage of 230V, whereas in reality the supply voltage is usually in the 240-250V range. A large margin of error for the rare occasion that the circuit is fully loaded.A most interesting thread.
Having absorbed and agreed with most of the conclusions, I have another EICR case in point - I spoke to NAPIT some years ago about an excessively long 30A RFC within an historic church building which was fine after a great many years of service.
The tech suppt. guy suggested that if the total load usually running is quite small, (eg. a fridge, the odd little charger and occasional hoover or kettle), then nothing to worry about - based on observation and common sense; note it and give it a C3. But then NAPIT are apparently less finicky than NICEIC!
Eh? Common sense?Feedback from the NICEIC… no improvement required… basically there is a VD which is minimal.. & the RM loading for that area it serves is minimal… The equipment being used have voltage tolerances & so would not be effected by VD…. Oh I say…
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