It looks like its about 150m long.
Are the legs routed such that you could potentially replace them with 4mm to reduce both VD and Zs to get past RFC design goals?
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Are the legs routed such that you could potentially replace them with 4mm to reduce both VD and Zs to get past RFC design goals?
We 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 cable?
I think you're thinking the same as me.
The circuit resistance we can't do much about, without at least some rewiring. We don't want to do that, cos it's a tonne of hassle, so let's try and change the design current:
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.
Your circuit, at just over 151m in length, has a volt drop of 17.8V for a design current of 26A (calculated using the basic VD calculation, with no adjustment factors). Way over what it wants to be.
If we lower the design current for your circuit to 16A, the volt drop is just 10.9V. Here is the calculation, based on the whole 16A design current being at the furthest point, midway round the ring:
half the length of the ring X half the design current X V/A/m
76 X 8 X 0.018
= 10.9V
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.)
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.
Zs limit for 20A B-curve is 1.75 ohms so that box is also ticked as above 1.3 ohms measured.
Recent versions of the regs all give the RFC supply OCPD as 30A fuse / 32A MCB but I have seen a few older systems with 20A rewirable fuses, I guess as the Ze was a touch too high at the time? In any case it is perfectly safe, even if not your "text book" RFC, as 20A protects the 2.5mm cable without worrying about current share between the two legs.
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.
Still, I would put in 20A MCB and call it a 20A circuit, then 16A design load, etc.
It is not addressing what folks will try to use it for, which depends on the area it covers and expected use-case, but it becomes a circuit consistent with safe CCC, VD, and Zs all being met without dependency on RCD for ADS.
Discuss Zs on ring main high in the UK Electrical Forum area at ElectriciansForums.net
Are the legs routed such that you could potentially replace them with 4mm to reduce both VD and Zs to get past RFC design goals?
OP
- Reaction score
- 650
No it was a barn conversion only a grd floor building…High ceilings etc
- Reaction score
- 2,874
We 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 cable?
- Reaction score
- 16,766
I think you're thinking the same as me.
- Reaction score
- 25,606
What did the design on the back of the cigarette box say.
OP
- Reaction score
- 650
Lounge area general Power Points… ref method C
- Reaction score
- 2,874
Would a design current of 16A be adequate for this space?
OP
- Reaction score
- 650
I’m pretty sure it’s not the actual 32 amp circuit…more volt drop…
What is the uplift value due to the RCD? (Before and after Zs values) as that can easily cause an otherwise good circuit to appear incorrect.
Although the relatively high readings (post #11) show it to be a long circuit, as others have indicated VD being a possible issue?
Although the relatively high readings (post #11) show it to be a long circuit, as others have indicated VD being a possible issue?
- Reaction score
- 2,874
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.
The circuit resistance we can't do much about, without at least some rewiring. We don't want to do that, cos it's a tonne of hassle, so let's try and change the design current:
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.
Your circuit, at just over 151m in length, has a volt drop of 17.8V for a design current of 26A (calculated using the basic VD calculation, with no adjustment factors). Way over what it wants to be.
If we lower the design current for your circuit to 16A, the volt drop is just 10.9V. Here is the calculation, based on the whole 16A design current being at the furthest point, midway round the ring:
half the length of the ring X half the design current X V/A/m
76 X 8 X 0.018
= 10.9V
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.)
- Reaction score
- 9,003
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.
OP
- Reaction score
- 650
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 thought it was 32A as for the MCB?
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.
Zs limit for 20A B-curve is 1.75 ohms so that box is also ticked as above 1.3 ohms measured.
Recent versions of the regs all give the RFC supply OCPD as 30A fuse / 32A MCB but I have seen a few older systems with 20A rewirable fuses, I guess as the Ze was a touch too high at the time? In any case it is perfectly safe, even if not your "text book" RFC, as 20A protects the 2.5mm cable without worrying about current share between the two legs.
- Reaction score
- 2,874
You'd think so, wouldn't you? But no, 26 amps: see table 7.1(i) on p74 of the OSG.
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.
Good point.
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.
Still, I would put in 20A MCB and call it a 20A circuit, then 16A design load, etc.
It is not addressing what folks will try to use it for, which depends on the area it covers and expected use-case, but it becomes a circuit consistent with safe CCC, VD, and Zs all being met without dependency on RCD for ADS.
- Reaction score
- 2
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!
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!
- Reaction score
- 5,636
I think that's at the end... 1.96ohms
- Reaction score
- 2,874
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.
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