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Discuss 16A off 32A ring in the Electrical Forum area at ElectriciansForums.net

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Pretty Mouth

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Interesting. So the installation I saw with a Wylex switch on the ring with a 16A fuse to a commercial socket outlet appears to fall in line with the regs, at least in part. Although, would a 16A commercial socket/plug comply or would it have to be permanently connected.

It looks like the rule of thumb that not higher that 13A off a ring is not true.
I would stop short of saying a 16A fuse definitely complied, but I couldn't say that it didn't comply either. The 16A breaker is mentioned in the OSG (in the 17th edition too, so it's not a new thing), being an IET publication we can assume it carries some weight.

The actual regulations concerning ring finals are surprisingly sparse. The design guide in app 15 will ensure compliance, but is not actual regulation, and deviation from it doesn't automatically mean non compliance.

A double socket is designed to run at 20A, so yes, it is expected that a load of greater than 13A will be placed at a point on the ring for at least some of the time.
 
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John-SJW

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What's the point of having a RFC on a16 Amp OCPD? just to accommodate a 16 Amp socket simpler to run the socket from a dedicated 16Amp circuit
That may not always be practical - such as ripping a house apart for one cable, or no spare slot in the CU.
 
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Lucien Nunes

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I can't answer 'how I get around the problem' because I've never encountered it, but it's an interesting theoretical question.

It might be acceptable for a 16A point load to be applied to a 32A RFC, especially if the diversity applicable or the manner of operation of a permanently-installed appliance can be foreseen and factored in. An oven that requires an average of 15A for a few minutes and then drops back to below 13A average, is not much different from one with a smaller element that draws 12A solidly for a longer warm-up time (It is different, though, because the heating in the circuit cables is proportional to I² but the heating in the oven is proportional only to I.)

We must distinguish between three possible kinds of problem arising from a large fixed load on a general-purpose RFC:

1) Risk of overloading cables, since on a 32A RFC conventionally In>Iz. Normally, this does not happen. If it does, it's very minor, which is why houses don't burn down when RFCs get broken near one end.

2) The load accounts for too great a fraction of Ib to allow the circuit to fulfil its role of supplying general purpose socket outlets. This is not specific to RFCs, it is a simple diversity conflict, that would occur equally on a 32A radial S/O circuit. If the circuit is intended to provide a useful amount of power elsewhere, a 32A circuit with a 16A load is probably better than two 20A circuits with a 16A load on one of them. The bigger the circuit, the better diversity works.

3) The accessories are not suitable for a 16A load. This is simply a practical consideration that is easily overcome by careful selection of equipment.

My view is that it is bad practice to burden a general-purpose socket-outlet circuit with large single loads unless their duty-cycle is known to be modest. I think this is actually a more significant consideration than the point loading of an RFC affecting current distribution.
 
westward10

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You can't help yourself thread closed.
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Okay at another members request I have reopened this.
 
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John-SJW

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I would stop short of saying a 16A fuse definitely complied, but I couldn't say that it didn't comply either. The 16A breaker is mentioned in the OSG (in the 17th edition too, so it's not a new thing), being an IET publication we can assume it carries some weight.
That indicates it does comply, but the appliance has to be permanently connected, not via a disconnecting device of any sort (a socket that is commercial or domestic). The picture you posted in your first post does state so.

But as other have noted, a look at the ring and its loading, it essential.

It was noted in previous posts that a 16A radial can have a 16A commercial socket directly off it giving a disconnection device.

I also noted in a previous post, that changing a 32A mcb for a 16A on a ring gives the same result, in that a commercial socket can be fitted directly onto the 16A ring. Of course the loading of the ring would need to be assessed, as per normal. I know my 32A ring could be dropped to 16A without any problems, as little is taken off it - TV, lap top, phone chargers and the occasional vacuum cleaner.

So, as been gleened so far:

1) A 32A ring can have a 16A appliance off it, with the appliance:
a) only being fixed;
b) with an isolation switch;
c) with a 16A fuse.

2) A 16A radial can have a 16A appliance off it with a fixed appliance or off a 16A commercial socket (the only 16A sockets available in the UK).

3) A 32A ring can be re assigned to 16A ring by a mere change of an mcb/rcbo. The appliance can be fixed or with a 16A commercial socket. As per the 16A radial.

Any constructive comments?
 
Lucien Nunes

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I would not connect a 16A load to a 16A general-purpose domestic socket-outlet circuit. When the 16A load is in operation at full current there is zero remaining capacity for other appliances, and when it drops back in duty-cycle there might be only a few amps. Since one would expect at least some load on such a circuit, small overloads might occur frequently that probably would not trip the MCB. This is an unsatisfactory situation that should be avoided to prevent premature failure.
There is no need to protect the circuit at 16A if the cables are installed with an Iz>20A which is a requirement of them being suitable for a 32A ring. Therefore it could be protected at 20A. Personally, I would rather it be protected at 32A.

c) with a 16A fuse.
Without meaning to come over as pedantic, it specifically states a 13A fuse or 16A MCB.

Note, I am not endorsing this as a 'good idea' (tm). There is a recommendation to connect loads of this size to dedicated circuits, which I think is sensible. But it does seem that a permanently-installed load that can operate correctly from a B16, that probably has a long-term cyclic consumption of less than 13A, may be connected to a 32A RFC if overall expected loading permits. As above, I don't like the 16A OCPD option.
 
Pretty Mouth

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Out of interest, what do you plan to use the 16A socket for?
 
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John-SJW

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Lucien,

A dedicated radial supply is the best of course, but I did not want to state the obvious. Just the options other options.

So, a 13A fuse or a 16A mcb

Yes, a 20A radial or ring gives a margin of wiggle room when a 16A appliance is connected.
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Out of interest, what do you plan to use the 16A socket for?
I am not planning anything. There are lots of general continental appliances that draw more than 13A. This must be a common occurrence.

Apart from running a new cable back to a CU, which may be very expensive and impractical (the lady of the house would not be amused if her nice house is ripped up), other approaches can be looked at. And as this thread has brought to light easier solutions are available and within current regs.
 
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pc1966

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If it were something that I had to do, I would have the RFC on 32A as normal and a separate isolation switch and 16A MCB 'FCU' just for the 16A socket.

As with the long thread on two rings on one MCB a couple of months ago, it is difficult to point to a 16A breaker-fed load on an RFC as being much of a danger compared to 20A from a double 13A-fed sockets.

But to me the main problem is when something very odd like this is done, the next person to fiddle with the system has a much higher risk of causing danger due to it not being correctly documents and/or the documentation not being read by said person.

One benefit of the RFC comes from the diversity possible from multiple loads below 13A along with a 30A fuse / 32A MCB supply OCPD. Reducing that to allow for a very unusual socket for domestic work seems wrong. Equally not having a 16A socket limited to 16-20A MCB is also a hazard, as even with a fixed load one might assume a higher chance of flex damage if it can be plugged in and out.
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Also I don't not think you can ever safely assume that a load plugged in to a socket is fixed. What you originally fitted it for might be, but next occupant might well decide to plug in a totally different appliance / extension cable to the socket. So the socket must be overload protected.
 
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John-SJW

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pc1966, your concerns are what people might do in the future.
That is merely a concern.

I saw a 16A tumble dryer off a 32A ring. A Wylex fused switch was on the ring with a 16A fuse. This switched a 16A commercial socket that was on the continental tumble dryer. They guy who fitted it, in his own home, said it was 100% safe. It was. He worked in complex commercial electrical switching and whatnot, so was fully understanding of electrical matters. He said if I leave this house I will install a 13A socket. By the OSG posted on this thread he half conformed to the current regs. They state a switch with a 16A mcb off the 32A ring is fine, but the appliance must be permanently fixed.

The OSG does make it easy to install an oven that draws over 13A but lower than 16A on a 32A ring. The point of an oven did come up in this thread. Of course the loadings on the ring have to be taken into account.

About 5 years ago a friend of mine renovating his house was told by me to buy an oven of 13A. I then saw a Meile oven they were smitten on that drew 15A or so, that was delivered. This was got around by splitting a ring, so not problem in the end. But the ring could have stayed by fitting a local switch and a 16A mcb for the oven with a permanent fix. All legal.

So, what has been determined is a 32A ring is not limted to just a maximum of a 13A appliance.
 
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DPG

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pc1966, your concerns are what people might do in the future.
That is merely a concern.

I saw a 16A tumble dryer off a 32A ring. A Wylex fused switch was on the ring with a 16A fuse. This switched a 16A commercial socket that was on the continental tumble dryer. They guy who fitted it, in his own home, said it was 100% safe. It was. He worked in complex commercial electrical switching and whatnot, so was fully understanding of electrical matters. He said if I leave this house I will install a 13A socket. By the OSG posted on this thread he half conformed to the current regs. They state a switch with a 16A mcb off the 32A ring is fine, but the appliance must be permanently fixed.

The OSG does make it easy to install an oven that draws over 13A but lower than 16A on a 32A ring. The point of an oven did come up in this thread. Of course the loadings on the ring have to be taken into account.

About 5 years ago a friend of mine renovating his house was told by me to buy an oven of 13A. I then saw a Meile oven they were smitten on that drew 15A or so, that was delivered. This was got around by splitting a ring, so not problem in the end. But the ring could have stayed by fitting a local switch and a 16A mcb for the oven with a permanent fix. All legal.

So, what has been determined is a 32A ring is not limted to just a maximum of a 13A appliance.
Ring split into 2 x 16A radials? Were both legs still able to cope with the other loads in addition to the cooker?
 
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