That is where, for non-domestic properties, things like the Construction, Design & Management Regulations (CDM) come into play as such limitations will be specified in the building's "handbook" which will be available to the new owner!
6.3.3??
- Thread starter Hager
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That is where, for non-domestic properties, things like the Construction, Design & Management Regulations (CDM) come into play as such limitations will be specified in the building's "handbook" which will be available to the new owner!
I would say so (others may disagree). As I've recently written to you, you wouldn't expect a 6A circuit supplying multiple kitchen sockets to be compliant, would you?
Some of the problem here may be that BS7671 probably encompasses design considerations to a much greater extent than do the essentially 'safety' regs under which you work.
Kind Regards, John.
It either does in a specific rule, or it doesn't. It may be implied and is good engineering practice.
But if it isn't written it isn't enforceable
That's always a consideration, but there is a limit to the extent to which one can reasonably take that into account. Don't forget that there is no significant safety issue, since the circuit will be protected by an OPD appropraie for the wiring - so the worst that happens is that the MCB operates when the hypothesised future night club tries to power their high wattage jiggley lights from the 6A circuit.
The same argument as yours would probably say that 16A or 20A radial circuiots supplying multiple circuits should not be allowed - on the basis that the property might subequently come into different use with lots of high loads being plugged into those sockets.
Kind Regards, John.
Oh that regulations were that clear! The Definitions section (Part 2) of BS7671 defines 'design current' as "the magntude of the current to be carried by the circuit in normal service".
Unfortuanately, it does not define "normal service"
Kind Regards, John.
Indeed - but you've done little more than change the words - we now need to define "expected"!
Let's make this less hypothetical. My kitchen has a ring final circuit supplying 6 double and two single 13A sockets - i.e. 14 x 13A 'outlets' in total. What, in your professional opinion, should be the "expected load" on that circuit? (noting that 14 * 7.5 = 105A
).Kind Regards, John.
433.1.1 is the regulation in question.
I have always read this to mean that the CPD must be selected to protect the circuit with regard to the design current AND the Current Carrying Capacity.
Now, however, it would also seem to mean that it (CPD) must be selected so that it should be protected from likely overload itself, (433.1).
This does seem to raise a lot of doubt with socket circuits - Rings and Radials.
Even a 2.5mm² 20A Radial to an integral garage with only one double socket is 'likely' to be overloaded quite easily - kettle and three-bar electric fire or similar, for example. Is this bad design?
There does not appear to be a simple answer which satisfies all of the regulations.
I have always read this to mean that the CPD must be selected to protect the circuit with regard to the design current AND the Current Carrying Capacity.
Now, however, it would also seem to mean that it (CPD) must be selected so that it should be protected from likely overload itself, (433.1).
This does seem to raise a lot of doubt with socket circuits - Rings and Radials.
Even a 2.5mm² 20A Radial to an integral garage with only one double socket is 'likely' to be overloaded quite easily - kettle and three-bar electric fire or similar, for example. Is this bad design?
There does not appear to be a simple answer which satisfies all of the regulations.
Indeed, but I think that most of us have already 'got there' (subconsciously), and are really considering the definition of 'design current' to be used when implementing that reg.
For Westie's benefit, that says that "every circuit shall be designed so that a small overload of long duration is unlikley gto occur". As you say, that is the crunch in terms of determining/defining 'design current'. As you go on to say ...
Exactly. That was one of the earliest issues I raised after I joined this forum, and it resulted in a lot of flak! Essentially, in order to 'comply' with 433.1 with a socket circuit,a designer has got to somehow use that crystal ball to ensure that the total current never exceeds In of the protective device - either that, or restrict ring and radial final circuits to a maximum of two single (or one double) 13A sockets!
Again,exactly. As I keep saying, one cannot undertake a proper design process for socket circuits - one has to guess and make 'reasonable informed judgments' about loads. It's just a type of 'diversity', and cannot be a science.
Kind Regards, John.
Let's make this less hypothetical. My kitchen has a ring final circuit supplying 6 double and two single 13A sockets - i.e. 14 x 13A 'outlets' in total. What, in your professional opinion, should be the "expected load" on that circuit? (noting that 14 * 7.5 = 105A
Domestic!!! oh you know how to give someone a headache
In a domestic situation there would have been a time where I would have factored diversity but these days all bets are off.
-electric kettle 3KW - ~12 amps
-toaster 0.5KW ~2amps
-Microwave 1KW ~4amps
-Washing machine 2KW ~8 amps
-dryer 2KW ~8 amps
-misc 0.5KW ~2 amps
TOTAL 36 amps. You could argue this situation could occur only around 3 times per day but that does not seem to be the trend any more with many people working from home, extended breakfast with various members of the family getting up at staggered times, washing machine running for 4 - 8 hours/day etc, etc.
In this case, the "standard" ring final OCPD of 32 amps seems very appropriate/reasonable but many households have one ring for the entire downstairs area!!!
You see this is one reason why I try to avoid domestic work. I just extended a ring final at a automotive refurbishment facility where I could predict the "expected socket load" to within 2 amps.
I would suspect that the term "reasonably practicable" will appear somewhere. Those two words are important and are legally defined.
In the case of the 20A spur to a double socket if it is to a "general" are where the designer has no means of knowing what would be connected that is one case.
If it were to, say a utility room and will supply a cold fill washer and a separate dryer 3kW & 3kW that would be an unacceptable design.
I had a discussion regarding this recently with someone, the final solution was 2 x 16A spurs to single sockets!
Regarding circuits, surely a circuit starts at the connection to a bus-bar so the CPD is part of the circuit and is covered by the circuit requirements!
In the case of the 20A spur to a double socket if it is to a "general" are where the designer has no means of knowing what would be connected that is one case.
If it were to, say a utility room and will supply a cold fill washer and a separate dryer 3kW & 3kW that would be an unacceptable design.
I had a discussion regarding this recently with someone, the final solution was 2 x 16A spurs to single sockets!
Regarding circuits, surely a circuit starts at the connection to a bus-bar so the CPD is part of the circuit and is covered by the circuit requirements!
On a common sense basis, I agree - but you have left 8 of my 13A outlets unusued - so what about fan heaters, vacuum cleaners - power tools (maybe even mower) etc to be used outside of kitchen door etc.etc.
In practice, one obviously applies 'diversity', whether consciously or not - but this does illustrate how impossible it is to 'properly design' when one is talking about domestic socket circuits.
In the depths of the really cold spell last winter, I found myself in my neighbours kitchen, witnessing two fan heaters running, the washing machine and dryer both on, and she boiled a kettle to make me some coffee; she could have made me some toast as well! I did stop and wonder whether that was all one circuit
Kind Regards, John.
Yes, I think EFLI worded that bit sub-optimally
As I explained, for your benefit 433.1 requires that design should ensure that an OPD's In should ensure that it is unlikley that there will be a small 'overload'of long duration. This is presumably to prevent people using, say, a 16A OPD with a known 17A or 18A 'expected load', even though one knows that one would probably get away with it. In other words, this regulation prevents one from using the 'leeway' inherent in the OPD (and much exploited by some cutout fuses!) to cover than short-term excursions of load above the device's In.Kind Regards, John.
Well - another three pages?
Circuit - An assembly of electrical equipment supplied from the same origin and protected against overload by the same protective device(s).
Exactly, and as I tend to say much too often, domestic electrical is a bit of a nightmare these days. I also say (far too often) that I rarely design ring finals anymore. I try to plan for radials and plenty of them.
In the depths of the really cold spell last winter, I found myself in my neighbours kitchen, witnessing two fan heaters running, the washing machine and dryer both on, and she boiled a kettle to make me some coffee; she could have made me some toast as well! I did stop and wonder whether that was all one circuit
I'll bet anything it was all on a single ring final with the 32amp MCB perspiring in its zone between max Ib & In, counting down the minutes to disconnection
But whatever design, in reality I wonder how many RFCs there are and what infinitesimal cases there are of the fuse or MCB operating through overload?
Yes we are aware of what limits we can "push" our kit too and over what limits we are in an "at our own professional risk" area. But if we didn't I would expect lots of cold Christmas Dinners every year!
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