I had a situation where I was re-wiring a house and one of the sockets was wired from the switchboard in 2.5. From this socket there was another socket wired in 1.5.
The second socket was in a position where it could not be rewired without running the cable on the surface with protection, the owner said they did not want this and were instead just happy to have the second socket disconnected.
Talking to some other electricians they said it is fine to have it wired like this as long as you are protecting the lower rated cable with you circuit protection at the switchboard.
I could not find anything that seemed to allow this, as section 2 states overload protection is required wherever there is a reduction in CCC and this situation doesn’t fit any of the exceptions.
Does anyone have any insight to whether you can reduce cable size between sockets like this or definitely not?
Overload Protection
Re: Overload Protection
Around the 60's and 70's, it was common to wire like that when only two outlets were allowed per circuit on a rewirable fuse. When anyone has added to an (old) existing circuit like this in the past, the rewirable fuse should have replaced with a circuit breaker (to allow more than 2 outlets), but often people haven't done that, resulting in a non-compliant circuit.
"Some other electricians" are correct, that as long as you provide overload protection for the smaller cable, that's all that is necessary. Because you are providing protection for the cable size after the reduction in size, it can go at the origin of the circuit, and no further protection is required at the point of reduction.
These days, it's common to have oversized cables for voltage drop on long runs to the first point on a circuit, then reduce the cable size for the remainder of the circuit, with the protection rated for the smaller cable size. Same with oversized consumer mains with a lower protective device. The protective device rating is lower (in part), so that the circuit can't exceed the maximum current for its designed voltage drop.
In your case, you would install a 16A MCB (or RCBO) to match the partially surrounded CCC of the 1.5mm² for domestic. I regularly use 16A MCBs for old circuits when replacing a switchboard for exactly this reason. I would only use 20A MCBs where I have installed new wiring, and know that the entire circuit is rated at 20A.
"Some other electricians" are correct, that as long as you provide overload protection for the smaller cable, that's all that is necessary. Because you are providing protection for the cable size after the reduction in size, it can go at the origin of the circuit, and no further protection is required at the point of reduction.
These days, it's common to have oversized cables for voltage drop on long runs to the first point on a circuit, then reduce the cable size for the remainder of the circuit, with the protection rated for the smaller cable size. Same with oversized consumer mains with a lower protective device. The protective device rating is lower (in part), so that the circuit can't exceed the maximum current for its designed voltage drop.
In your case, you would install a 16A MCB (or RCBO) to match the partially surrounded CCC of the 1.5mm² for domestic. I regularly use 16A MCBs for old circuits when replacing a switchboard for exactly this reason. I would only use 20A MCBs where I have installed new wiring, and know that the entire circuit is rated at 20A.
- Rating: 16.67%
Re: Overload Protection
First the rules as they are now for new work.
Correct that the default requirement [2.5.1.2]is overcurrent protection (both overload and short circuit) at origin of circuit; and at each reduction of CCC . Three Exceptions listed, with "pointers" to the relevant details (which are in following clauses).
In this case there's a decrease in CCC, but no protective device. Two different Exceptions apply.
While expressed as Exceptions to the underlying requirement; they are actually both so common as to be considered normal prqactice.
However it's worth following them through in order to understand the principles behind them.
For shortcircuit protection; the s/c protection for reduction of CCC is in an alternative position (Exception 2), i.a.w 2.5.4.3.
Specifically 2.5.4.3.2 condition 2.
In summary, if the device at origin is capable of providing s/c protection for the section having reduced CCC ; then adding another s/c device at the point of reduction isn't required.
For overload protection; the o/l protection for reduction of CCC is omitted (Exception 3), i.a.w. 2.5.3.4.
Specifically (b)(i).
The conductor with reduced CCC is on the load side of a change of CCC that is effectively protected by a device on supply side of origin of circuit.
More simply; if the device at origin is rated appropriately for the smaller cable, there's no need to add any additional device just for reduction of CCC.
(Why didn't it just say that? Because the principle applies to more situations than this simple case of reduction on CCC part-way along a subcircuit).
Another very common case for omitting o/l protection is (b)(ii); where the equipment supplied by the cable with reduced CCC isn't capable of causing an overload; plus the it has no branch circuits, and supplies no sockets.
For example, a light supplied in 1.0 mm2 connected to a circuit wired in 2.5 mm2.
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As DougP said; it used to be common practice to wire socket circuits like this, particularly in days of TRS.
When I was an apprentice (early '70s) ; it wasn't considered good practice.
These days, it's becoming common again.
Have to remember that the details of "the Rules" have been changed quite a bit over the years; so we can't expect older installations to comply with today's requirements.
ESR 113 says they only have to continue to comply with whatever requirements were in place when installed, and not have become electrically unsafe. It can be difficult to know exactly what the original requirements were; but that's seldom a real problem unless we start changing things.
We can maintain to original condition [ESR 59(3)(b)] .
We can also maintain to be i.a.w. "3000"; so that does let us try to apply today's rules to older installations.
Sometimes they fit. Can't tell whether they do in that case, 'cos we don't know the rating of the circuit protection, and we don't know the installation conditions.
But if we change anything, then today's Rules apply . Not just that the bit we work on then must comply to "3000"; but that what we do doesn't reduce the safety of any other part of the installation [ESR 13].
Altering overcurrent protection of existing circuits is very easy to get wrong, so great care is needed.
For example when 'upgrading' from SERFs to mcbs.
Correct that the default requirement [2.5.1.2]is overcurrent protection (both overload and short circuit) at origin of circuit; and at each reduction of CCC . Three Exceptions listed, with "pointers" to the relevant details (which are in following clauses).
In this case there's a decrease in CCC, but no protective device. Two different Exceptions apply.
While expressed as Exceptions to the underlying requirement; they are actually both so common as to be considered normal prqactice.
However it's worth following them through in order to understand the principles behind them.
For shortcircuit protection; the s/c protection for reduction of CCC is in an alternative position (Exception 2), i.a.w 2.5.4.3.
Specifically 2.5.4.3.2 condition 2.
In summary, if the device at origin is capable of providing s/c protection for the section having reduced CCC ; then adding another s/c device at the point of reduction isn't required.
For overload protection; the o/l protection for reduction of CCC is omitted (Exception 3), i.a.w. 2.5.3.4.
Specifically (b)(i).
The conductor with reduced CCC is on the load side of a change of CCC that is effectively protected by a device on supply side of origin of circuit.
More simply; if the device at origin is rated appropriately for the smaller cable, there's no need to add any additional device just for reduction of CCC.
(Why didn't it just say that? Because the principle applies to more situations than this simple case of reduction on CCC part-way along a subcircuit).
Another very common case for omitting o/l protection is (b)(ii); where the equipment supplied by the cable with reduced CCC isn't capable of causing an overload; plus the it has no branch circuits, and supplies no sockets.
For example, a light supplied in 1.0 mm2 connected to a circuit wired in 2.5 mm2.
-------------
As DougP said; it used to be common practice to wire socket circuits like this, particularly in days of TRS.
When I was an apprentice (early '70s) ; it wasn't considered good practice.
These days, it's becoming common again.
Have to remember that the details of "the Rules" have been changed quite a bit over the years; so we can't expect older installations to comply with today's requirements.
ESR 113 says they only have to continue to comply with whatever requirements were in place when installed, and not have become electrically unsafe. It can be difficult to know exactly what the original requirements were; but that's seldom a real problem unless we start changing things.
We can maintain to original condition [ESR 59(3)(b)] .
We can also maintain to be i.a.w. "3000"; so that does let us try to apply today's rules to older installations.
Sometimes they fit. Can't tell whether they do in that case, 'cos we don't know the rating of the circuit protection, and we don't know the installation conditions.
But if we change anything, then today's Rules apply . Not just that the bit we work on then must comply to "3000"; but that what we do doesn't reduce the safety of any other part of the installation [ESR 13].
Altering overcurrent protection of existing circuits is very easy to get wrong, so great care is needed.
For example when 'upgrading' from SERFs to mcbs.
- Rating: 16.67%
Re: Overload Protection
Hi,
There was no need, that I can see from your description, that you had to worry about the 1.5mm cable, unless the owner wanted you to (full rewire etc). Just because something exists in an installation doesn't mean that you have to upgrade it to todays rules. Unless it is unsafe, then the home owner should take steps to make it not unsafe safe, disconnection, re wiring etc.
If you were installing a new socket outlet in your example - extending the old 1.5mm cable, you would need to consider if the existing cable would be made less 'safe' then it currently was (so long as it wasn't unsafe). As long as your work didn't make it less safe you could leave it that happily.
Since you were going to connect the existing 1.5mm cable to a 2.5mm cable there would be nothing stopping you leaving it there connected. Of course the selection of MCB could affect the 'safety' of the 1.5mm wiring depending on load current capacity etc. But if you left the same rated MCB in the board you wouldn't be making that 1.5mm cable less 'safe' so it could be left there.
There was no need, that I can see from your description, that you had to worry about the 1.5mm cable, unless the owner wanted you to (full rewire etc). Just because something exists in an installation doesn't mean that you have to upgrade it to todays rules. Unless it is unsafe, then the home owner should take steps to make it not unsafe safe, disconnection, re wiring etc.
If you were installing a new socket outlet in your example - extending the old 1.5mm cable, you would need to consider if the existing cable would be made less 'safe' then it currently was (so long as it wasn't unsafe). As long as your work didn't make it less safe you could leave it that happily.
Since you were going to connect the existing 1.5mm cable to a 2.5mm cable there would be nothing stopping you leaving it there connected. Of course the selection of MCB could affect the 'safety' of the 1.5mm wiring depending on load current capacity etc. But if you left the same rated MCB in the board you wouldn't be making that 1.5mm cable less 'safe' so it could be left there.
Re: Overload Protection
Thanks guys for the help and background information.
I new there must have been something allowing this as Doug said this is a situation commonly seen.
AlecK section 2.5.3.4 b i was exactly what I was looking for. The problem was I had read it but never really comprehended it properly due to the way it is written.
I new there must have been something allowing this as Doug said this is a situation commonly seen.
AlecK section 2.5.3.4 b i was exactly what I was looking for. The problem was I had read it but never really comprehended it properly due to the way it is written.