Load limit of a radial circuit

[Bobthefish]

Hi,

I'm looking to add some sockets to my basement to supply a new workbench area. The existing wiring is on a dedicated 2.5mm radial circuit covered by a 32A MCB, and feeds a double socket and boiler (the boiler has a FCU).

I've been led to believe that provided the radial circuit covers an area less than 75m2, I should be able to add additional sockets by simply extending the radial within this area.

My concern is that the existing double socket is being used for a washing machine and tumble dryer, and that given the peak current of these, I might be asking for trouble adding any further appliances on the 2.5mm / 32A combo. I'll be running power tools (saws, sanders, etc), and unlikely to have more than one running at any given time, but my mitre saw is 2kw, so on the rare occasion that I'm sawing with the washing machine (2.4kW) and dryer (1.4kw) both on at the same time, I'd run the risk of exceeding the 27A the wire can handle, without tripping the MCB, or any of the 13A fuses. The existing socket is only about a metre from the consumer unit, and the extended sockets would be less than 10m away.

Am I worrying about nothing, or should I be considering upgrading to 4mm cable, or converting the radial to a ring?

Cheers,

 

[ban-all-sheds]

You are not worrying about nothing.

You must:

1. Increase the cable size to 4mm² (or possibly 6mm², depending on how it is installed), or
2. Change the breaker to a 20A or 25A, depending on what's available for your CU, and how the cable is installed, or
3. Make the circuit into a ring.

All of these options are notifiable in England and Wales, and all should be properly tested and certified.

 

[EFLImpudence]

I'm looking to add some sockets to my basement to supply a new workbench area. The existing wiring is on a dedicated 2.5mm radial circuit covered by a 32A MCB, and feeds a double socket and boiler (the boiler has a FCU).

2.5mm² has a maximum current rating (it could be less depending on the cable installation method) of 27A so that is wrong.

I've been led to believe that provided the radial circuit covers an area less than 75m2, I should be able to add additional sockets by simply extending the radial within this area.

The area is irrelevant.
You can have as many sockets as you like on a radial circuit but the current will be limited by the MCB which, in your case if a radial, should be 25A (possibly lower)

My concern is that the existing double socket is being used for a washing machine and tumble dryer, and that given the peak current of these, I might be asking for trouble adding any further appliances on the 2.5mm / 32A combo. I'll be running power tools (saws, sanders, etc), and unlikely to have more than one running at any given time, but my mitre saw is 2kw, so on the rare occasion that I'm sawing with the washing machine (2.4kW) and dryer (2kw) both on at the same time, I'd run the risk of exceeding the 27A the wire can handle, without tripping the MCB, or any of the 13A fuses. The existing socket is only about a metre from the consumer unit, and the extended sockets would be less than 10m away.

You seem to understand. You have to limit your usage.

Am I worrying about nothing, or should I be considering upgrading to 4mm cable, or converting the radial to a ring?

You could make it a ring and keep the existing cable.
You can have a 4mm² radial with 32A MCB.

The only way to gain more capacity is to have more circuits.

 

[ericmark]

As said what you have is wrong, it needs changing, however as far as notifying goes read it yourself, I personally would not consider it to be a new circuit, I would say your altering an existing, however since it is so wrong then I would also be asking what else is wrong in the house?

BAS list is a or list not an and list do any of the 3 options and it's OK.

There was a recommend floor area this was to stop the volt drop being exceeded and with a ring over loading near start finish of the ring, but today we measure the loop impedance we do not worry about floor area.

 

[John D v2.0]

Actually I suspect it's not wrong at present as long as the loop impedence is low enough for the fault current and the cable isn't buried in insulation.
Overload is taken account because a double socket plus an fcu doesn't exceed the CCC of the cable. Think of it as an unfused spur, but without the special ring rules for unbalancing.
Whether the person responsible for it was thinking that way I don't know...

 

[deadshort]

I would favour making it into a ring, you have the 32a mcb and the wiring is 50% done !

DS

 

[Bobthefish]

Thanks all.

It sounds like my concerns were well placed, and the guys who rewired the house a couple of years ago either gave us the bare minimum for the boiler and sockets, or just didn't think about it (it's not the first issue I've had with their work).

I think we'll probably end up making it into a ring, as a couple of people have suggested - many thanks for the advice.

Cheers

 

[ban-all-sheds]

I personally would not consider it to be a new circuit

I personally think you are wrong in each case. Using the same numbering sequence as my option list:

1. Completely replace all of the cable - new circuit.
2. Change the OPD - new circuit.
3. Change the circuit topology from radial to ring- new circuit.

 

[ericmark]

I think the phrase "New Circuit" is rather daft as a legal phrase, every time you switch on a light you make a circuit, and when you switch it off there is no circuit. So only word is "New" and if it's been there before then it's not new. It is another let the court decide, and it is unlikely to every go to court for them to decide, so it is another ground nuts law, which was the one quoted as being the worst written law.

 

[John D v2.0]

True, and even if you took a more strict definition of new circuit that it has to not have been there before, and it has to have a protective device, then every time we plug a new appliance in we're making a new final circuit.

This may seem a silly comment, but think what happens when you plug in a few extension cables to get to the bottom of the garden, 50m of 1mm flex "installed" by someone without testing the loop impedence. So many grey areas

 

[ban-all-sheds]

when you switch it off there is no circuit

Nonsense.

 

[mwatsonxx]

when you switch it off there is no circuit.

Very true, a circuit is not circuit when it is switched OFF, otherwise it wouldn't be OFF

 

[ban-all-sheds]

Very true, a circuit is not circuit when it is switched OFF, otherwise it wouldn't be OFF

Nonsense.

If what you and Eric say is right then there is no such thing as a circuit when being installed. An electrician can never install a circuit unless it is live and current is flowing. Assuming safe working practices, no Building Regulations notification of any wiring is ever required, because there will be no circuits. Turn off the appropriate MCB(s) and there are no circuits in bathroom zones.

If a circuit does not exist when switched off, then switching on brings it into existence. If something did not exist, and you then do something to make it exist, then you have created something new, so according to you and Eric you'd need Building Regulations approval every time you wanted to turn it on.

You and he are talking nonsense.

 

[ericmark]

You are of course correct it is a lot on nonsense, to make sense first you need to define what you mean by the words used, BS7671 does define what they mean by a circuit, but as soon as the Part P lot said that fitting a FCU does not form a circuit they in doing that showed they were not referring to a circuit as defined by BS7671 so you have to look at the words as used generally in the English language.

So when you switch on a light it makes a circuit, however it does no make a new circuit, that light has worked before. Very like the axe which has been passed down father to son for many generations and is 250 years old, it has had many heads, and had many shafts, but still considered as being the same axe. It does not really matter which bit of the old circuit remains, it is still the old circuit until every bit has been simultaneously removed and refitted so at some point there was nothing of the old circuit remaining.

This is of course nothing to do with electrics, it's the English language and its use. So the grey area is where you branch from a circuit remember we are not following BS7671 definition, so if you add a spur and new socket from old socket is that a new circuit? It has not been a circuit before, so yes in English it is a new circuit, but we are told this like fitting a FCU is not a new circuit.

As I said ground nuts, not being nuts taken from the ground, was considered as the worst written law, no one could work out what it really said. It is the same with Part P, we have loads of people telling us what they want it to say, but until a court rules on it, the law is simply not workable.

At a latitude of around 45° if I am static on the earths surface I am doing around 500 miles per hour, it will vary, so I am breaking the 30 MPH speed limit, but some where some one will have tried that on and the law will have relative to the earth in it some where. Some where some court will have defined when you are considered as speeding, we will have definitions like air speed and ground speed, using knots then it is ground speed because the knot is tied to minutes of movement around the earth. So if a hover craft is doing 100 knots, it is likely going slower than a jet doing 100 knots as closer to centre of earth. Some one defined the knot. However new circuit has not been defined so it is a meaning less phrase.

 

[EFLImpudence]

If you open a circuit breaker, do you have to repair it?