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https://www.askthetrades.co.uk/cgi-bin/yabb/YaBB.pl DIY Forum >> Electrical Questions >> Spurs, MCBs, and confused https://www.askthetrades.co.uk/cgi-bin/yabb/YaBB.pl?num=1101274475 Message started by HarryH on Nov 24^th, 2004, 5:34am

Title: Spurs, MCBs, and confused
Post by HarryH on Nov 24^th, 2004, 5:34am

Hi all. This looks like a nice place...(replace flannel) :-)

If I've got it right, the reason taking a spur from a spur is bad (unless protected by an FCU) is that the MCB in the CU is chosen assuming the presence of the two conductors in the ring, and that too much current could be drawn down the spur cables..yes?? If this is so, why can't you take any number of spurs from a socket in a radial circuit? I can't visualize why adding a spur in a radial circuit is different to just extending the circuit - sorry if this is stupid but it's like trying to visualize 11-dimensional Superstring Theory to me.

I'm not sure if that's even relevant, but what I've got is a radial circuit coming down from the loft to a wall socket from which a cable then crosses a doorway by going *under the carpet* (don't ask - wasn't my doing) to the next socket and then finally onto the fridge socket in the kitchen. I don't want to traverse the doorway by going under the floorboards because there's a gas pipe really close to were the cable would emerge under the boards. Can I use some kind of cable connector to split the cable in the loft to go down either side of the doorway? i.e. one 'arm' would supply one socket, the other, two sockets.

One other thing if I may: this radial circuit, plus two others which supply one socket in each of two bedrooms are all coming from one 16A MCB in the CU (I think it's all 2.5mm twin and earth cable). Is this bad - shouldn't it be a 20A MCB?

H

Title: Re: Spurs, MCBs, and confused
Post by Lectrician on Nov 24^th, 2004, 7:45am

A radial circuit can safely be wired as a star arrangement with branches everywhere ;)

Title: Re: Spurs, MCBs, and confused
Post by HandyMac on Nov 24^th, 2004, 7:48am

The technical explanation isn't really relevant because you could feasibly string 10's of sockets in an extended spur, each of which could draw miniscule amounts of current so it won't trip the protective device.

Adding a spur to a spur goes against the 16th edition wiring regs so it's a no-no.

You also need to think in terms of the protective conductor (the earth wire if you will). If you've got spurs on spurs then you will have the earth being made and broken at each spur using a terminal block connection. If you happen to break the earth at some point in the chain then none of the following spurs will have an earth - scarey!

HM

Title: Re: Spurs, MCBs, and confused
Post by sparkyjonny on Nov 24^th, 2004, 12:47pm

wrote on Nov 24^th, 2004, 5:34am:

One other thing if I may: this radial circuit, plus two others which supply one socket in each of two bedrooms are all coming from one 16A MCB in the CU (I think it's all 2.5mm twin and earth cable). Is this bad - shouldn't it be a 20A MCB?

H

Effectively you have a star arrangement here with three branches from the MCB. 16A is fine - if 20A is recommended, anything lower is even safer. It just limits your maximum load. Alternatively they could each be put on 16A breakers seperately, to increase the max load allowed. They may have been originally like this, then put all on the same breaker to allow another breaker to be used for a new circuit. Be cautious about upgrading 16A to 20A incase the 16A was fitted for a reason (many cables grouped together for a reasonable distance / thermal insulation / old 2.5mm cable with a 1mm CPC [earth]).

Title: Re: Spurs, MCBs, and confused
Post by JerryD on Nov 24^th, 2004, 5:13pm

wrote on Nov 24^th, 2004, 7:45am:

A radial circuit can safely be wired as a star arrangement with branches everywhere �;)

When you say 'star', does every leg of this star have to start from the mcb or could this star start from a jb somewhere along the radial circuit? ???

Title: Re: Spurs, MCBs, and confused
Post by Lectrician on Nov 24^th, 2004, 5:20pm

Anywhere, think of it as a tree with branches!

Title: Re: Spurs, MCBs, and confused
Post by HarryH on Nov 24^th, 2004, 7:43pm

I still don't understand why in a radial circuit the regulations put a limit on how many sockets can be connected to a branch (spur). I don't really understand the earth point either as wouldn't it be interupted anyway at each socket in the main radial circuit?

But if I understand Lectrician, it is ok for me to cut the cable in the loft, add a junction box, and have a branch going to one socket on one side of the doorwar and another branch - or is that the main trunk (?) going to the other side of the doorway to the two sockets; see, here again, I can't see the difference between a branch (spur) and the main trunk of the radial circuit.

Sparkyjonny, thanks, I did understand that. :-)

H

Title: Re: Spurs, MCBs, and confused
Post by L.Spark on Nov 24^th, 2004, 10:24pm

Quote:

But if I understand Lectrician, it is ok for me to cut the cable in the loft, add a junction box, and have a branch going to one socket on one side of the doorwar and another branch

Thats fine.

There are more issues with ring circuits that radials.

Radials are far more flexible and allow you to take sockets off here and there without problems, this is due to a MCB rating of just 20A

Title: Re: Spurs, MCBs, and confused
Post by ban-all-sheds on Nov 25^th, 2004, 1:25am

Quote:

If I've got it right, the reason taking a spur from a spur is bad (unless protected by an FCU) is that the MCB in the CU is chosen assuming the presence of the two conductors in the ring, and that too much current could be drawn down the spur cables..yes??

Yes.

Quote:

If this is so, why can't you take any number of spurs from a socket in a radial circuit?

You can.

Quote:

I can't visualize why adding a spur in a radial circuit is different to just extending the circuit

It's not

wrote on Nov 24^th, 2004, 7:43pm:

I still don't understand why in a radial circuit the regulations put a limit on how many sockets can be connected to a branch (spur).

They don't.....

Title: Re: Spurs, MCBs, and confused
Post by The_Trician on Nov 25^th, 2004, 1:35am

I hate radials when used to provide 13A socket circuits.
Spur off Spur off Spur etc ad finitum. Why not run just one moe cable back from the last socket and make a Ring?

Seen too many radials off 32A mcbs for my liking.

Cheapskates!

Get a full Ring put in you tight gits.

TT

Title: Re: Spurs, MCBs, and confused
Post by plugwash on Nov 25^th, 2004, 1:48am

rings do have thier place but i really can't see what is wrong with properly installed radials TT

the ring cuircuit was designed to allow two 3KW heaters at arbitary locations and a few small appliances on top of that whilst minimising cable use (iirc it was introduced in a time of copper shortage).

in some ways i think the way to go is a 20A radial per room possiblly also running the lights via a FCU off that same radial (though that does raise RCD related issues). This way when you want to do work in the lounge that involves the electrics you just turn off the breaker marked lounge and leave the rest of the place totally unaffected

Title: Re: Spurs, MCBs, and confused
Post by HarryH on Nov 25^th, 2004, 7:17am

So it looks like I can do what I want - thanks L.Spark.

The bad news is that some of this doesn't make any sense to me - err.. thanks everyone else. :-)

If, as Ban-All-Sheds says, a spur in a radial is the same as a socket on the end, then adding a spur to a spur would be the same as adding a socket on the end surely? But the regs don't allow the former according to Handy Mac? (Are you sure this has nothing to do with parallel universes etc.)

Regarding my assumed limitation on the number of sockets on a spur in a radial circuit, I got that from a Wickes leaflet (83):

Quote

"With either type of circuit, ring or radial, branch cables (spurs) can be connected to the main circuit cable to feed individual remote take-off points....

..The only limitaions are that each spur may feed just one socket-outlet (single, double or treble,), or one fused connection unit, ....

They seemed to have switched to talking about rings mid-paragraph without stating that.

H

Title: Re: Spurs, MCBs, and confused
Post by rabbit_rabbit on Nov 25^th, 2004, 9:09am

I may be wrong but if I recall the UK is about the only EU country that wires with rings - radials are the order of the day in most other EU countries.

RR

Title: Re: Spurs, MCBs, and confused
Post by ban-all-sheds on Nov 25^th, 2004, 10:08am

wrote on Nov 25^th, 2004, 1:35am:

Seen too many radials off 32A mcbs for my liking.

Nothing wrong with that.

In 4mm� cable....

Title: Re: Spurs, MCBs, and confused
Post by ban-all-sheds on Nov 25^th, 2004, 10:10am

wrote on Nov 25^th, 2004, 1:48am:

in some ways i think the way to go is a 20A radial per room possiblly also running the lights via a FCU off that same radial (though that does raise RCD related issues).

And issues of if you want to do the work when it's dark, you lose the lights in the room as well....

Title: Re: Spurs, MCBs, and confused
Post by ban-all-sheds on Nov 25^th, 2004, 10:38am

wrote on Nov 25^th, 2004, 7:17am:

The bad news is that some of this doesn't make any sense to me - err..

Welcome to BS7671.... ::)

Quote:

If, as Ban-All-Sheds says, a spur in a radial is the same as a socket on the end, then adding a spur to a spur would be the same as adding a socket on the end surely?

It is, for radials.

Quote:

But the regs don't allow the former according to Handy Mac? (Are you sure this has nothing to do with parallel universes etc.)

They do, for radials.

The basic problem is as you said at the start. A ring is wired with cable that can't carry 32A. The only reason it works is because it's a ring, and the current is shared between two cables to each socket (but you can still get problems with lots of heavy loads at one "end" of the ring). A spur only has 1 cable, so when wired with 2.5mm� is no longer safe to carry the full current that the breaker or fuse will pass, so you are limited to 1 socket, or many sockets via a 13A FCU, so that you can't draw that much.

With a radial, the cable is sized to support the rated current, so provided you stick to the same size cable (obviously :-/), it's safe to have as many sockets as you like on any branch. The only limitation is floor area served.

Quote:

Regarding my assumed limitation on the number of sockets on a spur in a radial circuit, I got that from a Wickes leaflet (83):

Don't take advice from the sheds. This is the guy that wrote that leaflet:

Title: Re: Spurs, MCBs, and confused
Post by HarryH on Nov 25^th, 2004, 12:16pm

:-)

And thanks for everyone's patience..

Title: Re: Spurs, MCBs, and confused
Post by sparkyjonny on Nov 25^th, 2004, 5:46pm

Much prefer radials for reason of power distribution, and ease of extending (branches).
From what I can see, the only advantage of rings is the reduced chance of no earth present at outlets, as a result of a broken CPC.

Title: Re: Spurs, MCBs, and confused
Post by Lectrician on Nov 25^th, 2004, 5:52pm

Rings will die soon.....along with bs1363 :o ::) >:(

Title: Re: Spurs, MCBs, and confused
Post by sparkyjonny on Nov 25^th, 2004, 6:21pm

And every plugtop, Lectrician?! What do you want, hard-wired appliances?!?

Title: Re: Spurs, MCBs, and confused
Post by plugwash on Nov 25^th, 2004, 9:03pm

what i don't see is why they don't make a 13A socket with a built in fuse for protection and shutters that are designed so a europlug will open them

harmonisation problem solved at least for small portable appliances

Title: Re: Spurs, MCBs, and confused
Post by sparkyjonny on Nov 25^th, 2004, 9:59pm

And a double socket with a 13A fuse, like the 3 gang ones available, if they cannot manufacture one which is rated at 26A.

Title: Re: Spurs, MCBs, and confused
Post by ban-all-sheds on Nov 25^th, 2004, 10:09pm

wrote on Nov 25^th, 2004, 5:52pm:

Rings will die soon.....along with bs1363 :o ::) >:(

No more rings? Not such a bad idea.

No more fused plugs? 'kin terrible idea.

wrote on Nov 25^th, 2004, 9:03pm:

what i don't see is why they don't make a 13A socket with a built in fuse for protection and shutters that are designed so a europlug will open them

How about because some appliances/appliance flexes need less than 13A?

Title: Re: Spurs, MCBs, and confused
Post by plugwash on Nov 25^th, 2004, 10:24pm

im not really convinced how much the smaller plug fuses achive

sure they are nice to have but nearly all modern appliances will have internal protection and so long as the flex is not insanely long the 13A fuse should deal with shorts ok

yes i know its been drummed into us from kids but no other wiring system does it

Title: Re: Spurs, MCBs, and confused
Post by ban-all-sheds on Nov 25^th, 2004, 11:22pm

Just because we're unusual it doesn't mean we are wrong.

Title: Re: Spurs, MCBs, and confused
Post by supersparky on Nov 25^th, 2004, 11:31pm

i dont see the point in harmonisation
i think its a load of balls

it wont help foreign sparks comming over here to know what they are doing cos they wont have the qualifications so there goes that idea

ss

Title: Re: Spurs, MCBs, and confused
Post by plugwash on Nov 26^th, 2004, 12:30am

the harmoisation of portable equipment makes a lot of sense both for manufactures and for customers.

the one part of portable appliances that hasn't been harmonised accross europe yet is the plug and socket system. For low current class 2 appliances the europlug is now usable over almost all of europe.

for higher current kit the combined schuko/french plug is also starting to replace local plugs in quite a bit of europe.

the UK plug system does have its advantages but I would say its days are almost certainly numbered.

fixed wiring is another issue but making the color code the same as that for portable kit was probablly a sensible move

Title: Re: Spurs, MCBs, and confused
Post by rabbit_rabbit on Nov 26^th, 2004, 8:50am

wrote on Nov 25^th, 2004, 11:31pm:

No, no SS ya have the wrong idea. Any illegal into UK will be allowed to work on electricity and gas. Its only the local Brits that have to get qualified and pay to join some Mafia club.

RR

Please Note: No racist comments implied or stated within the above statement. The author denies any and all any such racist implications under the Race Relations Act 1996 subsection A1, paragraph 4, sub-paragaph (iiib). The posting also adheres to the Data Protection Act for illegal immigrants 2003.

Title: Re: Spurs, MCBs, and confused
Post by scotspark on Nov 27^th, 2004, 2:53pm

I prefer radials these days as if a connection come loose or a cable is broken then you lose part of the circuit which highlights there is a problem where as with a ring the circuit will continue to work without showing up any effects.

saying that i still wire plenty of ring cct's

Title: Re: Spurs, MCBs, and confused
Post by sparkyjonny on Nov 27^th, 2004, 5:16pm

Yeah, the only advantage I can see of rings is the smaller cable size (slight cost advantage, but easier to work with).

Title: Re: Spurs, MCBs, and confused
Post by scotspark on Nov 28^th, 2004, 7:13pm

A 16 or 20 Amp radial can be wired in 2.5 all the same

It is rare that I wire a 4mm 32 Amp radial

one of the reasons that 4mm is so much more expensive than 2.5 is just the volumes produced,n i use thousands of metres of 2.5 but barely any 4 and with the increasing wattages of showers and Range cookers I am using more and more 10mm as opposed to 6