I have never seen a wire nut used in the UK, but they seem to be very common in the USA, why?
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Although very much the connector of choice in North America, the twist-on wire connector remains a largely untapped resource in the UK electrical market. Unfortunately many electricians still associate them with the poor-quality ‘screw-it’ wire nuts from the 1970s; manufactured from ceramic, these had a tendency to crack very easily and expose the wires, leading to safety hazards.
Though it looks like Ideal is still trying to tap UK twist-on wire connector market, with the new Twister® PRO (which to me looks just like a regular wire nut, aside from it being multicolored).
The Wikipedia article on twist-on connectors suggests they were outlawed in the UK 50 years ago. No references are given. It is suggested that 50 years ago the ("Scruit" brand) twist-on connectors were made of ceramic and were liable to crack and expose the wires.
UK Electricians are a conservative group. Vendors of novel products have to go to considerable length to persuade them that their products meet the relevant legislation.
An Ideal Industries Inc. catalogue says
Conforming to BS EN 60998 Twister® wire connectors fully comply with BS 7671:2008 (17th Edition Wiring Regulations).
However, it isn't uncommon to see strenuous efforts on the part of manufacturers to persuade UK electricians that novel products meet regulations. E.g. Wagobox and Ezybond earth fasteners Some of these novel products more or less disappear from the market within a few years, others succeed.
In the UK, electricians almost invariably use screw-terminals.
ruler is marked in cm with mm subdivisions
The white translucent screw-connector block in the foreground is a typical example. This one is rated for 5A. Other sizes are available for different current ratings.
An electrician left behind the other connectors in this picture. I suspect they were used for temporary connections during installation. I've not seen any in a completed installation (but I'm only a householder not an electrician, so have very limited experience)
The "Marr No.1" connectors at back right look superficially similar to US wire-nuts but are very different. The metal insert is removed and a grub nut is used to clamp the wires, then the plastic insulator is screwed over the connection.
The "Legrand 10mm2" connectors at top left may be for higher current ratings. Note that the plate that presses on the wires is arranged so as not to exert any twisting force on the conductors. Perhaps this avoids damaging stranded conductors. It certainly prevents the conductors being pressed to one side of the screw and therefore not being securely clamped in place.
An obvious advantage of these connectors is that the wires can be removed intact and should usually not need trimming before reconnection. I have the impression that in the US, wire-nut connections are often removed by cutting the wires, which may leave progressively less and less wire available in the junction-box or back-box for remaking connections.
When more than two wires are to be joined, a slightly different type of screw-connector is used:
Switches, junction boxes and ceiling roses often contain sufficient bult in capacity that you don't need additional connectors:
Easier-to-use connectors are becoming more common:
Some of these are "lighting connectors" used to join solid-core wires to stranded wires. They are all rated at 20A and at least 300V. They all incorporate a test-port and wire-trimming guide.
It may be that UK electricians looking for novel connectors prefer these to twist-on connectors.
I am a British Electrician and I have lived in the USA. when I first saw and used wire nuts in the USA, I was horrified.
Frankly, the wiring standards and codes in the USA are lax compared with those in the UK and the domestic wiring is pretty amateurish.
In the USA, 110v power outlets are allowed in bathrooms. Pull cord light switches are not mandated. In the UK, power outlets and wall mounted switches are not allowed.
In the USA, Existing 2 wire circuits that do not have earth are not required to be brought up to code even when the property is sold.
In the UK - you very very rarely encounter circuits with no earth.
In the USA, some older houses are wired with metal sheathed BX cable where the metal sheath is the earth. It has a very high earth resistance and crappy insulation.
In the USA, Wiring is radial spur and sharing of power and lighting on one circuit is allowed. There is a ridiculous amount of circuits in an average home. My own home had perhaps 40 breakers.
Connections are often made with wire nuts. And they aren't so great. More often than not the wire is damaged by either the pretwist or the action of the wire nut itself. The wires themselves are just jammed into a box in a tangle. It looks awful.
Some three wire circuits are wired with two phases sharing a common neutral. The theory is that the net current through the neutral is zero when both loads are the same. However, the power factor of the load can affect that. Also you get situations where some idiot has wired the different phases through different single breakers (rather than a dual breaker linked by a bar) and then only isolated one side. Person working on it gets a shock.
Romex cable in the USA has softer and is less rugged than the equivalent T&E cable in the UK. It is not mechanically tough and can easily be damaged.
2 pin plugs with no earth are allowed in the USA. In the UK all plugs are three pin.
three pin plugs in the USA do not contain a fuse of any sort.
Three pin sockets in the USA do not have shutters in them. Its possible for a child to stick a paperclip in a socket and get a shock. In the UK, sockets have shutters that are lifted by the long earth pin that makes contact first.
This is just a selection.
The main thing I liked about the USA electrical system was that the faceplates of sockets and lights could be swapped without the need to disconnect the wires. The rest of it, feels 50 years behind the UK.
Tester101 addressed your "why not in the UK?" question. As to the "why in the USA?", I believe it's because wire nuts are easy to use, cheap, and occupy little space in the box.
Even an amateur can create a good splice with wire nuts. They don't need to learn to pretwist or anything: just line the wires up, jam 'em in, and twist like your life depends on it (hint: it might). Then hold the nut and give each wire a tug to make sure the nut grabbed it good. Got it wrong? Try again, no muss, no fuss. Really mangled things? Cut the ends off, strip a bit more, and try again. It's a lot more forgiving and reliable than any similarly cheap alternative.
Not sure about "why not" in UK (although Tester101 seems to have a good explanation [as always]), but there is a trend in US, at least in the DIY community to use the new side-by-side connectors
The overall connection is wider, but I find it easier to flex the bundle toward the back of the box, especially when there are three or four wires. Also, there is tactile feedback that the connection of each wire is solid and less chance that one of the wires has a loose connection as it might in a twisted bundle.
Finally, it generally needs less of a stripped lead, which is helpful if you need to snip the end for a new connection. Sometimes cutting the twisted lead leaves a pretty short wire to work with.
Building Regulations Part P in UK does not specifically ban or recommend using screw terminals in the UK. It goes to mention, that during joing wires they should be joined in the terminal of a switch/contact or to use appropriately certified equipment that is relevant to the job!
There is also an a section for old building compliance that may be encountered during renovations and they show some bad and good ways of doing certain things but do not mention anything about joining wires.
I have noticed that in every electrical installation i work with in the UK there are always block connectors used. I even took apart some old lighting made in Birmingham in 1990 that used ceramic block connectors for joining 220V to ballasts.
When I did some electrical certifications in South Africa, examples of connecting wires used plastic block connectors. But, allot of light fittings I installed; for example ceiling fans - were supplied with screw nut thingies and I just used those. But other halogen type lights had block connectors.
It is interesting though that during a short period in Poland - most of the wiring i worked with (in pre-WWII buildings) they used some sort of soviet ceramic screw caps and connectors were scarce. Today however.. You can only buy plastic block connectors and those screw things are frowned upon. You cant even buy them at the whole sellers, just eBay if you desperate.
I have never seen any regulation banning the use of those connectors- But in my own experience I ALWAYS questioned how long will that wire actually last screwed in freely like that? And i preffered to use block connectors for piece of mind!
Wire nuts are not used in the UK because the old ceramic ones were banned many years ago for good reason; the same reason outlets are not allowed in bathrooms: 240 volts! In Florida at least, code requires an outlet within 3 ft of each wash basin so that the wife can hook up a hair dryer - separate bathroom circuit on a GFCI. It is nonsense to say US wiring and electrics generally are inferior to the UK - just different systems and different codes. I had 10 inspections by the city for a bathroom refurb recently, 3 of which concerned the electrical - all subs had to be licensed. A pity the UK is not so vigilant about work done on properties there.
The reason we don't use wire nuts is because they damage the cable when you undo them. I lived & worked in the uk as an electrician for nearly 14 years.
Most electricians that are considered to be master electricians have at least 3-4 years of college education. On top of that, we do regular courses to upgrade our knowledge when the electrical regulations get revised. eg 16th edition, 17th edition etc.
On top of this, you need to do a testing & inspection course.
This is essential as anytime, a house, flat/apartment or dwelling of any kind is sold or rented/leased, the electrics have to be tested.
We do not use wire nuts as twisted copper wires weaken ever time you untwist & re twist back together again. A terminal block is considered safer & better as you loosen the screw, pull out the wires, test, reinsert & wire up again.
Rental properties which change tenants would have to be tested regularly. Wire nuts would become a hinderance. Secondly like many countries using 220-250 volts, only qualified electricians can test & certify the condition of the electrical systems.
Most electricians that I have meet in california have a very limited knowledge of testing & fault finding. They normally want to rewire the house or upgrade the fuseboard/panel rather than locate & rectify the fault.
You will not find the answer in the electrical regulation book as they are more of a guide to good electrical practice than a how to guide. The regs book gave you the european guidelines but does not tell you how to execute the work.