two-way light wiring

[SparkyTris]

How would a professional electrician go about wiring a standard "staircase" light? That is, a single ceiling luminaire independently controlled by 2 SPDT switches, one at the top and one at the bottom.

I understand the principle, but can't work out a neat way of doing the communicating wiring/lamp supply using regular 3-core cable throughout.

 

[IanDB]

Look at the "For reference" post ot the top of the forum. More diagrams there than you shake a stick at. All professional too! Could it be anything but here, I ask myself?

 

[SparkyTris]

Thanks, that's a great help. By 3-core, I meant twin and earth, obviously.

 

[Lectrician]

Ah...we refer to cables as twin+earth, or 3 core+earth.

You need 3 core for 2way switching usually, although there are some other methods that can use 2 core, although in a new domestic dwelling you will usually find 3 core+earth used for 2way.

What exactly are you doing?

 

[SparkyTris]

I am working up to a major re-wire of a country cottage. I am a marine electronic engineer to trade, so understand the domestic wiring principles, but am not experienced in domestic installations. I'd prefer to get an electrician in to do the work but simply don't have the financial resources for this.

So I'm breaking the job down into bits and posting on forums whenever I come across a wrinkle in my plans.

The cottage was originally wired for lighting from a Lister+start-o-matic in about 1940 and this circuit still remains (with a wooden fusebox)! It was clagged on to the Wylex bakelite box when the AC mains arrived in 1964. The wiring for the lights is all thread-wound insulation, now disintegrating and sparks tend to fly when you knock into a hanging light. The socket circuits are of 1964 vintage and the insulation is perishing on them. So all in all, as a MIEE, I would say that it needs a full rewire.

 

[securespark]

As Lectrician says, the usual method these days is to wire one switch as a normal one way (with t & e), then run a three-core and earth from one to the other. Assuming the old colours, at the switch with two cables, the red of the twin & earth and blue of the 3 core would go to L1 and the black of the T & E and yellow of the 3 core would go to L2. Then the red from the 3 core would go to common.

The other end is simply wired as blue L1, yellow L2, red Common.

Of course the conductors should all be marked accordingly to show their function.

However, in your existing 1960's install, it is fair to say they did not do it this way. They probably did it as follows:

The live feed comes in at the common of the switch at the bottom of the case, along with two conductors for L1 and L2. At the top of the case, the L1 and L2 conductors attach to the top switch, along with a switchwire to common.

But there are other variations I've seen, including one involving junction boxes (!), where the switch feed, switchwire and the switch conductors all meet in a jb, and then go off to their respective switch locn's. Very messy!!

 

[ban-all-sheds]

So I'm breaking the job down into bits and posting on forums whenever I come across a wrinkle in my plans.

Good luck (not meant sarcastically)!

Two problems with that:

1) You might not realise that your plan is wrinkled - there could easily be things you miss because you just don't know what you don't know.

2) You shouldn't really do the design in bits.

Also - what you do with the knowledge is entirely up to you, but you do know about the new building regs, don't you...

 

[Damocles]

So does anyone reckon it is safer to follow the new part P and not do this job for lack of cash, or just get on with it?

 

[breezer]

So does anyone reckon it is safer to follow the new part P and not do this job for lack of cash, or just get on with it?

i would recommend an electrician, with the poster as "navvy" electronics teaches you how transistors, ic's resistors work and how to wire the supply. It does not teach you wiring a house.

Call me (insert word), but there are too many "faults" being picked up with the posters questions for my liking.

Years ago (i do mean 30+) i remember an electronics expert telling me that he had found a use for cassette tape boxes, he had "rewired" a house and used a cassette box as a junction box (he put choc bloc in it) then buried half way down the wall.

 

[SparkyTris]

BAS - The way I see it, there are no new building regs. As long as wiring is done according to BS7671, and notification is provided to LABC and certification is successfully obtained then you have nothing to worry about. I think the thing that everyone is upset about is paying a jobsworth to certify. In Scotland (where I live) we have had this legislation for 25+ years I think, and we seem to manage fine.

Breezer - can you identify any particular area(s) of my knowledge (going on prev postings) that need attention?

In any case, I have a tame electrician around the area who has very kindly agreed to check my work as I'm doing it. So I'm not completely on my own. I know that there's nothing like experience, but you have to start somewhere.

[/quote]

 

[breezer]

Breezer - can you identify any particular area(s) of my knowledge (going on prev postings) that need attention?

yes

3-core cable

By 3-core, I meant twin and earth, obviously.

I am a marine electronic engineer to trade

sorry but you did ask, i am being nice about it, and yes you have to start some where , but not a rewire.

like asking some one who can drive to drive a 40 foot articulated vehicle

 

[SparkyTris]

It's no cause for concern if all that I'm getting wrong is a bit of terminology.

Or was that a joke?

 

[SparkyTris]

I can drive a 40' artic! And from experience I can say, that if you can drive, and have a bit of C.S., then you can also drive a wagon! reversing & manouevering it is the hard bit.

I know what you're getting at though. But seriously, what special laws apply to mains domestics that don't apply to any other form of electronics?

laws as in ohm's law, kirchoff's law etc.

 

[plugwash]

its mainly a matter of being aware of what the beurocrats think is safe

also ideas of things like fault currents don't tend to come up much in electronics

and in mains wiring you can't consider wires to be zero resistance.

a rewire is quite a big job but i can't really see why its difficult. Indeed imo its probablly eiaser than working on existing installs because you know exactly what you have done rather than having to figure out someone elses f*ckups.

 

[ban-all-sheds]

BAS - The way I see it, there are no new building regs. As long as wiring is done according to BS7671, and notification is provided to LABC and certification is successfully obtained then you have nothing to worry about.

Err - that's what I meant - those are the new Building Regs...

Breezer - can you identify any particular area(s) of my knowledge (going on prev postings) that need attention?

Trouble is we don't know, and I imagine you don't know, what you don't know.

You seem to be firing off random questions as they occur to you - what if you don't think to ask about something important because you have no idea that it's important? For example (and apologies if I've missed you asking, or if you've not asked because you do know) - the type of supply you have makes a fundamental difference to the type of CU you have, and the sizes of main bonding conductors. It's quite conceivable for you to have no idea of that, and never realise you should look into it. It's like a lot of things - it isn't rocket science, the difference betwen types of supplies is easily understood, and the reasons for the different requirements that arise are easily understood, and they are easy to get right when you do the install. But you have to know that it's a topic you need to know about in the first place

also ideas of things like fault currents don't tend to come up much in electronics

and in mains wiring you can't consider wires to be zero resistance.

And heat-loss calculations, thermal factors, grouping factors...

The point that plugwash makes about resistance is a good example. I have an electronics (hobbyist) background, 'O' and 'A' level physics, and I knew all about Ohm's Law, Kirchoff's Law, Faraday's Law, Lenz's Law, resistance, capacitance, inductance, reactance, impedance yadda yadda yadda, but it wasn't until I started learning about electrical installations that I found out why borrowed neutrals were a dodgy idea, or that thick copper cables had such a high resistance that the lengths in an ordinary house could pose problems. I knew why there was an earth conductor and what MCBs did, but I didn't know about disconnection times, and why EFLI was so important, and why TT supplies needed RCDs.

The fact that you've got an electrician to help you is excellent news - pay him for design consultancy and testing and inspection, and you do all the hard graft. That way you'll both be happy, you can learn and he'll be able to sign off the installation from a supervisory capacity with a clear conscience. Involve LABC early enough and tell them what you are doing, and they might not charge you a whacking fee, unless you're confident of finishing before April, in which case you don't need to involve them at all.