Author Topic: Light switch plate & box depth  (Read 561 times)

Mrs Pingu

  • Who ate all the pies? Me
    • Twitter
Light switch plate & box depth
« on: August 21, 2020, 05:24:38 pm »
OK, now I have painted the not-very-great hall white, the light switches don't look white anymore, so I was thinking about replacing them with metal ones.

I have a couple of stupid questions:
1. The MK ones shown here https://www.screwfix.com/c/electrical-lighting/switches-sockets/cat830530?switchessocketsfinish=brushed#category=cat830530&brand=mk&switchessocketsfinish=brushed&switchessocketsproducttype=light_switch all state they require either a 16mm or a 25mm back box. So I opened mine up and poked a tape measure in the box and it came up with 40mm. So does that mean that these are no good for me, or do the figures quoted just mean that the back box must be a minimum of 16mm or 25mm in each case?

2. One of the light switches is for a light controlled by 2 different switches, I don't need a special switch for that do I?
Do not clench. It only makes it worse.

Re: Light switch plate & box depth
« Reply #1 on: August 21, 2020, 05:30:57 pm »
1. It means minimum 16mm.  40mm will just make it so much easier to connect the wires.

2. Yes. You need one described as 2-way. The first on on your link is ideal.

ETA - all those on your link are ok. Here's one which isn't because it is described as 1-way.
https://www.screwfix.com/p/mk-logic-plus-10ax-1-gang-1-way-light-switch-white-5-pack/54564

Kim

  • Timelord
Re: Light switch plate & box depth
« Reply #2 on: August 21, 2020, 05:34:29 pm »
I'd expect back box depth to be a minimum requirement - ie. how much the gubbins sticks out at the back of the switch.  Nobody ever regretted having extra room in a backbox.

"2-way" is electrician for 'SPDT'.  Which is what you need for two switches operating one light.  (They work fine as a single switch, too - just ignore the extra terminal.)


ETA: Crosspost with Neil C
Careful, Kim. Your sarcasm's showing...

robgul

  • Cycle:End-to-End webmaster
  • . . cyclist, Cytech accredited
    • Cycle:End-to-End
Re: Light switch plate & box depth
« Reply #3 on: August 21, 2020, 05:38:27 pm »
I obviously don't know what your house looks like but I think you might find that the brushed finish looks a bit "industrial" and polished chrome looks better - at least I think it does.   If you have a B&Q nearby go and have a look in there (I know it's the same outfit as Screwfix but you can browse which you can't do at there - at least not at the one here)

It's also worth looking on ebay or Amazon as they invariably have metal/chrome switches at much cheaper for the same brands (MK or Crabtree are my preferred)

TIP:  For the two way switches when you start to swap them - before you unscrew the teminals make a diagram with the numbers of the terminals on the switch plate and the colours of wires - it's easy to get them the wrong way round so the "here and there" switching doesn't work.

Mrs Pingu

  • Who ate all the pies? Me
    • Twitter
Re: Light switch plate & box depth
« Reply #4 on: August 21, 2020, 07:09:48 pm »
Ah, that's what 2 way means, excellent :)

Next questions:
3. I know I need an earth connection for a metal switch but that should already be there, yes?
4. The switch for the bathroom is a 2 gang which operates a light and a fan seperately. I just need a bog std 2 gang for this?
Do not clench. It only makes it worse.

Re: Light switch plate & box depth
« Reply #5 on: August 21, 2020, 08:02:49 pm »
3. I know I need an earth connecting for a metal switch but that should already be there, yes?

Assuming you have a metal mounting box the earth will be connect to that and the connection to the front plate comes through the screws.

Quote
4. The switch for the bathroom is a 2 gang which operates a light and a fan seperately. I just need a bog std 2 gang for this?

Yes. Maybe. Sort of.

Fans are meant to have a maintenance isolation switch for cleaning that disconnects live and neutral. If you have a separate one of those you're good. If the previous installer used some fancy light switch that acts as a double pole isolation switch, you'll need to replicate that.

If, as is infinitely more likely, they just used a standard two gang light switch, you're not doing any harm by doing the same.

Mrs Pingu

  • Who ate all the pies? Me
    • Twitter
Re: Light switch plate & box depth
« Reply #6 on: August 21, 2020, 08:41:15 pm »
The back boxes are made of plastic.
Do not clench. It only makes it worse.

Re: Light switch plate & box depth
« Reply #7 on: August 21, 2020, 09:16:51 pm »
There will probably still be  securing screw earth continuity. But if replacement switches have a separate earth terminal you will need a secondary earth bond from the back box to switch. You are now in the 'not like for like'  replacement which should involve electricians who can test and certify the installation. I consider myself as a component person with plenty of power electrical experience. However I  employ professionals 'cos if it goes wrong later the insurance assessor will be looking for the documents.

Re: Light switch plate & box depth
« Reply #8 on: August 21, 2020, 10:19:18 pm »
The back boxes are made of plastic.

Hopefully there'll be a terminal on the switch body. If not there are various bodges.

if it goes wrong later the insurance assessor will be looking for the documents.

If it survives the fire I'll point them at the inspection certificate from 2012, which contains absolutely no details of what the installation looked like at the time...

Re: Light switch plate & box depth
« Reply #9 on: August 21, 2020, 10:26:31 pm »
Don't forget longer screws for deeper back box.
Get a bicycle. You will never regret it, if you live- Mark Twain

Feanor

  • It's mostly downhill from here.
Re: Light switch plate & box depth
« Reply #10 on: August 21, 2020, 10:37:19 pm »
OK, now I have painted the not-very-great hall white, the light switches don't look white anymore, so I was thinking about replacing them with metal ones.

I have a couple of stupid questions:
1. The MK ones shown here https://www.screwfix.com/c/electrical-lighting/switches-sockets/cat830530?switchessocketsfinish=brushed#category=cat830530&brand=mk&switchessocketsfinish=brushed&switchessocketsproducttype=light_switch all state they require either a 16mm or a 25mm back box. So I opened mine up and poked a tape measure in the box and it came up with 40mm. So does that mean that these are no good for me, or do the figures quoted just mean that the back box must be a minimum of 16mm or 25mm in each case?

2. One of the light switches is for a light controlled by 2 different switches, I don't need a special switch for that do I?


1)As others have said, the depth is a minimum, to allow for the wiring.
If the existing box is deeper, that's fine.

2)The replacement switch needs to have 2-way capability. Called various things. Most light switches are.

PM me if there is any serious doubt and I'll come around.

Re: Light switch plate & box depth
« Reply #11 on: August 21, 2020, 11:00:37 pm »
Don't forget longer screws for deeper back box.
You can just keep the existing screws and use them with the new switches.
Quote from: Kim
Paging Diver300.  Diver300 to the GSM Trimphone, please...

Re: Light switch plate & box depth
« Reply #12 on: August 22, 2020, 08:36:04 am »
The back boxes are made of plastic.

Hopefully there'll be a terminal on the switch body. If not there are various bodges.

if it goes wrong later the insurance assessor will be looking for the documents.

If it survives the fire I'll point them at the inspection certificate from 2012, which contains absolutely no details of what the installation looked like at the time...

Our consumer unit electricians certificate is so vague it could apply to anything. 

Re lighting circuits, many older ones don't have an earth because they didn't think it needed one back then.  We just had all our lighting rewired to include an earth because without we shouldn't use metal light fittings - it was a good opportunity because thanks to the building inspector we also had to have a smoke alarm in all rooms except bathroom and and kitchen.  The kitchen has a heat detector.  We may be grateful one day..
Sic transit and all that..

Re: Light switch plate & box depth
« Reply #13 on: August 22, 2020, 09:52:08 am »
Metal switches should be earthed.

You should not use the screws as the only earth.

With a plastic back box, the screws aren't connected by the back box anyhow.

I've never wired in a metal switch, and I can't find a photo online, but I can't see how they could possibly meet the rules without having an earth connection.

House wiring has had earth wires to switches and lights for the last 40 - 50 years, so it's almost certainly just a matter of joining the earth wire that's already there to the earth connection on the switch.

If either is missing, get a man in.
Quote from: Kim
Paging Diver300.  Diver300 to the GSM Trimphone, please...

Kim

  • Timelord
Re: Light switch plate & box depth
« Reply #14 on: August 22, 2020, 11:53:58 am »
I once spent half an hour installing an earth terminal on some metal wall lights, while MIL was re-decorating Dr Biggles's office as he was out of the country.  Having successfully scrounged up some materials in their well-stocked boomer garage and made the modification, I came to re-fit the lights ....and discovered that Dr Biggles had, in the absence of an earth terminal on the lights, cut the earth wire of the plastered-in twin&earth flush with the wall.   :facepalm:
Careful, Kim. Your sarcasm's showing...