MK Grid Plus back boxes

[sparkwright]

Who knows - but what you suggest couldn't explain there being rear knockouts in the surface boxes but not the corresponding flush ones, would it ?

Well, a surface one is perhaps more likely to need a knockout out of the back - because if the wiring is surface too, the manufacturer has no idea if conduit or trunking is to be used. The only way trunking of mixed sizes can be used successfully would be by butting the trunking up to the box, and notching the back of the trunking, and the wall, and running the cables through the back.
Alternatively, a surface box could be placed over a smaller flushed in back box perhaps.
If flush, I would say it's more likely the cables will be flush too, and the cables would generally be run on the same side of the wall as the box itself.
Who knows? Clutching at straws.
But don't forget, making knockouts cost the manufacturer money.

 

[JohnW2]

Well, a surface one is perhaps more likely to need a knockout out of the back - because if the wiring is surface too, ...

Hmmmm. I would have thought the opposite - that, because (as you say), the wiring would be most likely to be 'surface' (whether clipped, in trunking/conduit or whatever), rear cable entry would be less likely to be needed with a surface box than a flush one In fact, I'm struggling a little recall when I last (if ever!) used rear cable entry with a flush box!

Who knows? Clutching at straws.

As above, I would personally suspect that your straw was a bit leaky

But don't forget, making knockouts cost the manufacturer money.

Indeed, but that certainly would not explain them putting knockouts in the boxes I would think are the less likely to need them, but not putting knockouts into the boxes which are more likely to need them. In any event,the double flush boxes already have 8 knockouts (on 4 different surfaces), so I doubt that one or two more would have much impact on manufacturing cost!

The surface boxes are, of course, somewhat more 'complicated', hence presumably more expensive to manufacture than the flush ones, with or without rear knockouts

 

[333rocky333]

Interesting.

I find that I partially lied. Although I have some on my shelf, I have very rarely used surface grid boxes, and the one I recently used (which reminded me of this issue) was certainly a 'flush' one. Having now looked more carefully, the surface ones (lower pair in pic below) are, indeed, as per the pics in your link - with one central knockout in the rear and fixing 'slots' (rather than circular 'holes')

However, in the case of the 'flush' ones (upper pair in pic below), there are definitely no rear knockouts in mine, and there are circular fixing holes (rather than slots). So, either things have changed (it may be quite some time since I bought these) or the ones I have (marked "MK") are (imperfect!) 'fakes')!

View attachment 379964

The upper boxes are mk but actually surface boxes there mk grid old style, now obsolete.
Standard deep knockout boxes were used then and even now for flush if needed.
There metal cover plate was much slimmer then.
Back in the day they were commonly used with surface steel conduit, hence likely limited need for rear knockouts.

The lower boxes are surface boxes too, but are mk grid plus which is still current.
The one knockout now possibly for option to have back to back switches in two rooms

 

[JohnW2]

The upper boxes are mk but actually surface boxes there mk grid old style, now obsolete.

I'm getting pretty confused! TLC are currently still selling them as flush boxes for Grid Plus ...

Standard deep knockout boxes were used then and even now for flush if needed.

I think I may have been dumb all my life, since it's never occurred to me that a grid frame would fit on an 'ordinary' backbox, if that's what you mean

Back in the day they were commonly used with surface steel conduit, hence likely limited need for rear knockouts.

Indeed, I've observed above that it would probably be rare for one to want rear cable entry into a surface box (give or take the one exception you mention below).

The lower boxes are surface boxes too, but are mk grid plus which is still current.

No disagreement about that!

The one knockout now possibly for option to have back to back switches in two rooms

Yes, i suppose that is one situation in which one might want rear cable entry into a surface box - but I can't thing of (m)any others!

Kind Regards, John

 

[333rocky333]

not sure why tlc say that, when i have time will post pic of them in an old 90s mk cataloque.
Not sure the new gridplus frame would actually fit inside that smaller box, have you tried that before you flush yours in?
We rip loads of them out both single and twin as when the grid switches break on the surface ones the new grid and frame may possibly fit but due to the shape the newer mk cover obviously wont fit neatly, therefore changing the box is desirable
Flushing them in is not ideal as you are then losing the advantage of adjustable lug

 

[JohnW2]

not sure why tlc say that, when i have time will post pic of them in an old 90s mk cataloque.

This is getting very confusing! TLC are currently selling both flush and surface ones which look identical to the ones I have, as per photos in my recent post ...

Not sure the new gridplus frame would actually fit inside that smaller box, have you tried that before you flush yours in?

In what I did recently, one of the frames I have ('on my shelf') certainly fitted into one of those ('smaller') 'flush' boxes (which appears to be what TLC, and others, are currently selling. What I can't tell you is whether the frames I have are the 'new' ones - how can I tell?

Flushing them in is not ideal as you are then losing the advantage of adjustable lug

I'm not sure that I understand that. Neither the 'flush' nor 'surface' boxes (per either TLC's photos or mine) have adjustable lugs. However, that doesn't seem to matter, since the frames (both ones I have and those being offered by TLC etc.) have slot for the fixing screw on one side - so facilitating adjustment even though the boxes don't have adjustable ligs. Am I missing something?

Kind Regards, John

 

[333rocky333]

The new grid plus frames are much chunkier, my mistake I forget both frame types have adjustment, so the back box adjustment like on the Appleby flush box is less critical.
As you can see by the chunkyness of the new grid plus frame, I feel you would not get much adjustment up/ down with that inside the so called flush box, whereas the older frame would have.
Though as you say, many on line are saying that mk 891 box is flush and grid plus, the newer range.
However most mk parts now, including gridplus, have the prefix "K" before the part number, which suggests this part number existed prior to the Gridplus range even existing.

The original gridswitch had two locating dimples and fits from the rear of the frame and the metal tags push on too lock.
The newer grid plus clips into the frame from the front and much squarer looking more like a euro module
The aperture in the cover plate is also. wider on the new gridplus compared with the older Grid

 

[JohnW2]

The new grid plus frames are much chunkier, my mistake I forget both frame types have adjustment, so the back box adjustment like on the Appleby flush box is less critical. ... As you can see by the chunkyness of the new grid plus frame, I feel you would not get much adjustment up/ down with that inside the so called flush box, whereas the older frame would have.

The amount of adjustment is fairly limited (although I'm not convinced that it is a lot less than it would be with any frame/box). The photo below shows the limits of what is possible. The slots themselves would actually allow for a bit more adjustment, but it is prevented by the bent-over bits of the frame at top and bottom hitting the inside of the box.

Though as you say, many on line are saying that mk 891 box is flush and grid plus, the newer range.

What is confusing me is that if, as you seem to have implied (I must try!), the grid plus frames will fit onto a 'standard' (galvanised steel) back box, why do 'special' flush boxes for MK grids (Grid Plus or not) actually exist?

The original gridswitch had two locating dimples and fits from the rear of the frame and the metal tags push on too lock.
The newer grid plus clips into the frame from the front and much squarer looking more like a euro module ... The aperture in the cover plate is also. wider on the new gridplus compared with the older Grid

Yes, I'm familiar with those 'old ones', and I have several installed in my house. However, I installed them 35+ years ago, and I don't think I've seen them being offered for sale ('unused') for very many years. As you say, even when installed they can be identified by the fact that the apertures on the front plate, and the switch dollies themselves, are a fair bit narrower than is the case with the Grid Plus now available.

 

[333rocky333]

Yes that is confusing, until now I was not aware they were called flush, they must have been both surface and flush back in the day, however the only reason I can think of is that the larger grid banks can be 4,12, 16 and even 24 gang,I don't think any such mk flush box existed back in the day for them, therefore the only way was to flush in what I knew as a surface box, similar to what you have done, or use a rival brands box.
To keep the range uniform, mk maybe intended using the older smaller surface box as well for that purpose as it was always smaller than the white cover or the flush Albany range covers.
When I find my pre gridplus mk book maybe that will shed more light.
I quess if you flush them in straight enough then not really an issue
I have in the past machined off the frames rear lip to sguare one up.
That's the grid plus current range frame you have fitted in your pic, obviously does fit so all good.

 

[JohnW2]

Yes that is confusing, until now I was not aware they were called flush, they must have been both surface and flush back in the day ...

Are you saying that, 'back then', the only ones were the 'smaller' versions, which are seemingly now called 'flush'?

When I find my pre gridplus mk book maybe that will shed more light.

Thanks.

That's the grid plus current range frame you have fitted in your pic, .....

Thanks for convirming. Other than for those which were installed decades ago (like some of those in my house), I haven't seen frames (or boxes) other than those for very many years.

.... obviously does fit so all good.

Yes, it fits. However (now just having tried and confirmed) that frame does, indeed, also fit a standard metal back box - which leads me to wonder why anyone (including me) bought any of these (presumably more expensive) 'special' grid back boxes. Am I missing something?

Kind Regards, John

 

[JohnW2]

What is confusing me is that if, as you seem to have implied (I must try!), the grid plus frames will fit onto a 'standard' (galvanised steel) back box, why do 'special' flush boxes for MK grids (Grid Plus or not) actually exist?

..... However (now just having tried and confirmed) that frame does, indeed, also fit a standard metal back box - which leads me to wonder why anyone (including me) bought any of these (presumably more expensive) 'special' grid back boxes. Am I missing something?

Has anyone got any thoughts about this?

Having seemingly spent a 'lifetime' thinking (probably through lack of thinking!) that I needed to use ('special') 'grid boxes' for flush mounting of MK grid systems, it seems that was probably never the case. I therefore wonder whether I'm missing something (i.e. some advantage of the flush grid boxes) and, as above, wonder if there is any reason why those boxes exist?

Kind Regards, John

 

[333rocky333]

however the only reason I can think of is that the larger grid banks can be 4,12, 16 and even 24 gang,I don't think any such mk flush box existed back in the day for them, therefore the only way was to flush in what I knew as a surface box, similar to what you have done, or use a rival brands box.
To keep the range uniform, mk maybe intended using the older smaller surface box as well for that purpose as it was always smaller than the white cover or the flush Albany range covers.

 

[JohnW2]

Thanks Rocky.

Yes, I obviously read what you wrote previously, and I suppose that it might explain why the flush boxes 'existed'. However, in the case of the 2-gang and 4-gang grid blanks (which accounts for most of what one sees domestically), I don't think it really explains why anyone (except due to ignorance, like me!!) bothered to buy 'special flush grid boxes', when 'ordinary' (presumably cheaper) back boxes would seemingly have done, does it?

Kind Regards, John.