When is an electrical 20mm fitting not a 20mm thread ?

[Andrew]

My house has a Coughtrey outside light that fits on the corner of the house such that the opel glass 'dome' (it points down) casts light along both walls.

It is fed by a cable that goes through the cavity and while the bayonet connector has a blue and brown wire (house built 1976) the internal light switch, connected into the downstairs lighting circuit has a pair of 1mm/sq red/black T&E cables with the neutrals joined by a choc block. The cables were obviously done while the house was built but after 1st/2nd fix but before the ceilings and downstairs partition walls were done. The cable to from the light switch run inside the cavity and the feed out to the light is 1mmT&E. Therefore there must be a joint somewhere inside the cavity somehow. I have the kitchen ceiling down so I can see where the 2 red/black cables pass through holes in the inner leaf into the cavity at the corner where the outside light.

This annoys me and because the light fitting has a 20mm? metal plug close to the Y arms of the unit and underneath I was hoping to screw a 20mm gland into this socket and run some new cable in that way.

I got a pack of 20mm glands from toolstation and took the

20mm plug from the light unit with me and they looked to be an identical thread but annoyingly they wont engage with the thread in the socket on the light.

How does todays 20mm electrical connectors (thinking of metal conduit) differ from what was used in the mid 1970's ?.

[Steve Walker]

Having done a quick google search, my first hit came up with these lights and a question about the thread. It appears to be 1/2" BSP as the person asking there tried a plumbing fitting in it.

[Andrew]

Hmm. Ok, I have big box of copper and brass plumbing bits so I will investigate. I still need?? to have a gland of some sort to make a weather proof seal, even though it is on the underside.

Did electricians use 1/2 inch bsp for conduit in 1976 ?.

[Steve Walker]

I've no idea, but it would have been something Imperial.

[charles]

It would have used an imperial conduit thread. ½ inch, possibly

[Andrew]

I've just tried an assortment of brass plumbing fittings and 1/2 inch or 15mm bsp is noticibly bigger in diameter.

The 20mm metric thread on the glands I bought earlier look identical when compared side-by-side and with threads 'engaged' when held paralle but they just wont screw in while the Coughtrey metal plug is a sloppy fit in the 20mm gland locknut, but it does hold.

[Andrew]

Even in 1976 ?. I'll have to wait for the carboot season to come around and see what I can pick up.

The alternative is a rubber bung of some sort that fits snuggly in the threaded hole and has a hole in it for the cable.

It's annoying, this coughrey light was obviously very popular, and useful because it illuminates both walls of the corner. I see them everywhere, and being cast aluminium with a baked on enamel finish they are long- lived. Unfortunately they went bust over 10 years ago.

[Andrew]

Scroll down for the picture posted by Rambo1152 on

17/dec/2019

he says his FIL fitted it, who was an *electrician*. Note the lack of a gland or indeed anything to prevent chafing where the cable enters the 20mm? threaded hole !

I wonder if he connected up the earth wire ?.

[charles]

I even installed on on our Village Hall.

[Harry Bloomfield, Esq.]

Andrew formulated the question :

Perhaps older than that, but 3/4" Imperial thread. You used to be able to get quite short bits of tube, 3/4 outside, 20mm inside and 20mm inside and 3/4 outside as adaptors during the switch.

[Tricky Dicky]

If it is 3/4? thread then perhaps one of these might be the answer

I imagine in older commercial properties there might be considerable imperial conduit which would be expensive to strip out especially if it required a small addition so I think the above is a useful way to bridge the imperial to metric.

Richard

[Harry Bloomfield, Esq.]

That's one version of them, the other one adapts the other way round. I thought that by now, they might have been discontinued.

[John]

...

Can you measure the thread pitch or at least compare it to the 20mm

If it is a 3/4 inch imperial fitting the outside diameter is 19.05 mm with a thread pitch of 16tpi

20mm metric has a pitch of 1.5 mm so about 17 tpi

The other possibility is PG13.5 thread with OD 20.5 and 18tpi

[Andrew]

It might be easier to re-cut the 20mm thread on the plastic gland to a 3/4 thread :-). Something else to consider, but I don't have a

3/4 die of a suitable thread type.

remember all I am trying to do is fit a grommet or gland to the (underside) tapped hole in the corner arm to prevent chafing of the supply cable.

[Andrew]

Holding them side by side they look identical. I don't have any way of measuring them accurately. The existing plug in the coughtrie arm is a sloppy fit in the plastic locknut of the gland, but does hold so the difference is marginal but enough prevent the plastic gland plug from screwing into the coughtrie socket.

[newshound]

In that sort of case I sometimes use something like hot melt glue or sanitary silicone to build up a sort of solid "gland". Perhaps with "not quite fitting properly" pieces to make it look neater.

[Andrew]

This could be perfect for the job, but it's in New Zealand

[Tricky Dicky]

See my post above, exactly the same is available here in the UK, both types male 3/4? to 20mm and female 3/4? to male 20mm at your local CEF and I should imagine at most electrical suppliers.

Richard

[Dave Plowman (News]

They changed from imperial to metric in the 70s (roughly). And 3/4" very difficult to tell from 20mm at a glance.

[Fredxx]

And 1/2" BSP is ~21mm

3/4" is 19mm