Steel Conduit - How to Install ?

Question

Looking around the internet I've found very little to help me understand how electrical steel conduit is installed.....but I'm sure someone on Element14 knows the answers

The question relates to the 20/25mm galvanised steel conduit often used in the UK - this comes as 3m lengths with a external make thread on one end and a female coupler on the other. Unless the fittings are 3m apart the conduit requires cutting and a new thread to be formed, which I already have tools for. The outlets/fixings also come with pre-cut threads; and example of the 90 degree box is shown below:

Whilst I can envisage fixing one of the box outlets to the required position and screwing in a piece of cut/pre-threaded conduit I wonder how the next fitting is added. Assuming that is a cut end which I have added a thread to this second fixing would need rotating onto the conduit. Because the first fixing is on the wall there would be no clearance to rotate the second fixing without putting strain on the existing system, this is especially true when the next fitting has additional bosses on it or as the system design grows.

Maybe the entire system is designed and then mounted to the wall ? I would imagine that could result in a very heavy setup plus inaccuracies in getting the fittings where required.

Any pointers would be gratefully read, thank you

genebren · Answer

This seems like a tricky installation. In the US, none of the conduit is threaded. Here we use slip on compression fittings (with simple spin on nuts inside the fixture) to attach the conduit to fixtures. What you outlined sounds doable, but this video shows some techniques that might make this a little easier https://www.youtube.com/watch?v=I-sIJ7Gznbk Best of luck!

beacon_dave · Answer

When it gets tricky you can use 'running couplers' to join two sections together. Overcut the thread long enough so you can screw the entire coupler (plus lock nut) onto it then offer it up and wind the coupler back half-way onto the thread you want to join to and finally tighten the locknuts.

shabaz · Answer

Hi Rod, According to sibling, there should be saddles holding it off the wall, and apparently, that provides sufficient clearance to screw on those fittings. I may have got it wrong in my explanation to him though!

chunkasmaximus · Answer

Thanks! I'm still very new so looking around and finding my way, lots of interesting stuff on here! It seems I may have finally found a place where I can 'talk shop' to my hearts content! I know what you mean about the drain hole, and no matter how well things are sealed, at some point and more often than not, you will find some degree of condensation. I put it down to two choices: 1. Add a drain hole, and hope insects and spiders don't find it and stay away, or... 2. Let it potentially build up with moisture and water... I'll take the dry stuff every time! Having said that, I suppose it depends on what is inside the enclosure that needs a drain hole. If it's just a right angled terminal box (like your picture in your post) or similar, I wouldn't bother. If it's an enclosure for some control gear etc, then I would be very careful about environmental factors - like sealing the screw holes, perhaps silicone along the top edge and sides, drain hole etc etc. Whether there is a drain hole or not, it ultimately comes down to this: maintenance. All electrical systems should be maintained and accessible where appropriate. So if there's a chance something could be damaged by possible water vapour or other external influences, check it every so often! You can't beat a good old nose around every once in a while to make sure it's in good working order

chunkasmaximus · Answer

As with everything in life Rod H, practice makes perfect! Donald Lane's answer pretty much sums it up. You just have to work methodically and in one direction - don't try starting at two different points and meeting in the middle! I've not looked at what advice or videos are available, but as an electrician who has used the stuff on and off since college, the best advice I can sum up is the following (in no particular order): 1. Practice and experiment joining different sections of conduit, boxes, accessories etc in small sections (using off cuts etc), then dismantle and reassemble. This will enable you to get used to doing things in the most efficient order. There's nothing worse than getting to a point, only to realise you have to undo it all. This will also help you with the planning of slightly larger scale installs, so you'll be less reliant on others for help. 2. Use all the available fittings for your conduit system to make your life easier. Like already suggested in other comments, use fittings like running couplers, nipples, lock rings, bushes and the like. This will give you more flexibility and make assembly a little easier. 3. Make the conduit bender your best buddy. A floor standing one is ideal, with a built-in vice if you can. It will be painful and it will take time but, the more you use it the less joints you'll have, the easier it will be to pull cables, and the neater it will look. It really is an art form, and if you plan to work with steel conduit long term, this is definitely a worthwhile investment. 4. Use the right saddles to give you clearance from the wall. There will be enough space to get your grips, spanners etc around your workpiece and get it fitted. As long as the piece you're attaching to is securely fixed, you would be hard pushed to do any damage - it's steel! 5. Lots of accessories, boxes, equipment etc can be connected to conduit systems, but don't worry if it's not obvious or straightforward at first. You can almost always adapt most things to suit, so don't be shy to make new holes or add conduit adaptors or the like. 6. Experiment with it. There is no hard and fast rule to how you should install or use it. As long as the system is correctly supported throughout its length, and cable entries are protected with bushes/glands, design it how you see fit. And finally, wear gloves. Burrs and metal splinters in soft skin are no laughing matter! Use cutting compound for threads (if you don't already), and a good round or half round file is essential too. Hope this helps.

three-phase · Answer

The PVC is cheaper and does not require the same amount of tooling as steel conduit but it isn't as robust mechanically. It is generally easier to assemble as it has quite a bit more flexibility, and is all push fit instead of threading everything together. It is important to glue joints together that could be pulled apart. i.e. if you have a straight section of PVC conduit between two boxes that are screwed to the wall, then the joints cannot separate. If the conduit has a right angle in it, then the joints can be pulled apart, even when secured with the saddles. In this case it must be glued, which isn't a problem, it is just that if it ever needed alterations, then it gets cut off and discarded, whereas the steel conduit can be unscrewed and reused. One thing to be aware of with PVC conduit, it that if installed outside in sunlight, it will expand and contract and if extra saddles aren't used, it can end up twisting or warping between the saddles, horizontal sections can sag under these conditions. If you have a lot to do, then working with steel conduit is well worth learning and there is a lot of satisfaction in the end result, even if there is a bit of frustration in getting there. If it is a one off job, then PVC would be more cost effective. The only other deciding factor is the mechanical robustness required. If the installation is in areas where there is risk of mechanical damage, then steel is the way to go. Do remember though that steel conduit is extraneous metal work and therefore must be earthed. Kind regards

three-phase · Answer

You can get conduit back boxes that have an outlet on the back and usually one on the side, but it is quite rare that they are used. You have to drill an oversize hole in the wall, usually 25mm for a 20mm conduit to be able to fit them flush. Below is a link to one example;

https://www.ukelectricalsupplies.com/norslo-conduit-box-prem-terminal-and-back-outlet-galv-20mm.htm

What most end up doing is as described by Dave Ingles. You would drill a 16mm hole through the wall, or 10mm if the cable is small enough to go through and then a larger 20mm hole through the back of a standard conduit box. Just make sure that the hole in the conduit box is de-burred thoroughly and then centred over the hole in the wall. Filling the hole in the wall with some silicone sealant will prevent water ingress and help keep the cable centralised.

If you are using T&E then you already have an extra layer of protection from the outer sheath of the cable as well as the inner insulation of the actual cores. Pulling T&E through conduit can be hard work if there are a lot of bends to go through.

You can get square galvanised adaptable boxes as well, some with knockouts and some you drill to suit your application with step cutters or hole saws, they probably don't look quite as nice as a round conduit box and are notoriously hard to seal up and stop water ingress.

Kind regards

chunkasmaximus · Answer

Hi Rod H,

There are no knock-outs on those boxes. If you want or need 'rear-entry' then you need to make a hole and fit a male bush into a female coupling - much like any other conduit termination you would do onto an enclosure for example. The bush goes inside the terminal (circular) box with the male thread protruding out the back, you then screw on your female coupling, and add your piece of conduit as normal.

If you can't get conduit to go through the wall for whatever reason, then you can omit this, and instead install a female bush over the top of your male bush and then run your cable through. You may need a lockring to go over the male bush in order to secure it before adding the female bush.

The female bush has a smooth inner edge and hence won't chaff your cable.

Hope this helps. Sometimes the image in my head doesn't translate to words on a screen!

chunkasmaximus · Answer

I know what you mean about the hole in the wall..

You would struggle to get a grommet on there, the metal is usually a bit too thick for the grommet to sit properly. You could try something like grommet strip (https://www.cablecraft.co.uk/halogen-free-black-grommet-strip-12742 ) which tends to be more flexible, but again I think the metal thickness may not allow it to sit properly.

Making the hole in the box shouldn't be too difficult, the material tends to be soft enough to use with step drill bits or hole saws. Cutting fluid will certainly help. If you struggle to do it, it may help to fix it to some secure scrap wood, then drill through it. You should end up with a nice clean hole.

You can then use a short male bush with a female bush to keep the rear protrusion minimal. You would probably only need to chip away a very small amount of brick/mortar to allow flush mounting of the box.

beacon_dave · Answer

If the hole in the box is oversized and centred on the cable entry when fitted, then the cable won't be able to touch the 'sharp edge' (which you have already carefully deburred of course...)

If there is enough gap between the cable and wall you could perhaps consider using a rubber sleeve bushing instead of a normal grommet to allow a flush mount. Something of the style: Product LinkProduct Link

You could also perhaps cut a grommet in half or use a nylon bush and bond it inside the box to keep it in place, or press in a soft brass grommet which would dress the edge of the hole with minimal protrusion to the rear.

Not sure wat the reg's have to say though.

Steel Conduit - How to Install ?

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