Getting a 110v transformer installed - basic questions

[Onoff]

23 minutes ago, Great_scot_selfbuild said:

Thanks - I should have made clear - it’s for all trades (we’re a way off from electricians having much to do on the house tbh and so it’s more for hte ring beam and timber frame build first.

 

I’d noted that the small yellow boxes have many comments advising that they’re not much use for more than a single tool in use. I’m not sure what the builder had in mind being ‘installed’ but I would guess something like these 10kVa (pic attached). I’ve not yet found detail on how these are connected?

 

Am I better off having a waterproof cabinet, consumer unit and sockets with multiple small transformers? (They’re far easier to pick up for not much ££).

 

 

 

 

I was offered two of those, for nothing on site after a project had finished. All quite legit, I was just that well in with the contractor. I dithered and the scrappy took them. 

[Great_scot_selfbuild]

On 22/08/2025 at 23:46, Adrock said:

And? I assume that's because they don't expect you to fully utilise the full rating of the transformer. You linked to a 3.3 and 5kva plug in transformer.

 

5kva at 230v gives you well over the rating of the plug top fuse, over 20A. I've also experienced them tripping 16A type C MCBs upon plugging them in over the years.

 

Anyhow, I'm being pedantic. Apologies. 

 

For the OP, you're right, stick a few metal clad sockets on a 32A RCBO and away you go.

@Adrock I’m a little puzzled by the idea that these products would be sold not to be fully used - a bit like a 4-socket extension not being able to be used for 4 plugs at the same time.

 

See my reply a few minutes ago - would you be suggesting a temporary waterproof cabinet, Consumer Unit (is that the same as ‘32A RCBO’?) and metal plug sockets, to then run multiple small yellow transformers off?

 

In my reply to @Nickfromwales I explained my challenge of where this is actually located.

[Great_scot_selfbuild]

6 minutes ago, Onoff said:

 

I was offered two of those, for nothing on site after a project had finished. All quite legit, I was just that well in with the contractor. I dithered and the scrappy took them. 

Do you have any pictures of how they’re installed by chance?

[Onoff]

1 hour ago, Great_scot_selfbuild said:

Do you have any pictures of how they’re installed by chance?

 

The bigger ones like you show usually need a 3 phase input which isn't a problem on commercial sites.

 

You can get smaller ones like this for instance that run off of a 230V single phase supply. Look at the description. You would need a kiosk, temporary or otherwise with a 5OA Type D MCB in. That then feeding a suitably sized armoured cable to power the site transformer. Cable size is basically dependant on length of run. 

[Adrock]

2 hours ago, Great_scot_selfbuild said:

@Adrock I’m a little puzzled by the idea that these products would be sold not to be fully used - a bit lit a 4-socket extension not being able to be used for 4 plugs at the same time.

 

See my reply a few minutes ago - would you be suggesting a temporary waterproof cabinet, Consumuer Unit (is that the same as ‘32A RCBO’?) and metal plug sockets, to then run multiple small yellow transformers off?

 

In my reply to @Nickfromwales I explained my challenge of where this is actually located.

 

I'd probably look at getting the big transformer you linked to if the site is going to be that heavy. It's also useful for rigging up 110v lights and such, which can be left plugged in permanently.

 

But there again I'm not really into domestic building yet, my 20 years so far have been in healthcare and bigger building sites (electrician and then project/contract managing).

 

Increasingly, in my experience anyway, 110v tools are not widely used apart from those that require a bit more oomph and even then they're being replaced by battery alternatives. 

 

Maybe provide 230v socket outlets and let the trades provide their own  small transformers. Is site lighting your responsibility? 

 

That was a bit waffley.

[SteamyTea]

27 minutes ago, Adrock said:

Increasingly, in my experience anyway, 110v tools are not widely used apart from those that require a bit more oomph

It is more to do with safety. They are also isolating transformers, so inherently safer.

The reason 110V was chosen, so I was told by an electrical engineer who trained in the 1940s, was that the equipment was cheaper as the USA had a much bigger and efficient manufacturing base, and the War Debt to them was huge, so a bit like the Ethanol agreement with Trump, we agreed to buy their equipment.

[ProDave]

Also, the 110V secondary is usually centre tapped and the centre connected to earth.  So if you did come into contact with a live wire the most it would be is 55V which is little more than a tingle.

 

By the way who actually says they are going to use a 110V transformer or indeed any mains powered tools.  I have worked on several new builds including 2 of my own, all timber frame and I don't recall much use of wired power tools.  Plenty of wired chop saws etc but they are sited away from the actual build a little, and later in the build perhaps a large drill to drill a core through a wall.  But most day to day stuff is all cordless tools now.

 

So what I am trying to say, is mains power available next to the build is probably enough.

[andyscotland]

41 minutes ago, SteamyTea said:

It is more to do with safety. They are also isolating transformers, so inherently safer.

 

 

33 minutes ago, ProDave said:

Also, the 110V secondary is usually centre tapped and the centre connected to earth.  So if you did come into contact with a live wire the most it would be is 55V which is little more than a tingle.

 

All true, although also this was more of an issue before RCD protection on 240v supplies was available/common.

 

Obviously still inherently safer if the voltage to earth can only ever be 55V, as you're already in a safe state rather than relying on a device to operate.

 

The other side benefit of 110v is that they use ceeform connectors which are more robust than 13a plug/socket and can be waterproof etc for use outdoors.

 

However you can just as easily run 240v on ceeform if you choose.

 

33 minutes ago, ProDave said:

By the way who actually says they are going to use a 110V transformer or indeed any mains powered tools.

 

I'd agree, in the past 110 was very common when your options were mains or hand power. But much more common to see battery tools now.

 

Also any trades that do have 110v tools will also almost certainly have their own standalone 110 transformer in the van. So long as there is a mains supply they can access, they can pop their own transformer there, run their own 110v cable to where they're working, and crack on.

 

So I'd suggest in the first instance:

- check which if any of your trades are going to use mains (110v or 240v) tools and if so where, and if they actually need you to provide a 110v supply.

- if people are likely to need to temporarily run power from a mains socket to somewhere else on site, perhaps invest in some brightly coloured 16A cables (this kind of thing, camping/caravanning or theatre/event suppliers will have loads of options) with adaptors to/from 13A to allow them to do so safely.

[Nickfromwales]

6 hours ago, Great_scot_selfbuild said:

Thanks @Nickfromwales we have lots of metal clad sockets for charging already and the people on site are all part of the same team at the moment. 

 

The challenge I’m trying to wrap my head around is where the builder would have this temporary connection positioned as the cable pops out of the ground exactly where it needs to rout up and into the plant room - if we install a temporary cabinet, CU and sockets there, then it’ll be right in the way. I’m wondering if a generator may still be needed if there’s plug-in tools needed until we at least have the cable up and in the footings of the house.

If you have 230v power already there, and there’s more than 1 breaker feeding them, then I’d just have all the 110v stuff plugged in as and when required. Leave the trannys outside and just run 110v leads to the point of use. 110v can be extended for miles, 230v not so much.
 

Have one tranny dedicated to lighting, and then invite trades needing 110v to arrange their own portable tranny and that’s usually the end of it.
 

The only time I see these multiple outlet, fixed 110v banks is on commercial sites. I’ve not seen one on a domestic site in ages, and last one I saw was on a huge site. These are mostly for when there’s a huge amount of demo and major construction.

 

We did a basement dig out and underpin last year and for the ground workers we just bought 2 standard portable 110v units which was suffice for the breakers, 3 storeys of temp lighting, power tools etc and all fine; we put a 230v CU in with 3x 32a RCBO’s feeding 2x 13a metal-clad double sockets each and nothing tripped afaik.

[joth]

9 hours ago, Great_scot_selfbuild said:

I’m a little puzzled by the idea that these products would be sold not to be fully used - a bit like a 4-socket extension not being able to be used for 4 plugs at the same time.

My car was sold as having a top speed of 140mph but I very rarely use that.

 

[Nickfromwales]

5 minutes ago, joth said:

very rarely

lol.

[Great_scot_selfbuild]

On 24/08/2025 at 12:39, Nickfromwales said:

If you have 230v power already there, and there’s more than 1 breaker feeding them, then I’d just have all the 110v stuff plugged in as and when required. Leave the trannys outside and just run 110v leads to the point of use. 110v can be extended for miles, 230v not so much.
 

Have one tranny dedicated to lighting, and then invite trades needing 110v to arrange their own portable tranny and that’s usually the end of it.
 

The only time I see these multiple outlet, fixed 110v banks is on commercial sites. I’ve not seen one on a domestic site in ages, and last one I saw was on a huge site. These are mostly for when there’s a huge amount of demo and major construction.

 

We did a basement dig out and underpin last year and for the ground workers we just bought 2 standard portable 110v units which was suffice for the breakers, 3 storeys of temp lighting, power tools etc and all fine; we put a 230v CU in with 3x 32a RCBO’s feeding 2x 13a metal-clad double sockets each and nothing tripped afaik.

 

We have our consumer unit at the garage (point of mains electricity arrival). I could be wrong, but I don't think we have more than one breaker as there was a discussion about multiple circuits in the garage and we decided more than one circuit there was overkill (background - if we have a chest freezer there then we didn't want it cutting out if use of tools in the garage caused the power to trip, but then again I'd know about it as I'd be in the garage and would be able to reset and sort it out, so we didn't really need two separately protected circuits).

 

See attached photo (need to get one of the CU with the cover lifted up, as I'm not sure whether there is an RCBO (would that be standard to install? it didn't come up in discussion - electrician is good and understood our needs, but ultimately I didn't know what to ask.

 

We have plenty(!) of sockets in this garage -  are you saying we could plug yellow 110v transformers in here and run them as far as we need? (from a power/current perspective, dealing with the safety of leads separately (we could ty-wrap cables to the fencing that leads up to where they may be used, as a possible option).

[Nickfromwales]

42 minutes ago, Great_scot_selfbuild said:

 

We have our consumer unit at the garage (point of mains electricity arrival). I could be wrong, but I don't think we have more than one breaker as there was a discussion about multiple circuits in the garage and we decided more than one circuit there was overkill (background - if we have a chest freezer there then we didn't want it cutting out if use of tools in the garage caused the power to trip, but then again I'd know about it as I'd be in the garage and would be able to reset and sort it out, so we didn't really need two separately protected circuits).

 

See attached photo (need to get one of the CU with the cover lifted up, as I'm not sure whether there is an RCBO (would that be standard to install? it didn't come up in discussion - electrician is good and understood our needs, but ultimately I didn't know what to ask.

 

We have plenty(!) of sockets in this garage -  are you saying we could plug yellow 110v transformers in here and run them as far as we need? (from a power/current perspective, dealing with the safety of leads separately (we could ty-wrap cables to the fencing that leads up to where they may be used, as a possible option).

How far away from there > house? 

[Great_scot_selfbuild]

1 hour ago, Nickfromwales said:

How far away from there > house? 

@Nickfromwales approx 50m from the CU, or 40m from nearest 3-pin socket. Could run a direct line over the protected tree gap and save quite a few metres though. Photo to help set the scene.

[Nickfromwales]

5 hours ago, Great_scot_selfbuild said:

@Nickfromwales approx 50m from the CU, or 40m from nearest 3-pin socket. Could run a direct line over the protected tree gap and save quite a few metres though. Photo to help set the scene.

As the crow flies plz?

[Nickfromwales]

5 hours ago, Great_scot_selfbuild said:

Photo to help set the scene.

Are you being held against your will to create modern-day dinosaurs by any chance?

[Great_scot_selfbuild]

6 hours ago, Nickfromwales said:

As the crow flies plz?

Here's a site plan view

[Nickfromwales]

11 hours ago, Great_scot_selfbuild said:

Here's a site plan view

I think you need to get a decent 230v supply over there first as last tbh. Leave 5m of slack at the house end, coiled up at your then TBS, and uncoil to move it to the room it'll reside in for eternity as the build progresses. That's just too long / adverse a run to be taking extension leads etc back/forth.

[FuerteStu]

On 22/08/2025 at 23:39, Nickfromwales said:

It's got a 13a plug though, hasn't it..... lol.😘

And this can happen when 5 people are plugged into splitters and extensions..