going 3phase from single (residential)

[JohnW2]

It's a house the circuit lengths arnt going to be that long ...

Mine's 'a house', too - but, as I said, for example, a ring final from the vicinity of the origin of the installation to the the top floor could well be of the order of 100 metres in length, hence perilously close to the limit (in terms of voltage drop, and also Ze if it were a TN system) for the 'usual' 2.5mm² cable - and with top floor (and roofspace) lighting circuits potentially up to at least 35-40 metres, with corresponding potential problems.

... it's not usual for industrial circuits to be hundreds of meters long.

The laws of physics don't suddenly change because the circuit is in an industrial setting. If the circuits were 'hundreds of metres long', one would require cables coinsiderably heftier than are used in corresponding 'regular domestic' circuits.

Kind Regards, John

 

[virgilns]

Now that makes much more sense and does explain why the larger TP boxes are so monstrously big!

Thanks for the input gents. Will take that to my spark (when I find one) and see what he says

 

[virgilns]

Also not sure 6way would be anywhere near enough mcbos?? Three floors of lights 3 floors of sockets (I'd rather keep them on separate rings for later work...) Oven(s), Garage 2 phase...and 3 phase to lathe...got to be at least 10? No shower though....

The secret is in the terminology A "6-way" 3-phase DB has 6 sets of 3-phase 'positions', each of which you can think of as 3 single-phase 'ways' for single-phase circuits. Hence, a 6-way 3-phase board has space for installing 6 x 3-phase circuits, 18 x single-phase circuits, or intermediate situations between those two extremes (e.g. 2 x 3-phase circuits plus 12 x single-phase ones).

Kind Regards, John.

Thanks, John.

In a previous post it was reccomended to use RCBOs...why RCBOs rather than a main RCD with circuits protected by MCBs? is it just so if a circuit trips due to a fault it only vuts out THAT circuit and not the rest of the house/board??

 

[ban-all-sheds]

Yup.

Also makes it less likely to trip in the first place as you have less cumulative leakage.

Xpole RCBOs seem to be getting cheaper too.

 

[virgilns]

So you just have a main isolator and run all circuits off RCBOs...

Now as the house wiring seems to be of questionable parentage, would this be a good idea...?

Install new TP dist board next to existing CU...wire garage TP & SP to this

Have power co come in and fit new TP Meter and have them connect old CU and new TP board to meter.

As each section of the house is renovated have it rewired back to the new board, distributing the load as evenly as poss across the three phases.

Once house is 'done' and all old wiring removed, get power co back in to disconnect the old CU and bin it.

 

[ban-all-sheds]

So you just have a main isolator and run all circuits off RCBOs...

Yup.

I don't know, but I'd not be surprised if you should be seated and with buttocks clenched when looking at the prices of 3P RCBOs....

Now as the house wiring seems to be of questionable parentage, would this be a good idea...?

Install new TP dist board next to existing CU...wire garage TP & SP to this

Ask your electrician, as he can see it all and you can discuss your requirements properly.

get power co back in to disconnect the old CU and bin it.

They won't do that.

 

[JohnW2]

I don't know, but I'd not be surprised if you should be seated and with buttocks clenched when looking at the prices of 3P RCBOs....

You may well be right, the saving grace being that, as far as I can make out, the OP would require only one 3-pole one (for the garage supply).

However, I do wonder whether this discussion is not perhaps getting a bit carried away with itself. It appears that there is currently a single CU fed from one phase which is adequately suppying all of the (largish, but not enormous) house, and the only new feature required is a 3-phase supply to the garage. If that's the case then (largely as per the OP's original suggestion), why not just leave the house supply as it is (updating CU and protective device if required) and simply add a feed of all 3 phases to the garage (with appropriate fuses, RCDs and MCBs {or the one 3-pole RCBO}).

The reason the discussion drifted away from that simple and 'obvious' approach seems to have been a desire to 'balance the load across phases', but I really think that it a red herring. The house supply would remain on just one phase, just as at present (and just as in the vast majority of UK residential installations), and adding a 'separate'3-phase supply to a different part of the property would not make that any less satisfactory. As for the 3-phase supply to the garage, that will only be 'as balanced across phases as it is" no matter which of the discussed arrangements is used.

So what it the real perceived advantage (if any) of moving away from the 'simple/obvious' route and running the whole house off all 3 phases (and a 3-phase board)? Am I missing something?

Kind Regards, John

 

[dhutch]

Am I missing something?

I dont think your missing a lot. Other than the fact the OP has stated in due coarse that he is going to be re-wiring the whole house.

However, aside from 10 bedroom stately homes I dont see the load from even a largeish house being much larger than that of a smaller house. They are all going to have one cooker/over/washingmachine/drier/etc and while if there is going to be a 3P supply and more then one CU in the building there is a argument for 'might as well' theres also and arguments for leavig ti well alone if its perfectly fine without.


Daniel

 

[JohnW2]

Am I missing something?

I dont think your missing a lot. Other than the fact the OP has stated in due coarse that he is going to be re-wiring the whole house. However, aside from 10 bedroom stately homes I dont see the load from even a largeish house being much larger than that of a smaller house. They are all going to have one cooker/over/washingmachine/drier/etc and while if there is going to be a 3P supply and more then one CU in the building there is a argument for 'might as well' theres also and arguments for leavig ti well alone if its perfectly fine without.

Indeed - I agree with all that. The arrangement I've described in my house at least partially results, historically, from a 'might as well approach'. Whilst the distributed sub-mains to multiple CUs does make some sense in terms of reducing final circuit lengths (and various issus of 'convenience', and perhaps cost), this is really becasue of the large physical size of the property, not the load. Given the presence of 3 phases, it makes sense on the 'might as well' basis to use them all, but I imagine that, load-wise, all the sub-mains could probably be satisfactorily fed from just one phase.

Kind Regards, John.

 

[plugwash]

For option 1, out of interest why do you consider a TP distribution box so expensive ?

It's not so much the cost of the box itself (about £80 or so from a quick look) as the cost of adjusting things to accomodate it and the fact that you pretty much have to use RCBOs since a RCD incomer would be crazy and I don't think split three-phase boards exist.

 

[virgilns]

John, you have a valid point. I don't 'need' to balance the house across three phases but I AM looking to have maybe two rings per floor.....

Also, seems like I may as well eradicate the existing 'unknown qty' CU in the process...

Yes, the tin box is around 100quid and the SP rcbos seem to be about 25quid...as opposed to mobs at a fiver...

Now could I just run the SP rcbos for the house, about 13 or 14 circuits and install a TP rcd for the TP cable, run it to the garage with either a small TP distribution board and mcbs or for the one off, a small surface mount TPN socket for the lathe (just need to get a suitably rated mcb in there...

So the house is all SP rcbos, the feed to the garage isTP rcd, and the distribution in the garage is TP mcb....

Or have I now lost the plot....

 

[dhutch]

Now could I just run the SP rcbos for the house, about 13 or 14 circuits and install a TP rcd for the TP cable, run it to the garage with either a small TP distribution board and mcbs or for the one off, a small surface mount TPN socket for the lathe (just need to get a suitably rated mcb in there...

So the house is all SP rcbos, the feed to the garage isTP rcd, and the distribution in the garage is TP mcb....

Assuming your planning to run the whole house back to the 3P distrobox that sounds about right, although I would have thought you would just have a MCB on the supply to the garage and then have the RCBO in the garage CU, but im not a spark and I might be talk rubbish.

I like the idea of having more than one CU for a large house, but for three stories its probebly the tipping point at which there no overriding reason to do that as not.

Daniel

 

[JohnW2]

John, you have a valid point. I don't 'need' to balance the house across three phases but I AM looking to have maybe two rings per floor.....

That may or may not be 'sensible'/necessary but it's the actual loading which matters, regardless of the number of circuits (you could have one circuit for each socket, if you wanted, but that, in itself, would not mean any more load). Is there a specific reason why you want two rings per floor? - 'upstairs' rings are usually only lightly loaded.

Also, seems like I may as well eradicate the existing 'unknown qty' CU in the process... Yes, the tin box is around 100quid and the SP rcbos seem to be about 25quid...as opposed to mobs at a fiver... Now could I just run the SP rcbos for the house, about 13 or 14 circuits ...

Sure, that's essentially what I was saying, BUT ... if you are going to replace the CU, you could replace it with a dual RCD one and then use (much cheaper) MCBs, rather than RCBOs. The only downside (compared with RCBOs) is that a fault on a circuit would then trip the RCD serving several circuits - but that is the arrangement that the majority of houses have.

...and install a TP rcd for the TP cable, run it to the garage with either a small TP distribution board and mcbs or for the one off, a small surface mount TPN socket for the lathe (just need to get a suitably rated mcb in there...

Essentially, yes. The only addition to that is that you would need overcurrent protection (probably a TPN switch-fuse) as well as the RCD to protect the cable to the garage - otherwise the only thing protecting it would be the supplier's service fuses. Assuming you were running armoured cable to the garage, the RCD could be at either house or garage end ...or you could use a (probably very expensive) 3-pole RCBO at the garage end.

Or have I now lost the plot....

Not at all - see above.

Kind Regards, John

 

[virgilns]

John, you have a valid point. I don't 'need' to balance the house across three phases but I AM looking to have maybe two rings per floor.....

That may or may not be 'sensible'/necessary but it's the actual loading which matters, regardless of the number of circuits (you could have one circuit for each socket, if you wanted, but that, in itself, would not mean any more load). Is there a specific reason why you want two rings per floor? - 'upstairs' rings are usually only lightly loaded.

Also, seems like I may as well eradicate the existing 'unknown qty' CU in the process... Yes, the tin box is around 100quid and the SP rcbos seem to be about 25quid...as opposed to mobs at a fiver... Now could I just run the SP rcbos for the house, about 13 or 14 circuits ...

Sure, that's essentially what I was saying, BUT ... if you are going to replace the CU, you could replace it with a dual RCD one and then use (much cheaper) MCBs, rather than RCBOs. The only downside (compared with RCBOs) is that a fault on a circuit would then trip the RCD serving several circuits - but that is the arrangement that the majority of houses have.

...and install a TP rcd for the TP cable, run it to the garage with either a small TP distribution board and mcbs or for the one off, a small surface mount TPN socket for the lathe (just need to get a suitably rated mcb in there...

Essentially, yes. The only addition to that is that you would need overcurrent protection (probably a TPN switch-fuse) as well as the RCD to protect the cable to the garage - otherwise the only thing protecting it would be the supplier's service fuses. Assuming you were running armoured cable to the garage, the RCD could be at either house or garage end ...or you could use a (probably very expensive) 3-pole RCBO at the garage end.

Or have I now lost the plot....

Not at all - see above.

Kind Regards, John[/quote
Only reason for splitting the circuits per floor is easier install and maintenance with the added advantage of only part of the floor is out should a circuit trip. Its not a load or an area issue (although one floor area would be close to the 100sq meters rule)
If i were to distribute the circuits over the 3 phases i presume its better to keep the distribution floor by floor rather than front of house back of house due to the potential of having the two phases potentially close together providing an increased risk of accidental contact with 415volts


Ah, yes...missed the overload protection for the cable...rcd surely though better at the house end of the cable as it then protects from damage to the the cable and the distribution in the garage, rather than just the distribution in the garage...

 

[Adam_151]

Ah, yes...missed the overload protection for the cable...rcd surely though better at the house end of the cable as it then protects from damage to the the cable and the distribution in the garage, rather than just the distribution in the garage... icon_eek.gif icon_eek.gif

No, dont RCD protect submains. Any thing that damages the buried SWA and comes into contact with a core will also be in contact with the armourings and will result in disconnection by the over current device