Running 2 phase in swa

[stay away from the grove]

Due to an increase in load for a subman, I now need 2 phases for an outbuilding. Original design was 63a supply, but now an electric shower and another ac unit has been added due to a design error.
Questions are, can you get a 2phase mcb to fit a schnider PoN board? Are you allowed to supply 2 phases in one cable switched by single pole mcb via an isolator with labels warning of two sources of supply? Can I use a 3 phase mcb with L3 spare? (99% sure I cant)
Submain is a 3 core, fed via isolator via 3 phase 24 way db, with L3 spare.Unless I can find a safe method to use what was the earth core as a phase, the load will trip its MCB.
The house is supplied with a 2 phase supply I managed to get architects to agree to let me utilise due to the demand for the house. Client does not seem to think its important to consult them with changes, but even if he did they don't seem to have a clue what they are doing. My manager doesn't seem to give a dam as he's leaving, and it's me that's going to be left in the lurch with no-one with the expertise or know how in the office.
Any advice except walking of site welcome!

 

[sinewove]

Due to an increase in load for a subman, I now need 2 phases for an outbuilding. Original design was 63a supply, but now an electric shower and another ac unit has been added due to a design error.
Questions are, can you get a 2phase mcb to fit a schnider PoN board? Are you allowed to supply 2 phases in one cable switched by single pole mcb via an isolator with labels warning of two sources of supply? Can I use a 3 phase mcb with L3 spare? (99% sure I cant)
Submain is a 3 core, fed via isolator via 3 phase 24 way db, with L3 spare.Unless I can find a safe method to use what was the earth core as a phase, the load will trip its MCB.
The house is supplied with a 2 phase supply I managed to get architects to agree to let me utilise due to the demand for the house. Client does not seem to think its important to consult them with changes, but even if he did they don't seem to have a clue what they are doing. My manager doesn't seem to give a dam as he's leaving, and it's me that's going to be left in the lurch with no-one with the expertise or know how in the office.
Any advice except walking of site welcome!

Assuming your supplying 2 phase and neutral supply you could use a 2 pole or 3 pole mcb mcb with L3 spare,subject to the armour being suitable for the earthing system.You would never switch 2 phases with a single pole device.

 

[Lucien Nunes]

Can I use a 3 phase mcb with L3 spare?

Yes, or a 2-pole one. As above, check your SWA is OK for earthing / EB without the 3rd core as earth, and also current rating. The 3-phase 3-wire rating will now apply as the neutral current can equal the line current.

 

[davesparks]

Questions are, can you get a 2phase mcb to fit a schnider PoN board?

Yes, you can get 1,2,3 and 4 pole MCB's for those boards.

Are you allowed to supply 2 phases in one cable switched by single pole mcb via an isolator with labels warning of two sources of supply?

That would be a bad idea.

Can I use a 3 phase mcb with L3 spare? (99% sure I cant)

Yes of course you can.

The house is supplied with a 2 phase supply I managed to get architects to agree to let me utilise due to the demand for the house.

What has the electrical installation design got to do with the architect? They draw buildings, not design electrical installations.

 

[stay away from the grove]

Yes, you can get 1,2,3 and 4 pole MCB's for those boards.


That would be a bad idea.


Yes of course you can.


What has the electrical installation design got to do with the architect? They draw buildings, not design electrical installations.

They employed an electrical architect to design the job, allocating circuits and giving me a db schedule and cable sizes. However they don't have a clue. Garage was to supply a 10mm to pavilion, and they said it was 16m to garage(existing). On inspection the armour clearly stated 10mm. So now its from the house, and after they added 1 32a AC unit they increased it to 50mm for volt drop, 70m run. I calc it at 25mm, border line 35mm. Explained I can't terminate a 50mm into a mcb so now I'm feeding an isolator in 25 or 35mm if it fits to reduce it. No idea their plans when it gets to the pavillion.....
70mm supply cable from mains which I laughed at, it only moved 7m away and said won't fit into board. Was changed to 35mm.
The house went from 1 existing 6a AC unit to 8 ranging between 10 to 20 amps. They also added 2 tesla chargers and an huge hot plate hob on an island. Added to the steam oven and oven the load became ridiculous.
AC company designed a 3 phase unit for the KW heating needed to pavilion, so had to change to 2x 32a single phase units. And as I said they now added a shower.
Jobs a complete mess with the client changing his mind or adding to it with no consultation, and my manager Is as useful as a chocolate fireguard. It's down to me to point out the problems.
Once phase 2 was opened the architect decided to alter circuits already installed to reduce circuits, to which I said its to late.

 

[stay away from the grove]

Yes, you can get 1,2,3 and 4 pole MCB's for those boards.


That would be a bad idea.


Yes of course you can.


What has the electrical installation design got to do with the architect? They draw buildings, not design electrical installations.

I'll speak to the wholesaler next week, I just wasn't sure if you could get a 2 phase mcb for that board.

 

[stay away from the grove]

Yes, or a 2-pole one. As above, check your SWA is OK for earthing / EB without the 3rd core as earth, and also current rating. The 3-phase 3-wire rating will now apply as the neutral current can equal the line current.

Yes swa is oversized 50mm which is reduced at bother ends to 25 or 35mm, depending if 35mm fits an mcb

 

[pc1966]

I'll speak to the wholesaler next week, I just wasn't sure if you could get a 2 phase mcb for that board.

You might find it is easier to get 3P MCBs off the shelf and just leave the 3rd phase unused. However, you ought to look at the overall load on the supply DB to decide which phase to leave unused so it is tolerably balanced and all phases in-spec from the supply point of view.

You absolutely cannot use the armour as anything other than CPC!

The biggest issue you are likely to have is the armour might not be enough for meeting the CPC requirements. Good news is 3C in 50mm or 70mm has enough steel to be acceptable for PME bonding (10mm copper equivalent for most supplies, as I doubt you are above 100A 3-phase so unlikely your supply PEN, if it is TN-C-S, is above 35mm) but you might have issues of meeting end of sub-main Zs especially if you are looking at a D-curve MCB for better selectivity.

From an old Prysmian sheet on SWA to BS5467 they list 50mm 3C with conductor R1 as 0.387 ohm/km but armour as R2 is 2.0 ohm/km. If you cannot meet acceptable end of sub-main Zs for the necessary supply current MCB then you are looking at putting in something like a 100mA or 300mA delay RCD at the sub-main supply end to achieve that (e.g. alternative to isolator you already have). However, that does free you to spec the supply MCB choice of current & curve for cable overload/adiabatic limit. For 50mm that is big! Thus getting something sensible in terms of fault selectivity.

 

[brianmoooore]

Is this a 3 phase supply? When I saw the title. '2 phase', I assumed what is commonly known as split phase, since 2 phase is the correct term.
Might just be me, since I'm far more familiar with 2 phase than 3.

 

[pc1966]

Is this a 3 phase supply? When I saw the title. '2 phase', I assumed what is commonly known as split phase, since 2 phase is the correct term.
Might just be me, since I'm far more familiar with 2 phase than 3.

That was my initial thought as well.

 

[pc1966]

Just a couple of other thoughts:

• You might want to limit the sub-main supply MCB to 63A C or 50A D so you have some selectivity with the (assumed) 100A supply fuses.
• You can use a normal 4P RCD for 2 phases and N, but check which phases as sometimes the self-test button diverts L1 to L2 (or L3) instead of to N, and not having that work is a definite no-no.
• Same for the DB at the end. You might chose to have two SP boards if layout works better, or you could use a 3P board with just one phase unused. While you often get "single phasing" kits to make a 3P DB in to SP, they are unlikely to be manufacturer-supported for 2+1 sort of arrangements, so safer to just have one unused and a label or notes as to why.

 

[westward10]

You might find it is easier to get 3P MCBs off the shelf and just leave the 3rd phase unused. However, you ought to look at the overall load on the supply DB to decide which phase to leave unused so it is tolerably balanced and all phases in-spec from the supply point of view.

You absolutely cannot use the armour as anything other than CPC!

The biggest issue you are likely to have is the armour might not be enough for meeting the CPC requirements. Good news is 3C in 50mm or 70mm has enough steel to be acceptable for PME bonding (10mm copper equivalent for most supplies, as I doubt you are above 100A 3-phase so unlikely your supply PEN, if it is TN-C-S, is above 35mm) but you might have issues of meeting end of sub-main Zs especially if you are looking at a D-curve MCB for better selectivity.

From an old Prysmian sheet on SWA to BS5467 they list 50mm 3C with conductor R1 as 0.387 ohm/km but armour as R2 is 2.0 ohm/km. If you cannot meet acceptable end of sub-main Zs for the necessary supply current MCB then you are looking at putting in something like a 100mA or 300mA delay RCD at the sub-main supply end to achieve that (e.g. alternative to isolator you already have). However, that does free you to spec the supply MCB choice of current & curve for cable overload/adiabatic limit. For 50mm that is big! Thus getting something sensible in terms of fault selectivity.

50.0 would only apply with rounded conductors and not sectoral.

 

[pc1966]

50.0 would only apply with rounded conductors and not sectoral.

There is quite a lot about this design, or lack of, that would need studying to be sure of the correct answer.

I based mine on the Prysmian table listing armour as 78mm which is near enough 8 * 10mm copper and, given the larger area to dissipate heat, going to be able to match a 10mm conductor on CCC for open-PEN.

Not clear if it has to, new build might be all-plastic plumbing so no need for extraneous part bonding anyway!

 

[pc1966]

For reasons best known the old data sheet seems no longer on-line. here is my copy as it is handy for info:

 

[westward10]

GN8 gives 92 for circular conductors and 78 for not circular or compacted circular, I have always considered three core 50.0 as inadequate.

 

[pc1966]

GN8 gives 92 for circular conductors and 78 for not circular or compacted circular, I have always considered three core 50.0 as inadequate.

Good point, I had not though to look in that appendix B.

It is odd though that while it covers the adiabatic factors, there is no open-PEN equivalent for selecting. I had always assumed steel as 8 times copper for conductivity so 10mm Cu ~= 80mm Fe, in which case 78mm is pretty close. A check on the specific conductivity has the 78mm armour as 91.5% of the specification for 10mm copper, so the I2R heating under prolonged overload would be just under 10% more than the copper wire. However, 50mm^2 3C SWA is around 30mm diameter so able to shed heat faster than 10mm^2 copper that is about 7mm overall diameter.

 

[pc1966]

Looking at CCC for similar conductors, say 25mm 6491X and 240mm 6491X, it seems the likes of Table 4D1A has ratings for CCC that are about 29% higher than constant I2R loss for column 2, and 34% more for column 8, suggesting that the approx 2.7 times difference in cable diameter is related to the roughly third greater ability to dissipate heat.

Not as much as I expected, but interesting to look at anyway!

 

[stay away from the grove]

Is this a 3 phase supply? When I saw the title. '2 phase', I assumed what is commonly known as split phase, since 2 phase is the correct term.
Might just be me, since I'm far more familiar with 2 phase than 3.

The supply is 2 phase from cut out fuses to meter, and meter to isolator. From isolator to DB is 4 core 35mm. 415v between phases. Its a large domestic dwelling, however the last refurb by a footballer wasn't as large as the new owners. The pavilion as its called has been added, about the size of a single floor of a house, for use as a gym and golfing simulator with a toilet/shower room.

 

[brianmoooore]

If it's a 2 phase supply, the voltage between phases is 460 volts (480 in old money).

 

[stay away from the grove]

GN8 gives 92 for circular conductors and 78 for not circular or compacted circular, I have always considered three core 50.0 as inadequate.

Whoever designed the circuit had a calculator malfunction. I calculated 25mm but only just for the load.
The installation method is buried in ground, 70m run with a max supply of 63a. Voiced concen about the size as it wont fit a mcb but was ignored, and the solution given to add an isolator, then 35mm tails to the mcb. At the other end I'm going to suggest a 3 phase db, with L3 spare like the house. As mentioned above, the db is a schnider PoN board, which was why I asked if you can get a 2 phase mcb to fit as its not something I've had to do before.
I will be passing this on to my so called manager as a possible solution. He can do the calculations, its above my pay grade, just I actually give a damn.
Just 109 lighting circuits to terminate and test now....