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!

 

[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....

 

[Lucien Nunes]

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.

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

The implication from the OP is that he has two out of three lines of a 3-phase supply, hence the 400 / 415V between phases. This is not unheard of in areas that have 3-phase 4-wire street mains but 3-core service cables from the 3-wire DC days. We have it in our street; two lines, two cutouts and at one time, two meters, because when the area changed over to AC the distributors were relaid in 4-core but the service cables were not.

This is why I prefer to call 230-0-230 (460) by the name 'split phase' instead of two-phase, to avoid the ambiguity. What the OP appears to have, and what I have in my house, I call 'two-out-of-three phase' to indicate that they are 120° apart and there is a third phase in the supply network that is not present at the origin.

IMO and strictly speaking, 2-phase means a system where the phases are 90° apart, which was used in the early days of polyphase AC instead of 3-phase. Both systems offered a rotating field vector so induction motors were self-starting and had no torque-zeroes, but 3-phase was more economical of copper and became the preferred system. I believe that Croydon still has some 2-phase 90° distributors serving single-phase customers, using the old triple-concentric cables. They are fed from Scott-connected transformers in the substations to spread the load across a 3-phase HV supply.

FWIW my very first post on this forum about 15 years ago was on these definitions. Someone was saying that a single-phase welder with a 400V input was '2-phase' when what they meant was that it connected between two lines of a 3-phase supply to get the 400V and confusion reigned.

 

[stay away from the grove]

The implication from the OP is that he has two out of three lines of a 3-phase supply, hence the 400 / 415V between phases. This is not unheard of in areas that have 3-phase 4-wire street mains but 3-core service cables from the 3-wire DC days. We have it in our street; two lines, two cutouts and at one time, two meters, because when the area changed over to AC the distributors were relaid in 4-core but the service cables were not.

This is why I prefer to call 230-0-230 (460) by the name 'split phase' instead of two-phase, to avoid the ambiguity. What the OP appears to have, and what I have in my house, I call 'two-out-of-three phase' to indicate that they are 120° apart and there is a third phase in the supply network that is not present at the origin.

IMO and strictly speaking, 2-phase means a system where the phases are 90° apart, which was used in the early days of polyphase AC instead of 3-phase. Both systems offered a rotating field vector so induction motors were self-starting and had no torque-zeroes, but 3-phase was more economical of copper and became the preferred system. I believe that Croydon still has some 2-phase 90° distributors serving single-phase customers, using the old triple-concentric cables. They are fed from Scott-connected transformers in the substations to spread the load across a 3-phase HV supply.

FWIW my very first post on this forum about 15 years ago was on these definitions. Someone was saying that a single-phase welder with a 400V input was '2-phase' when what they meant was that it connected between two lines of a 3-phase supply to get the 400V and confusion reigned.

What you said is what the property has, 2 out of 3 lines entering the property. As an installation electrician I've never came across this before, therefore apologies for the wrong terminology. So from now on it's best to call this a 2 line supply?

 

[Lucien Nunes]

I'm just being pedantic, ignore me. Two phases out of a three phase supply. Or two-phase, no-one thinks about 'real' 2-phase any moreunless they work for the DNO in Croydon (if that's still in use).

 

[brianmoooore]

no-one thinks about 'real' 2-phase any moreunless they work for the DNO in Croydon

Or in my village and the surrounding area.