Convert 3 phase into 3 single phases

[Mr Shaun Roche]

Hi there,
I have currently started working on a farm and they are having a new 100a 3 phase supply installed.
This is being terminated in an external enclosure. They then want to carry this 3 phase 150 meters to a barn and at this point split it into 3 single phase supplies. Is this even possible?
Surly the return on the neutral would have to be capable of carrying 300a if they are split to 3 seperate phases there would be the potential for 300a to flow back down the neutral. I know in 3 phase balanced loads in theory there should be nothing flowing back down the neutral but this can't be the case if they are then split to 3 single phases.
Just after some theory to fill in a blank space iv never had explained to me. Thankyou

 

[Strima]

If you balance the loads on three phase they cancel each other out on the neutral.

For example of you have two phases loaded at 3Kw and one loaded at 1Kw potentially you only have 8.5 amps on the neutral. This is a very rough and simplified example.

This is why we try to balance single phase loads on three phases as much as possible, also the DNO like their phases to be balanced for generation purposes.

No doubt one of our more experienced and eloquent members will explain in fully shortly.

 

[Mr Shaun Roche]

Thankyou Strima, I was thinking once they have taken say each phase and add a neutral to it. For instance say 25mm tails live and neutral x3 to get 3 single phase systems out of the 3 phase each at 100 amp. Surly the return on the 3 phase neutral needs to carry that 300 amps as on single phase the neutral is relied upon to return what is taken. I think I'm waffling and mincing my words but hope someone understands it

 

[Sintra]

In your scenario the highest neutral current possible is 100A. That’s if 1 phase is fully loaded ant the other 2 have no load. Add a load to the other 2 phases and the neutral current reduces.

 

[Mr Shaun Roche]

Thankyou Sintra, so the phase loads would still cancel each other out (assuming their balanced) even though the 3 phase has been split to three single phases.

 

[davesparks]

They then want to carry this 3 phase 150 meters to a barn and at this point split it into 3 single phase supplies. Is this even possible?

That's going to be a pretty big cable, or probably 2 cables if you've got any sense!
What size cable have your calculations come up with?

 

[Mr Shaun Roche]

Iv come up with 50mm cable for it in the sense that I haven't taken into account that 300a may pass down the neutral although from what I'm being told this is not the case. 3P&N so 4 core swa buried in ducting at 600mm, no other cables with it, distance 150m. Had to up the size of cable by one due to volt drop.

 

[davesparks]

Iv come up with 50mm cable for it in the sense that I haven't taken into account that 300a may pass down the neutral although from what I'm being told this is not the case. 3P&N so 4 core swa buried in ducting at 600mm, no other cables with it, distance 150m. Had to up the size of cable by one due to volt drop.

Are you sure? By my calculation that gives a volt drop of around 14V, which is way too much for a distribution circuit!

300A won't pass down the neutral! I think you may need to get someone with a bit more knowledge and experience in designing this kind of installation to help before you end up making a very expensive mistake!

At a rough estimate, assuming the final circuits connected to this are all quite short and include lighting 150mm will give around 2% volt drop.
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Are you sure? By my calculation that gives a volt drop of around 14V, which is way too much for a distribution circuit!

300A won't pass down the neutral! I think you may need to get someone with a bit more knowledge and experience in designing this kind of installation to help before you end up making a very expensive mistake!

 

[Let me think]

Thankyou Sintra, so the phase loads would still cancel each other out (assuming their balanced) even though the 3 phase has been split to three single phases.

If you haven't looked at this stuff in while, then it can take sometime to get your head back around it. If don't use it regular enough, then it tends to get a little murky...

Worth having a look at https://www.electronics-tutorials.ws/accircuits/ac-waveform.html to get back upto speed on the theory. Many other great site out there, if you don't want to buy/dig-out the books

BTW I'm in the process of putting together an aid-memoire for 3-phase i.e. basic theory revision sheet with key formula sheet - I've skip out how formulas are actually derived, as we tend to be more interest in how to use the formulas rather than where they came from, plus it takes ages to type up the steps. Just let me know, if you would like a copy (when I'm down). Also happy to share with you what I have at the moment.

 

[Mr Shaun Roche]

Thankyou Let me Think that would be very helpful.
Dave sparks I went away and did cable calc on the spot. I'm not completing this work anyway. I may over look and ask questions when they do, do it. I was more intrigued as to how you go about using 3 phase as three single phase submains and what they entails and in the back of my head I had the stupid notion that 300a would have to pass down the neutral in this situation. Still learning and generally only do domestic single phase at the moment but want to learn more.

 

[Lucien Nunes]

It's no different to taking single-phase final circuits from a 3-phase DB, or single-phase house service cables being tapped off a 3-phase distribution cable in the street. The neutral cable from each single-phase submain can contribute up to the full line current into the shared neutral junction, but because the three submain currents are out of phase, they can't all do so in the same direction at the same time. One might be sending a positive current into the junction while another is sending a negative current, cancelling or partly cancelling the first current out. When the loads are equal the neutral current in the 3-phase supply cable is zero; when one or two phases are fully loaded and the other(s) unloaded, the neutral current is at its maximum and equal to the line current.

Distribution cables sometimes have a reduced neutral, taking into account that a large number of small loads are never going to be completely unbalanced, therefore the neutral current will never be as high as any one line current. There's one exception and that is for loads that generate heavy zero-sequence harmonics e.g. 3rd harmonic. The 3rd harmonic currents in all three lines are in phase with each other, so they do add in the neutral rather than cancel out.

Mathematically speaking, the three single-phase currents do add, but they add vectorially, not algebraically. Spend an hour learning to use phasor diagrams and all will become clear!

 

[pc1966]

Iv come up with 50mm cable for it in the sense that I haven't taken into account that 300a may pass down the neutral although from what I'm being told this is not the case. 3P&N so 4 core swa buried in ducting at 600mm, no other cables with it, distance 150m. Had to up the size of cable by one due to volt drop.

Check your Zs resulting from that and supply protection times, etc, as the armour will be much poorer as a conductor and you might need to run something like an additional 16mm copper earth in parallel. Or go 5-core but that will cost significantly more.

 

[Mr Shaun Roche]

Thankyou Lucien ill look more into Phasor diagrams as I have touched on them qualification route wise but I find they try to cover these things in one lesson and then want to move on. I'd prefer to spend a quick trying out different scenarios etc. That's one for me to do in my free time. And harmonics is defiantly something I need to read up on.

Hi pc I know currently the single phase system supplying the house is earthed through rods at each consumer unit and I'm pretty sure UKPN specified for a rod to be installed next to where they want to install the 3 phase head. I think I remember the spark doing it mentioning each building will have an earth rod and submains were going to be run earthless underground, dnt qoute me I was catching info he was passing to the land owner.

 

[Let me think]

Thankyou Let me Think that would be very helpful.
Dave sparks I went away and did cable calc on the spot. I'm not completing this work anyway. I may over look and ask questions when they do, do it. I was more intrigued as to how you go about using 3 phase as three single phase submains and what they entails and in the back of my head I had the stupid notion that 300a would have to pass down the neutral in this situation. Still learning and generally only do domestic single phase at the moment but want to learn more.

Work in progress attached. Worts and all....Used old colour scheme for phases as looks better in diagrams

 

[davesparks]

. I think I remember the spark doing it mentioning each building will have an earth rod and submains were going to be run earthless underground, dnt qoute me I was catching info he was passing to the land owner.

They won't be run 'earthless' and if they are they will be extremely dangerous.
They may however have a seperate TT earthing system for each building, but each submain must still have a suitable CPC.