joining two feeds into one consumer unit.

[jouleflow]

I have two 40A swa cables from my main house consumer unit. This consumer unit serves the whole house and the two swa cables serve two consumer units in an extension. One is upstairs and has very little demand and one is downstairs with a higher demand.
My question is:
Can I add the upstairs swa into the downstairs consumer unit allowing more demand and an appropriate t+e cable back to feed the upstairs cu.
Basically I want to join the two feeds together either in the downstairs cu or another way.

This will be done by a qualified sparky but I just want to know if it is feasable before I hire him.

Thanks

Greg

 

[brianmoooore]

Yes, of course, eddy currents.
My excuse is that I've been working all day up in the air in my cherry picker, with the sun beating down. Brain more than a little frazzled.

 

[jouleflow]

Worst case is that one phase is fully loaded and others are carrying no current

so you now have 2 steel armoured cables carrying the current in one direction each.
the eddy currents will be horrible, it is for this reason that you can’t get single core swa cable.
edit,
single core aluminium wire armour is available and is ok because there is no inductive coupling.

Could this happen anyway with 4 core?
Only one cable will be carrying current in one direction with the other cable carrying the neutral and earth the opposite direction.

 

[James]

Yes, of course, eddy currents.
My excuse is that I've been working all day up in the air in my cherry picker, with the sun beating down. Brain more than a little frazzled.

Find a cold beer, it will cool your faulty brain and make things much clearer!!

 

[James]

Could this happen anyway with 4 core?
Only one cable will be carrying current in one direction with the other cable carrying the neutral and earth the opposite direction.

In a 4 core you would have 3 phases and a N conductor.
the current will balance out to be equal.
outer armour provides the earth and there should be no current or just a negligible amount.

 

[jouleflow]

Will I need 5 core swa for 3 phase? or is the armour always used for the earth?

 

[DPG]

Will I need 5 core swa for 3 phase? or is the armour always used for the earth?

I think you really need to get an electrician in for this work.

 

[Electro-tech]

I have two 40A swa cables from my main house consumer unit. This consumer unit serves the whole house and the two swa cables serve two consumer units in an extension. One is upstairs and has very little demand and one is downstairs with a higher demand.

I agree with DPG, you obviously have sufficient funds to contemplate a 3 Phase DNO install & employ an electrician, so without any further information on demand over 3 Consumer Units including the Main Unit I think you need professional help.

 

[jouleflow]

I have an Electrician ready to soon start the install before the 3 phase is connected. I am just trying to discover which solutions I can suggest & he can utilize. Sketching a several proposals will give the information of what is installed and what is required as a client.
I don't intend to do any work myself.

 

[DPG]

I have an Electrician ready to soon start the install before the 3 phase is connected. I am just trying to discover which solutions I can suggest & he can utilize. Sketching a several proposals will give the information of what is installed and what is required as a client.
I don't intend to do any work myself.

Leave the electrician to devise the best solution. Easy.

 

[jouleflow]

How will I know if it will be the best solution for me, unless I do some research.
I've 3 solar inverters to also consider integrating lol.

 

[DPG]

How will I know if it will be the best solution for me, unless I do some research.
I've 3 solar inverters to also consider integrating lol.

Research - yes. But asking what specific type of cables you need, that is down to the sparky doing the work.

 

[jouleflow]

Thank you for all your replies and patience. I am probably a bit too inquisitive, possibly down to 50+ years as an engineer.

All the decisions will be made by the electrician and all of which I will respect.

The main query I still can't understand is the theory of using my two buried existing 3 core swa cables to supply the 3 phase from the new 3 phase meter to the new distribution board. One swa containing the 3 lives and the other containing the neutral. My understanding is although all the 3 live wires will be the same length and in the same direction, it is still not possible.

Which of the scenarios below prevent one swa for lives and the other for neutral, assuming the rating is within spec.

1. 3 phases only in swa, create unacceptable eddy currents and is never used.
2. 3 phases + neutral in swa, don't create unacceptable eddy currents.
3. 3 phases only in swa is ok but only if the load over all 3 is always balanced.
4. 3 phases only in swa will tolerate a small imbalance.
5. The neutral wire can't be used alone in swa.
6. The neutral wire needs to be encased with the lives to provide a current in the opposite direction to work.
7. The neutral wire needs to be the same length as the live wires.

Thanks in advance for any advice.

 

[Electro-tech]

I think maybe the answer is based on how the SWA cables are correctly terminated into each Consumer Unit, I assume All metal C/U's. Also, what you are asking is a bit unconventional, to use the 2 existing SWA cables as 3phase you will need to be able to isolate together the main live conductors, which means line & neutral conductors. I would like to see how you achieve this. Makes nonsense of cable resistances too. Is there a way that you can rewire a circuit or circuits that are causing the high loads & connect to the less loaded C/ Unit?

 

[pc1966]

1. 3 phases only in swa, create unacceptable eddy currents and is never used.

It is used for loads without a neutral (like your #3 case).

So if you had a delta-star transformer to generate your L1-3 & N arrangement in the far building it would be acceptable, but that would cost you far more than just putting in a new cable!

You do see separate L and N cables but they are normally alluminium armour to avoid the eddy current losses (and higher fault impedance which can limit disconnection times on over current protection alone).

If you can't get enough single-phase on the existing cable then faffing about with various ways to work around it are usually a wast of time as they are either not in keeping with one or more regulation, or they end up far more expensive than the simple and obvious approach.

If you do put new cable in then put it in some twinwall duct - or two. Then you can replace it later if you need to! Also it would allow you to put in a fibre network cable as well so you have as much bandwidth as you could ask for in the future.

 

[jouleflow]

It is used for loads without a neutral (like your #3 case).

So if you had a delta-star transformer to generate your L1-3 & N arrangement in the far building it would be acceptable, but that would cost you far more than just putting in a new cable!

You do see separate L and N cables but they are normally alluminium armour to avoid the eddy current losses (and higher fault impedance which can limit disconnection times on over current protection alone).

If you can't get enough single-phase on the existing cable then faffing about with various ways to work around it are usually a wast of time as they are either not in keeping with one or more regulation, or they end up far more expensive than the simple and obvious approach.

If you do put new cable in then put it in some twinwall duct - or two. Then you can replace it later if you need to! Also it would allow you to put in a fibre network cable as well so you have as much bandwidth as you could ask for in the future.

Thanks pc.

The 3 phase supply will now be directly from the grid.
It looks like each phase isolator will also have to isolate the neutral. I guess the neutral could be wired in series over the 3 isolators to isolate it by any switch? I expect that will be too simple.