joining two feeds into one consumer unit.

J

jouleflow

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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
 
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parallel cables are allowed in the regs, however there are some pre requisites that need to be met.
it is worth asking a spark to quote but it may not be worth the effort and you might not save anything in the end.
 
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As above, it is possible but has significant further implications for cable protection, isolation, and selectivity between protective devices if a board fed from another board.

Probably you would be better to look at having the SWA for the highly loaded board replaced with something bigger.
 
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That is one of the many issues you may have.
the terminals at both ends will have to be suitable to accept both cables.
the cable run should be the same length, the same size and ideally the same route.

it is not quite a simple as you might imagine.
 
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Thankyou all for your help, it seems that it is not such a good idea.
Can I ask if another solution is viable.
The downstairs newish metal cu has about 16 ways. It has one 100A main isolator and two double rcd's - one 80A & the other 63A.
Can this cu be used as two individual ones if I add another 100A isolator?
So the original feed supplies one half of the board and then splice into the upstairs feed to independly supply the other half. I could then balance the loads accross the board.
 
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When you say you have 2 off swa cables that are 40A
what size are the cables?
i.e. 3 core 4 square mm?
 
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jouleflow said:
They are 3 core with an outside diameter of 20mm. I assume they are 10mm sq.
Click to expand...
If they are 10mm then they would usually be rated at least 60A (depends on thermal environment, etc), however, there may be other factors limiting the capacity such as the voltage drop if very long, or fault disconnection time depending on how it is powered (MCB or fuse, your supply earth loop impedance, etc).

It would be worth getting a professional electrician in to asses the origin supply characteristics, the cable & route, and sub-main protection, as it might be simply a case of changing the feed to a 60A switched-fuse arrangement for the board needing more power.
 
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I'm now considering 3 phase installation from the grid.

Could I use the 2 buried existing 3 core -10mm sq swa cables in combination for the 3 phase supply from the new meter/cu/fuses to a 3 phase distribution board about 6 metres away in a new outbuilding?

i.e. one cable to carry 3 live & the other to carry the neutral and earth.
 
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The requirements for conductors in parallel, which is what you're proposing here, insist that the cables must be of equal length (so that the resistance of each is the same otherwise you don't get equal current sharing). As I'm guessing this won't be the case, by the time you've bashed about making them just so, I strongly suspect it would have been quicker and cheaper to just upgrade the existing cable.
 
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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.
 
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James said:
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.
Click to expand...
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.
 
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brianmoooore said:
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.
Click to expand...
Find a cold beer, it will cool your faulty brain and make things much clearer!!
 
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jouleflow said:
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.
Click to expand...
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.
 
Last edited:
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jouleflow said:
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.
Click to expand...
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.
 
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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.
 
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jouleflow said:
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.
Click to expand...

Leave the electrician to devise the best solution. Easy.
 
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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.
 
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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?
 
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jouleflow said:
1. 3 phases only in swa, create unacceptable eddy currents and is never used.
Click to expand...
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.
 
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pc1966 said:
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.
Click to expand...
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.
 
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jouleflow said:
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.
Click to expand...
Each single-phase DB coming off it will need its own independent DP isolator (i.e. the normal main switch).

However, you would be well-advised to have a 4P isolator off the meter(s) tails so you can safely do any work further down-stream without getting the DNO/billing company involved. This sort of thing:

Product - Electrium

Product
www.electrium.co.uk www.electrium.co.uk
 
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jouleflow said:
Yes, I have discussed various parts of the whole circuit. I'm interested to see if this is possible as it will save a lot of digging of trenches and can utilise the same cables. If I have to buy another cu for a different phase that's ok.
Click to expand...

Your electrician would be better buying the equipment. Then he is responsible for. He will also probably not offer any warranty on items you have bought yourself.
 
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jouleflow

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joining two feeds into one consumer unit.
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