Sub Mains Query

[steviez]

Hi,

I am trying to understand the common practices that electicians use day to day on certail installations. Im in collage at the moment but my mind is thinking far ahead

If you had the following install spec how would you best distribute sub mains:

3 phase supply in to a new building (100amp per phase)
4 sub mains needed at 63amp per sub main (all three phase)
one phase would be for internal use and the remaining three for out buildings.

How would you distibute it? I have never seen it done before.

Thanks

 

[sparkticus]

Hi,

3 phase supply in to a new building (100amp per phase)
4 sub mains needed at 63amp per sub main (all three phase)
one phase would be for internal use and the remaining three for out buildings.

Quite common practice is to feed each phase & N sub-main (after the meter) into a 3Ph&N switchfuse then SWA (4 core) submain to a consumer unit for final circuit connection.

Henley blocks after the meter to allow for connections into the switch fuse boxes.


EDIT, wait did you mean single phase to each outbuilding?

If so then single phase switch fuse units > SWA 2 or 3 core > consumer unit > final circuits. One phase would need to support two sub-mains. You could try choosing the two lowest loads but not always reliable in a domestic situation. Again, you would need Henly blocks after the meter to allow for multiple tails from blocks to switch fuses.

 

[steviez]

Hi,

3 phase supply in to a new building (100amp per phase)
4 sub mains needed at 63amp per sub main (all three phase)
one phase would be for internal use and the remaining three for out buildings.

Quite common practice is to feed each phase & N sub-main (after the meter) into a 3Ph&N switchfuse then SWA (4 core) submain to a consumer unit for final circuit connection.

Henley blocks after the meter to allow for connections into the switch fuse boxes.


EDIT, wait did you mean single phase to each outbuilding?

If so then single phase switch fuse units > SWA 2 or 3 core > consumer unit > final circuits. One phase would need to support two sub-mains. You could try choosing the two lowest loads but not always reliable in a domestic situation. Again, you would need Henly blocks after the meter to allow for multiple tails from blocks to switch fuses.

Well this is only pure insight in to how the job is done so both types are good to know. I was looking in to the three phase side becuase my understanding is that a 63amp sub main via 3phase would be a far smaller cable to run that 63amp single phase. So just wanted to see how they both were done.

 

[sparkticus]

I was looking in to the three phase side becuase my understanding is that a 63amp sub main via 3phase would be a far smaller cable to run that 63amp single phase. So just wanted to see how they both were done.

Not if you are actually talking max CCC per phase. In such a case a single phase conductor supplying a load requiring say 30 amps would be no bigger than the L1 (or L2 or L3) conductor supplying 30 amps per phase. True that if you just look at the load then a machine/appliance with a total load of 30 amps could be supplied with 3 phase conductors of smaller CSA than single phase conductors assuming a balanced load. (and assuming the load could be configured to accept either single for three phase)

 

[steviez]

I was looking in to the three phase side becuase my understanding is that a 63amp sub main via 3phase would be a far smaller cable to run that 63amp single phase. So just wanted to see how they both were done.

Not if you are actually talking max CCC per phase. In such a case a single phase conductor supplying a load requiring say 30 amps would be no bigger than the L1 (or L2 or L3) conductor supplying 30 amps per phase. True that if you just look at the load then a machine/appliance with a total load of 30 amps could be supplied with 3 phase conductors of smaller CSA than single phase conductors assuming a balanced load. (and assuming the load could be configured to accept either single for three phase)

I was just thinking of 60amp total supply over the three phases so it would give you 60amp for the out building supply but distibuted over the three phases.

 

[Adam_151]

No one ever expresses it like that... so what you mean is a 20A TP supply... thats not very much and discriminatation with protective devices might be an issue, along with the fact that you'd not be able to run a ring final for sockets, etc

 

[steviez]

No one ever expresses it like that... so what you mean is a 20A TP supply... thats not very much and discriminatation with protective devices might be an issue, along with the fact that you'd not be able to run a ring final for sockets, etc

So that would be wrong? whats the way to do it then?

 

[Wontdothatagain]

How would you distibute it? I have never seen it done before.

Tails into an isolator and then into an 6 way MCCB board, SWA or approipriate cable from each device to the sub boards.

I just dont like sub mains off MCB's

 

[sparkticus]

I was just thinking of 60amp total supply over the three phases so it would give you 60amp for the out building supply but distibuted over the three not ophases.

That is more or less a 20amp three phase supply which could be useful for a 3ph 3 wire(without neutral) or 3ph 4 wire(with neutral) appliance/machine but not much use for single phase applications in the domestic world.

 

[steviez]

I was just thinking of 60amp total supply over the three phases so it would give you 60amp for the out building supply but distibuted over the three not ophases.

That is more or less a 20amp three phase supply which could be useful for a 3ph 3 wire(without neutral) or 3ph 4 wire(with neutral) appliance/machine but not much use for single phase applications in the domestic world.

So for this type of situation is it best to run a single phase supply to each out building rarther than a three phase one?

 

[sparkticus]

So for this type of situation is it best to run a single phase supply to each out building rarther than a three phase one?

If you only have single phase requirements then absolutely only a single phase sub-main. If you were installing a three phase oven in one outbuilding then it would probably make sense to run a 3Ph&N sub-main only to that outbuilding, run the oven on 3ph&N and sockets/lights from only one of the phases but all from a single consumer unit (you just configure/populate the consumer unit with a three phase MCB for the fixed 3ph oven and single phase RCBOs for the sockets & lights.

Alternatively you could simply run a separate single phase supply for the sockets/lights. It all depends upon loading, voltage drop, customer requirements etc, etc.

 

[Paul_C]

a 20A TP supply... thats not very much and discriminatation with protective devices might be an issue

As demonstrated by load-balancing issues which can arise in some Continental homes, where it's not especially unusual to find a 20 or 25A 3-phase supply for the whole house.

 

[ban-all-sheds]

Im in collage at the moment

 

[steviez]

Im in collage at the moment

Do you ever contribute anything helpful to this forum apart from your strangeness? Im in my first year!

 

[ban-all-sheds]

Yes, often.