Installing 3 Phase supply to new 5/6 Bed house

[davesparks]

Sorry folks
Maybe I have been to long with Pierre today, I have no calculations to back any of my posts up ,but the point I am trying to get across is that most large houses are supplied with a single phase 100a , which seems to be adequate for their needs , adding a couple of bedrooms should not add a ridiculous amount of maximum demand, divide this up over three phases and you should not exceed much more than 40a per phase on balanced ,load this is based on diversity to calculate VD ,and not the CCC of the cable
PS I Am going to bed now before I suffer the same fate as Paul the magpie

Yes 100A single phase would almost certainly be fine, but that is not the same thing as 40A three phase when the installation will be made up of single phase loads.

 

[davesparks]

Your added problem with regards to the cable size is that it will have to be based on the neutral current unless you can perfectly balance the phases. So you might end up going up two sizes anyway.

Do you have any technical backup to that statement? Because it sounds somewhat fishy on the surface of it when you consider that no imbalance situation will cause the neutral current to be higher than any of the phases.

The only time neutral current will become a problem is when third order harmonics are present

 

[Kamikaze]

Do you have any technical backup to that statement? Because it sounds somewhat fishy on the surface of it when you consider that no imbalance situation will cause the neutral current to be higher than any of the phases.

The only time neutral current will become a problem is when third order harmonics are present

I have recently returned from the pub and my statement is based on information gathering dust in the dirty crevices of my mind. I’m back at work tomorrow, so will look it up then. From memory it’s one size up for a 20% imbalance and two sizes up for a 40% imbalance.

 

[davesparks]

I have recently returned from the pub and my statement is based on information gathering dust in the dirty crevices of my mind. I’m back at work tomorrow, so will look it up then. From memory it’s one size up for a 20% imbalance and two sizes up for a 40% imbalance.

I think you are getting confused with something else, there is no such thing with basic three phase circuits.

 

[HandySparks]

I have recently returned from the pub...

Explains it all. Quit now before digging a deeper hole! :lol: :54: :6:

 

[Kamikaze]

Either way, I would be funny as hell to watch him wrestle with a 185mm into a 60-80A main switch.:biggrinjester:

 

[davesparks]

Either way, I would be funny as hell to watch him wrestle with a 185mm into a 60-80A main switch.:biggrinjester:

It's ok, he's using a hager switchfuse, everyone knows they are the best of the best when it comes to industrial switchgear and ability to terminate oversized conductors !


Methinks the op may not be used to working with the larger sizes of cable, hence the rapidly reducing design current through the thread.

 

[Martin Kerr]

A high spec 5/6 bed house (the sort of house than might be down a 100m drive) will often have a lot more appliances than typical.

I've worked recently on one such house, e.g. the kitchen has 2 built-in ovens, built-in warming drawer, built-in steamer, built-in microwave, 6-ring hob. The plant room has a couple of big water tanks with backup heaters in case the gas boiler fails. Every bedroom has an en-suite (though depending, the showers might not be electric). It needed a lot more than 100A.

Yes SJD but we are still working on diversity, any ones guess as to how many appliances are fully loaded at the one time

 

[Kamikaze]

It's ok, he's using a hager switchfuse, everyone knows they are the best of the best when it comes to industrial switchgear and ability to terminate oversized conductors !


Methinks the op may not be used to working with the larger sizes of cable, hence the rapidly reducing design current through the thread.

What I can’t understand is why they couldn’t build the house nearer to the services.:biggrin:

 

[davesparks]

What I can’t understand is why they couldn’t build the house nearer to the services.:biggrin:

Or more to the point, how it can make more sense to install this distribution circuit than to have the service installed at the house in the first place?

 

[dmelec]

Thanks for your replies.
Here's my calculations based on 80 amp 415v supply. Distance 135m. Buried in the ground
Total load 44.34Kw
Consumed Load in KVA divided by power factor 44.34/0.8 =55.42
Full load current 55.42x1000 / 1.732x 415
55,425 / 718.78 = 77.10 Amp
Ground temp correction factor 0.95
Soil correction factor 1.05
Cable depth correction factor 1
Cable group correction factor 1
Total derating factor 0.99
50mm 4 core cable current carrying capacity= 116 amps
Resistance =0.8
Reactance = 0.14
Total derating current of 50mm 116 x 0.99 = 114.84
Volt Drop
1.732 x 77.10 x (0.8 x 0.8 + 0.14 x 0.6) x 135 x 100 / 415 x 1000 = 1.95

Haven't done these sort of calculations for a while so sorry if there are any mistakes

 

[Kamikaze]

Or more to the point, how it can make more sense to install this distribution circuit than to have the service installed at the house in the first place?

It’s to make it easier for the local scrap cable recyclers to strip out the tails and leg it before the owners can make it down the drive.:angel_smile:

 

[Simlec]

I don't get your volt drop (mine is higher) but your next problem is your disconnection time with your chosen cable and fuse combi.