Cable size - 3 Phase supply - Single phase loads

[fjr1300]

Hi All

Got a 3 phase 20A per phase supply coming in, feeding 2 gites. The owner needs the 3 phase supply to also feed a new house 66 meters away. The house will only single phase loads and probably be unbalanced most of the time

Now the tables I have, show that for a single phase - the wire needs to be 25mm sq (30Amps max - 83 meters max) but i'm thinking about the neutral wire.

If each phase is limited at 20A and the single phase loads were taking 20Amps - then would the neutral current be 60Amps ?

Is my thinking correct ?

lee

 

[ipf]

If it's balanced it's zero

 

[telectrix]

sounds like 20A per phase is nowhere near enough. be struggling to feed a house alone. if the phases are balanced, there is 0 ( zero) current in the neutral.

 

[telectrix]

and, as a matter of interest , what are gites? is this wolverhampton?

 

[fjr1300]

First, gites in France are holiday homes.

Second, the loads will NOT be balanced - they are all single phase (lights/sockets/heaters/cookers ...etc)

Third, In France you are limted to the amount of power you can have 60Amps (12kw) single phase normally, 90Amps in you live near a town. Three phase is normal in the country side.
You pay a standing charge based on the max KVA you are supplied with - so its would be nice to have 60Amps per phase but the standing charge is around 800€ per year - thats without using any electric.

So most people try and live with as little as possible KVA.

lee

 

[Knobhead]

As I recall tariffs in France are based on a maximum demand of a set current, this doesn’t mean it can’t be exceeded. (The supply contract is for a maximum current, exceed this and you pay a penalty).
You have calculated a 25mm cable for the total load of three gites each taking there allotted 20A (single phase). The neutral needs to be of an equal size to the phase conductor. So a 4 core 25mm cable will be suitable.
Because it’s a 3 phase supply the neutral currents don’t add together, they cancel each other out in the neutral conductor. So basically as long as the neutral is the same size as the phase conductor you will be OK.

If you want to prove it for yourself
In=√(Ia²+Ib²+Ic²)-((Ia*Ib)+(Ia*Ic)+(Ib*Ic))

Say
Gite A takes 16A
Gite B takes 8A
Gite C takes 13A

The current in the neutral (In) will be 7A

 

[plugsandsparks]

If i understand you correctly just two phases are currently in use for the two gites and you need to add a third gite using the phase that is not being used.

If this is the case you are well out of balance at the moment so bringing in the third phase will improve the balance and hence reduce the neutral load. Problem is with all of this is that the loads are bound to vary all over the place so your neutral load equally will be up and down. Have you tried clamping it and putting on some typical loads?

 

[plugsandsparks]

Ha - Tony has done the math - as he said

 

[fjr1300]

Thanks Tony

Just to get my head around the "canceling" part, is that because one phase may be at the top of the sine wave (+), therefore phase to neutral current flows and on another phase it's on a negative part of the sine wave, current flows neutral to phase and thats the canceling in action

In France, EDF fit "Disjoncteur de Brachements" which are effectivly 500mA RCBO where they set the max current - exceed the current and it trips - all power lost so you can't overload the system

lee

 

[fjr1300]

The house is going to use all Three phases, so it's definitly going to be out of balance and probably will cause tripping problems later when the owner have people in the gites

To make things even more silly the EDF Disjoncteur de Brachements (load limiter) is in the bedroom of one of the gites - if it trips the owners have to disturb the people to reset it. - They don't want to pay the cost of a new supply being put in. - ho hum !!

lee

 

[Knobhead]

OK lets take it as if there is just two gites.
Gite A 16A
Gite B 8A

The total in the neutral would be 13.8A
So still less than the largest phase current.

 

[Knobhead]

To make things even more silly the EDF Disjoncteur de Brachements (load limiter) is in the bedroom of one of the gites - if it trips the owners have to disturb the people to reset it. - They don't want to pay the cost of a new supply being put in. - ho hum !!

lee

Eeerr yes, could prove embarasing :love:

 

[Knobhead]

Well that something else I’ve learnt to day. I knew about French maximum demand tariffs but I was under the impression you got nailed for excess with a penalty tariff.

The “Disjoncteur de Brachements” is similar to systems used around the Nottinghamshire coalfields where pit villages would be supplied by the colliery. They were taken out in the 50’s.

Must have been weired, one village could be on 240V 50Hz, another on 110V 25Hz, a bit futher down the road 100V DC.

Until last year there was a web site run by an ex MEB engineer in which he described the cange of of Mansfield to 240V 50Hz from 220V DC. He had order a new motor for the potato peeled in a town centre chippy. He got the rating wrong, the spud peeler was slinging spuds all around the shop!

 

[Engineer54]

In France, EDF fit "Disjoncteur de Brachements" which are effectively 500mA RCBO where they set the max current - exceed the current and it trips - all power lost so you can't overload the system

These units aren't RCBOS they are MCBs with adjustable trip settings incorporating an earth fault facility set at 500mA. We have exactly the same units in Cyprus...

I can't see how your limited supply of 20A per phase is going to adequately supply 2 Gites and a a new house. Time for your customer to bite the bullet and upgrade his EDF max KVA supply me thinks...lol!!

 

[fjr1300]

Going to put in a 40A/phase rated cable so that they can up the supply later.

For the house to have its own supply, EDF will start charging from 1500€ (and upwards) and the "Consuel" (Bit like a building inspector for electrical) will want to inspect the house later. - People try to avoid any Consuel inspections.
The idea behind the inspections is great, make sure everything has been installed per the regs and is safe - BUT (and it's a big but), some how a load of "wouldn't it be nice" items got added
EG.
All bedrooms must have telephone points - they've not heard of wireless phones :smilewinkgrin:

All phone points must the RJ45 type - most phones in the stores have the big French connections
Cable also has to be Cat 5 or 6 rated - 100M of old phone wire 20-30€ - Cat 5/6 out here 150-200€
(Thank god for CPC.co.uk - 300M = £60)

You must have a comms box next to the consumer unit - this has TV/phone lines coming in and RJ45 patch panel. Prices start at 160€ and got to 1000€. Great is you were building and office but not a 2 bed holiday home.

And I could go on and on especially about DCL light connectors but I won't bore you

I have perfect respect for the regs when they are there for safety reasons but when you add a load of other "but it would be nice if" under the "Safety label" it does make people start to ignore bits and some of those bits ARE there for safety

OK Rant over

Lee