Electrical calcs help

[Norman Verona]

We have a 3 phase supply of 18 kw. We use the 3 phases individually.

Can someone tell me how many watts (or amps) we get from each phase?

Thanks

ps we are 220v so  calculate we have 81 amps in total.

[Speed 8]

Do I understand you correctly by stating you use each phase as a separate 220V line and you don't use the phased 380V?

[Norman Verona]

Exactly. It was like that when we moved here. There's one phase (cable) to the house, one to the garage and the third to the gites. I want to run the gites and garage of one cable and use two in the house.

[SimonT]

Power in a three phase system is (square root of three) x V x I Cos theta where:

Root 3 is about 1.73

V is volts

I is current

cos theta is your loads power factor. If this is a purely resistive load then it will be 1, if it's capacitive or inductive then it will be less than 1.

So if you had a purely resitive load in a star configuration (e.g. you have a neutral at the star) with a line voltage of 380V and the load was 100R the power would be:

phase voltage is 380v/1.73 = 220V

phase current = 220V/100R = 2.2A

Power per element is 220 x 2.2 = 484W

Total power is therfore 3 x 484W = 1.452kW

For Delta connected loads the equations are different

So, in your case assuming all phases are balanced AND it's star connected and the power factor is 1 you'll get about 27A per phase

18000/3 = 6000W

6000/220 = 27A

Hope this helps!

(Hopefully someone else can check this

[SimonT]

Oooh just thought. Be careful that the two live cables you intend to run to the house are not connected together! They are both the same potential but 120degrees out of phase to each other!

[DerekJ]

NO Norm, Don't do it!

First off the 3 phase calc is only valid if you are using 3 phases to a single source. If you are only using a single phase, then use P=I*V holds true.

But DO NOT take 2 phases into the house unless you are VERY careful. You can end up with 415V between two sockets fed from different phases. Then, if you have a neutral fault then very nasty things can happen (depending on what the fault is).

If you use one phase in one part of the house and the second phase in a second part then it may be OK when you install the wiring. But remember that several years down the line, someone can modify the circuit and nasty things can happen.  

220V hurts, but 415 hurts like b*******y (I know)

Glad Lynne is recovering.

Cheers

DerekJ

[Norman Verona]

Thanks, that's what I reckoned - 27 amps.

We have two cables in the house but one goes straight out again. I will disconnect this from the cable that goes to the gites and use it solely for the cooker which requires 32 amps. Should be OK as we'll never have two ovens, the grill and the hotplates on at the same time. The other phase can serv the lighting and sockets.

All I need do is copy the current (notice the pun) connections.

Some time next week as I've plumbing and stud walls to finish tomorrow. Saturday and Sunday are clearing up some of the mess so the plasterer can get to the walls (he tells me it'll help)

[DerekJ]

Thanks, that's what I reckoned - 27 amps.

We have two cables in the house but one goes straight out again. I will disconnect this from the cable that goes to the gites and use it solely for the cooker which requires 32 amps. Should be OK as we'll never have two ovens, the grill and the hotplates on at the same time. The other phase can serv the lighting and sockets.

All I need do is copy the current (notice the pun) connections.

Some time next week as I've plumbing and stud walls to finish tomorrow. Saturday and Sunday are clearing up some of the mess so the plasterer can get to the walls (he tells me it'll help)

So, if I remember correctly,  that's the hot wire to be connected with a bond clamp to the cold water pipe.....

[Plumber Boy]

NEVER MESS WITH ELECTRICS or GAS..unless its your trade.!!

Its not just you and your family you can harm..but those around you too.!!

Jon

[Norman Verona]

Derek, Funny you mention that. Previous owner wired the house before me. The earth was bonded to the cold copper pipe coming from the outside. But the copper pipe was connected to a plastic pipe!. I'm running an earth wire to a ground spike.

Jon, I agree which is why I've a certified plumber/gas fitter installing the boiler and the electrics have been checked by an electrician. Both are French. However I've run the pipes and cables as per French regs.