3 Phase calculations.

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juniorspark

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During my time at college we covered three phase theory although not a great deal in circuit design, most of my apprentiship has been working on domestic rewires and feel im lacking in this area a little (installation side is fine but the working out side has always been done for me)

I use a chart which manages the steps for circuit design on single phase (calculate IB, apply diversity select IN etc) was wondering if anyone had some worked examples of doing the same for three phase circuits or any info for that matter?

think the process is mainly the same with the exception of getting the design current and loading per phase which im unsure on.

 
mainly working out mcb sizes and design current.

if power = 1.732 x V x I x COS Then i assume i can rearrange the formula to find the design current, but would this be per phase or to be divided accross the phases?

 
Sorry Jnr,

I'll have a look at this later and post some stuff for you!

Have been sidetracked for a few days!

---------- Post Auto-Merged at 18:38 ---------- Previous post was made at 17:39 ----------

Junior,

Off the top of my head before I go on to do some work!

The design calculation procedure is the same as for single phase once you have the phase current.

The phase current/loading per phase is the same thing.

As long as you don't get into Y/D connections then you won't be worried.

First thing is it a balanced 3ph load, motor, machine with no N connection etc. or is it 3 1ph loads, DB or 3 perhaps!

Lets go with balanced 3ph load, say a 3ph immersion heater.

We will ignore the control for now.

You need to be sure you either do the calc in base units, Volts, Watts and Amps or add in the relevant conversion factors, i.e. 1000 for kW.

Ph current is given by the relationship P=IxVx (Cos phi)

I=current, V=voltage, Cos phi=pf=power factor, normally <1.

We will take pf as 1 as for simplicity I have specified a resistive load, thus we can leave it out.

All loads will have capacitive and inductive components, just for certain loads we can ignore them.

For high currents even the cables will bring in capacative & inductive affects, check out the tables in the brb.

This is rearranged to give you I = P / (Vx(cos phi) as Cos phi =1 in our case this becomes

I = P/V

Now for 3ph we use square root of 3 x ph to ground voltage.

thus sq root 3 x 230 (work this out and it is 400 ish!)

So you could use 400 rather than bring the root 3 in to simplify things until you understand it.

Given a load of 10kW the phase current would be using our short cuts

P=10kW=10000W, V=400, I=?

I=P/V=10000/400=100/4=25A (ish).

This will be the current flowing in each phase of the BALANCED 3ph supply.

As we have stated purely resistive load then no other components to be considered.

Please remember this is just an example!

So you could select a 25A 3 pole breaker, though you may want a bit of headroom so go for a 32A and then follow the rest of your 1ph design calc crib sheet.

An immersion should not overload, however, 3ph tends to indicate ind/comm, there is a habit of adding loads so unless the supply is for a known dedicated load and you have run through the full accuracy calc then you may want to allow some slack.

Any use?

 
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