3 Phase Question

[Bez]

Hi All,

The Question.....

We have a water/immersion heater tank,

Rating 3Kw

Phase : 1/3

Voltage : 230/400

Now I take it from this the tank can be connected to either a single or 3 phase supply.

Now if we assume a cable run of 10m from the board what would be the cable size and breaker size if the tank were connected using a 3 phase supply.

Any calculations must be written down please gentlemen.

It's a theoretical problem just to help me along in my quest to gain knowledge

 

[kme]

I don`t think anyone will actually DO the math for you.......

but we will provide the info to help.

I`m assuming you can work it out for a single phase 3KW load? So we`ll just concentrate on the difference with TP.

Your cable size will be given by BRB appx 4 - the actual size will depend on the correction factors to be applied, the same as any other circuit.

MCB size is quite easy - your 3KW load is spread across three phases; so uses 1KW per phase. Convert to amps, and go to the next size MCB readily available. This will then provide the minimum current-carrying-capacity for your cable - work in any correction factors, ensure v.drop is within limits, and you`ve sussed it!

If you want to actually DO the math, and post your workings on here, as a sort of test, we can tell you if you get the right answer, or where you went wrong

Hope that helps you?

KME

 

[Andy™]

I don`t think anyone will actually DO the math for you.......but we will provide the info to help.

If you want to actually DO the math, and post your workings on here, as a sort of test, we can tell you if you get the right answer, or where you went wrong

as above

 

[Evans Electric]

Reckon a 1.5 X 3core will do it .

 

[Bez]

Here goes,

1000w / 400v = 2.5A

three or four core SWA Ref. Method C (Clipped Direct)

1.5mm csa carries 18A

All protected by a 6A triple pole MCB

 

[Evans Electric]

Why were you asking then , Bezzel ?

 

[Bez]

Just a little confused about the power distribution in the 3 phases. I may have this all a..e about face but isn't there only 2 of the 3 phases supplying power at any one time due to the 120 Degree differance in wave?

Please enlighten as this does confuse me.

 

[Sidewinder]

Bez,

Your maths is not quite right, you may have a square root needed somewhere?...

 

[Evans Electric]

Yup , Sq.root of 3 required Bezzell .

 

[Bez]

Bez,Your maths is not quite right, you may have a square root needed somewhere?...

Now that's really put the cat in!

Tell me more.

 

[kme]

EH?

No mate - they`re 120 degrees out with respect to one another; so, although all three will be at different points in their cycle; they can all quite happily be supplying at any given time. Even a single phase sinusoidal supply drops to 0V 100 times a second - what`s the difference?

The actual power (voltage) in any one phase at any given moment is, to all intent and purpose, irrespective.

KME

 

[Sidewinder]

C'mon Bez, get your books out and look up the 3ph formula?...

We'll point you along but there is no way are we gonna hand it to you on a plate, FFS I don't even do my kids homework for em, I make them do it themselves and just give them pointers.

 

[Bez]

C'mon Bez, get your books out and look up the 3ph formula?...We'll point you along but there is no way are we gonna hand it to you on a plate, FFS I don't even do my kids homework for em, I make them do it themselves and just give them pointers.

Your reply was fine without the FFS, there's really no need.

Silly me for thinking the forum was a place where knowledge could be gained by those wishing to gain it, given by those willing to share it.

Cheers!

 

[Manator]

Your reply was fine without the FFS, there's really no need.Silly me for thinking the forum was a place where knowledge could be gained by those wishing to gain it, given by those willing to share it.

Cheers!

His comment is tongue in cheek, and refers to the example given by him regarding the help he gives his children.

If you were given a direct correct answer what would you learn? The reason why posters will not answer direct is because, if we did then the OP would learn nothing.

When you ask for help it is always given but you have to work for it, otherwise its pointless.

 

[kme]

Which I think I tried (obviously fruitlessly) to convey in first reply :coat

 

[Bez]

Which I think I tried (obviously fruitlessly) to convey in first reply :coat

I'm sorry KME, your reply was helpful and for that I'm grateful, I just don't see the need for the unhelpful comments, whether tongue in cheek or not.

Sometimes you can stare at a book and re-read things many times over before the penny finally drops, but I will go back and try to make sense of it, albeit with slight apprehension about posting a question on the subject again.

 

[kme]

I hope not! There isn`t a need to be apprehensive about posting a question regarding the part of a topic you can`t "get your head around". I think it was more the inference that you wanted to be shown ALL the "working out" by someone else, that rattled a few cages.

There was a phrase I was taught years ago, by an RN CPO(comms). It was aimed at radio communications(naval) ; but works equally well in everyday life.

"You don`t have to know all the information. You only have to know where to look to find that information."

I like to think we`re a bit like that - directing people to the appropriate location to find what they need.

Don`t be upset by our offhand or tongue-in-cheek comments - they aren`t intended to cause offence.

KME

 

[Chr!s]

I thought you two were supposed to be bez mates:^O

 

[Bez]

Bez,Your maths is not quite right, you may have a square root needed somewhere?...

Gotcha now I think.

3 Phase Current - I=P/V root 3

Thus

I= 3000/400 X 1.732

I= 12.99/3 = 4.33A per Phase