question

[NWDS]

morning all, just a quick question

This is how its wrote,

A single phase load @ 400v ( 3ph ) AC is rated @ 6.4kW and has a power factor of 0.8, calculate the design current.

Answer is 11.5 amp

So

1000/(1.732*400*0.8)= 11.5 amp

1.732*400 = 692.8

692.8*0.8 = 554.24

1000/554.24 = 1.804

Were am I going wrong ????.

Thanks in advance

 

[NWDS]

I think I have it,

6.4kW = 6400

so 6400/554.24 = 11.5

The answer/formula written stated 1000/554.24.

just goes to show that some things can be written wrong.

NWDS

 

[msb2003]

Although you've got to the right answer, I think you've applied the calculations wrong!!

In the first post you've applied the pf to the voltage.

Take 6.4KW as the true power

true power / pf = apparent power

6400W / 0.8 = 8000W

Voltage bit correct

400v x 1.732 = 692.8v

apparent power / voltage = current

8000W / 692.8v = 11.5A

I think that makes sense!!

 

[NWDS]

you bet it has, but again that is how it was shown.

NWDS

 

[the doctor]

hi

i am sure that does not sound right to me.

if i am reading the op's question correctly it is a single phase load of 6.4 kW at a pf of 0.8....

that would be then an apparent power of 6400/0.8 which would give a figure of 8000VA or 8kVA.

if this is a single phase load then the VA would have to be divided by the voltage of 230V (400/1.732) so.... 8000/230 would give a design current of 34.78A

someone put me out of my miseryBlushing

 

[msb2003]

if this is a single phase load then the VA would have to be divided by the voltage of 230V (400/1.732) so.... 8000/230 would give a design current of 34.78A

someone put me out of my miseryBlushing

I agree with your logic and think its a poorly worded question.

As the answer was given as 11.5A I simply worked it get the required answer!!

 

[the doctor]

hji msb

if the question in the book was correct you could wire a 6.4kW 8kVA cooker/load in 1mm twin and earth.....we would make fortunes

 

[Andy™]

A single phase load @ 400v ( 3ph ) AC is rated @ 6.4kW and has a power factor of 0.8, calculate the design current.

its either SP or TP?

 

[NWDS]

Morning all, I wrote the question word for word as it had been presented, thw site it was on was swindon-massive.

NWDS

 

[the doctor]

it is possible to wire a 400V load as a "single phase" by connecting across two lines. In this case the design current would be 8000/400 which would give 20A so i cannot figure out where the answer came from:_|

 

[the doctor]

right! ive got it now!

6.4kW or namely 8kVA connected on a three phase load.

It is easier to picture as 3 equal loads on single phases so.....

8000/3 = 2666VA/ phase or line so the design current per line would be.....

2666/230=11.59A

 

[NWDS]

Thanks Doc.Applaud SmileyApplaud SmileyApplaud SmileyApplaud SmileyApplaud SmileyApplaud Smiley

NWDS