3 phase volt drop

[anthanth]

(sorry this went into a few forums it should not have -my bad)

Any help would be great

A volt drop calculation that i could do with some help with is the 3 phase VD

I fully understand the single phase vd calculations but just wanted to get a confirmation when using 3 phase.

e.g

Say a large load of 30Kw is 200m from the point of distribution and you need to find out the volt dropped.

Do you use 30000/230 - 130 Amps or

do you use 30000/415 - 72 Amps????


Now lets say using a 50mm conductor the differences are below.

230 ---- 0.84 x 130 X 200 (m) /1000 = 21.84 volts dropped
415 ---- 0.84 x 72 x 200 (m) /1000 = 12.0 volts dropped

At 415 v this would be approx 3% vd!!

Have a got this correct or would some body like to correct me?

Just looking for some help................

many thanks

 

[ACDC]

3 phase calcs are different to single phase calcs.

Without my books with me, ill try and remember the calcs.(im sure if im wrong, someone will shoot me down)

Design Current Ib = 30000/400*1.732 = 43.30amps (Cos phi, efficency e.t.c assumed to be 1)

You say a 50mm conductor, is this swa, singles, multicore SWA???? ill assume 0.84 is correct for now

so @400v 0.84*43.30*200/1000 = 7.27volts.

Your allowed 3% VD for lighting circuits so 12v in a 400v system
Your allowed 5% VD for other loads so 20v in a 400v system.

 

[telectrix]

now i do it a simpler way. as long as the load is balanced...... 30kw is 10kw per phase, so 43.48A per phase. then as above..

(43.48 x 0.84 x 200)/1000

 

[anthanth]

Hi there, thanks for the updates. Ok it is a SWA cable in the ground.

I understand the workings above but would like to check why i came out with 72 amps and
you came out with 43.48.

It seems to do with the 1.732 that i missed. (30000 / 400 X 1.732) instead of my 30000/400

Can you let me know where this came from and why is it used??


Thanks again for the info

getting there................

 

[Knobhead]

it’s √3 of the line voltage so 400V / √3 =230.9V

 

[Guest123]

http://www.electriciansforums.net/e...al-regulations/47059-cable-calc-template.html


A good basic all rounder for cable calcs.

Obviously other factors also need to be considered when it comes to more complicated loads such as motors etc but for everyday calcs it's fine.

 

[anthanth]

Ah ha ok. Many thanks for the input above. Just to clarify for my own sake with an example......

say there is a 70 Kw load and it is fed from a supply 400m away, if you were to use a 50mm swa cable then is the below calculation correct??


70000/400 * 1.732 = 101 Amps (per conductor - correct?
so

0.84 X 101 X 400/ 1000

= 33.9 volts dropped


(i used 0.84 from table 6D2 in OSG however the table 4J4B in the regs gives very different firgures, which i assume to be to do with reactive and resistive factors)

So do i have the above correct. Obviously the volt drop would to too high for lighting and normal loads so a larger cable would be needed but the calculation was all that i was after.


Thanks to all for input thus far.......

 

[anthanth]

ah ha ok yes i was using the wrong tables using the 4E4A it is slightly different (SWA table)

50m cable, 70 kw load, 400m

0.86 X 101 X 400 / 1000 = 34.7 V

Oui or non?

 

[ACDC]

it looks good, although you do realise you are using the table for 90`c cable... only use this cable if you comply with the 2 notes written next to the table.

As far as the calcs go, looks fine to me from a basic point of view.

Good link from Lenny above.

 

[anthanth]

why is 400 v used as the line voltage and not 415??

 

[telectrix]

blame the european bograts. its nominally 400, same as single phase is nominally 230

 

[Knobhead]

When I started it was 440V then 433, 415 and as now 400.
If Brussels get their way it will be 380V.
View attachment 10550
One of the transformers I’ve fitted. NOTE the sign.

 

[Engineer54]

In truth the fact is ..... it's still 415V/240v, ...the 400V/230V is just an imaginary voltage to please the harmonisation crap we've been saddled with....

 

[bomjac]

Duplications deleted.

Common industrial voltages in use in the UK, all 3ph are 400V, 415V, 440V, 460V and 480V. Go to Norway and they insist on using 690V if an installation is above a certain size also.

Probably a tutorial question that was posed back when 415/240V was the norm.