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Votlage Drop for Unbalanced three phase

fiftyhertz
Hi all,


Hoping someone can help so i can forget about this and get to a beer garden..


For a balanced three phase voltage drop, the three or four core mv/a/m figure can be used from the releavnt table to determine the voltage drop.


However, for an unbalacned system ie a three phase submain feeding a DB, which supplies single phase loads gets more confusing. After reading previous posts, it seems the best way to deal with this is to calculate the single phase drop from each phase to the final circuit. However, this is where i am finding conflicting infromation, calculations for electricians and designers states that:

"By inspection of the extract from Table 4D4B below, it can be seen that the three- and four-core cable voltage drop per amp per metre (mV/A/m) is times the two-core (mV/A/m). The converts to three-phase; the division by two is necessary as there is assumed to be no voltage drop in the neutral."  Hence, where the 0.866 figure comes from which i have read about.


However, the Amtech handbooks simply divides the Z values in the table by root 3, without the division of 2 to get a single phase volt drop. Which makes more sense to me, as there is likely to be current in the neutral of the three phase system that needs to be accounted for. 


Example:


4 Core 3P+N 25mm2 cable supplying a distribution board 10m long, L1 110A, L2 80A, L3, 80A:

From table 4D4B z = 1.5


VD 1L1 = ( mv x L x I)/ 1000

VD = ((1.5/1.732) x 10 x 110)/1000

VD = 0.96v for the drop of the loads between L1 and Neutral


Is this correct, doesnt seem quite right to me.


Thanks



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  • 0

    gkenyon:

    Now, if all that's too much, there is an easier way if you only want the worst-case volt-drop you could imagine, although this could potentially be a huge over-estimate.


    (a) Take the largest line current for each phase you would envisage at the DB. Take the biggest one.


    (b) Calculate the single-phase voltage drop. This will give you a larger result than the above calculation - an over-estimate, but very easy calculation.


    (c) Calculate the three-phase voltage drop using this largest line current as the line current and assume it's balanced. This will give you the three-phase (line to line) voltage drop.


    (d) Calculate the final circuit voltage drops from each of the circuits downstream, added to the relevant single-phase and three-phase values at the DB calculated in (b) and (c) respectively.




    Is this saying then that the total VD on, for example, L1 on an unbalanced 3ph distr cct with single ph final cct, is the single ph VD in the distr cct + the 3ph VD in the distr cct + the single ph VD of the final cct, all on L1?


    Is there an example of calculating VD for unbalanced three ph system in the EIDG, i can't seem to see it? 


    Also, have they used the correct methodology in this article then? They seem to be calculating VD in the 3ph distribution cct for balanced and unbalanced systems, but don't make it explicit, yet use the 3ph mV for a balanced system. Sorry if I've misunderstood it.


    professional-electrician.com/.../


    F

     

Reply
  • 0

    gkenyon:

    Now, if all that's too much, there is an easier way if you only want the worst-case volt-drop you could imagine, although this could potentially be a huge over-estimate.


    (a) Take the largest line current for each phase you would envisage at the DB. Take the biggest one.


    (b) Calculate the single-phase voltage drop. This will give you a larger result than the above calculation - an over-estimate, but very easy calculation.


    (c) Calculate the three-phase voltage drop using this largest line current as the line current and assume it's balanced. This will give you the three-phase (line to line) voltage drop.


    (d) Calculate the final circuit voltage drops from each of the circuits downstream, added to the relevant single-phase and three-phase values at the DB calculated in (b) and (c) respectively.




    Is this saying then that the total VD on, for example, L1 on an unbalanced 3ph distr cct with single ph final cct, is the single ph VD in the distr cct + the 3ph VD in the distr cct + the single ph VD of the final cct, all on L1?


    Is there an example of calculating VD for unbalanced three ph system in the EIDG, i can't seem to see it? 


    Also, have they used the correct methodology in this article then? They seem to be calculating VD in the 3ph distribution cct for balanced and unbalanced systems, but don't make it explicit, yet use the 3ph mV for a balanced system. Sorry if I've misunderstood it.


    professional-electrician.com/.../


    F

     

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