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Votlage Drop for Unbalanced three phase

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

Parents

0 fiftyhertz over 5 years ago

gkenyon:

Sorry ... got diverted onto something else

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

No, page 65 para 2 explains why only a balanced option is used.

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/.../

This follows the examples given in the EIDG - which assumes three-phase volt drop to the find DB and single-phase for the single-phase final circuit.

However, as I've said, if the system is so badly imbalanced (which may or may not cause other problems in the installation), then you can't really follow the broad guidance in the EIDG.

Graham, firstly there is some excellent information in the above posts, however the one thing i dont quite understand is that previously you mentioned that three phase and single phase volt drop were two diffirent enterties and therefore for a single phase volt drop at a final circuit we should not mix and match as the example in proffesional electrician. As anyhting in the neutral from the final circuit imbalance will also be in the neutral of the submain. I understand that if the final circuit was three phase circuit such as a motor there will be no drop in the neutral so we can use the three phase volt drop in the submain for that calc.

In addition, for the EIDG page 65 states that it assumes the fourth core should not be loaded if balanced. But if unbalanced and calcultaing three phase should we not calculate the three phase volt drop and the add the drop in the neutral if it unbalanced? Would this be correct

Thanks again
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0 fiftyhertz over 5 years ago

gkenyon:

Sorry ... got diverted onto something else

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

No, page 65 para 2 explains why only a balanced option is used.

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/.../

This follows the examples given in the EIDG - which assumes three-phase volt drop to the find DB and single-phase for the single-phase final circuit.

However, as I've said, if the system is so badly imbalanced (which may or may not cause other problems in the installation), then you can't really follow the broad guidance in the EIDG.

Graham, firstly there is some excellent information in the above posts, however the one thing i dont quite understand is that previously you mentioned that three phase and single phase volt drop were two diffirent enterties and therefore for a single phase volt drop at a final circuit we should not mix and match as the example in proffesional electrician. As anyhting in the neutral from the final circuit imbalance will also be in the neutral of the submain. I understand that if the final circuit was three phase circuit such as a motor there will be no drop in the neutral so we can use the three phase volt drop in the submain for that calc.

In addition, for the EIDG page 65 states that it assumes the fourth core should not be loaded if balanced. But if unbalanced and calcultaing three phase should we not calculate the three phase volt drop and the add the drop in the neutral if it unbalanced? Would this be correct

Thanks again
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Votlage Drop for Unbalanced three phase

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