calculating volt drop

[ecowarrior]

verification of Volt drop (domestic) needs to be calculated under load conditions (I assume) - which you cant really do - so presumably one calculates this from R2+ Rn with the cable temp figs adjusted at what 60 degrees? I havnt given it much thought until today. I have just checked the voltage at the various outlets and recorded them open cct. If this is a calculation what is the rating value of the conductors and what is the formula or do u estimate the length of the conductor by the resistance of it and work back from that? ?- or have I got it completely confused

 

[EFLImpudence]

verification of Volt drop (domestic) needs to be calculated under load conditions (I assume) - which you cant really do - so presumably one calculates this from R2+ Rn with the cable temp figs adjusted at what 60 degrees?

It would be R1+Rn.

For socket circuits, the rating of the MCB.

For other circuits the actual load can be used.

It would normally be done at the design stage when the length would be known and from the listed values for the conductors.

I havnt given it much thought until today. I have just checked the voltage at the various outlets and recorded them open cct.

What does that mean?

If this is a calculation what is the rating value of the conductors and what is the formula

The voltage drop values of the various conductor sizes are listed (mV per metre) and the formula is mV x A x m (voltage drop per metre x load x length).
These values are given at the operating temperature of the given cable.

or do u estimate the length of the conductor by the resistance of it and work back from that? ?

You could for already installed cable.


There is a maximum voltage drop allowed of 5% (11.5V) for power and 3% (6.9V) for lighting circuits.

 

[bernardgreen]

Which is more important ? Voltage drop or power dissapated in the cable ?


Could the voltage drop limits imposed be a way to limit the amount of heat generated in the cables in a simple way that does not require complex calculations.

The alternative would be to specify maximum safe heat disapation in the cable and its resistivity per metre at various temperatures. Calculations could then be carried out to determine the maximum current the cable could carry before overheating. This more complex method would be appropriate where cable size and / or current loading changes along the cable route.

 

[Alarm]

Does not taking into account bends, covering,clipping, environmental conditions and grouping already exist?

 

[ban-all-sheds]

Could the voltage drop limits imposed be a way to limit the amount of heat generated in the cables in a simple way that does not require complex calculations.

Not unless the load is a constant power type (e.g. SMPSU) which will attempt to draw more current to compensate for lack of voltage. A normal load will draw less current as the cable length increases, not more, so a longer cable will heat up less, not more.

Try this simple test.

Take 2 pieces of 1mm² cable, one 1m long, the other 1km long.

Which gets hotter when connected between L & N at the cutout?