Voltage Drop When Load Is Applied Question

[Davisonp]

Hi,

I came across a situation recently while investigating a problem where measuring voltage at a set of 4 sockets read 243 Volts with no load, and when a 21 Amp load is applied, the voltage then reads 230 Volts.

Is it normal to have such a large voltage drop under load? Or does it imply the supply cable to these sockets is undersized?

Thank you.

 

[Wilko]

Hi - over simplified it indicates the impedance of the supply is about 0.62Ohms, which is made up of the house wiring and the suppliers distribution network and overall your value is fine. By design we aim to keep below 7% V drop within the final outlet cct (say 16v) at designed load (say 32A). This V drop would be from your side of the meter to the socket (ie not including suppliers cable to your house) . Cheers, David.

 

[Wilko]

Hi - sorry I had a brain fade above - the design value is 5% , so about 11.5V . Sorry for that ! Cheers, David.

 

[Knobhead]

Hi,

I came across a situation recently while investigating a problem where measuring voltage at a set of 4 sockets read 243 Volts with no load, and when a 21 Amp load is applied, the voltage then reads 230 Volts.

Is it normal to have such a large voltage drop under load? Or does it imply the supply cable to these sockets is undersized?

Thank you.

What was the voltage at the supply when the circuit was under load?

 

[Davisonp]

What was the voltage at the supply when the circuit was under load?

I did not measure the voltage at the supply, just at the load point.

I think I have been over thinking this. It is a 5% drop between the intake voltage source and the load point, not 5% drop between unloaded and loaded voltage. I was just wondering if from experience that big a volt drop is usual for 21 amps load or a sign of an undersized cable.

 

[Leesparkykent]

I did not measure the voltage at the supply, just at the load point.

I think I have been over thinking this. It is a 5% drop between the intake voltage source and the load point, not 5% drop between unloaded and loaded voltage. I was just wondering if from experience that big a volt drop is usual for 21 amps load or a sign of an undersized cable.

What size cable and what was the R1+R2?

 

[Davisonp]

What size cable and what was the R1+R2?

Don’t know the cable sizes (comes off a sub distribution board). I Did not measure R1+R2 (I was investigating an earth leakage problem and the voltage drop is just something I noticed and lodged in my mind.

 

[Richard Burns]

Overall the volt drop is not too bad but it would tend to indicate that the socket circuit was wired in something like 1.5mm², but this is theoretical and any additional resistances from joints may cause this level of volt drop.
That said for a basic length socket circuit in 2.mm² unless it is on a 16A breaker (taking 21A!) the volt drop is too high for the circuit as it should be designed to be within limits.
A more likely scenario than being wired in small csa cable is that there is a high (ish) resistance joint somewhere that is causing the additional volt drop.
The above points about R1+R2 and the supply voltage are relevant to determining the cause, normal R1+R2 may mean a loose supply cable as demonstrated by a drop in the supply voltage.
If the supply voltage is stable then the likelihood is that the R1+R2 ( or rather the R1+Rn) is higher than it should be.