Voltage drop

Monkeyblaine · May 11, 2013

Is there a voltage drop from DNO transformer to incoming supply? For a standard 100A supply to a house so they design cable to with stand no VD as so at maximum demand there will be no VD. It is then up to us to design circuits as to have as little as possible so that 230v remains everywhere. Of we design badly would the company's VD calcs be wrong as less voltage more current. And I'm I right in thinking overload on a cable would result in a higher VD to that circuit as they must be linked.

TaffyDuck · May 11, 2013

The point where the supply enters the house system is beyond your control but you will find that it is usually well above 230V. More likely is 235-250V so you have a considerable amount of spare capacity. As the current flowing in the cable increases the temperature of the cable increases and this causes voltage drop to increase. In other words VD is a product of the action of current flowing not a cause.

Richard Burns · May 11, 2013

There is a voltage drop from transformer to incoming supply, because there is resistance in the cable, however the supply company work to ensure that the voltage supplied to houses / industry remains approximately constant.

You would tend to find that for a row of houses where the start of the row is close to the transformer and then (assuming they were all using the same supply) the voltage at the first house would be higher than that at the end of the row.

In the UK, the declared voltage and tolerance for an electricity supply is 230 volts -6%, +10%. This gives an allowed voltage range of 216.2 volts to 253.0 volts.

Where a new housing estate is built that will increase the demand on the supply and cause a drop in voltage.
The DNO therefore then needs to assess it's supply characteristics and ensure that the additional demand does not drop local voltage below the above limits.
On a temporary basis there may be high demand industry that may cause local fluctuations in the voltage.

BS7671 uses the "nominated" voltage of 230V as a standard reference voltage so that all calculations will be consistent. UK equipment is designed to be able to work (in the main) safely with the possible range of voltage available.