# Why isn't there a voltage drop across the Main Wires?

As far as I know the power to my home is directly coming through a Step-Down transformer, so why isn't there a voltage drop and how are all the houses hooked up to the same transformer getting the same voltage?

Same is the question with Voltage Inverters which use a Step-Up Transformer. They also don't contain a "22VAC" Voltage Regulator.

If the answer is related to Amps, then how do we see the same voltage drop across a 2 Amp and 4 Amp DC Power Supply for the same load?

• What makes you think there's no voltage drop between the transformer and you? – Nick Johnson Jun 5 '15 at 7:10

There is a voltage drop. Part of designing a distribution network is to ensure the voltage drop is within a tolerable range, at the maximum expected load.

Let's consider a simplified example.

Suppose that your house power supply is 240 VAC nominal, delivered over a 10mm² two-core cable (single phase power supply.) Your maximum demand is 20 amps. We want no more than 5% voltage drop. What is the maximum length of the supply cable?

Looking up Australian Standard AS3008.1.1:2009 Electrical Installations - Selection of Cables we find some useful tables giving the resistance of various kinds of cable.

Table 34 tells us that 10mm² conductor has a resistance of 2.23 Ω/km at 75° C.

The voltage drop on the cable is given by: 20 amps × 2 ways × 2.23 Ω/km × length (km). The permissible voltage drop is 5% of 240 VAC = 12 V. We can work out that the maximum cable length is 135 metres.

To answer the second part of your question - "how are all the houses on the same transformer getting the same voltage" - the houses are all getting slightly different voltages. However, so long as they are all getting a voltage "close enough" to the nominal voltage, there is no problem.

• I should add that 'conductor' refers to copper conductor. Cables are usually copper in Australia. Aluminium is more commonly used for aerial conductor, where its light weight is helpful. – Li-aung Yip Jun 5 '15 at 16:30