Amps and Voltage

Hello,

I am struggling to understand the concept of equipment, voltage and ampage. For example, if we are using a three phase cable at 400v, and the cable has a maximum current capacity of 119A. Will the equipment draw only the voltage and ampage it needs, and if so what occurs with the left over voltage?

Pretty basic question, but i understand harder principles just trying to get my head round this.

 

Cheers

In the simplest of terms, the voltage delivered to the load will be 400v as long as the current the load draws is less than the maximum current capacity. The current drawn for a purely resistive load will be governed by ohm's law -- current = voltage / resistance.

So, there is no left over voltage, but there is left over current capacity. This left over current can be used by a second load in parallel with the first one.

All of this is made more complex by three phase and non-resistive loads but the general concept remains the same.

Example: For a single phase system, all voltages and currents measures as RMS values...
An electric heating element with a resistance of 4 ohms is connected to a 400v power source. The current is 400V / 4 Ohms = 100A.

That has cleared most things up for me, so thank you for that however ohms law states that:
For a three phase system - 200A = 400/2

For a single phase system - 115A = 230/2

So we can see that a three phase system uses higher ampage, is this correct? And if so, why?

Cheers

You can't use ohm's law like that for three phase.

For a single phase system, 230V / 2 ohms = 115A, you have a single 2 ohm load in a single phase system, so that is your total current and wattage can be calculated using V * A = 27450W

For a three phase system, you have three 2 ohm loads, but the duty cycle changes. With a 230V, 115A load (per phase), you end up with 45812W total power.

If you change the voltage to 400V without changing the load resistance you will get nearly double the current/power. This is why most 110V appliances will fail catastrophically when connected to 230V but 230V appliances often just run at reduced power when connected to 110V.
There are online calculators that will help with figuring these values out, try http://www.rapidtables.com/calc/electric/Amp_to_Watt_Calculator.htm or google three phase power calculations or similar.