Cable Questions - Voltage Drop

Hi Friend,

Your question is in complete. Is the cable of Copper or Aluminium conductor? What is the length of cable? ( 2x120+70 ) not clear. Give more details please.

The cable you mentioned should consist of 2 core of 120sqmm for lines and one core 70sqmm for earth.

You can get all the information you need from cable manufacturer's data sheet and normally they will show you in the data sheet how to calculate voltage drop.

Dear for calculating the cable size following factors are involve

1- Lenghth of Cable

2- Load connected with cable

3- Type of Load (Motor or Lightining Load)

4- Ambient Temprature of Room

But in your question I donot understand anything please provide me detail.

For a very close approximation, (no skin effect) you must know the following about the load:

1- Three Phase (use 1.73) or Single Phase (use 2)

2- Length of conductors (in feet)

3- Resistivity constant "K" of the conductor (Cu use 12.8 or Al use 22)

4- Amps carried by the conductors

5- Circular mils of the conductor (use tables, and unless conductors are paralleled, use the mils for only one of them. These are the current carrying conductors only, no grounding)

Then use the following for 3 phase-

Voltage drop (in volts) = (1.73) (K) (amps) (Distance in feet)/ circular mils of conductor

For single phase-

Voltage drop in volts = (2) (K) (amps)(distance in feet)/(circular mils of conductor)

Remember there will be no voltage drop until there is a load on the circuit. If you take a meter out and check the circuit voltage under load, the measured voltage will be remarkably close to what is calculated. Convert to percentages as required for branch or total voltage drop. For reference use UGLY's, better yet, Robert B. Hickey's "Electrical Engineer's Portable Handbook" is excellent.

A good rule-of-thumb is when the circuit distance in feet equals the nominal voltage, you should consider voltage drop. As an example, if the nominal voltage is 480 and the circuit distance is 480 feet. The voltage drop will be close to or exceed recommended amount.