What is the max voltage drop allowed?

You may have heard of the term. Voltage drop is the known phenomenon where the voltage at the end of a run of cable is lower than at the start. Any length or size of cable will have a resistance, and running a current through this resistance will cause the voltage to drop. As the length of the cable increases, its resistance increases in proportion; so voltage drop is particularly a problem with long cables runs, for example in larger buildings or on larger properties such as farms.

Australian Standards require the total voltage drop from the point of supply (i.e. where the power enters the site from the grid) to anywhere in the installation is kept below 5% of the full line voltage. Voltage drops higher than 5% are liable to cause issues such as dim or flickering lights, electric motors running hot and potentially burning out and heating elements heating poorly.

Remember that the 5% maximum is from the point of supply – so if you’re adding a new cable from an existing switchboard, part of this 5% will likely have already be taken up in the cables feeding this switchboard.

What can I do about it?

To lower the voltage drop in a circuit, you need to increase the size (cross section) of your conductors – this lowers the overall resistance of the cable. Of course, larger cable sizes increase cost, so it’s important to calculate voltage drop and find the optimum conductor size that will reduce voltage drop to safe levels while remaining cost-effective.

How do I calculate voltage drop?

To accurately calculate the voltage drop for a given cable size, length, and current, you need to accurately know the resistance of the type of cable you’re using. However, AS3000 outlines a simplified method that can be used.

The table below is taken from AS3000 – it specifies ‘Am per %Vd‘ (amp metres per % voltage drop) for each cable size.  To calculate the voltage drop for a circuit as a percentage, multiply the current (amps) by the cable length (metres); then divide this number by the value in the table.

For example, a 30m run of 6mm2 cable carrying 3 phase 32A will result in 1.5% drop: 32A x 30m = 960Am / 615 = 1.5%.

AS3000 Table C7

Example

A cable from a main switchboard to a feed mill is 160m long. The feed mill requires a 415VAC, three phase, 80A supply. As there is already a voltage drop before the main switchboard, we need to limit the voltage drop of this new cable to 3%. What size cable is required?

80A x 160m = 12800Am / 3 (%) = 4267 Am per %Vd.

Looking up this value in the table, 50mm2 is the smallest suitable size.

Voltage drop, in the word itself, it means itself, here the drop is nothing but a loss and it says the loss in the voltage or reduction in the voltage is called voltage drop.

Why is voltage drop important in the electrical system?

Yes, due to the voltage drop, the power supply to the circuit get reduced, resulting lights to flicker or burn dimly, heaters to heat poorly, equipment damage, improper working, less output, less efficiency, loss of money by paying excessive electricity bill, motors to run hotter than normal, speed drop, small motors do not start and changes to burn out.[wp_ad_camp_1]

How voltage drop arise?

There are many reasons for voltage drop.

Easy to understand about Voltage drop:

Simple and easy understand about voltage drop is garden hose pipe. Consider Voltage as the water pressure supplied to the hose pipe and Current (flow of electron or charges) as water flowing through the hose pipe. And the internal resistance of the hosepipe is determined by the type and size of the hose – same as electrical conductor’s length, MOC (Material of construction) and size.

Must Refer: What is the difference between resistor and resistance

Allowable Voltage Drops:

The National Electrical Code recommends the voltage drop should not be exceeded more than 3 % from the source to utility. Example if you have a circuit voltage of 240 AC and you have a one light at 100 meter long. You have planned to give power supply to the lights, here the supply at the light terminal should receive 233 Volts and the drop should not exceed 7.2 Volts.

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Standard utility allowable voltage drop:

For 480 volts Power system the output of the transformer secondary side should be a minimum of 480 volts and 430 V should be at utility end.

Also see:

Note: The changes in the transformer’s incoming, the output voltage also reduce.

For 240 volts Power system the output of the transformer secondary side should be a minimum of 240 volts and 200 V should be at utility end.

For 120 volts circuit system the output of the transformer secondary side should be a minimum of 120 volts and 110 V should be at utility end.

The voltage drop permissible limit in India:

Maximum allowable voltage drop in India in various area

Learn More:   What is Ohms Law?

Voltage variations in 33 kV and 11kV feeders should not exceed the following limits at the farthest end under peak load conditions and normal system operation regime.

• Above 33kV (-) 12.5% to (+) 10%.
• Up to 33kV (-) 9.0% to (+) 6.0%.
• Low voltage (-) 6.0% to (+) 6.0%

The same range cannot be maintained in rural area, because rural area has many interconnections, power to be transferred for long distance, and power theft etc. In these cases, 11/0.433 kV rather than the usual 11/0.4 kV distribution transformers can be used.

Fair use Ref: Energypedia article

Voltage Drop Calculation For DC (direct current) power system:

In DC power system, we can calculate voltage drop across the conductor by using basic ohm’s law formula. Also, using Kirchhoff’s circuit laws (Voltage and current) You can find, the sum of the voltage drops across each component of the circuit is equal to the supply voltage.

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The resistance of the conductor can be calculated by using the mathematical expression of given conductor’s size, length of the conductor, cross-section of the conductor, and Material conductor made up off.

Note: inductor act as a short circuit for Direct Current.

Voltage Drop Calculation for AC (Alternating Current) power system:

Consider a conductor which carries alternating current, two elements are perfectly doing this action. One is resistance and another one is reactance. In an Ac power system the  opposition to the current flow occurs because of resistance and reactance typically it is called impedance. Here, the vector sum of oppositions to current flow from both resistance and reactance is called impedance. Here impedance is denoted by variable Z. SI unit of Impedance is Ohm.

The total impedance of the circuit is depending upon the frequency of the Alternating current and magnetic permeability. The Voltage drop in AC circuit can be calculated from Ohm’s Law.

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Case Study Regarding Voltage drop:

I had worked in a power plant and we had an issue whenever we start 250HP Motor (three phase, 440V and 50 Hz), the plant light starts flickering. Also, the lighting distribution feeder (MLDB) was installed in the same panel. We had planned to rectify that issue. We measured the feeder voltage when the motor starts rotating. At that time, we saw that the feeder voltage drops from 440 V to 380 volts for 7 sec. So we identified The feeder voltage dropped from 440 Volt to 380V, due to that, we had an issue with the lighting system. To correct that problem, we changed the incoming supply of the lighting circuits. The issue permanently solved. Created separate feeder for all lighting circuits…or use separate lighting transformer.

See here: What is the purpose of Lighting Transformer

MLDB: Main Lighting Distribution Board.

Also see:

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