How To Calculate The Maximum Demand and The Total Connected Load?

One of the most basic electrical calculations that an electrical engineer should know is to calculate the Total Connected Load (TCL) and Maximum Demand (MD).

The one million dollar question: What is Total Connected Load and Maximum Demand?

Total Connected Load (TCL) is the mechanical and electrical load (in kW) that will be connected (or to consumed) for that particular area.

The Maximum Demand (MD) is the total kW that actually contributes the total power used in one time after applying the diversity factor based on the Total Connected Load calculated.

Example 1
Let say, your own bedroom is having 1no. 2hp air-conditioning split unit, 4nos. of single phase socket outlet and 1no. 2X36W T8 Florescent Tube.

I’m using the Excel program since I can easily manipulate all the values in case fine-tuning values need to be made in future.

Then, your Total Connected Load and the Maximum Demand will be as per above;

By looking into the figure above, the Total Connected Load and the Maximum Demand is 2.57kW and 1.36kW respectively.

How The Calculation Works?
1. The unit for TCL and MD is in Watt. Therefore, all the loads need to convert into Watt.
a) 2hp a/cond split unit = 2 X 746W = 1492W
b) 2X36W Flou. Fitting = (2 X 36W) + Ballast Wattage = 80W
c) 13A Socket Outlet = 250W (this is rather subjective since some engineer putting 300W/nos)
2. Determine the Diversity Factor (DF) for the respective load. The DF is the percentage of load that will contribute for the total of the Maximum Demand. For the above example, I’m expecting the split unit will contribute 6o% of the total MD, lighting will be 80% and socket outlet is 40%.
3. Multiply the Diversity Factor with the Connected Load to get the Maximum Demand.
4. Summing-up the individual MD to obtain the Total MD.

Why I Need To Calculate The TCL and MD?
By calculating the TCL, you’ll know the total load connected for a particular area and also you can determine the sizing of cables. But, the most important thing is by having the TCL, you can determine your MD. This MD will be declared to the utility provider for the purpose of meter deposit and utility bill.

How To Calculate The Voltage Drop?

What is Voltage Drop?

The term “Voltage Drop” is essentially refer to the reduction in voltage in an electrical circuit between the source of power to the end load where the source of power supply to.

How This Voltage Drop Happen?

By law of nature; Energy cannot be created and it also cannot be demolish. However, energy can be transform from one form of energy to another form of energy (ie: solar energy transform to electricity).

Cable will carry the electricity. The voltage drop in cable will happen by the time dissipation of heat by the cable itself by means of transforming of electrical energy to heat.

Why you need to calculate the voltage drop?

The main reason that you need to calculate the voltage drop is due to the IEE Wiring Regulation spelled that the voltage drop of cable should not be more that 4% between the main incoming supply points to the terminal of any fixed machine. So then, you have to calculate the voltage drop in order to meet this wiring regulation.

A significant drop in voltage could be the negative result of an incorrect conductor size in a cable.

How To Calculate The Voltage Drop?

Calculation of the voltage drop is given by the formula below

Where ;                mV/Am = millivolt drop per meter per ampere of the cable

                                                (this information is given in the table based on IEE Wiring Regulation)

                                I = Current in the cable (in Ampere)

                                l = Distance of cable (in Metre) 

Example1

Based on IEE Wiring Regulation 16^th Edition (Table 4D2B), the mV for 1X4C 50 sq mm Cu cable will be 0.81

Voltage Drop (Volt) = (0.81/1000) X 100 Amp X 100 metre = 8.1 Volt @ 1.95% of voltage drop.

Based on the calculation above, the percentage of the voltage drop is only 1.95% out of 4%. However, please take note that, the 4% is actually between the main incoming supply points to the terminal of any fixed machine. Therefore, the voltage drops from the Sub-Switch Board to the final connected load (ie. motors, lightings or socket outlets) need to be calculated as well to meet the wiring regulation.

How To Calculate The Power Factor Correction

Let says, there is a small Multipurpose Hall with the total loads as per tabulated below and you are being assigned to design the power factor correction.

Before designing the power factor correction, you should know what’s the purpose in correcting the power factor and how to do the correction of power factor?

The Theories

Looking into the description of the load, the biggest loads are contributed by the mechanical equipments which are pumps and motors.

Bear in mind that pumps and motors are the inductive loads.  These inductive loads will increase the value of the Reactive Power (KVAR). The Power Triangle below illustrated the relationship between the KW, KVA and KVAR.

From the power triangle above, the power factor are derives as per below formula;

In order to obtain an efficient systems;

a)      The power factor should reach to 1.

b)      The angle θ should reach to 0.

c)      The capacitive load needs to be added in the system.

Why Power Factor Needs To Be Improved?

Money is the major reason. By improving the power factor, you’ll actually can:

a)      Reduce the Maximum Demand and electricity bill

Higher KVAR will lower the power factor. Meaning that, more power or Maximum Demand (KW) is needed to counter the increase of the inductive load (KVAR). The more power that being consumed, the more utility bill that needs to be pay.

b)      Avoiding the power factor penalty

The utility provider will impose the power factor penalty should the power factor in your system falls below their minimum power factor value.  

c)      Increase system capacity

Let say, 1000KVA transformer @ 80% power factor will produce 800KW of power. If we increase the power factor to 90%, that same transformer can deliver 900KW of power. The increase KW will generate more load can be added to the systems thus increasing the system capacity.

d)     Reduce system loses and voltage drop level

The more power consume, the more heat will dissipate in the systems.  This heat is actually the electrical loses in your power system and it will increase the voltage drop level.

Ok... we go to the basic formula of three phase power;

For the power factor of 0.8, the current that required will be 17.4A. If you increase the power factor to 0.9, the required current is only 15.5A. Based on this current comparison, the lower power factor will increase the value of current in the systems. Due to this increase in current, more heat will generates along the feeder cable thus increase the electrical loses in the system.

Now, you already knew the theories and the reason behind the power factor correction. The next question is how to design the power factor correction.

HOW TO DESIGN THE POWER FACTOR CORRECTION?

First and foremost, in order to reduce the inductive load (KVAR) (some people called correction of power factor) in the system is by adding the capacitive load in your system. This is due to the capacitive load is inversely proportional to the inductive load.

Let says your initial Power Factor (PF) = Cos Ө1 = 0.7

Final Power Factor (PF)   = Cos Ө2 = 0.85 (Based on your final required value)

Therefore;        Ө1= Cos^-1   (0.7)   = 45.57°

                        Ө2= Cos^-1 (0.85) = 31.79°

KW = Maximum Demand = 78.63kW

Vector Diagram (Final)

The added KVAR will be based on the below formula:

KVAR (Added) = KW (Tan Ө1 - Tan Ө2)

                             = 78.63 [Tan (45.57) – Tan (31.79)]

                             = 78.63 [(1.02) – (0.62)]

                             = 31.45 KVAR

Conclusion

The design KVAR should be more that the calculated KVAR.  Therefore, the added KVAR shall be based on 35KVAR.