In this post, First; We are going to see how to calculate an Electrical load of a machine. & Than after that the method of selection of an MCB is been discussed.
This post will flow step after step :
Electrical Load
Electrical Load is actually a ‘power rating of electrical machine’. Electrical power is consumed by the load to deliver an output. Electrical load is been represented with the term Wattage, in short “Watt“.
There are 2 types of power to operate an electrical load. That are; AC(Alternating current) and DC(Direct current). While AC power is further classified as “single phase” & “multiphase“
MATHEMATICALLY–
- DC power = Voltage * Current
- Single phase power = Voltage*current*cosφ
- Three phase power = √3*Voltage*current*cosφ
- In case DC – In an area, Voltage is a fixed parameter. It is the current which actually defines the consumption by the load.
- In case AC, Both current & power factor defines the power consumption.
Power factor is the cosine angle in between voltage & current. Since only AC power contains the angular term That’s why power factor only appears in AC power. The power factor varies in a range 0-1-0.
The term power factor interprets how efficiently electrical power is been consumed. As a consumer, We have to pay for entire energy input which includes losses too. Low power factor represents high losses. Power factor 1 is the maximum value which is known as unity power factor at which maximum output is delivered.
Name plate of 3 phase 0.75KW Induction motor is shared below. With rated voltage 415, rated current 1.8amps(Max.), Rated power factor 0.8. Rated power is 0.75KW !
Simple Calculation for an Electrical load
CASE DC
Let suppose
- Fan = 80W
- Light = 40W
Both the loads are DC power operated. With the voltage rating of 24V DC.
DC power=V*I
- Fan Current = 80/24= 3.33 Amps
- Light Current = 40/24= 1.66 Amps
- Total power(wattage) = 120 Watt
- Total current = 4.99; 5Amps (approx.)
Case AC(Single phase)
Now, Again same load rating but this time with Ac power.
- Fan = 80W (1 phase)
- Light = 40W (1 phase)
Both the loads are AC operated with standard voltage rating of 230V AC single phase. Power factor for fan 0.85. Power factor for light 0.95.
AC power= V*I*cosφ & I= P/(V*cosφ)
- Fan current = 0.40 Amps
- Light current = 0.18 Amps
- Total power = 120 Watt
- Total current = 0.58 Amps
Case AC(Three phase)
Now, load rating again remains the same but this time with 3 phase Ac power. Voltage rating 400V AC
- Fan = 80W (3 phase) (pf=0.85)
- Light = 40W (3 phase) (pf=0.95)
3 phase AC power= √3*V*I*cosφ & I= P/(√3*V*cosφ)
- Fan current = 0.13 Amps
- Light current = 0.060 Amps
- Total power = 120 Watt
- Total current = 0.190 Amps
We have the basic calculation for Electrical current. Now we are going to know about the selection of MCB for particular type of load.
Selection of MCB
MCB– Miniature circuit breaker. MCB is a type of switch cum safety device. This safety device provides protection form overload & short circuit conditions.
There are multiple parameters to select an MCB. Such as :
- Number of poles
- Rated current
- Rated Voltage
- Power type
- Curve type
- SCCR
Definition of different parameters of MCB
| SN | PARTICULARS | UNITS | DEFINITION |
| 1 | Number of poles | – | Parallel MCB circuit. Only circuit means single pole, 2 parallel circuits means 2 poles, 3 parallel circuits means 3 poles. |
| 2 | Current Rating | Ampere | It is the rated current of MCB. Such as 6Amps, 10Amps, 63Amps. |
| 3 | Voltage Rating | Voltage | Rated voltage of MCB. Such as 400 Volts |
| 4 | Power Type | AC or DC | Rated power type of MCB. |
| 5 | Curve Rating | – | Curve rating defines the actual trip current range; proportional to rated MCB current. There are different Curve rating expressed in alphabet such as B, C, D, K. |
| 6 | Short circuit current rating | Kilo Ampere | It is the maximum current interrupted by an MCB without getting damaged. Generally expressed in k Amps. Such as 6K Amps, 10K Amps, 25K Amps, 36K Amps |
Selection of different parameters of MCB
| SN | PARTICULARS | UNITS | DEFINITION |
| 1 | Number of poles | – | Selection depend upon number of phases & circuits. Such as 1-phase requires single pole, 2-phase requires 2 poles, 3-phase required 3 poles |
| 2 | Current Rating | Ampere | Selection depend upon rated load current. Rated MCB current should be selected wisely. It neither be too high or nor be too low. High rated MCB do not trips in fault situations while low rated MCB trips in normal working conditions. That’s why MCB current is selected slightly higher than the rated system current. Surge current, high starting current is also needs to be considered. That’s why MCB rating is selected 1.25 times of rated system current (minimum). For example- Rated load current is 10 Amps. 10*1.25=12.5 Amps. Near by rated MCB 16 Amps. |
| 3 | Voltage Rating | Voltage | Considered equal or more than the system voltage. MCB with 400 Volts rated can be selected for 120 Volts, 230 Volts, 400 Volts. |
| 4 | Power Type | AC or DC | Depend upon system power type AC or DC. |
| 5 | Curve Rating | – | This rating is selected based on the load type. It also considers the place where it is going to be connected, such as homes or industries. B-curve rating MCB has tripping current range is 3 to 5 times MCB current rating. B- curve type is selected for homes. C-curve rating MCB has tripping current range is 5 to 10 times the MCB current rating. C-curve type is selected for industries. |
| 6 | Short circuit current rating | Kilo Ampere | Short circuit current rating of MCB is selected wisely according to load type & placement of MCB such as home or industry. If lower value is selected, MCB can get damaged permanently in short circuit fault situations. Which can be harmful for the system & humans too. 3KA – house 6KA – heavy loads at house 10KA – commercial or industrial 36KA – commercial or industrial |
Correct MCB selection for a load
Next, In a tabular form. MCB rating is selected for practical load type. In this table a load is selected. Than according to the load MCB all the parameters are selected.
| Load Type | Load (Watts) | System Voltage | Power type | System power factor | System Current (A) | 1.25*system current | Recommended rated current of MCB | Curve Type | SCCR | Load example | Pole |
|---|---|---|---|---|---|---|---|---|---|---|---|
| Resistive | 500W | 120 | DC | – | 4.16 | 5.2 | 6A | B | 6KA | Heater | 1 or 2 |
| Resistive | 500W | 230 | AC | 1 | 2.17 | 2.717 | 6A | B | 6KA | Heater | 1 or 2 |
| Inductive | 2000W | 230 | AC-1 | 0.8 | 8.7 | 10.87 | 16 A | C | 10KA | Single phase motor | 2 |
| Inductive | 3000W | 400 | AC-3 | 0.8 | 5.41 | 6.76 | 10 A | D | 10KA/16KA | 3 phase IM motor | 3 |
Practical examples
1. Load calculations & MCB selection For single phase
An image of name plate of Induction motor is shared below. After that load calculation & MCB selection is discussed.
- Rated Power = 1.5KW
- Rated Voltage = 230 V AC
- Phase = Single
- Frequency = 50 Hz
Now, Calculating the current
- P= V*I*cosφ
- I=1500/(230*0.8) (cosφ=0.8 Standard Inductive load)
- I=8.15 Amps
For rated current of MCB, 1.25*Rated load current = 10 Amps (approx.)
- Nearest rated MCB, 16 Amps
- Number of poles, 2
- Curve type, C (Inductive)
- System voltage, 230V AC
- System power, AC
2. Load calculation & MCB selection for 2.2KW 3 phase Monoblock pump
- Rated Power= 2.2 KW
- Rated current= 4.7 Amps
- Rated Voltage= 415 Volts
Since this is Induction motor, taking, 1.25 times of rated current (4.7 Amps)= 5.8 Amps
Rated nearby current of MCB = 10 Amps, Not considering 6 Amps which is pretty much close.
- Curve type, D
- Number of poles, 3
- System voltage, 415 V AC
3. Load calculation & MCB selection for 3 phase 5.5 KW Induction motor
- Rated Voltage = 415 V AC
- Rated Power = 5.5 KW
- Rated RPM = 2900
- Rated current = 10.2
Since this is Induction motor, A high pressure pump taking current 1.5 times of rated current; Which is 15.3 Amps.
- Nearby; MCB rating ; 20 Amps Not considering 16 Amps which is pretty much close.
- Curve type should be D
- MCB poles = 3 poles
Common Mistakes to Avoid
- Underrated MCB → frequent tripping
- Overrated MCB → fire risk ⚠️
- Ignoring starting current
- Wires with Improper or without lugs
- Loose connection at MCB terminals
Final Tips
- Always consider future load
- Use branded MCBs
- Follow wiring capacity