All electrical equipment and devices consume power and, of course, draw electric current. Calculating current is one of the basic calculations in electrical engineering.
I have built a free android app calculator to make it easy for you to make electrical calculations.
Electric Current formula
Current formula for DC, AC 1 phase and 3 phase current are:
- I (DC) = V/R
- I (1 phase) = P / (V * cosΦ)
- I (3 phase)= P / (√3 *V * cosΦ)
Where,
I : is the current
P : is the power
V : is the voltage
cosΦ : is the load power factor
For more information about load power factor, read my article here.
Now, let’s move forward and have examples.
AC Current formula Example
AC loads are divided in two, single and three phases loads.
The electric current formula is almost the same for both, except using (√3) in case of three phase loads.
Note that, you need to convert load power from horse power to watt, before you use it in the equipment.
Fortunately, I have created an android electrical calculator that can calculate AC current of HP and KW loads without implementing HP to W conversion. Fast electrical calculator, is my app on Google play, install it now 100% Free
Single phase current calculations
Single phase loads are the loads working on one phase and neutral power source, 1 phase loads are like single phase induction motor, domestic loads and lighting fixtures.
To calculate current for single phase loads we use the following electrical power formula:
P = V * I * cos Φ
Then, I = P / (V * cosΦ)
Where
P : Electrical power in Watts
V : voltage in volt
I : current in amps
cos Φ : is the power factor of the load
In instance, We have a 1 phase induction motor of 10 HP, operating voltage 110V and power factor of 0.8
To Implement the current formula, we should convert HP to Watt, 10HP = 10*746 = 7457 Watt
I = 7457/ (110*0.8) = 84.7Amp
Note, if the load is a resistive load, like heaters, its pf is unity i.e = 1
If the load above is a 7457 Watt heater, its current will be I = 7457/110*1 = 67.97Amp
Three phase loads calculations
Three phase loads are the loads working on one three power source.
3 phase loads are like, three phase induction motors and most of industrial loads.
To calculate current for three phase loads, we use the same mentioned formula but with a little change as follows:
P =√3 * V * I * cos Φ
Then I = P / (√3 *V * cosΦ)
Example,
Assume we have a three phase induction motor, power rating 15 HP, voltage 400V and power factor 0.85, calculate the motor current!
Convert motor power from HP to W, 15HP = 15*746 = 11185.5 Watt
Motor current I =11185.5/ (√3 *400*0.85) = 19.46 Amp
Note, if the load is a resistive load, like heaters, its pf is unity i.e = 1
If we have a three phase heater of 1118.5 Watt, 400V its current will be,
I (Heater)= 11185.5/ (√3 *400*1) =16.14A
Online Current calculator for AC Loads
Electrical4uonline design engineer made a simple online electrical calculator for AC loads, try it below
DC current calculation formula
Direct current is the current with direct value and magnitude, i.e no changing in direction or value like AC current, DC current source like batteries and dynamos.
Using Ohms law mentioned above we can easily calculate DC current as follows
I=V / R = P/V
P : DC power in watt
I : DC current.
V : DC voltage.
R : Electrical resistance.
DC current calculations example,
A DC motor, 25 KW, operating voltage 240 DCV what is its current?
I = (25*1000)/240= 104.2 Amp
Electric Current formula in Physics
As discussed above, that current is the flow of charge-carrying particles. Therefore, the first electric current formula in the term of charge may be written as.
If the flow of current is variable, then the amount of current during a specific interval will be
I=dQ / dt
The symbol of current is I, while the SI unit of current is Ampere. One Ampere of current is the motion of 6.25 × 108 electron particles in one second.
The unit of charge is coulomb and time is second. Therefore, the corresponding unit of current (Ampere) will be (Coulomb/Second).
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