AC DC Full Load Current Calculation Formula

Full Load current calculation of AC and DC machine:

What is Full Load Current:

Full Load Current is nothing but a maximum allowable current. The input current to the machine exceeds the full load current means, the electrical machine may get damage. Due to the excess current flow, the machine produces additional heat (Because of P=I² * R)., This may lead to damages to the insulation or winding of the electrical equipment. Hence, Operating a machine below full load current increases the life span of the electrical equipment.

AC Loads (Alternating current) :

Full Load current calculation 3 Phase Motor:

In most of the Three-phase systems, electricity consumption happens through star and delta connection. The input power (P) to the system is the same, irrespective of the connection.

Power in KW (Kilowatts)

V= Voltage +/- 10 % in Volts

I= Full load current in Amps

Cos pi = power factor

Three Phase Power P = 3 V*I* Cos pi

KW = output power in Watts……. All are given at the nameplate details.

As we said, the full load current of the three-phase system is depending on the type of connection.

Here

Iph => Phase Current

Iline => Line Current

For Star Connection the Full Load Current Iline is equal to Iph

Iph = Iline

For Delta Connection the Full Load Current Iline is equal to 1.732 times of Iph

Iph/1.732 = Iline

Hence Three Phase full load current I is equal to

I= P/(1.732*V*Cos pi)

Full Load current calculation Single-phase Motor:

P = V * I * Cos pi

V= Voltage +/- 10 % in Volts

I= Full load current in Amps

Cos pi = power factor

KW = output power in Watts……. All are given at the motor’s nameplate details.

• I =P / V * cos pi Amps

Full Load current calculation Three-phase Heater coil:

Power formula  KW

V= Voltage

I= Full load current in Amps

KW = output power in Watts……. All are given on the heater’s plate details.

• Iinput =P / 1.732 * V * I Amps.

Full Load current calculation Single phase heaters:

Power formula  KW

V= Voltage

I= Full load current in Amps

KW = output power in Watts……. All are given on the heater’s plate details.

• I = V X I Amps

Calculate through resistance:

1. Measure the resistance R of the heater coil using multi meter.
2. Use this formula

P = I^2 * R

• I = square root of (P/R)

Also see: How to Calculate Voltage Drop

Full Load current calculation DC machine (DC Motor & DC generator):

DC => Direct Current

P= V X I

• I = P/V amps, Where

V = E ± Ia Ra ± Is Rsh + brushes drop (shunt machine)

V = E ± Ia (Ra + Rsh) + brushes drop (series machine)

V = Supply Voltage

E = back emf

• Also see: Difference Between Voltage and EMF

Ia = armature current

Ra = armature resistance

Is = Field current

Rsh = Field resistance

Back EMF e = (pi * N * P * Z / 60 A)

Pi= Magnetic Flux

N= speed of the machine

P = number of poles

Z = number of conductors

A = number of parallel paths

P = A for lab winding

A= 2 for wave winding

Myths About Full Load Current:

1. Full Load Current For aluminum cable is o.8 per square meter
2. For Copper Cable is 1.2 per square meter
3. 3 phase 415V, at 0.8 pf, 1 Hp motor full load current = 1.3 Amps.
4. 1 Phase 230V, at 0.8 pf, 1 Hp motor full load current = 4 Amps.