AC DC Full Load Current Calculation Formula

Full Load current calculation of AC and DC machine:

The full load current is used to design the protection system for the electrical equipment.

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 Motor Loads (Alternating current) :

AC loads consist of resistive loads, inductive loads. Resistive loads are water heater, room heater etc. Inductive loads are induction furnace, single induction phase motor, three-phase motor etc.

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.

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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
Hence, Three-phase motor full load Current I = P / (3 * V * Cos pi)

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

Look at the above formula, the three-phase full load current is equal to Power divided by the 3 times of product of line to neutral voltage and power factor.

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)

Here three-phase full load current is equal to the power divided by 1.732 times of line to line voltage and power factor.

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Full Load current calculation Single-phase Motor:

Single-phase motor full load current I is equal to power P divided by the power factor times of line to neutral voltage.

P = V * I * Cos pi

Full load current I = P / (V x Cos pi) Amps

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.

Full Load current calculation Three-phase Heater coil:

For three-phase, full load current for the resistive load is equal to the three-phase power divided by 1.732 times of voltage. Here power factor will be unity for resistive loads.

As you know the Power formula,

P = 1.732 x V x I

Full load current I,

I =P / 1.732 * V Amps.

V= Line to Line Voltage

I= Full load current in Amps

If you consider the line to phase voltage means, the full load current formula become,

I =P / 3 * V Amps.

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

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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.

P = V X I Amps

Full load current for the single-phase heater will be,

I = P / V Amps

Calculate through resistance:

  1. Measure the resistance R of the heater coil using multimeter.
  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

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

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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.

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