100A AC Load Monitoring Circuit using Arduino, Watt Limit, LCD, Alarm, Auto Shutdown

In this project we are going to make one very useful grid power protection circuit. This will be like a power guard system which we can use in homes or apartments to monitor the wattage of the load and also to cut off the load automatically if it starts taking too much wattage, more than what we allowed. This circuit is very good if we want to protect our wiring or prevent overloading in rented flats or inside any inverter output etc.

What this project will do?

Let us first understand that what exactly this project will be doing for us. So this Arduino project will do these main things:

First it will read the current of the AC load using a 100A current sensor, like ACS758 or SCT-013.

Then it will multiply the current with the voltage to calculate the power in watts. Because as we know Power = Voltage × Current, right?

After that, it will show this power value on an LCD display with I2C so we can see how many watts are being used.

Now we will have two push buttons, one for increasing and one for decreasing the power limit in watts. So we can set the max wattage we want to allow.

Then if the load starts taking more than this wattage limit, then system will start warning using a buzzer alarm.

After the limit is crossed 3 times, then system will switch OFF the relay and cut off load automatically to protect.

All the process like watt, limit, shutdown etc. will be shown on the LCD screen so that we can know what is going on.

Parts we need for building this project

Let us now see what parts we must collect to build this full circuit.

Complete Circuit Diagram

How to connect everything

Now let us try to understand how we are going to connect all these components together with Arduino:

We take the output pin of ACS758 current sensor and connect it to A0 analog pin of Arduino.

Then we take SDA and SCL pins of LCD I2C module and connect them to A4 and A5 of Arduino respectively.

We take the push button for UP and connect one side to D2 pin, and the other side to GND. We also connect one 10K pull-up resistor to make it work correctly.

Same way, we connect DOWN push button to D3 of Arduino.

Then we connect the IN pin of relay module to D7 pin, so that Arduino can control the relay.

For the buzzer, we connect positive side to D8 and negative to GND.

The ACS758 sensor VCC pin we connect to 5V, and GND to GND.

Load wires must go through the sensor so that it can sense correct current.

How this circuit works in step-by-step logic

Now let me explain the main working logic of this project in a super simple way:

First when we switch ON everything, Arduino starts reading the analog value from the current sensor.

This analog value tells how much current is flowing in the load wires.

We already know the voltage of AC mains, like 220V or 110V, so Arduino multiplies Current × Voltage to get the Power in Watts.

Arduino then shows the live wattage on LCD. So we can see how much power the load is taking right now.

Now we can use UP and DOWN buttons to set a Watt limit, like 500W or 1000W or whatever we want.

This limit value is also shown on LCD, so we can know what limit is set.

Then if the load power crosses this watt limit, then Arduino will ring the buzzer to give warning.

Arduino will count how many times the power crossed the limit.

If it happens 3 times then Arduino will turn OFF the relay and the load will be shut down to avoid damage.

LCD will now show “LOAD SHUTDOWN” so we know it is OFF.

This is how the full system works in loop.

Full Arduino Code

Now here is the full Arduino code which we can upload directly into the Arduino UNO board.

#include <Wire.h>
#include <LiquidCrystal_I2C.h>

LiquidCrystal_I2C lcd(0x27, 16, 2);

const int currentSensorPin = A0;
const int buttonUp = 2;
const int buttonDown = 3;
const int relayPin = 7;
const int buzzerPin = 8;

float current = 0.0;
float voltage = 220.0; // Fixed AC voltage
float power = 0.0;
int wattLimit = 500; // Starting default limit
int warningCount = 0;
bool relayOn = true;

void setup() {
  pinMode(buttonUp, INPUT_PULLUP);
  pinMode(buttonDown, INPUT_PULLUP);
  pinMode(relayPin, OUTPUT);
  pinMode(buzzerPin, OUTPUT);

  lcd.begin();
  lcd.backlight();
  lcd.setCursor(0, 0);
  lcd.print("AC Load Monitor");
  delay(2000);
  lcd.clear();
}

void loop() {
  readCurrent();
  power = current * voltage;

  lcd.setCursor(0, 0);
  lcd.print("Pwr:");
  lcd.print(power, 1);
  lcd.print("W  ");

  lcd.setCursor(0, 1);
  lcd.print("Limit:");
  lcd.print(wattLimit);
  lcd.print("W ");

  if (digitalRead(buttonUp) == LOW) {
    wattLimit += 50;
    delay(300);
  }
  if (digitalRead(buttonDown) == LOW) {
    wattLimit -= 50;
    if (wattLimit < 0) wattLimit = 0;
    delay(300);
  }

  if (power > wattLimit && relayOn) {
    warningCount++;
    beepWarning();
    if (warningCount >= 3) {
      digitalWrite(relayPin, LOW);
      relayOn = false;
      lcd.setCursor(0, 1);
      lcd.print("Load SHUTDOWN   ");
    }
  }

  if (relayOn) {
    digitalWrite(relayPin, HIGH);
  }

  delay(1000);
}

void readCurrent() {
  int sensorValue = analogRead(currentSensorPin);
  float voltageReading = (sensorValue / 1023.0) * 5.0;

  float offset = 2.5;
  float sensitivity = 0.04;

  current = (voltageReading - offset) / sensitivity;
  if (current < 0.3) current = 0; // remove noise
}

void beepWarning() {
  digitalWrite(buzzerPin, HIGH);
  delay(300);
  digitalWrite(buzzerPin, LOW);
}

A few Ideas to Improve the Project (Optional)

If we want to save the watt limit even after power goes OFF then we can use EEPROM in Arduino to store it.

We can add one reset button to turn ON the load again if it goes OFF after shutdown.

If we want real-time voltage also then we can use ZMPT101B voltage sensor and multiply real voltage × current to get full accurate power.

We can also add LCD scrolling or extra rows to show more messages like warning, counter value etc.

What happens after shutdown?

So in our current code, after 3 warnings, Arduino cuts the relay by doing digitalWrite(relayPin, LOW);, and then sets relayOn = false;.

Now because relayOn = false, Arduino will never turn ON the relay again even if the power becomes zero.

Then every time the loop runs, this line:

if (relayOn) {
  digitalWrite(relayPin, HIGH);
}

will not activate because relayOn = false. So Arduino keeps the load OFF permanently.

So is this shutdown permanent?

Yes bro, this shutdown is permanent in our current code. Once it shuts down the load after 3 warnings, it will stay OFF forever.

This is actually good for safety because it means some human must check what went wrong, fix the overload and then manually reset the system.

Can we reset the overload lock just by switching OFF/ON the power?

Yes we can do that easily.

So what happens is:

• When Arduino is running, it stores all values like warningCount, relayOn, etc. only inside RAM (temporary memory).
• This RAM gets erased the moment you cut the power to Arduino.
• So now when we switch ON power again, then Arduino starts from the top, like fresh baby, with all values reset to default.

Conclusion:

So friends, this is how we can build one smart and powerful AC Load Monitoring System using Arduino with full wattage calculation, limit setting, warning alarm, and auto shut down feature. We used 100A sensor so we can handle big current loads also. And the LCD shows everything live.

If anybody wants to protect their AC wiring or apartment or inverter from overload, this is the best project to try.