What Is a Main Circuit Breaker and How Does It Work?

A main circuit breaker is the primary overcurrent protection device found in an electrical service panel. Its purpose is to protect the home’s electrical wiring from damage due to overloads or short circuits. Understanding what a main circuit breaker is and how it works is important for any homeowner.

What is a Main Circuit Breaker?

A main circuit breaker, also known as a main breaker or main disconnect, is the large circuit breaker that controls power flow from the utility service to the rest of the electrical system in a home. It is the first line of defense against electrical overloads.

The main breaker is connected to the service entrance wires that provide electricity to the home. These incoming service wires connect to the main circuit breaker, which then distributes power to all the branch circuit breakers in the electrical panel.

Key Features of a Main Circuit Breaker:

  • It is the primary overcurrent protection device found after the utility meter.
  • Larger amperage rating than other breakers in the panel. Often 100 amps or more.
  • Shut off switch for cutting all electric power to the house.
  • Mounted separately from the branch circuit breakers.
  • Connected to thick incoming service entrance wires.
  • Main disconnect point required by electrical code.

The main breaker serves as a central shutoff switch to cut power in emergencies or for maintenance. It also provides overload and short circuit protection for the service panel and whole house wiring.

How Does a Main Circuit Breaker Work?

A main circuit breaker works by monitoring the amount of current flowing through the system. Here is a look at how it operates:

1. Continuous Monitoring

The main circuit breaker continually monitors current flow from the main service wires. It is designed to trip and shut off power if electrical overload conditions occur.

2. Electromagnetism

The breaker uses an electromagnet to control the flow of power. Higher current flow induces a stronger electromagnetic field in the breaker’s trip mechanism.

3. Bimetal Strips

Bimetal strips help sense the current flowing through the breaker. Heating of the strips due to higher currents causes them to bend and trigger the breaker trip.

4. Trip Lever

If the current exceeds the amperage rating, the electromagnetism and heated bimetals will engage a trip lever. This instantly flips the breaker off into a “tripped” position.

5. Power Cut Off

With the breaker tripped, contacts are separated and power flow is stopped. This protects wiring and equipment from damage.

6. Manual Reset

The main breaker remains off until manually switched back to the “On” position. This ensures the overload issue is resolved before power is restored.

In summary, the main circuit breaker provides continuous monitoring of current levels. It trips automatically during an overload to stop the flow of hazardous levels of electricity.

What Happens When a Main Breaker Trips?

When a main circuit breaker trips and shuts off, power will cut out to the entire house or facility. This sudden loss of electricity can be disruptive, frustrating and sometimes dangerous if the outage occurs at an inopportune time.

Here are some key things that will occur when the main breaker trips:

  • All lights, appliances and equipment will abruptly turn off.
  • Electrical outlets will stop working.
  • Furnace, refrigerators and other essential appliances will shut down.
  • Security systems, smoke alarms and electronics will lose power.
  • Sump pumps will be unable to operate, risking flooding.
  • Loss of heating or air conditioning until power is restored.
  • Elevators or other critical equipment may get stuck.

The main breaker trip will create a blackout situation for the entire building until it can be reset. Tripping during severe weather or at night can make the outage more hazardous. It’s crucial to identify and resolve the trip cause before switching the main breaker back on.

What Causes a Main Circuit Breaker to Trip?

There are several potential causes for a main service panel breaker tripping:

Electrical Overload

Too much current flow through the main breaker can cause overheating and tripping. Major appliances, HVAC systems or other equipment drawing too much power can create overload issues.

Short Circuit

A short circuit occurs when electricity takes an unintended detour around an appliance. This bypass draws very heavy currents that instantly trip the main breaker.

Damaged Breaker

Excessive heat, age and corrosion can damage the breaker internals over time. This causes the trip mechanism to malfunction and nuisance tripping may occur.

Loose Wiring

Loose incoming service wires or bus bar connections in the breaker panel can create points of high resistance. These hot spots will trip the main breaker.

Faulty Appliance

An appliance with a shorted motor, bad wiring or other electrical fault can suddenly start drawing very heavy currents. This abnormal load may trip the main breaker.

Lightning Strikes

A lightning strike or power surge coming in from the utility service may instantly overload and trip the main circuit breaker.

When the main breaker trips repeatedly, it’s vital to have an electrician troubleshoot and find the root cause of the problem.

Main Breaker Ratings and Sizes

Main breakers are available in a range of amperage sizes to match different electrical service needs:

  • 60 amp – Typical for smaller homes or garages with limited electric demand.
  • 100 amp – Minimum size for most modern homes with moderate power needs.
  • 150 amp – Used in larger homes with all electric appliances, central A/C etc.
  • 200 amp – Provides capacity for very large homes with premium amenities and electronics.
  • 400 amp – Only needed for the largest estates, industrial sites or apartment buildings.

Larger main breaker sizes cost more initially but provide capacity for future expansion. They also run cooler and are less prone to nuisance tripping.

Choosing the right main breaker size involves calculating the home’s estimated electric load and allowing room for unforeseen demand. Most electricians recommend at least 100 amp capacity for new homes.

Main Breaker Upgrade

Homes with frequent main breaker tripping may require a breaker panel upgrade. This involves replacing the existing main breaker and panelboard with a new larger capacity system.

Reasons for upgrading the main breaker include:

  • Existing breaker is outdated or underrated for the home’s needs.
  • Need to add more branch circuits to handle added electrical loads.
  • Existing 100 amp breaker trips too easily. Upgrade to 150 amp or 200 amp for more capacity.
  • Correct chronic tripping issues due to undersized main breaker.
  • Home additions or remodeling have significantly increased the electrical demand.
  • Existing fuse panel needs upgraded to modern main breaker panel.
  • Correct faulty old breakers that fail to trip when necessary.

Main breaker upgrades typically cost $1000 to $2500 and are completed by licensed electricians. The work involves replacing the main panelboard, breakers, and service wires. Permits are also required for this type of electrical service upgrade.

How to Reset a Tripped Main Breaker

When the main breaker trips and cuts power to the home, the issue must be resolved before resetting the breaker. Here are the steps to safely reset a tripped main circuit breaker:

  1. Locate and inspect the main breaker in the service panel. Verify it is clearly switched to the “Off” position.
  2. Unplug major appliances and equipment in the home prior to resetting the breaker.
  3. Check for any signs of electrical damage, burnt wiring, sparks or smoke. Call an electrician if any hazardous conditions exist.
  4. Thoroughly check the house circuits to determine the cause of the overload.
  5. Correct any identified overload issues before proceeding.
  6. Flip the main breaker switch to the “On” position to restore power.
  7. Monitor the breaker carefully over the next day. Turn off again if it trips again.
  8. Call an electrician if the main breaker continues to trip for no obvious reason.

Resetting the main breaker is only the first step. Additional troubleshooting is needed to uncover the root cause of the tripping to prevent potential electrical fires or damage.

Main Breaker Safety Tips

Here are some vital safety precautions regarding main circuit breakers:

  • Clearly label the main breaker to avoid accidentally switching off power.
  • Only flip main breaker off/on with dry hands for safety.
  • Keep area around panel free of clutter and obstructions.
  • Call an electrician for any signs of burning smells from panel.
  • Do not overload circuits. Balance high loads across multiple circuits.
  • Address any frequently tripped breakers immediately before a fire hazard develops.
  • Keep outdoor panel covers closed and locked to prevent access by children.
  • Have old Federal Pacific or Zinsco type panels replaced, as they are prone to failing.
  • Conduct occasional thermal scans of the main lugs and wires to check for hot spots.

The main circuit breaker is the first line of defense against electrical fires and shock hazards. Understanding how to properly maintain and operate it is key for home safety.

Frequently Asked Questions About Main Circuit Breakers

What is the main difference between a main breaker and branch circuit breakers?

The main breaker is significantly larger, rated for higher amps, and is directly connected to the incoming service entrance wires. Branch breakers have lower amp ratings and distribute power to individual circuits only.

What should I do if water leaks onto the main circuit breaker panel?

Immediately shut off the main breaker and call an electrician. Water leakage onto a panel is an electrocution hazard. The affected breakers and wires will need replacement.

Why does my main breaker keep tripping with nothing plugged in?

Frequent unexplained tripping may indicate a faulty breaker or loose/damaged service wiring. Contact a qualified electrician to inspect and repair the electrical panel.

Is it safe to regularly use the main breaker as an on/off switch?

No. Constantly cycling the main breaker stresses the electrical system. Frequent on/off switching should be avoided unless necessary.

Can I upgrade my main breaker myself?

Main breaker upgrades require working on service entrance wires. This very dangerous work must be done by licensed electricians only. Never attempt a DIY breaker upgrade.

How often should I have my home’s main breaker inspected?

Electricians recommend having the main breaker thoroughly inspected every 5 years or so. Thermal scans can help spot loose wire connections before they fail.

Why does my main breaker make a buzzing or humming noise sometimes?

This may indicate a loose wire connection on the breaker lugs causing minor arcing. Have an electrician inspect and tighten all connections.

What is the minimum amp rating for a kitchen circuit breaker?

Kitchens require dedicated 20 amp small appliance branch circuits. The main breaker must be large enough to supply these and other required home circuits.

Can I leave the main breaker off for several days while on vacation?

No, you should leave it on. Certain appliances like refrigerators and sump pumps need continuous power. Also, the sudden surge when turning back on can damage electronics.


In summary, the main circuit breaker is a crucial safety component protecting the home from electrical hazards. It works by continuously monitoring current levels and automatically shutting off power if overload conditions occur. When sizing the main breaker, it is important to consider future expansion needs. Upgrading an undersized or outdated main breaker can prevent nuisance tripping and enhance home electrical safety. With proper maintenance and operation, the main breaker will provide many years of reliable service.