What Size Breaker Does My Generator Need?

Why the breaker matters

The breaker protects the wiring and connected loads from overcurrent. Without it, a fault could overheat the circuit and cause damage. When your generator feeds power to your home through an inlet box and transfer switch, the breaker in the panel is what stops dangerous current from reaching your house wiring. A correctly sized breaker also prevents the generator from being overloaded by allowing only the current it can safely supply.

Your generator has its own built-in breaker that protects the alternator. The breaker on the house side of the connection protects the feeder wiring and branch circuits. Both are needed, and both must be sized correctly. Generator overload protection explains how these protective layers work together. The built-in breaker on the generator is usually a small thermal or magnetic breaker that trips if the alternator draws too much current. The panel breaker, however, is the one that protects the permanent wiring in your home, and it must be selected based on the generator's output and the wiring method.

Find the continuous amp rating

Generator spec sheets list a running watt rating and a starting (peak) watt rating. For breaker sizing, always use the running watts, which is the continuous output the machine can sustain. The peak figure is a surge that lasts only a few seconds, and using it would leave you with an undersized breaker that trips under normal loads. For example, a motor or compressor may draw a surge when starting, but the breaker is not designed for that short interval; it is sized for the steady load.

To get the continuous amps, divide the running watt rating by the generator's output voltage. The result is the current the generator can deliver hour after hour. For example, a generator rated at 5,000 running watts on 120V draws about 41.7 amps, while the same wattage on 240V draws about 20.8 amps. This is why you must check the voltage that matches your transfer switch and inlet box. Most portable generators can supply both 120V and 240V, but the transfer switch and the feeder wiring are often single-voltage, so you need to use the voltage that your circuit is actually supplying.

Some spec sheets show a separate 'continuous' or 'duty cycle' rating that may be lower than the headline running watts. Use the lower number if it is listed. Generator rated vs max watts and generator duty cycle explain these differences in more detail. If the generator has a derated output at altitude or high temperature, you may need to adjust the amp rating downward, which would allow a smaller breaker, but it is safer to base your calculation on the maximum continuous output the generator can produce under normal conditions. Always check the manual for any derating guidelines.

Steps to size the breaker

Follow these steps to choose the correct breaker size for your generator connection:

• Locate the running watt rating on the generator spec plate or in the owner's manual. It is sometimes labeled 'rated watts' or 'continuous watts'.
• Determine the voltage you will use for the connection. This is usually 120V or 240V, depending on your transfer switch and inlet box. Check the rating of the transfer switch and the wiring to confirm.
• Divide the running watts by the voltage to get the continuous current in amps. Use the formula: amps = watts / volts.
• Round up to the nearest standard breaker size. Standard sizes include 15, 20, 30, 40, 50, 60, 70, and 100 amps. For example, if your calculation gives 41.7 amps, you would round up to 50 amps.
• Verify that the transfer switch and the feeder wire are rated for this current. The wire must have an ampacity equal to or greater than the breaker rating. If the wire is smaller, you must downsize the breaker to protect the wire, even if that limits your generator output.
• Install the breaker in the panel following the panel manufacturer's instructions and local electrical codes. If you are not comfortable with this work, hire a qualified electrician.

Voltage and feeder wire considerations

The voltage at which you feed power matters because current is inversely proportional to voltage for a given power. Doubling the voltage halves the current, which means you can use a smaller breaker and smaller wire. For example, a 7,000-watt generator running at 240V draws about 29 amps, so a 30-amp breaker is sufficient. The same generator at 120V would draw about 58 amps, requiring a 60-amp breaker and heavier wiring.

The feeder wire size must match the breaker rating. If you install a 50-amp breaker, the wire must be rated for at least 50 amps. If the distance from the inlet to the panel is long, voltage drop becomes a consideration, and you may need a larger wire even though the breaker stays the same. Generator cord gauge and length provides more detail on wire sizing. Always consult the National Electrical Code (NEC) or a licensed electrician for the exact conductor sizing requirements.

The transfer switch also has a maximum current rating. If you connect a generator that can deliver more current than the switch is rated for, the switch could overheat or fail. For example, a 30-amp transfer switch should not be used with a generator that produces more than 30 amps at the selected voltage. How to size a transfer switch covers this in depth.

Common breaker sizes and typical generators

To give you a quick idea, here is a table of typical breaker sizes for common generator wattages at two different voltages. These are approximate and based on a perfectly resistive load; you should always calculate using your generator's exact running watt rating.

Safety note

Portable generators produce carbon monoxide, which is deadly. The CPSC advises never using a generator inside homes, garages, or other enclosed spaces, and placing it outside far from windows, doors, and vents. Install a battery-operated CO alarm in your home. Always follow the safety instructions in your generator's manual. The CPSC also notes that a single generator can produce as much carbon monoxide as hundreds of cars, so placement is critical.

What to pick for your generator size

Questions

Can I use a larger breaker to get more power from my generator?

No. The breaker protects the wiring and must match the generator's continuous rating and the panel's capacity. A larger breaker would let the circuit draw more current than it is designed to handle, creating a fire hazard. The generator's own overload protection is set to its rated output, so increasing the breaker size does not increase the generator's capability.

Does the generator's built-in breaker replace the one in the transfer switch?

No. The built-in breaker protects the generator's alternator. The breaker in your transfer switch or panel protects the house wiring. Both are needed for different parts of the circuit. If you use only the generator's breaker, a fault in the house wiring could go unprotected, potentially causing an electrical fire.

What if my generator produces both 120V and 240V?

Use the voltage that matches your transfer switch and inlet box, and size the breaker accordingly. The continuous amp rating is different at each voltage, so check both if you have a dual-voltage generator. For example, a 5,000W generator draws 41.7A at 120V but only 20.8A at 240V, so the breaker needs to be sized for the voltage you actually use.

How do I know the continuous watt rating of my generator?

Look for the 'running watts' or 'rated watts' on the spec sheet. It is lower than the starting or peak watts. If a 'continuous output' is listed separately, use that value. Many generators also have a data plate that lists the rated current at both voltages, which can be used directly without calculation.

Can I use a breaker that is smaller than the calculated size?

You can, but it will trip when the generator is loaded to its full capacity. That is not necessarily unsafe, but it will limit the usable output. If you have a 5,000W generator and a 30A breaker at 120V, the breaker will trip at 3,600W. You would need to keep loads below that level. It is better to match the breaker to the generator's rated output.

Do I need a special breaker for a generator circuit?

Usually a standard single-pole or double-pole breaker is fine, depending on whether you are connecting 120V or 240V. The breaker must be rated for the type of panel you have. Some panels require GFCI breakers for outdoor circuits, but that depends on local codes and the specific installation. Consult an electrician for code requirements.