Generator Inrush Current and Locked Rotor Amps Explained

What is inrush current?

Every time an electric motor starts, it needs an extra burst of power to get its shaft turning from a standstill. That brief demand, called inrush current or starting current, lasts only a fraction of a second, but it can be much higher than the current the motor draws once it reaches full speed.

Generators reflect this in their specifications. Most portable and standby units list two numbers: a running (continuous) watt rating for steady loads and a surge (starting) watt rating for the short bursts that motors need. The difference between the two exists largely to cover inrush current. For a fuller explanation of how these ratings relate, see running watts vs starting watts, and for the basics of watt ratings, see what generator watts mean.

Locked rotor amps and why they matter

Locked rotor amps, or LRA, is the term for the current a motor draws when its rotor is held stationary, which is close to what it draws the instant power is applied. At that moment the motor has not yet built up any back voltage, so the current is limited mainly by the resistance and inductance of the windings. The result is a current spike far above the motor's normal running current.

Manufacturers of compressors, pumps and other motor-driven equipment often stamp an LRA figure on the nameplate. That number is useful when you size a generator, because it tells you the heaviest starting current the equipment will ask for. Comparing the LRA of a well pump or air conditioner against a generator's surge capability is a reasonable way to judge whether the unit can start the load.

LRA is not the only factor. The duration of the surge also matters, since a generator must hold its voltage and frequency while the motor accelerates. A motor that starts easily and reaches speed quickly is friendlier to a small generator than one that labors for a long moment.

Which appliances have the biggest starting surges

The largest inrush currents in a typical home come from appliances that cycle compressors: refrigerators, freezers, air conditioners and heat pumps. A compressor starts against high pressure in its plumbing, so the motor has to work hard for a moment before the system settles into steady operation.

Pumps are another common source of starting surge. Sump pumps, well pumps and sewage pumps all use induction motors that draw heavily at startup. Power tools with large motors, such as table saws and air compressors, also have noticeable inrush current, although their running watts are usually modest.

Resistive loads such as incandescent bulbs, space heaters and electric ovens do not have a meaningful inrush surge. Their power demand rises smoothly with the voltage, so they stress the surge rating of a generator much less than a motor of the same running watts.

How to size a generator for starting surge

The practical rule is to add up the running watts of everything you expect to power at the same time, then add the margin needed for the single largest starting surge that will occur while the others are running. If you plan to start a refrigerator while the lights and a furnace fan are on, the surge rating must cover the refrigerator's starting demand on top of the already-running load. For a step-by-step approach to the whole load picture, see how many watts you need.

Sequence your starts. Starting one large motor at a time is much easier on a generator than starting two at once, because the surges are not combined. If your layout lets you switch on the well pump and the refrigerator separately, the required surge rating is lower than if both must start together.

For a generator that must run a compressor surrounded by other loads, choose a unit whose surge rating leaves clear headroom. In practice this usually points to a model a size class or two above what the steady load alone would suggest. Browse the best portable generators, the best home standby generators and the best 5001 to 9000 watt generators for units that balance running and surge output.

Run the generator outdoors and away from openings

Generators produce carbon monoxide (CO), an odorless, colorless gas that can be deadly. CPSC warns that one portable generator produces as much CO as hundreds of cars, and that CO can incapacitate and kill within minutes. Never run a generator inside a home, garage, basement, crawlspace or shed, even with doors and windows open.

Place the generator outside and well away from windows, doors and vents, with the exhaust directed away from the home. CPSC and CDC both advise keeping at least 20 feet of distance from the house. Install battery-operated CO alarms on every level of the home and outside sleeping areas, and check the batteries regularly.

For more on running a generator safely while it is loaded, see generator safety checklist and carbon monoxide safety.

Start heavy loads one at a time

Starting a motor is the most stressful moment for a generator. To keep the engine from bogging down and the output voltage from dipping, start the largest motor first, let it reach running speed, then add other loads. This avoids overlapping inrush currents that could exceed the generator's surge rating.

If a motor fails to start or the generator labors, reduce the load before trying again. Drop the air conditioner, heater or other large draw, start the motor, and then restore the other loads. This makes the unit's job easier and keeps frequency and voltage steadier for the appliances that stay on.

For generators with electric start, see electric start considerations. For more on keeping the generator healthy under load, see how to load test a generator.

What to pick for your use

Questions

Why does a motor draw more current when it starts?

At the instant of startup, the rotor has not yet built up any counter voltage that normally limits current. The motor's windings appear almost as a short, so the current is limited only by their resistance and inductance. That causes a spike far above the running current.

What is the difference between inrush current and locked rotor amps?

Inrush current is the general term for the extra current a motor draws as it accelerates to full speed. Locked rotor amps (LRA) is the current the motor would draw if the rotor were held still, which is the maximum the motor can demand. LRA is a useful spec for comparing starting difficulty.

How much surge capacity do I need for a refrigerator?

Add the refrigerator's starting watts to the running watts of everything else you plan to have on. The generator's surge rating must cover that combined total. Because compressor motors have the largest surges, it is safest to allow clear headroom above the steady load.

Can I start two large motors at the same time?

It is possible but not recommended. Each motor's inrush adds together, which may exceed the generator's surge rating. Starting them sequentially reduces the peak demand and prevents the generator from bogging down.

Are there loads that do not need surge capacity?

Resistive loads like incandescent lights, space heaters and electric ovens draw nearly the same current from start to finish. They do not create a meaningful surge, so only the running watts matter for sizing.