Don't Let Generator Sizing Trip You Up

Prior to heading to the rental center, confirm requirements for voltage, amperage and single- or three-phase load. Neglecting to have any of this information in hand could result in bringing back the wrong size unit. For example, a customer asking for a “200-amp generator” could actually need a unit ranging in size anywhere from 30 to 166 kVA, depending on the specific load requirements.

In addition, make sure the generator has enough receptacles, and that the load will not exceed the amp rating of each receptacle. Ensure power cords are in good condition and sized correctly according to the generator operator manual. “Choosing incorrect power cords can overheat and overload both the generator and the tools powered,” says Bernier.

When Over-sizing is Unavoidable

Running loads at less than 30% of generator rating can potentially cause wet stacking. Yet, running small loads can’t always be avoided.

When this is the case, Bernier recommends running the generator with a full rated load for two to four hours to clean out the engine. This should be done once or twice a year; after the job is completed; or when the generator is returned from rent. Owning or having access to a load bank that matches or exceeds the size of your generator is a convenient way to accomplish this.

Operating the generator in cold ambient conditions where it may be difficult to achieve normal engine operating temperatures (190° F) can have the same adverse effects as over sizing. Bernier suggests increasing the load if possible, or periodically using a load bank to minimize the risk of wet stacking.

NOTE: Restricting the generator set air flow to raise engine temperature is another possible cold weather solution. However, Bernier cautions that the air restricting device must be thermostatically regulated, and the air flow must be fully restored once engine temperature reaches 200° F.