3000W Chinese Gensets Info.

VDOCAD wrote:
Professor:
What I do not understand very well is, why is it that running the A/C from a 15 amp branch circuit is no problem(I tested it, the A/C starts right up), but it may be an issue from a portable generator that puts out 29 amps out of the RV outlet. If the "kick in the pants" is the amps, it seems the generator has plenty to offer.

Good question. To answer it properly I need to spend a moment talking about circuit breakers and 15 amp branch circuits.

A residential type circuit breaker has both a magnetic and thermal trip mechanism. For the magnetic trip mechanism to work, we need sufficient current in a coil of wire around an iron core to create an electromagnetic field strong enough to pull in a release lever that opens the circuit. For the thermal trip mechanism to work we need to have enough current flowing in a bi-metallic strip inside the breaker to make it distort and hit the trip lever. The circuit breaker is designed to protect the circuit it feeds, not you. Therefore there is a lag or delay in the time it takes to trip a breaker.

In the case of the 15 amp breaker in your house, the buss bar the breaker is attached to is most likely being protected by a 200 amp main breaker, and that main breaker is most likely being protected by a 1,000 amp breaker at the transformer. All have a time delay or trip lag.

When your AC starts on the 15 amp residential branch circuit, the 15 amp breaker allows a "surge" of current to pass that is only limited by the available current. We have hopefully established that the available current can be 1,000 amps or more for a brief period of time (usually 80 to 200 msec). This available current surge is enough to get the air conditioner compressor motor past the initial locked rotor current demand without triping the breaker or exceeding the available current the circuit can offer.

A gasoline driven generator simply does not have a current reserve comparable to a residential circuit. When an electric motor or other device needs a high surge of current, the generator craps out because the demand is beyond the capacity. This is why many manufacturers recommend buying a generator that has a surge rating that is at least 2-1/2 times the rated wattage of the appliance(s) you want to run. Fortunately, the start or locked rotor currents on many devices have dropped due to improvements in design. Thus, we can often fudge on the 2-1/2 times rule.