SIZING GENERATOR TO START AND OPERATE CENTRAL AIR CONDITIONER
AND ELECTRIC MOTORS



Motor driven appliances (such as refrigerators and air conditioners) for initial start-up require larger amounts of current than when they are running. This is because induction motors initially act like a short-circuited transformer. The maximum start up current is referred to as "Locked Rotor Amps" (LRA) because at the first moment when the rotor is at standstill it appear as if it is locked. This current will drop significantly when motor accelerates to about 75% full speed. The LRA is typically 3 to 8 times continuous operating current (called full load amps, or FLA). Note, this does not equate to 3 to 8 times real (active) power because the power factor of a starting motor is low (<0.5). That's why when selecting a home generator for starting requirements, you must consider primarily its current surge capability.
By the way, refrigerators may have lesser overall ratio between LRA and FLA because of the resistive heaters that are periodically connected to defrost the freezer.

HOW TO FIND LRA


An appliance nameplate usually has either starting amps or code letter designation. If you know the code letter you can obtain approximate initial kVA/HP ratio from NEC 2014 Table 430.7(B). After you calculated kVA, for single-phase coils you just multiply it by 1000 and divide by nominal voltage. If you can't find neither nameplate LRA nor code letter, the inrush amps can be measured. You will need a clamp-on meter with Peak (surge) capability. To use it, you will have to clamp a single wire in the cable that feeds your device. For a cord-and-plug appliance, this can be done with an AC line splitter. Alternatively, you can take an extension cord, carefully remove a few inches of the external sleeve and pull black or white lead out of the bundle. For a hardwired unit it may be a bit more complicated-- you would need to figure out how to reach a single string-- just don't try to do it by yourself unless you have proper electrical training. Anyway, here is a basic measurement procedure. First of all, power off your unit. Set the multimeter to Peak reading, put on line-worker rubber gloves and enclose the wire within the clamp. Finally, turn on your unit and take the reading.
For a chart of starting and running currents of single-phase 240VAC central air conditioners depending on their size, complete step-by-step genset sizing procedure for standby and portable setups and much more get my Home Generator ebook.


SIZING GENERATOR TO START A MOTOR


Once you know LRA, you can pick a genset. However there is one lesser known detail you need to know. Most guides will tell to pick a model with surge current matching inrush current of your motor. Well, with such an advice you may wind up with twice larger generator than you really need. The case is that nameplate LRA is given for full voltage starting. In reality, when you start a motor from a generator, the current surge causes voltage dip. When voltage drops, the current is reduced proportionally. Most residential appliances can start with 30% voltage sag, that is at 30% lower currents. As the result, starting volt-amps could be 0.7*0.7=0.49 of nominal. For example, a typical 5-ton (5HP) a/c has 145 LR amps at 240VAC. At 30% voltage dip it would require (145*0.7)*(240*0.7)=17,052 VA to start. Commercial application normally allow only 15% drop, in which case you would need to deal with 0.85*0.85=.72 of nominal starting kVA.

By the way, the HP numbers for air conditioners may be confusing for some. Indeed, technically, 1 ton refrigeration is 4.7 hp or 3.5 kW. However, in case of air conditioners, electricity is used to pump energy from a cold area to a hot area. With a typical efficiency, 1 kW of electric power can transfer 3 to 4 kW of cooling. That's how 5 ton a/c can have 5 hp motor.

Genset manufacturers often specify their models' surge wattage capability, but unfortunately, they rarely state LRA capability. The chart below shows typical data for standby generators.

Generator's Rated Power (kW)710131416171820
Surge Current Capability at 240VAC 1 Ph
(LR Amps @ 30% Voltage Dip)
466395102117125133145
Example. Let's find what generator you need for a 5-ton air conditioner. At nominal voltage such an a/c would initially draw around 145A. But at 70% voltage it would need only 145*0.7=101.5 A. From the genset chart above we see that to provide such an inrush current you need a system rated 14kW or greater. Note that during steady state operation such an a/c will consume about 6 kW. So, you would have up to 8 kW available to run other devices in your home.
If you have several motor driven loads, the calculation gets a bit more complicated. You will need to find the load with the greatest difference between surge and running amps.
REVIEW AND RATINGS TABLE OF STANDBY GENERATORS FOR HOME USE
Then add that difference to the total running current of all appliances. This will give you net surge current requirement of your backup system assuming multiple devices rarely start up at exactly the same time. See our generator sizing guide for details. If you are buying an automatic system without an "intelligent" load control, be aware that after detecting a service interruption it may try to activate all your motors simultaneously. With such a system you'll need a genset with the capacity to provide the total starting current. Otherwise, the motors may trip the genset's circuit breaker or can overheat and even burn out. Alternatively, you may choose to set your standby system to manual mode. Then in an emergency you could first turn on the central a/c and then all other loads sequentially.





If your genset's surge current capability turns out to be less than needed to start your central a/c, it may require some form of assisted starting. You may need to install a "hard start" kit, which is quite cheap. It is basically a large capacitor in series with a relay. A two-wire device has to be connected with "piggy-back terminals" parallel to the existing "run capacitor" (these terminals may be marked RUN). Such a device usually has a solid-state relay. It is basically a PTC material that rapidly increases in resistance as it is heated when an electric current passes through it. As the result, it disconnects the start capacitor from the circuit soon after power is applied. The PTC material then remains hot from the "trickle current" that continues to flow through it as long as there is voltage. Note that when the power is disconnected from the motor, the solid state material begins to cool down, which takes one to two minutes. If AC is re-applied during the cooling off period, the hard start capacitor may be ineffective because it is still disconnected. Other designs use a potential relay with voltage or current sensing to determine when to disconnect the cap. They have three wires that have to be connected to Common, Start and Run terminals. Be sure to remove power for 10 minutes before you connect a hard-start device!