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 appears as if it's 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 states either starting amps or code letter designation. If you know the code letter you can obtain nominal starting kVA
STARTING/HP ratio from NEC Table 430.7(B). After you calculated kVA
STARTING, for single-phase coils you just multiply it by 1000 (to convert kVA to VA)and divide by nominal voltage. For eaxmple, for 240V appliances:
LRA=1000*kVASTARTING/240. For more details see our
LRA calculator. If you can't find neither nameplate LRA nor code letter, the inrush amps can be measured. If you are technically inclined you could do yourself. You will need a
clamp-on meter with Peak (surge) capability. To use it, 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. Most likely your high wattage device goes to a separate breaker. You would need to open front panel of the main circuit box and locate the wire that goes to this breaker. Just please, 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 sizing procedures for standby and portable gensets, a chart of typical starting and running currents of single-phase 240VAC central air conditioners depending on their size, 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 important
lesser known detail you need to know. Most other guides will tell you to pick a model with surge current matching
nominal 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 nominal LRA or the one calculated from the code letter is given for
full voltage starting. In reality, when you start a large motor from a generator, its initial current surge causes voltage dip. When voltage drops, the current consumed by the motor is reduced proportionally. Most residential appliances can start with up to 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. To be clear, it does not mean you need a 17kW device- you just need a device that can provide that much of surge power. Note that commercial applications normally allow only 15% drop, in which case you 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 only 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 just 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 Steady State Power (kW) | 7 | 10 | 13 | 14 | 16 | 17 | 18 | 20 |
Surge Current Capability (240VAC 1 Ph)
(Amps @ 30% Voltage Dip) | 46 | 63 | 95 | 102 | 117 | 125 | 133 | 145 |
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. Such a device could initially provide (240*0.7)*102=17,136 VA. Note that during steady state operation such an a/c will consume only about 6 kW. So, you may 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 surge amps of each such load one at a time.
Then add these values to the total running current of all remaining appliances. This will give you net surge current requirement of your backup system. See our generator sizing
guide for more 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 disconnect part. 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 removed from the motor, the solid state material begins to cool down, which takes a few minutes. If AC voltage is re-applied during the cooling off period, the hard start capacitor may be ineffective because it is still effectively 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.
Note. Be sure to remove power for some 10 minutes before you connect a hard-start device- the capacitor may still keep the charge!
Above analysis and examples are based on typical numbers. They are provided "as is"for reference only and
don't constitute a professional or a legal advice- see complete Disclaimer linked below. Refer to your product specification and operational manual for all decisions.