The maximum electrical load for generators and their actual electrical load ratings are generally not the same thing!
For advertising purposes, the maximum electrical load is usually quoted. However, this electrical load is meant only for a very short duration (start-up loads) and is not indicative of the actual sustained electrical load the generator is designed to handle. The maximum load duration varies from manufacturer to manufacturer, and some manufacturers won’t even specify a duration or cycle time for the maximum load.
The Kubota owner’s manual is a good example. The AE6500 generator has a maximum 6,500-watt output, but it’s rated at 5,400 watts.
The owner’s manual doesn’t explain the rating system, nor does it provide any warnings. The owner’s manual only warns not to exceed the current rating identified for each receptacle.
Because of this ambiguity, you need to help your customers add up their electrical load requirements to ensure they purchase a generator with a rated load that exceeds their electrical needs. You first must understand that there are two types of electrical loads: resistive and inductive.
Resistive loads are pure resistance loads. Examples are lights, toasters, welders, radios, televisions, and electric heaters. Simply put, the resistance remains constant, so the current and wattage remain constant when a constant voltage is applied across the resistance.
We can generally think of inductive loads as windings typically used in motors. Depending on their design, electric motors draw different amounts of current per horsepower developed. Therefore, inductive loads are given a letter designation rating that relates to the kilowatts needed per horsepower or Kilovolt Amp (KVA)/horsepower (hp.). Inductive loads normally burn out generators due to their start-up current draw, so I will teach you how to understand them.
Every induction motor greater than 1/20 hp has a code letter on its name plate (see Chart 1).
V 22.4 and up
Note: 1 VA = 1 Watt or 1,000 VA = 1 KWatt
This letter indicates how high the inductive currents will be during start-up. This “motor code” is a National Electrical Manufacturers Association (NEMA) standard for induction motors which relates the locked rotor (or start) KVA required per horsepower of running power (1 KVA = 1,000 Volt Amps). Starting a Code “G” motor at 65 percent of the rated voltage is standard in the generator industry. This standard began with the military specification for generators.
When a motor is first started, the rotor essentially stalls, and extremely high currents (far in excess of the rated currents) flow. These currents are nearly 90 degrees out of phase with the generator voltage, so the motor produces very little rotational torque. As the motor begins to turn, these high currents begin to drop off and come into phase until the motor comes up to speed, at which point the motor current levels off.
If a capacitor or inductor is used as a load in an AC circuit, current flows, but very little power is delivered to the load. Inductive or capacitive loads tend to shift the current out of phase with the voltage. That is, when the voltage across the load is peaking, the current flowing at that time is very low, and vice versa.
The following charts will prove very useful in determining your customers’ electrical needs and in recommending the correct size generator to meet those needs.
Chart 2 gives the approximate starting load for electric motors.
Average Starting Watts Required
Motor Approx. Universal Repulsion Capacitor Split Phase
HP Rating Running Watts Motors Induction Motors Motors Motors
1/8 275 400 600 850 1,200
1/4 400 500 850 1,050 2,000
1/3 450 600 975 1,350 2,700
1/2 600 750 1,300 1,800 3,600
3/4 850 1,000 1,900 2,600
1 1,000 1,250 2,300 3,300
Note: These are not absolute values. Actual values should be measured with a Clamp type induction pickup meter.
Chart 3 lists wattage requirements for both resistive and inductive electrical appliances.
Air [*] Conditioner
(12,000 Btu) 1,700+
(20 amp) 500
Belt Sander (3″) 1,000
Chain [*] Saw 1,200+
Circular [*] Saw 1,000+
Coffee Maker 1,000
Compressor [*] (1 HP) 2,000+
Compressor [*] (1/2 HP) 1,400+
Curling Iron 700
Freezer [*] 500+
Disc Sander 1,200
Electric Nail Gun 1,200
(one element) 1,500
Electric Skillet 1,250
Furnace [*] Fan (1/3 HP) 1,200+
Hair Dryer 1,200
Hand [*] Drill (1″) 1,100
Hand [*] Drill (1/4″) 250+
Hedge [*] Trimmer 450+
Jet [*] Pump 800+
Lawn [*] Mower 1,200+
Light Bulb 100
Oil [*] Burner on Furnace 300+
Oil [*] Fired Space Heater
(85,000 Btu) 225+
Oil [*] Fired Space Heater
(30,000 Btu) 150+
Paint [*] Sprayer, Airless
(1/3 HP) 600+
Paint [*] Sprayer, Airless
Refrigerator [*] 600+
Submersible [*] Pump
(1 HP) 2,000+
Submersible [*] Pump
(1/2 HP) 1,500+
Sump [*] Pump 600+
Table [*] Saw 10″ 2,000+
(*.)denotes appliances with motors requiring additional start-up loads . Loads in Watts (Amp*Volts)
They will help you and your customers add up appliances on each of their circuit breakers, and help determine which appliances require generator support.
Chart 4 is very important.
Maximum Allowable Cable Length
Current @120 @240 #8 #10 #12 #14 #16
Amps Volts Volts Wire Wire Wire Wire Wire
2.5 300W 600W — 1,000 ft 600 ft 375 ft 250 ft
5 600 1,200 — 500 300 200 125
7.5 900 1,800 — 350 200 125 100
10 1,200 2,400 — 250 150 100 50
15 1,800 3,600 — 150 100 65
20 2,400 4,800 175 125 75 50
25 3,000 6,000 150 100 60
30 3,600 7,200 125 65
40 4,800 9,600 90
Note: These are minimum wire sizes. Add 25% more or select the next larger size wire. Remember, the larger the number, the smaller the wire!
Power loss in electrical wiring and extension cords often causes generators to overload and burn out. Heavy, industrial power cords are an inexpensive investment to prevent burning out a new generator.
Inspecting the generator’s set-up will help decide any warranty claims.
It is wise to sell or purchase a generator rated at least 15 to 25 percent greater than the anticipated electrical needs to compensate for unexpected or forgotten loads and to remain below the continuous rated load of the generator.
How to use Chart 4
Example 1: A generator’s continuous output is 4,800 peak watts combined load. A customer needs to install the generator 95 feet from a water well. What size extension cord should be used?
Answer: First, find 4,800 watts under the load column. Next, look for 95 feet under the cable length column. If the load is 220 volts, the chart calls for a No. 10 extension cord, since the No. 12 wire’s maximum distance is 75 feet. To ensure adequate current carrying capacity, use No. 8 wire.
If the load is 120 volts, the chart calls for a No. 6 extension cord, because the No. 8 wire’s maximum distance is 90 feet. To ensure adequate current carrying capacity, use No. 4 wire.
Example 2: A 1-hp., capacitor motor, water pump is to be run off a mobile generator. The wire will only be 5 feet long, so there is no measurable loss. What size generator is needed in this application?
Answer: First, have the customer bring in the pump’s specification sheet before you do any advising.
Use Chart No. 2 for an average motor’s power use. Since a 1-hp. capacitor motor requires 3,300 watts, the customer needs a generator with a rating at least 25 percent greater than 3,300 watts.
Example 3: An air compressor (220 volts) with a 3,800-watt starting load is to be run on a 100-ft., No. 10 wire, extension cord. What size generator is needed?
Answer: Find 3,800 watts in the load column, and read the current draw. The current requirement is 20 amps. This requires a generator to produce 20 amps x 220 volts = 4,400 watts. A 5-KW generator is the least expensive choice for intermittent use. A 6.5-KW generator is probably even better for a continuous application.
For start-up loading the generator, recommend adding one load at a time, starting with the largest motor to the smallest. Next, add the resistive loads in priority of their use, and never load a generator to the point where the rpm drops, or it will not produce the 60-cycle current needed for most applications.
Paul Dilger is a retired professor of agricultural engineering at Cal-Poly State University. For OPE certification training materials, visit Dilger’s web site: imslo.com.