The choice of generator extension cord is very important, It must be flexible yet strong, durable yet versatile, and of course, there are electrical concerns to be accounted for, as well.
1. Generator extension cord
The most common way to use a portable generator is to place it outdoors and plug the extension cord into the selected appliance through an open window or door.
The use of generator extension cords can solve the problems of traditional extension cords:
• You can only restore power to appliances with cords. Hardwired items, like furnace fans, well pumps and ceiling fans, don't have cords. Therefore, you can't power them with an extension cord.
• If the cords are too long, the resulting power drop may damage the generator and appliances. And, if they are placed under rugs or carpets, heat can build up and spark a fire.
• It's time consuming stringing a bunch of extension cords together. The power could be restored by the time you get everything set up.
2. Generator extension cord into house
Like any internal combustion engine, a generator engine exhausts carbon monoxide gas, which can give you a headache, knock you out or even kill you. This is easy to avoid, though: Don’t run a generator in your garage or porch, and keep it at least 10 ft. away from your house.
Buy a Standby Generator if You Can Afford It
A standby generator, unlike a portable generator, is permanently connected to your electrical system and goes on automatically when the power goes out.
The difference in cost between a portable generator and a standby unit may not be as great as you think. Remember, a portable unit requires either expensive extension cords or a transfer switch. Standby units can run on less expensive natural gas, which will save you money in the long run.
Hooking Up Generator to House: You'll Need Heavy-Duty Extension Cords
Remember, if you decide not to install a manual transfer switch, you'll need a lot of expensive, heavy-duty extension cords. Using undersize cords presents a fire hazard and can damage motors as well as stress your generator. To run a refrigerator, depending on how energy efficient it is and how far from the generator, you'll need at least a 12-gauge cord. A 50-ft. 12-gauge cord will set you back about $50. Multiply that by five or six and you can see that a transfer switch starts to sound like a better deal.
3. 100 ft generator extension cord
Extension cords contain copper wire through the center that varies in thickness.  The thickness is rated by American Wire Gauge (AWG).  Most common are AWG 16, AWG 14, AWG 12, AWG 10, and AWG 8, with the lower the number, the thicker the wire, or bundle of wire.  The thicker the wire, the more watts (and amps) you can expect to power through it. For example, a thin standard cord for a wall lamp which you’ll have in your home, which doesn’t need much power is probably AWG18 wire with only a two pronged plug.
In order for your electrical device to operate efficiently and safely, it must get the full power that it needs.  You could even damage the device.  If the resistance due to the cord thickness or length keeps the device from getting what it needs, you have to increase the thickness of the wire (lower number AWG), or shorten the cord. The lower the gauge AWG number (thicker the wire), the longer the cord you can use to supply power to identical devices.  Take a look at the table below:

Let’s say you are going to run an appliance with a 10 Amp draw on 120 Volts.  That equates to 1200 watts if you remember the formula that Watts = Volts x Amps.  If your extension cord is #16 gauge wire, you can have a cord up to 50 feet.  For the same appliance, if your cord is #10 gauge, your maximum length is 250 feet.  Now very few households have extension cords this long, but on-site work can often mean that the portable generator will be located 100’s of feet from the power need.  And lower gauge extension cords are very expensive. 
4. Portable generator extension cords
Extension cords come in different gauges and lengths. However, not all of them are compatible with the portable generators.
To use your generator efficiently, it is essential to choose the right power cord; otherwise, there is a high chance of damaging the generator as well as the devices connected with it.
The control panel of the generator consists of numerous standard 120V outlets, and one or more different high-current receptacles.
The National Electrical Manufacturers Association (NEMA) is the generic standard for AC power connectors in America and some other countries that define about 150 different types of outlets.
They have a varied combination of contact blade widths, shapes, orientation, and dimensions that make them unique for different types of devices and their needs.
Therefore, you must be careful before buying an extension cord, because a mismatch between the plug of the cord and the receptacle on the generator can cause a lot of inconveniences.
Do not worry much, though; only a few standard outlets are used in case of most portable generators.
Now, NEMA wiring devices are made in current ratings from 15 to 60 amperes and voltage ratings from 125 to 600 volts. Consider the NEMA 5-15R, which the most common outlet found in the US households.
Here, ‘5’ represents a 125V two-pole, three-wire configuration, ‘15’ the rating of the device in Amps, and ‘R’ represents receptacle (‘P’ is used for a plug).
The NEMA devices can be divided into two categories – straight-blade, and twist-locking.
The straight blade configuration is mostly used for lighter, general-purpose devices. The most common examples are the 5-15R and 5-20R. The twist-lock connector is basically for the transfer switch and used for heavier, commercial-purpose devices. They have a letter ‘L’ before the name to differentiate from the straight-blade connectors. The L14-30 is predominantly used in the US.
Let’s just give you a brief idea of how an outlet looks like. The figure on the left shows a four-prong 120/ 240V L14-30 outlet, where lines 1 and 2 are electrically ‘hot’ wires. Next, we have a Honda generator showing some of the different outlets commonly used in the US.

5.Heavy duty extension cord for generator
Conntek 20602 NEMA L14-30 Generator Power Cord, 50 Feet
This fully molded generator cords feature a 4-Prong locking configuration commonly found on most medium to large size portable generators and transfer switches. The cable jacket STW remains flexible in various weather conditions with heat-resistance up to 221° F. These cords are built with heavy duty 10AWG wire, which is great for contractors, mobile businesses, and home emergency power system applications.