( This article has been updated to present more information and clarity. )
The best extension cord for your generator is one that is specified to handle the power that’s being delivered between the generator itself and the device or devices plugged in.
I suspect that some people don’t know that it does make a difference having the right or best extension cord (or cords) when powering appliances and other devices.
What difference does it make?
If the extension cord is ‘too small’, it will potentially restrict the power necessary to safely operate the loads that are plugged in. The cord will heat up.
What do I mean by ‘too small’?
I mean the internal wires themselves. Wire is rated in “gauge”. Thick wire has more capacity for power than thin wire (logical).
Don’t let the thickness (appearance) of the cord fool you! While generally speaking a thicker appearance (fatter cord) will probably mean the wires inside are heavier gauge, it’s not always the case. It could just be a thicker outer jacket.
Check the gauge!
There are LOTS of extension cords that are #16 and #14 gauge. What you want is at least #12 gauge wire conductors in that cord. Check the package. Somewhere on the label it will indicate this!
For wire, the smaller the gauge number (#) rating, the more power it can handle. So #12 can handle more power than #14, etc..
NEC Ampacity Limits
The NEC (National Electrical Code) sets limits on the amount of Amps (current) that wire conductors may carry (for safety / overheating). There are variations depending on wire insulation/jacket, temperature, use.
The following is from NEC Table UL486E for copper conductors, not more than 3 in cable.
WIRE GAUGE MAX AMPS (Max 3 bundle) (Table: UL486E) 14 AWG 15 Amps 12 AWG 20 Amps 10 AWG 30 Amps
Note: Extension cord specs vary (materials and other factors). Check the label!
Why is #10 gauge Extension Cord Better?
There are a number of factors that contribute to the ratings for extension cords and they vary somewhat. However generally speaking here are some specifications when using Ohms Law when applied to the NEC limits listed above.
14/3 extension cord
15 amps maximum
1800 watts @120 volts
12/3 extension cord
20 amps maximum
2400 watts @120 volts
10/3 extension cord
30 amps maximum
3600 watts @120 volts
Note: The “/3” simply means there are three wires within the insulation of the extension cord. One is for ground (green), one for “hot” (black), and the other for neutral (white).
How does length affect extension cord ratings?
Voltage drop. Most typical extension cords for outdoor use come in 25 foot and 50 foot lengths. All wire has some resistance (depends on conductor type and other factors). The longer the wire (extension cord) the more resistance (and voltage drop).
With regard to generator extension cords, I wouldn’t sweat it. In most cases you will be 50 feet or less. When you’re looking at 100 feet (or more!) then this becomes potentially problematic. In these cases it’s better to use heavier gauge (#10 better than #12, etc..).
Can I use 2 extension cords with my generator?
If your generator puts out more power than the rating of one extension cord, and if your generator has multiple household style outlets, you might use more than one extension cord. Just don’t overload your cords.
For example, one of my generators is rated for 3500 watts. It has several conventional outlets (and a high power multi-prong outlet). I might use two 12/3 extension cords to deliver power inside to my various appliances if need be.
I use one of these on my portable generator to split the high power output into (2) 10/3 lighted outlets:
Use Outdoor rated extension cords
Some are rated for indoor only use while others are designed for outdoor use. For use with a generator, you definitely want outdoor rated. The differences are in the insulation material, strength, and the ends themselves (plug & receptacle design).
Warning signs of overloaded extension cords?
– extension cord becomes warm, very warm, or hot
– the generator circuit breaker keeps tripping
– your appliance is making a funny noise
So what is the best extension cord for a generator?A good general purpose choice is a 10/3 (#10 gauge) outdoor rated.
10/3 will perform better than 12/3 and NEC specs indicate power handling up to 3600 watts. Additionally there will be less voltage drop under load. Theoretically a longer run.
Tip: My experience has been that you are not going to find a 10/3 extension cord with a household style plug/outlet that lists its rating higher than 15 amps (or 20 amps with a T-blade plug) due to regulations (even though the #10 wire inside may be capable of handling the NEC rating).
You can purchase specific 30 amp generator extension cords / cables.
Here’s one with 10 gauge wire that converts a 30-amp generator locking outlet to four 20-amp household outlets and extends your power up to 25 feet: