Voltage Drop | Recommended Size | Prices |

The National Electric Code (NEC) sets limits on the amount of current that cords are permitted to carry. This is for safety, to prevent overheating the cord. The limits for common cords are given in the chart below:
Copper wire
size (AWG)
16 10 amps
14 15 amps
12 20 amps
10 30 amps
AWG - American Wire Gauge

AWG general rules of thumb - for every 6 gauge decrease, the wire diameter doubles and for every 3 gauge decrease, the cross sectional area doubles.
Source: Ampacities for wire

Voltage Drop:
As important as overheating is voltage drop based resistance due to size and length of the cord.
Remember your physics? Ohm's Law, or V = I R1, says current (amperes) (I) is potential difference (volts) (I) divided by resistance (ohms) (R).
E.g. If you apply 120 volts to a circuit with a resistance of 10 Ohms you will get 12 amps of current.

Most of the resistance in electrical appliances, motors, light bulbs, etc. is in the appliance, but the wire in your house and extension cords also increase resistance.

Temperature and resistance: Resistance goes down in colder temperature. Ohms per 1000 ft.
AWG 32°F 77°F % change
10 0.9203 1.018 10%
12 1.463 1.61910%
14 2.327 2.57510%
16 3.700 4.09410%
Source: Table of Equivalent AWG Wire and Resistance per foot | interfacebus.com

Long runs of wire can have an significant effect on the voltage delivered to the appliance. A 100 ft. cord will have twice the resistance of a 50 ft. cord.

Recommended Extension Cord Size (120 v)

extension cord size, gauge chart, true value hardware

Home Depot
Length Gauge
16 14 12 10
25, 50 13 AMP 15 AMP 20 AMP 20 AMP
100 ft 10 AMP 13 AMP 15 AMP 20 AMP
150 ft 7 AMP 10 AMP 15 AMP
Home Depot Gauge Chart

extension cord size, gauge chart,  ace hardware

Maximum voltage drop based on recommendations
Store 14 ga 12 ga
True Value 100' 7% 5%
Home Depot 150' 7% 5%
Ace 150' 10% 7%

Voltage drop in the cord often doesn't allow the power tools to run at full speed. This causes them to heat up and can inflict damage to them.

Electrical equipment is designed to operate at within a given voltage range, typically no less than 10% and no more than 5% from it's voltage rating.
Source: Calculations Voltage Drop | Mike Holt Code Resources

Note 1: There is some voltage drop in your house wiring. 6% with a 13 amp motor from my circuit breaker to my garage with 14 gauge - 15 amp circuit.
There was a 9% drop (breker box to garage) with a 12 amp load in a friend's house.

                          Amps   Voltage drop with 10 amp load
Outlet by Circuit breaker 10.6   119 to 115  3%
Outlet in garage           9.9   115 to 107  7%
Note 2: Freezing weather will reduce voltage drop by about 10%. See below.

Buying Guide: Extension Cords | at The Home Depot
What Size Extension Cord Do I Need? | D.I.Y. by Example
Electrical Extension Cord Safety | Univ. of California (safety.ucanr.org
Length of wire/ amp draw

Calculate Extension Cord Voltage Drop

Some sites say,
"When voltage gets low, the current must increase to provide the same amount of power and this increase in current can cause the motor to overheat.
Source: The Highs and Lows of Motor Voltage | Design content from Electrical Construction & Maintenance (EC&M) Magazine
This was not true in my tests with a leaf blower.
A 12 amp rated blower drew 10.8 amps when connected to an outlet next to the circuit breaker and 10 amps when connected to an outlet in the garage with 14 gauge wire (a 15 amp rated circuit). An 8% drop. This is higher than it should be given the length about 40'
I suspect the above increased current refers to motors with voltage/current regulation circuits which do not exist on most home tools.

How to Select the Right Extension Cord

Looks can be deceiving! You can't tell how much current a cord can handle by how thick the cord is! The reason is that cords are made with different amounts of resistance to physical damage.

Two cords of noticeably different thickness may have exactly the same copper wire (and therefore current capacity) inside them. Notice in the photo of examples at right that one of the #16 AWG cords is thicker than the #14 AWG cord below it. See "Cord Marking" below for more details.

Cord Marking
Extension cords are marked continuously along their length with several items of information. The most important marking item is the one that tells the size of the wires in the cord. This marking usually appears in a form such as "14/3," which indicates a three-conductor #14 AWG cord.

The marking item that usually follows the wire gauge tells us the type of cord, meaning the physical construction. The two types of construction likely to be found are "S" (hard service) and "SJ" (the J is for "junior," a lighter duty, less-rugged version of the type S cord. (Both types are shown in the photo.)

Additional letters may follow the S or SJ. The additional letters indicate other performance characteristics. Here are some:
Letter What it indicates
T Thermoplastic insulation, instead of a rubber-type material.
E Thermoplastic elastomer insulation (more rubber-like than thermoplastic)
O Oil resistant.
W Moisture and sunlight resistant.

Source: Choosing and Using Extension Cords | home.mchsi.com/~gweidner

1. In most physics books, "I" is used as the symbol for electric current because in the early scientific days it was known as Electrical Intensity. When the facts about electric current and electrons were discovered the letter C had already been used for Capacitance so the letter I was retained.
E stands for Electromotive Force.
In the International System of Units (SI) the notation is V-Volts, A-Amperes, Ω-Ohms.

Other Considerations:
Flexibility in cold weather. The kind of insulation will affect flexibility.

Prices for a 50 foot cord
Store 10ga 12ga 14ga
Home Depot $94 $50-61 $35-40
Lowes $70 $45
Ace $47 $26
True Value $65 $38
Harbor Freight $20(s)
(s) - Sale
Note: Multiple outlets are convenient for connecting multiple tools like drills, chargers, ..., but inconvenient for use with something like a leaf blower.

Wire specs
Residential wire
Wire Current Ampacities NEC Table 310-6

last updated 28 Mar 2014