How Wire Size and Length Affects Power and Voltage Loss

Determining available power use on Wire sizes is a relatively straight forward calculation, albeit with different variables that can affect the voltage drop as well as the amperage rating.

I am not going to get into the technical aspects of actually making the calculations there are places like Paige Wire Calc or Wikipedia for  wire gauges  that go into this in detail. For this discussion I’ll keep it simple and practical.

For single-phase circuits that you will find for your needs in household and small construction tools the wire size needed is a linear formula involving the conductor type such as aluminum or copper and their own respective resistivity, the number of amps, the length and the allowable voltage drop.

The size of wire is directly proportional to the voltage drop allowed and the length. What this means is that if your wire is rated for 20 amps at 100 feet you’re allowed a 3% voltage drop. If you double that length your size of wire needs to double in area (Not to be confused with gauge) to retain that same voltage drop.  See our post on Voltage Drop for more info.

Power Available Based On Wire Size and Length

On the other had a specific wire size can provide so much power in watts or amperage. For example let’s look at a 6 gauge wire and the typical selection for long runs: (Disclaimer, this is an example and used for teaching purposes only, you should consult the manufactures recommendations for extension and power cords or a certified electrician for your specific situation in determining your exact needs)

For A number 6 copper wire:

At 15 amps use up to 200 feet

At 20 amps use up to 150 feet

At 30 amps use up to 100 feet

At 50 amps you may only get 25 feet and likely would be looking at a #8 wire

So as you can see the various lengths are dependent upon the amount of power you will draw.

In the practical since if you were to use two 100 ft cords rated at 30 amps you may only be able to utilize 15 amps.

In wattage (V*A) that’s (115V * 30A = 3450W and consider that you get up to an additional 3% plus voltage drop to 110V * 15A = 1650W. This is over simplified, but you should begin to understand the practical application of using long extension cords.

If you really want to push long runs you need to consider buying a rated cord for the service.

Our recommendation is to understand your worst case scenario and purchase cords accordingly. If you’re going to double up for specific service, plan on only being able to use half your power source.

Here are basic recommendations for getting what you need.

20 Amp Service Extension Cords

Coleman Cable 09208 12/3 SJTW Twist To Lock Extension Cord, 20-Amp, 50-Foot, Yellow

Yellow Jacket 2885 12/3 Heavy-Duty 15-Amp SJTW Contractor Extension Cord with Lighted Ends, 100-Feet

30 Amp Service Extension Cords

Coleman Cable 01912 25-Feet 10/3 Generator Power Cord with L5-30P Plug and 3-Outlets

Generac 6388 100-Foot 30-Amp Generator Cord with NEMA L14-30 Ends for Maximum 7,500 Watt Generators

Conntek Marine Shore Power 10/3 30 Amp Cordset with Light Indicator (Yellow, 50-Feet)

50 Amp Service Power Cords

Camco 55195 50 AMP 30′ Extension Cord with PowerGrip Handle

Conntek 1450SS2-15 15-Foot Temporary Power Cord, 50-Amp 125/250-Volt, NEMA 14-50P Generator Plug to CS6364 Locking Connector


 Realize that you need the correct wire size and length for your service needs, shopping for the right rating with length will eliminate any issues with overloading the wire or generator. If you’re doubling length you will be reducing your voltage and power available. Make sure you use the proper size for your service application.

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