Extension Cord Size Calculator

Ampacity

First, each AWG gauge has a certain Ampacity. In simple terms, it’s the max current a wire can handle. It’s dependent on the ambient temp, the wire material and the temp rating of the wire. Think of this as the max current a wire could handle at any distance.

The NEC sets these values.  We used the UL general use reference chart when building our calculator.

Voltage or Power Drop

The second issue we need to be aware of is the fact that the wire in the extension cord itself has resistance.  That matters because the longer the run of wire (or extension cord, in this case), the less power that will get to your electrical string trimmer, lawn mower, hedger or other tool.

This doesn’t always matter that much. For a few percent of voltage drop, most equipment won’t be damaged, and you won’t notice a performance hit.  It will matter as the percentage amount of the drop increases .  Motors and other electrical devices are designed to work at certain voltage levels.  As the voltage drop increases, the motor usually ends up getting hotter and hotter.  If it gets hot enough, the motor will see a reduced life – or might even burn out.

What Value To Use For Voltage Drop?

The National Electronic Code uses 5% as the max allowable voltage drop for a combined feeder and branch circuit built into your house. The branch itself isn’t supposed to exceed 3%. For light appliances and garden tools, the number will vary.  This manufacturer of lighting equipment says their fixtures can handle a 10% max voltage drop.

I wish I had an absolute number to use, but there really isn’t one.  I can’t imagine a scenario in which you’d have trouble damaging your equipment using a 3% max drop number, but this goal is tough at higher distances and medium amp loads.  It is the number you’d get many times if you take the recommendation used in the manuals and back out the implied voltage drop.  So if you are one that likes to err on the conservative side, use a 3% number.

Likewise, unless you specifically know the equipment can handle it I wouldn’t go over a 5% voltage drop for the particular extension cord you are using.  It’s not something scientific, but allows you to have some buffer for manufacturing defects, material impurities, etc.  I have a 4% number as the default in the calculator for just this reason.

Be aware – the higher the number you use for allowed voltage drop, the more likely you are to damage your power equipment from insufficient voltage.

Other Caveats

This is a simple calculator for use by most homeowners looking for an extension cord for their corded lawnmower or weed eater. To keep it easy to use, we left out some other variables such as temperature. We also assume you are using a copper wire (not aluminum). We also assume you aren’t pulling any more than 20 amps

Know Your Circuit Capacity

The last item you must know is that is the amperage of the circuit breaker for the outlet/circuit you are plugging into.  There are two scenarios you need to be aware of:

• If the rated amp of the tool is higher than your circuit breaker or fuse, you’ll blow the breaker.  That’s a feature, not a bug – it means you shouldn’t be running that tool on the circuit.  Also, be aware of any other equipment you have plugged in on this circuit.  If need be, unplug those other items while you use your equipment to bring the total amps under the circuit’s handling capacity .
• If your cord is rated at a lower amperage than the circuit, the breaker won’t always protect you.  Here’s the scenario – you have a 18 amp chainsaw and you are using a AWG 16 gauge 25 ft extension cord.  This is on a 20 Amp circuit.  Your cord will overheat and possibly ignite before your breaker will switch off, since the breaker is fine with 20 amps headed down the wire.  You extension cord isn’t cool with that – it really shouldn’t handle more than 13 amps.  You are likely looking at the wire getting hot – if you are unlucky hot enough to melt the insulation.

The solution to these issues is easy – just buy a 12 AWG extension cord for 15 amp circuits, and a 10 AWG one for 20 Amp ones.  You will trade off having a heavier cord bundle to lug around and increased cost if you do this, but you won’t have to fiddle around with worrying about tool amperage and distance

What Do The Letters On The Extension Cord Mean

If you look at the outer jacket of any decent extension cord, you’ll notice numbers and letters printed or embedded into the cover.  These are there to give you information about the extension cord.  The letters tell you about the jacket, while the numbers are the gauge and wire count.

The first number will be the wire gauge; the second will be how many conducting wires are inside. For our purposes, you’ll be looking for cords with “/3” on them – you need the ground wire when operating outdoor power equipment.

Here’s a table of the letters and their meanings:

You want to make sure you buy an extension cord with a W letter, since you will be using it outdoors. If you plan on using it in really hot or cold conditions, go with an E as well.

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