I have some new corded equipment coming in, and until I know where its permanent placement will be in my workshop, it will be connected via an extension cord. But, there’s a problem – what size extension cord will I need?
Short Answer: consult with your user manual.
For an extension cord that will be used with a 110V power tool or high-powered device, I prefer full 15A-rated extension cords of the following sizes:
25-foot extension cord: 14 AWG (or 12 AWG)
50-foot extension cord: 12 AWG (or 10 AWG)
For up to 12A, I could go with 14 AWG or 12 AWG for a 100′ extension cord. For 220V 20A tool use I’m going with 10 AWG.
The lower the AWG value, the thicker the cable and wire conductor diameters.
Selecting an extension cord shouldn’t rely on any guesswork.
Which extension cord you could and should use depends on 2 things:
- Current draw of the tool or device
- Desired cord length
The voltage and plug styles are also important, but that’s independent of the cord size.
Let’s say you have two sizes of water pipes – one has a 1/2″ inner diameter, and the other a 3/4″ diameter. Which pipe is more restrictive? Electrical cords are similar in concept.
When you have higher electrical current moving through a narrower cord, you get voltage drop and heat.
How do you know how much current a tool draws from an electrical outlet? This information will usually be listed on the tool itself and in the product manual.
Motor and power tools can be tricky since the current draw and electrical load can sometimes spike. Generally, equipment makers will offer recommendations as to their tools’ different power needs. Refer to their guidelines, and if in doubt consult with an electrician or other authority.
Extension Code Size Charts
What brought all this up is that I have a machine coming in that requires a 220V outlet with 20A circuit protection. I have one 220V outlet in the corner of the workshop, with in-wall wiring rated for 20A and a 20A circuit breaker.
The tool itself is rated at 9A per motor specs. Since the equipment is 220V and doesn’t come with a plug built-in, I could potentially use 15A or 20A plugs with it. My wall outlet can work with 15A or 20A plugs.
I could dig up calculation charts and best-practice guidelines, but that might still involve some guesswork and judgement calls. It’s easier to refer to manufacturer’s extension cord size charts, especially since their experts (and legal teams) have triple-checked the accuracy of their recommendations.
The equipment manufacturer recommends a 20A circuit, and so I’m going by that, rather than motor specs. Extra overhead is a good thing, although the downside is that the higher gauge cords are thicker and heavier.
This is the chart Powermatic provides in their user manual for extension cord guidance.
The tool still hasn’t shipped yet, and so I could wait until it’s here to see what the motor plate says, but I’d rather have as much as possible prepared before that.
I have read a few examples of cord bottlenecks where motor start-up can be affected. I assume that if start-up (inrush) current is higher than the typical operating current, you’re going to get a higher voltage drop, and this could affect start-up operation.
When in doubt, it’s usually better to size-up to a thicker gauge wire. With wiring, the lower the gauge number, the thicker the conductor size. Meaning, a 10 gauge wire has thicker conductors than 12 gauge wire and can handle higher current.
Going with a cord and plugs that are rated for 20A removes any doubt here.
Even it were suitable to go with 15A plugs, something that’s not clear until I see the motor plate of the new equipment, that’s still on the border of 10 and 12 AWG recommendations.
Additionally, I have just one piece of 220V equipment coming in right now. But what if I buy a table saw next? Or a stationary air compressor?
I’m going with 20A plugs to be future-proof, and so the cord needs to be able to handle 20A of current.
Plus, I’m making the assumption that the 20A of circuit protection extends to the minimum recommendation between wall outlet and device wiring.
So, this means 10 AWG. This way, I don’t have to make assumptions about whether a cord rated to 15A is sufficient or not. If myself or anyone else ever sees the cord with 20A cord ends, they don’t have to question whether it can be used with 20A devices.
I could probably get away with using 12 AWG cable and 15A cord ends, but setting it up for 20A means I don’t need to build a separate 20A-rated extension cord later on. Making sure the extension cord is rated for 20A ensures there’s no guesswork here.
As for the 220V vs 110V, there’s not much of a difference since the cord is rated to 600V. Even if I went with lighter duty cord, the voltage rating would be 300V. I went with the standard 600V rating for greater durability. The 300V-rated cord is very slightly less expensive (~5%), but has a ~13.8% smaller diameter and is ~16.7% lighter.
If going by the cord ratings, brand I’m looking at rates their 10 AWG to 30A, and their 12 AWG to 25A based on manufacturer specs. They’re simply quoting NFPA 70 NEC Table 400.5 (A)(1) specs for these ratings. Their 14 AWG cord is rated to 18A.
I’ve seen some online sources recommend that you can use 14 AWG for loads up to 20A, but that’s simply incorrect, as the maximum NEC rating is 18A.
You can’t go by cable rating alone, because length matters, and you can’t go by self-calculations such as voltage drop because there are other factors such as NEC guidelines.
I’m going by the manufacturer’s guidelines, which is usually a safe default. 10 AWG can be more difficult to work with, but I also ensured I went with plugs that are rated for it.
Let’s say I wanted to buy an off-the-shelf extension cord for general workshop use. Going by the Powermatic chart above, any cord carrying 15A current would call for a 12 gauge cord (at least).
Different equipment makers have different requirements when it comes to extension cord use.
Flexzilla – the brand of extension cord shown above – says their 14-gauge 25′ extension cord has a 15A load rating. They also have a 14-gauge 50′ extension cord rated at 15A and a 100′ cord rated at 13A.
There’s a disagreement here. According to Powermatic’s recommendations, 14 gauge would be insufficient to carry a 15A load. The cord is UL-approved and presumably safe to use as rated.
So what do you do? Generally, I follow the tool-maker’s recommendations and step up to a higher wire gauge if in doubt.
Some brands base their recommendations on different things. When in doubt, I go with the higher-gauge recommendation.
Here is the extension cord chart Dewalt gives for one of their corded table saws.
This particular Dewalt 120V table saw is rated at 15A. So, according to their chart, can you use a 14-gauge 25′ extension cord? Yes. Can you use a 14-gauge cord that’s 50′ long? No, or at least they don’t recommend it, they recommend a 12-gauge cord.
Bosch’s recommendations for their corded miter saw are similar to Dewalt’s.
I follow a similar rule of thumb for myself and bought a 14-gauge 25′ extension cord and a 12-gauge 50-foot cord. Since I bought them for general purpose tasks, rather than specific tools, I know that I can safely use them between any 15A tool or device and any 15A-rated wall outlet.
Some tools aren’t recommended for use with 100′ extension cords, presumably because the voltage drop would be too low. Keep in mind that there is also wiring between the outlet and the circuit breaker, and that contributes to a voltage drop as well.
There might be the option to step up to 10 AWG cables for longer 15A extension cords.
I also made sure the extension cords I purchased were UL-listed, which I take to mean they went through 3rd party safety testing.
You can buy extension cords of different sizes. There’s a UL-listed 100-foot cord with 16-gauge cable, and it’s rated at 10A. For specific uses, that’s okay. But if you go by any of the above charts, all of the brands above would recommend higher-gauge cables to be used with the specified equipment.
If you have an extension cord that physically fits between a heavy duty power tool plug and your wall outlet, are you going to remember whether the cord is under-rated or not?
Speaking personally, I don’t know what I might want to use any particular extension cord for, as usage needs can change under different circumstances, and so I try to ensure any of my cords can handle any of the loads that might be plugged into them.
If I’m not home, and my wife needs to plug in a wet/dry shop vacuum, I don’t want there to be a situation where the wrong extension cord could potentially create a fire hazard.
So, I size my extension cords to match the device form factor or circuit protection.
Basically, for 110V 15A cords I go with 14 AWG for 25′ length and 12 AWG for 50′ lengths. For my new 220V 20A cord, I’m going with 10 AWG for a 35′ run. This is all based on 3-conductor cables.
When in doubt, consult your device user manuals. If still in doubt, consult with an electrician or other electrical safety authority.
Charts like those above remove the need for guesswork.
I’ve have seen color-coded charts where they make “light duty,” “heavy duty,” and “extra heavy duty” recommendations, but you don’t need much more than a current vs. cord length chart.
Is it safe to go with a lower-rated cable? Possibly, but why take any risks? Improper extension cord sizing could at the least affect tool performance, and at worst start a fire.
One thing to keep in mind is that it’s not always clear what brands are basing their recommendations on. Powermatic’s chart, for instance, does mention that their recommendations are based on 5V max voltage drop at 150% of the rated current, I assume because their tools can have high inrush voltage spikes.
Since Dewalt’s chart has different values for 120V and 240V, they’re likely looking at a maximum voltage drop percentage. Since 240V is 2X 120V, a fixed-value voltage drop is 1/2 the percentage at 240V than at 120V.
You could also make your own Ohm’s Law calculations based on conductor size and voltage drop limits, but that’s not always the best approach as discussed above. Are you sure you have the correct wire gauge and resistance value table? Are there any other considerations involved such as NEC guidelines?
It’s usually good advice to use the shortest extension cords possible, and to uncoil them for use to help with heat dissipation.
There are a lot of good brands out there, but here is what I use for my 110V 15A-rated extension cords:
I have used Yellow Jacket-branded extension cords and power blocks with great experiences.
Do you guys have any recommendations regarding extension cord brand or size selection?