Extension Cord Question

[Michael O'Sullivan]

I have had several people claim that using an extension cord (particularly one rated for fewer amps than the motor is rated for) is bad for the motor. Is this true? I can see how it would be bad for the cord and a potential fire hazard, but I would like to know if this could fry a motor to boot.

[Eddie Darby]

Not just the capacity of the extension cord, but also the distance.

[Steve Rozmiarek]

I have had several people claim that using an extension cord (particularly one rated for fewer amps than the motor is rated for) is bad for the motor. Is this true? I can see how it would be bad for the cord and a potential fire hazard, but I would like to know if this could fry a motor to boot.

Just my observations, I have three Dewalt Emglo portable air compressors, that are extreemly touchy about this sort of thing. One of them will not run off an extension cord, and two won't from the generator. Don't know why these things are so sensative, I have not come across anything else that we have that behaves this way. I can't imagine that this is good on the compressors motors either.

[Dave Falkenstein]

A light weight cord heats up, but it also causes a voltage drop. You are then running your tool or motor at a reduced voltage. Oversimplifying it, Watts (power) equals Volts times Amps. If you reduce the voltage, then the amperage rises to supply the same power. Low voltage and high amperage is bad for the tool.

[Sam Yerardi]

In the simplest electrical terms, one of the reasons is that the extension cord, being made of copper, has electrical resistance. When a motor is first started, there will be a surge of current flow that can be (depending on the motor and the application) significantly greater than the running full load current that the motor is designed to operate at. The wiring supplying eletrcial power to the motor must be capable of supplying this current ABOVE the rating of the motor. The surge current can be greater, but the surge current typically lasts seconds or less, depending on the size of the motor and its load. This is one reason for using time-delay fuses or circuit breakers. However, when either the length of the supplying cable (i.e., extension cord) is too long (resistance is higher), or the wiring is too small for the current load (can't handle the current draw), the harder you are making it on the motor to operate at it's rated starting and full load current value because the motor is demanding the normal amount of current but the supplying cable can't provide it. The motor will still try to do what it is designed to do. This will overheat the windings in the motor, leading to windings that will either short out when the varnish insulating the wires in the motor melts, or a winding opens up if the time period of overheating is long enough. In terms of voltage rather than current, it is as Dave indicates in his posting. The extension cord acts as a resistor essentially, causing a voltage drop across it that robs the available voltage for the motor. As dave says, it is a question of power, and the wiring system you are providing can't provide the necessary power the motor is requiring.

[Mike Cutler]

Michael

Yes they can fry the motor.

When you see the rack of extension cords, you'll notice that there is a pretty big price discrepancy. Because they're all colored orange, or yellow, everyone just picks the cheapest one, because they're all the same, right?

But they are not. The $7.00, 50 foot extension cord is probably 16AWG wire, the one that is $15.00 -$20.00 is probably 14AWG wire, and the one that is $30.00 is 12AWG wire. It doesn't help that the manufacturers package them all in an outer jacket that appears the same size either. There are other factors that effect the price also. Jacket insulation, conductor insulation, type of exterior sheathing protection, to name a few.

Rule of thumb. Pick an extension cord that is, at a minimum, equal to or a larger AWG than the device it is supplying.

[Sam Yerardi]

I'll add one thing to Mike's post. Even if the extension cord's AWG is the same as the motor's pigtail or wiring cable, you could still have a problem if the extension is excessively long, i.e., 100 ft or greater. My rule of thumb is use the same gauge as Mike says, but keep the length as short as possible. Even then, you could still have a problem depending on the motor and the application. For small hand tools, drills, etc., I wouldn't expect to see a problem but you could..

[Peter Quinn]

Dave nailed it. Under rated extension cords are a bad idea for those reasons. A very good 25" 10GA extension cord is maybe $35? Cheaper than my cheapest power tool. I tend to run any extension cords a size over, not a size under, and only run extension cords on bigger tools when when neccessary.

I believe at the same gauge solid strand romex can sustain more heat and presents less resistance (which if I am not mistaken is a factor in voltage drop) than multi strand flexible cable.

I have a makita 15A sliding miter saw, and attempting to run it on cable either too long or too small produces a noticable and audible reduction in motor speed and performance.

[Sam Yerardi]

... presents less resistance (which if I am not mistaken is a factor in voltage drop) ...

Peter,
Resistance plays a fundamental part in the the voltage drop. There is a basic law of electricity (Ohm's Law) that expalins what you are talking about. Being an electrical engineer I could go on all night and bore you to death with math but it comes down to this:

Voltage = Current * Resistance

And there are umpteen variations of this.

[Ben Cadotte]

Not only can you burn up your motor, you can cause a fire! Almost every manual I have for my electric tools has a page near the front that list minimum cords to use and has a chart for minimum cord based on distance.

[J. Z. Guest]

Not only can you burn up your motor, you can cause a fire! Almost every manual I have for my electric tools has a page near the front that list minimum cords to use and has a chart for minimum cord based on distance.

I'm glad some folks read those!

I'm an electrical engineer at UL, and that table is in there by our requirement, for the reasons noted above.

Steve, I suspect the reason your Emglo compressors are so touchy about it is that they are working against a resistance that is variable. The inrush current to a compressor that has to pump that piston against a compressed air load is quite hard to overcome when the motor is starting from 0. Now if you rob some power from that, it becomes even harder.

DeWalt probably put the strongest motors and the most aggressive compressor heads in there that they could manage, and it is not tolerant of being starved for power.

Peter, I've never seen a 10 AWG extension cord in the store, at least not for 120 V. Do you order them?

[Steven Wilson]

I make my own extension cords. I use to do remote audio recording as well as had my own sound systems and bands and I learned early that reliable, clean, power is key. I bought a 500' spool of SJOOW (IIRC) 10ga cord a few years ago at an auction (much cheaper than the Borg) that I use for extension cords. With the right connectors and strain relief they are a pleasure to use.

[Dan Clark]

I have a 100' 10g extension cord. I believe that I paid a little more than $100 for it. It was pricey. I think that's why the big box store don't carry them.

Dan.

[Ben Cadotte]

I have 2 10ga cords from borgs. 1 - 25' and 1 - 50'. I think they were $36 and $64 5 years ago if I remember right. Both of them are Yellowjacket brand.

Thing is there is like 1 carton for them on the shelf. Maybe 2-3 cartons for 12 ga. But like 40 for 14 or even worse 16 gauge extension cords. Why do they even make 100' 16 gauge extension cords??? Guess everyone wants cheap.

[Ben Cadotte]

I'm glad some folks read those!



I learned a lesson about 20 years ago. Working as an apprentice carpenter. Was cleaning up for the day. I went to unplug an extension cord that a compressor was connected too. I nearly burnt my hand it was so hot. The rubber / plastic around the plug was almost pliable. Not just hot but very hot. So I got a prime lesson about using the right size cord.

[Mike Cutler]

I'm glad some folks read those!

I'm an electrical engineer at UL, and that table is in there by our requirement, for the reasons noted above.



Jeremy

Can I ask you a professional work related question concerning extension cords?

Does UL test, or any manufacture endorse, the practice of plugging multiple extension cords into each other to increase the length?
This is an ongoing debate we've had for years at work. My position is that we shouldn't be daisy chaining extension cords together as a general practice.

[Al Willits]

"""""""
Peter, I've never seen a 10 AWG extension cord in the store, at least not for 120 V. Do you order them?
"""""

I've seen them at industrial supply houses, but I made my own, I needed several outlets along the lenght of it, so I found the wire at one of the borgs, along with all the outlets and boxes in their electrial section.

I think they called it Power Cord if I remember right.

Al

[Peter Quinn]

Jeremy...I buy them from an industrial/commercial discount tool seller in my area. They stock 10AWG-16AWG in different lengths (25'-100'), I tend to use either 10AWG or 12AWG not longer than 25', beyond that I'm looking for a closer outlet.

Funny side note relative to start up current. When my father in law (electrical engineer for an industrial laundry facility) wired my 1951 Dewalt GR 14' RAS (industrial model), he bypassed the old capacitor start on the motor and put in a modern electronic starter. When setting up the new starter he checked the momentary amp draw to "tune in" the new starter...91A! This is a 17A/220V motor and apparently draws 91A for a few milliseconds...I was amazed.

[J. Z. Guest]

Does UL test, or any manufacture endorse, the practice of plugging multiple extension cords into each other to increase the length?

This is an ongoing debate we've had for years at work. My position is that we shouldn't be daisy chaining extension cords together as a general practice.

You're right Mike. The guys who say it is OK to do so are just telling themselves that so that they don't have to track down longer cords. However, I don't think you'll win an argument with these guys unless you can teach them Ohm's Law and physically show them a plug that's too hot to touch. ;) They'll just keep fighting with their experience. ("No fires yet!")

UL does not endorse this, we discourage it. Ditto for any kind of daisy-chaining, such as plugging extension cords into outlet multipliers or power strips, etc. Lots of contact resistance is added, not to mention wire resistance. Points of higher resistance become a load and end up getting hot.

I highly doubt any manufacturers endorse this practice. If UL saw it in their manual, we would make them change it or remove the UL Mark.

Regarding the 10 AWG extension cords, I think I'm going to make myself a short 12 AWG one. (small shop, I only need about 25' max) In my experience, the commercial-grade screw-on plugs have a much lower impedance connection to the plug blades than the ones that are spot-welded & molded at the factory by the millions.

[Sam Yerardi]

Mike,
Just as a standard practice in this application I would not advise daisy-chaining extension cords. One reason is the added resistance, obviously of the additional cords, but also the potential added resistance of each plug connection if a good electrical contact is not being made. Ihave seen in some instances where there can either be a bad mechanical connection or corrosion on the contacts leading to these types of failures.

[Tom Godley]

HD sells them:

I went to get some 10G wire to make up a couple of extension cords - it was much cheaper to by an extension cord and cut the ends off.

I needed to make a few 240V with 20amp twist lock ends.

I found that they were unloading the 100' 10g 125V twist locks for 45 bucks!. I found two out of the packaging and asked the manager if he would take $70 for both -- sold.

Normally the 10G black wire in the spool that they sell is over $1.25 a foot - HD wanted a little less than $50 for the 50' 10g extension cord with conventional ends.

[Mike Cutler]

Jeremy

Thank you for that answer.
I don't know that I'll ever get the practice stopped, but we have suceeded in getting 12AWG cords as the only ones that can be drawn from stock. I've seen some frightening uses of extension cords.

[J. Z. Guest]

Mike, sadly, it's one of those things where it will probably not get stopped until something burns up.