Danger of the Extension Cord

[SpaceCow]

I have been told by several people that using an extension cord of a large length is bad for appliances. Recently, one told me that he doesn't run his air compressor on an extension cord because "the longer the cord, there harder the compressor has to work." I doubt this guy is very knowledgeable about electricity, so I wanted to ask if this is somehow true.

As a beginner, the obvious issue I see with extension cords is that putting too much current through them will heat them up due to the higher resistance of the long wire., which could be a fire hazard. The voltage drop from the cable would also cause the voltage to the appliance to be lower. As far as I know, under-volting most appliances will not damage them. Is this correct?

Is there anything else that I missed that would cause an extension cord to make a motor "work harder" or be damaged?

Lastly, I have small welder that works on 120V and under 15A.  I want to use one or two extension cords with to extend my reach. How would I determine whether it is safe. Can I just try it and see if it works? This is a very intermittent load, and its under 15 amps. I can't imagine a cable melting from that. Should I measure the resistance of the extension cord(s) and use Ohm's law to calculate the power dissipated when 15 amps are pushed through it? What would be a maximum power dissipation for an extension cord?

Thanks for the help.

[ZeTeX]

Buy a proper 120V 16A rated cord and try it out. what's the worse that can happen? surely the wire wont melt.
"As a beginner, the obvious issue I see with extension cords is that putting too much current through them will heat them up due to the higher resistance of the long wire., which could be a fire hazard. The voltage drop from the cable would also cause the voltage to the appliance to be lower. As far as I know, under-volting most appliances will not damage them. Is this correct?"

I'm not an expert but I'm pretty sure you are correct.

[madires]

Yep, it's about the extension cord's resistance. Select one which is specified for the required power, i.e. has the proper wire gauge. The wire's resistance limits the power a machine or tool can draw. In some cases this is handy to prevent the breaker being tripped, e.g. for a large angle grinder.

[ebastler]

SpaceCow, looks like you have a solid understanding of the issues. Everything you wrote makes sense, and Zetex and madires have added good information. If you use a properly rated extension cord, its resistance should not be excessive ad should not cause problems.

It's a good idea to feel the temperature of the cord, and especially the plug connections, after running a high-powered device for 5 minutes or so. Bad contacts (in cheap or worn-out plugs/jacks) can occor, and can cause excessive resistance and heating.

One aspect that has not been mentioned: The extension cord should not be coiled up (as shown in your photo) when running high power levels through it. As the heat cannot dissipate, this could cause a fire hazard; the cord is rated for free air excange. Hence, if your cord is longer than you need it in a particular situation, do unwind it and spread it out a bit. Particularly important for cords on storage reels!

[Jeroen3]

There are three reasons exension cords are dangerous:
1. The cable is damaged. Either the isolation or the wires.
If the current carrying wire is damaged, there will be a higher resistance at that point. Thus a hotspot, probably melting the isolation and shorting it.
If the earth is damaged, your equipment is not protected against earth faults anymore.
2. The plug is worn out. Makes your socket burst in flames.
3. Your using it on full rating when wound up. It will get hot, and melt. This is probably the most common cause of extension cords malfunctions. Apart from people cutting it with a garden trimmer.

[Ian.M]

Some appliances can be damaged by sustained undervoltage.  e.g. anything with a switched mode PSU will typically draw more current at lower voltages, and anything with a high power motor is more likely to stall, and may overheat if its fan cooled. However with a little common sense, nearly anything can be run at the far end of a good quality 100' lead without significant problems - just don't put the fan heater on the same lead as a welder with a switching PSU!

[Vtile]

The most significant feature of extension cord is not mentioned.

If it is low quality and made from too small wire (did I just broke my signature) it adds a lots of resistance between the safety fuse and the device you are using. If everything goes as it should NOT to go, you have a short circuit that will not trip the fuse quick enough and the permanent wiring in your house will melt in worst case.

[ebastler]

The most significant feature of extension cord is not mentioned.

If it is low quality and made from too small wire (did I just broke my signature) it adds a lots of resistance between the safety fuse and the device you are using. If everything goes as it should NOT to go, you have a short circuit that will not trip the fuse quick enough and the permanent wiring in your house will melt in worst case.

Sorry, but that does not make sense to me. If the current flowing through the series connection of in-house wiring, extension cord, and short-circuited device remains below the rating of the fuse, it should certainly be within the rating of the in-house wiring, so nothing should melt there.

One can probably construe cases where the connected device short-circuits, the extension cord's resistance limits the current so that the fuse does not trip, and the device and extension cord run very hot. But as long as the device is not used unattended, I don't think this is a serious threat.

[Zero999]

I have been told by several people that using an extension cord of a large length is bad for appliances. Recently, one told me that he doesn't run his air compressor on an extension cord because "the longer the cord, there harder the compressor has to work." I doubt this guy is very knowledgeable about electricity, so I wanted to ask if this is somehow true.

As a beginner, the obvious issue I see with extension cords is that putting too much current through them will heat them up due to the higher resistance of the long wire., which could be a fire hazard. The voltage drop from the cable would also cause the voltage to the appliance to be lower. As far as I know, under-volting most appliances will not damage them. Is this correct?

Is there anything else that I missed that would cause an extension cord to make a motor "work harder" or be damaged?

Lastly, I have small welder that works on 120V and under 15A.  I want to use one or two extension cords with to extend my reach. How would I determine whether it is safe. Can I just try it and see if it works? This is a very intermittent load, and its under 15 amps. I can't imagine a cable melting from that. Should I measure the resistance of the extension cord(s) and use Ohm's law to calculate the power dissipated when 15 amps are pushed through it? What would be a maximum power dissipation for an extension cord?

Thanks for the help.

All perfectly valid but what's missing is why.

The longer the extension lead, the higher the voltage drop, leaving less voltage for the appliance. Many loads such as switched mode power supplies and induction motors actually draw more current, as the voltage is increased. The amount of heat dissipated in the motor winding is equal to I²R so more current will cause it to run hotter. Another compounding factor is metals have a positive temperature coefficient, causing the resistance to increase with temperature.

Another issue is safety. As mentioned above, the extension lead should not be coiled up and used at its full rating. It's also susceptible to damage. Another problem is the increased resistance of the earth conductor. If a phase to earth fault occurs, the voltage on the earth conductor needs to remain below 50V (to avoid the risk of shock) and blow the breaker fast enough to avoid fire. If the extension lead is too long, then the earth conductor's voltage can rise to an unsafe level, under fault conditions and the breaker might not trip in time to avoid a fire.  Fortunately if you're using an RCD/GFCI then it will trip before the earth conductor's voltage becomes dangerous but a phase to neutral short could still cause a fire, if the breaker doesn't trip in time.

To mitigate the above problems, the longer the extension lead is, the thicker the cable needs to be.

See the following link below for more information but look at the regulations in your jurisdiction.

http://www.industrialextensionleads.co.uk/maximum-extension-lead-length-3-w.asp

[bson]

The extension cord should not be getting noticeably warm.  Here's an AWG table, as you can see 10awg is rated for 15A power transmission.  Its resistance is 1ohm per 1000ft, which is 0.1ohm for an 100ft extension cord, or a little less for an 80ft industrial length (this is a common reel size).  15A rms * 0.1ohm = 1.5V rms drop and ~40 25W dissipated.  This is utterly negligible no matter how you have it coiled or reeled. In reality this is extremely conservative since any actual extension cord will be multi-core and downrated vs their awg.  For example 14awg is rated for 5.9A transmission, and there might be three such cores in an extension cord rated for 10A use.

http://www.powerstream.com/Wire_Size.htm

[bitwelder]

[rstofer]

I wouldn't be surprised if the User Manual for the welder talks about extension cords.  Many appliance/tool manuals do.

The entire problem is voltage drop caused by wire resistance.  About the biggest extension cord you can get is #12 AWG and it will probably work fine for your 15A welder out to 100 ft.  Probably...  Not guaranteed...  When you start stacking two extension cords, things might get a little iffy at 200 ft.  But it's the welder that will have the problem with low voltage, not the cords and certainly not the circuit breaker or fuse.

Or, maybe the idea that the resistance is so high that a short circuit won't trip the breaker.  The breaker should trip at 10x nominal so a 20A breaker should trip at 200A instantaneously.  Every other kind of trip is 'long time' and has to do with long time overcurrent.  Possible, but not likely.  So, the question is, could you have enough resistance to preclude creating a 200A short circuit.  120V / 200A = 0.6 Ohms down and back and, as luck would have it, the resistance is 0.685 Ohms for #12 AWG.  There is a slight possibility that a short circuit won't trip the breaker.  But it will trip pretty soon on 'long time'.

I would just try it and see how things work out.  The duty cycle for these small welders is pretty low and the cord has plenty of ventilation.

If you want to do this professionally then there are some heavy duty extension cords in the marine industry but they are pricey:

https://www.westmarine.com/electrical-cord-sets

You can also buy the cable and add your own cap and body.  The problem is, most 20A devices won't accept really large diameter cable.  You will probably want to buy a couple of adapters and use the standard marine connectors (or something very similar).

https://www.westmarine.com/adapters-plugs-outlets

Motors on extension cords will have a hard time starting because they draw around 6x full load to start.  Maybe more...  The voltage drop will also cause the motor to overheat.  Not using an extension cord with an air compressor is probably good practice.  That said, I have done it with my little DeWalt pancake compressor.  I won't do it with my bigger compressor.

[tautech]

OP
There's welders of different types and they behave differently on extension leads.
Older transformer sliding choke types didn't much like the voltage drop from long leads and one needs to crank up the output some to compensate.
New Inverter types are more tolerant.

I have leads that are reserved for welder use and outlets that I work from have been uprated with heavier than standard wiring all to minimise voltage drop and the resultant heating of plugs, sockets, switches and the welder itself. Yep, I melted some outlets and particularly their switches.
Keep an eye on your supply "system" for heating until you know that it can manage the load.
Another old trick was to use the outlets from your cooking range as the range normally has the heaviest wiring supplying it and the outlets themselves have short heavy wiring.
Welder mains leads are normally quite heavy but it's a good idea when using leads of any length to go at least 1 gauge size larger to minimize voltage drop.

Your buddy with the compressor is on to it. 

[Vtile]

....

Or, maybe the idea that the resistance is so high that a short circuit won't trip the breaker.  The breaker should trip at 10x nominal so a 20A breaker should trip at 200A instantaneously.  Every other kind of trip is 'long time' and has to do with long time overcurrent.  Possible, but not likely.  So, the question is, could you have enough resistance to preclude creating a 200A short circuit.  120V / 200A = 0.6 Ohms down and back and, as luck would have it, the resistance is 0.685 Ohms for #12 AWG.  There is a slight possibility that a short circuit won't trip the breaker.  But it will trip pretty soon on 'long time'
.....

This have another quirk, the entire property connection (from local HV transformer) might have so much impedance in lines that the whole house is on the edge already.. So one must take account also the lines before the house.

Well, I wrote my warnings.   

[CatalinaWOW]

i am trying to refresh 40 year old memories here so can't vouch for the full accuracy, but saws, compressors and other tools often have induction motors.  As the available voltage drops the lag angle increases, changing the power factor.  So now at a given delivered power you have a significantly higher circulating current.  Further heating the wires and increasing the voltage drop.  The effect is very noticeable and can be seen on job sites where saws start slowly and bog down easily at the end of long cords.

The extra circulating current is where the potential for damage to the tool comes.  Just watch your tool.  If it is performing noticeably differently get a better or shorter power cord.

[Elf]

I have stopped using "off the shelf" extension cords as the wire and connectors are usually of poor quality. There is nothing magic about the wire in an extension cord (versus, say, all the other building wiring between you in the panel), but I think poor quality and excessive length of extension cords has led to the warnings about not running tools off of them. Higher resistance and the resulting voltage drop (from small gauge wires) causes motors to draw excessive current and heat up (see: http://www.motorsanddrives.com/cowern/motorterms12.html ).

I just make all of my 15 or 20 amp extension cords out of 12/3 or 10/3 SOOW-type cord and good Leviton or Hubbell industrial or hospital grade connectors. Much better than what you find at the store.

I was always told to coil high current extension cords in a "figure eight" pattern rather than just a loop, to avoid overheating. I think the implication was that it would avoid some inductive effects, but it seems that advice is only valid for single conductor cables (e.g. large camlok single phase cables for higher current / higher voltage applications). Given an average two or three wire extension cord, I would think the magnetic fields would cancel each other out?

[Zero999]

I have stopped using "off the shelf" extension cords as the wire and connectors are usually of poor quality. There is nothing magic about the wire in an extension cord (versus, say, all the other building wiring between you in the panel), but I think poor quality and excessive length of extension cords has led to the warnings about not running tools off of them.

There are some differences: the cable inside the house is often solid core and will be much thicker than most extension leads.

I was always told to coil high current extension cords in a "figure eight" pattern rather than just a loop, to avoid overheating. I think the implication was that it would avoid some inductive effects, but it seems that advice is only valid for single conductor cables (e.g. large camlok single phase cables for higher current / higher voltage applications). Given an average two or three wire extension cord, I would think the magnetic fields would cancel each other out?

You're right about the magnetic fields cancelling, even for a single phase circuit, the live and neutral conductors run parallel so their fields cancel.

Even if the magnetic fields don't completely, they're not responsible for heating the cable anyway. Only resistive losses cause heating of the cable.

[ptricks]

One thing you can do to  see how your extension cord is performing is to plug in an incandescent bulb at the end along with whatever you are powering , if the light dims when you run the appliance, then you are using a cord that is under sized for the load. Motors like on compressors and power tools are really bad about needing a large start up current that many cords cannot supply.

[Vgkid]

When it comes to large loads,the smaller the guage,the better. With my air compressor will charge the tank, carry it where I need it, then plug it into the extension cord(or not, for smaller tasks). When it comes to making an extension cord, I often use the cables of salveged outdoor gear.

[GreyWoolfe]

I have stopped using "off the shelf" extension cords as the wire and connectors are usually of poor quality. There is nothing magic about the wire in an extension cord (versus, say, all the other building wiring between you in the panel), but I think poor quality and excessive length of extension cords has led to the warnings about not running tools off of them. Higher resistance and the resulting voltage drop (from small gauge wires) causes motors to draw excessive current and heat up (see: http://www.motorsanddrives.com/cowern/motorterms12.html ).

I just make all of my 15 or 20 amp extension cords out of 12/3 or 10/3 SOOW-type cord and good Leviton or Hubbell industrial or hospital grade connectors. Much better than what you find at the store.

I was always told to coil high current extension cords in a "figure eight" pattern rather than just a loop, to avoid overheating. I think the implication was that it would avoid some inductive effects, but it seems that advice is only valid for single conductor cables (e.g. large camlok single phase cables for higher current / higher voltage applications). Given an average two or three wire extension cord, I would think the magnetic fields would cancel each other out?

I also made 2 30 ft extension cords from 12/3 and heavy duty connectors for my generator.  I made them just long enough for my particular application.  1 runs the refrigerator and the other connects to a heavy duty surge protector for plugging in lights and fans.  Better to have larger wire extensions than smaller.  The 2 extensions never heated up anywhere, including at the connectors for the 24 hours I had to run the generator after Hurricane Matthew.