We have a couple of items (Food Dehydrator and Air Compressor) that clearly state that an extension cord should NOT be used to connect them to a power supply.
Any ideas why?
Is it because you need to choose an extension cord that is of the correct rating for that item and choosing the wrong extension cord could result in overheating?
Or is there another explanation?
Perplexed from Clapham!
pH
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Unfortunately that is the manufacturer’s or suppliers advice. As the purchaser the risk is all yours if you do not follow it.
Is the requirement well founded?
That would require some detailed design information and manufacturing IP, put to a suitably indemnified electrical engineering consultant. There are potentially damaging or high risk of injury fault conditions that are more likely with the use of an extension lead. Whether the risk is greater for the two examples mentioned, hope this clarifies.
P.S.
Not a recommended solution, but worth knowing. It is possible to buy heavy duty extension leads.
EG Deta 50m 2.5mm 10A Extra Heavy Duty Extension Lead - Bunnings Australia
Be careful though as there is no standard that defines heavy duty. One products extra heavy duty product may be another suppliers light duty. Look at the wire size when comparing products. The longer the extension lead the larger the wire size.
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Not being a sparky nor a power distribution focused EE there are two main issues and a few potential/theoretical minor ones.
The most obvious is an under-rated extension cord, easily identified by it heating up when the device is in use, as well as the labelled rating. Another issue are the male and female plugs that might not get seated perfectly and lose connections beget heat. In both cases a fire can start. Some devices also hit a high peak demand when they start-up that could be much more than their stabilised (rated) draw that could impact the cord. Heavy duty compressors would be poster products for that.
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These are high power devices that draw a fair bit of current. If that current is drawn through a longer wire with dry connections there is the risk of ohmic heating (especially if the cord is coiled up or covered in some way) or possible sparking at the joints if they are not well made. A newer or more heavy duty cord may mitigate the risk.
Another possibility that I am less certain of is if the devices have voltage regulation. There will be a voltage drop over the cord. That drop might cause the regulator to adapt in exciting ways. As they say on Wikipedia “this topic needs the attention of an expert”.
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Many thanks - your replies have clarified the matter!
Something for Choice to investigate?
pH
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It’s been easier for the supplier to say no rather than invest in determining if an extension lead could be safe, IE minimum wire size specification, maximum length and any other restrictions.
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How so? Perusing Choice articles reveals a repetitious statement of the form
Don’t plug it into a powerboard or extension cord. Heaters use a lot of energy, which can cause the extension lead or powerboard to overheat …
‘Do not use extension cords for [certain devices]’ seems to cover it, does it not? What more is there to add?
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This website will provide your answer…
The longer a wire (cord), the higher the resistance and the greater current required to meet the requirement of the appliance. Drawing higher currents poses a number of risks, overheating is one…
Most domestic extension cords (and powerboards) aren’t designed for high load appliances to hang off the end of them.
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There are some answers about the reason but the information about ratings and selection of cords for purpose is not so relevant. None of my cords have the gauge of the wire stamped on them as described, so distinguishing “heavy duty” that way is not so easy. Also in Oz some heavier cords (for caravans etc) have a different plugs and sockets that do not fit a standard Oz 3-pin power point.
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Information on it’s rating would have been included on labelling/packaging on purchase, and possibly on the product itself.
You have highlighted why manufacturers state not to use extension leads for high load devices. The average consumer won’t know the rating of the wiring/cord/flex and if they did, what it means. This is why the general advice is provided not to use any extension lead/powerboats when using some products.
These are 20 Amp plugs. Neutral and active (and sometimes earth) are larger preventing insertion/use with a standard domestic 10 Amp socket. A 10 Amp plug will go into a standard 20 Amp socket, but not the other way around.
We have a standard 20 Amp socket in our laundry, specifically for our dryer which is rated at 4500W (about 19 Amps) and as a result has a 20 Amp plug.
Edit: this photo shows the difference between a standard 10 Amp (upper in photo) and 20 Amp (lower in photo) plugs…
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Most commonly caravans use 15 Amp. And yes there are 20Amp as well.
‘Caravan Power: Why RVs Use 15-Amp Electrical Circuits - GoRV
A range of industrial and power tools also use 15 Amp and some 20 Amp 230V single phase GPO’s. This may include household ovens, although many are now hard wired and come without plugs. For greater power 3phase is the norm.
https://www.accesscomms.com.au/australian-mains-plug-variants/
P.S.
Only one comment on looking at a US based non standards guide on copper wire gauges for extension leads. Australian household electrical standards and supply are different to those applied in the USA. YMMV.
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Plugs have a wider earth pin to prevent insertion into a 10 Amp socket. I see that the RV website indicates there is a
safe and legal way to plug a van into a 10A power source is to use an Ampfibian power adaptor, a device that contains an RCD to prevent an electrical overload. When plugged in, the Ampfibian will provide 10A to the RV. If current exceeds 10A/2400W, the unit will cut the current off
This would mean that one would only be able to draw 10 Amps, where if most things being used in a caravan (cooker, aircon, fridge, water heater) would potentially cause overload. To work, one might need to juggle what is turned on at any one time.
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Those larger pins also provide a larger contact area, because you need it for higher amps. If you think of electrical current (amps) in terms of the number of electrons, your need double the electrons flowing around the circuit when you double the amps. They travel at the same speed, so you need more space for them to “roam”. There are mathematical formulae for these things, but the fact is, size matters… You can assume that the 15 amp plug is sized the electrical need, not only to avoid plugging in 10 amp cables.
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As others have mentioned, the real issue behind the warning particularly applies to appliances with motors that draw a brief but high starting current. Theoretically you could plug a heater drawing 10A into a chain of uncoiled 10A rated extension leads and nothing bad would happen save that the voltage drop would significantly reduce the heater’s output. With a compressor, the reduced voltage might mean that the motor stalls and burns out. How do I know this … 
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My guess would be the same as Dock Tony.
The compressor probably draws a high current to overcome air pressure and get it going at startup. This probably only lasts for a few seconds, BUT if an extension cord is used, there is a risk that during startup, the voltage drop (loss of voltage) caused by the extension cord will result in not enough power getting to the compressor motor, and the motor might struggle and strain and in doing so, might burn itself out.
An extension cord presents resistance, and thus a voltage drop at 3 points.
The plug, the socket and the cord itself.
I have also come across similar warnings in some instances for refrigerators, for the same reason.
I can’t answer for the air fryer.
BB
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I cannot see any issues with a short extension cord, say 3 meters. The resistance in that length is negligable. The cable run from fuse box to the power point is in most cases much longer than the extension cord and will contribute to any voltage drop more.
Now long extension cords, say 50 meters or more, is another matter. There will be a noticable voltage drop, and that drop due to resistance will be given off in heat. That heat could be a danger if the cable is coiled up and cannot be dissipated.
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A 3 metre ext cord may have negligible actual cable resistance, BUT, using it inserts another plug and socket into the chain of power, and thereby adding resistance, and thus a loss of power to the device.
The proof of this is:
have you ever noticed that when using extension cords, sometimes the plugs and sockets get warm? That warmth is generated by power being dissipated by the resistance of the connections between the electrical contacts inside the plug and socket.
One factor affecting this would be oxidation of the electrical contacts in those plugs and sockets. Another factor is loose connections within the plug and socket - wire clamps not screwed down tight enough or have worn loose over years of use.
I had an experience using one of those high pressure water sprayers. Using a long extension cord, it had difficulty getting started.
I ended up having to clean all the connections in the plugs and sockets in order to overcome the problem.
BB
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Hi syncretic: While I understand you point, the term “ohmic heating” is incorrect. This refers to a cooking process where a current is passed through food, and that current causes a temperature rise due to the resistance (ohms).
What you are referring to is the temperature rise in an electrical circuit due to the current flow. This temperature rise is generally due to resistance in the circuit, measure in "ohms’.
It depends on context. Another term for the same thing is resistive heating. Modern usage seems to mainly use “ohmic heating” to refer to heating food. However, it is exactly the same physical principle regardless of whether it is current passing through food or a copper wire. It is heating due to current passing through an ohmic resistance, ie a substance that obeys Ohm’s law.
As well as being examples of the same principle some references also name them as the same thing, eg Wikipedia:
Perhaps it would have been less distracting if I had said resistive heating.
The main reason an appliance manufacturer would declare that an extension cord must not be used to connect their appliance to power is to cover themselves against ‘damages’ if something goes wrong. This is because most people do not really understand the dangers of electricity and they do silly things. The various earlier comments highlights the confusion about extension cords.
To clarify some incorrect points made earlier:
All extension cords have a current rating relevant to the plugs at each end. So, an extension cord with 10-amp plugs, which can be connected to a standard 10-amp power point, will be able to carry 10-maps. (Same deal for extension cords with 15-amp or even 20-amp plugs - they have larger wire of course.).
The “duty” of an extension cord is about where the extension cord can be used. A “heavy duty” cord is for a workplace, or where there is a possibility of heavier than normal use like a workshop. “Ordinary duty” cords are for domestic use, they are fine for any sort of use inside the house. The “heavy duty” extension cord generally has thicker insulation to give it more protection, which is why the are heavier.
Some qualifications for the use of extension cords are: If you have a log run, get a longer extension cord, do not connect two together, and always uncoil the extension cord to prevent the mutual heating effect. An overheated extension cord is dangerous.
If you have to connect an appliance using an extension cord be mindful of the load - current in ‘amps’, or power in ‘Watts’.
An electric motor will draw more amps at start than when running. In the old days when I was designing motor starters we used the rule of thumb of a start current being 7-times the run current. But before you get to concerned this only lasts a few seconds, and remember your refrigerator, your washing machine and your dishwasher have electric motors and they continue to run fine - probably they will not have an extension cord though!
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