Thread: Query - Running Air Compressor on extension lead

Originally Posted by Jim Ferrous

G'day all,

I often see the statement "You shouldn't run an air compressor on an extension lead" its gotta' be straight to a wall socket.

My query is if your shed is say 15 metres from your house and you run a heavy duty rated extension lead from a socket to the air compressor what's the difference if a sparky ran a wire the 15 metres to your shed and then you plugged the air compressor into the wall socket?

There is no difference PROVIDED the heavy duty extension really is Heavy Duty and the pugs and sockets are clean and not oxidised.

The screwed terminals inside GPOs general make for a lower resistance connection better than the spring loaded thingies used inside GPOs. When they are clean they should be fine but if they are covered with a dull layer of oxidation, then they can cause problems.

What passes for HD extensions at Running may or may not be HD. Most so-called HD cables from bunnings use 1.5mm copper cored cables. If you want really be sure the cable is HD then use a 2.5mm core cable. The 15A welder I have specifically says, any extension should use 2.5mm cable.

Most so called HD extension leads are only HD plastic covering.

Only buy leads that quote a copper thickness, if they don't quote copper don't buy. All other labels are from the marketing department and mean FA.

The biggest difference is a licensed electrical contractor sparky will know the load limits of the electrical cabling being installed and hopefully comply with legislations, whilst your average weekend warrior may not and may choose to run an extension cable not fit for purpose and end up overloading the circuit.

The warning is there to protect your equipment AND reduce the liklihood of you burning your house own.

Thanks for the replies so far all.

15amp heavy duty cables at Bunnings are stated as 1.5mm cores -

https://www.bunnings.com.au/hpm-20m-...cable_p4430369

The thickest HPM make are all 1.5mm core -

https://hpm.com.au/extension-leads/

It also depends on the current draw of your compressor, if its a small one chances are that cable will be fine. I believe that cable is designed for caravans, I'm not a sparky so wouldn't know the difference between the current draw of a typical caravan vs compressor.

If you want 2.5mm core cable you'll need to get one made up, bunnings do sell the cable. see link below.

https://www.bunnings.com.au/olex-2-5...metre_p4430139

Thanks for the link Tony, appreciated.

This is from the compressor's user manual -

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2.5mm cables are used for up to 20A connections so sometimes the extension cords use round pins and sockets like 3 phase plugs.

I'm not telling or suggesting anyone do this but I made my own (6m long) from 2.5mm cable and 15A plugs and sockets.
I only plug it into a 15A socket which has its own breaker.
It's always OK to over rate a cable.

There are 3pin 20A plugs and sockets available
ege https://www.sparkydirect.com.au/p/72...--43820tr.html
https://www.sparkydirect.com.au/p/72...-143920tr.html
Notice the prices, and as well as larger earth pins, they also have larger active and neutral pin sizes.

Screen Shot 2019-04-08 at 3.24.51 pm.png Screen Shot 2019-04-08 at 3.25.08 pm.png
You can plug your compressor into a 20A socket but you cannot plug the 20A plug into a 10A or 15A socket - that's why I used 15A plug/sockets.

Depending on your pressure requirements your other alternative is to get a 15m long x10mm ID compressor hose which won't lose too much pressure.

Just saw your compressor specs. While should run OK from a 1.5mm x 15m extension, given it is 15m I would still prefer to run it from a 2.5mm extension.

I just remembered that the last time I made up cables with 2.5 mm wire the wire would not fit into the standard 10A three pin plug or cable socket. I solved the socket problem by using a 20A cable socket, the normal 10A plug will fit in, but on the plug end you have to do some electrical gymnastics to get the cable connected to the plug top.

I live on a farm and the reason that I went to all that trouble was that I was making up a 50m and two 25m extensions and didn't want any significant voltage drop on motor start when you plugged them all in one long line.

The other way around the problem is to just use a long air hose instead of a long extension lead.

Originally Posted by Jim Ferrous

G'day all,

Is it right that in the U.S running at 120v they need even heavier duty extension leads (is that correct)?

Is this where the "You shouldn't run an air compressor on an extension lead" came from?

probably, in the US with a 2400 watt load current would be 20amps @ 120volt compared to our 10amps @ 240volts. It’s all about transmission losses in a cable called P=I^2R. In the case above for a cable resistance of one ohm is 20x20x1 or 400 watts loss in the US compared to our 10x10x1 or 100 watts loss to the tool given off in cable heat and is why cables get warm and should be used uncoiled and power companies use very high voltages on transmission lines on long distances. So heavier duty cables over long lengths will be more efficient due to the lower resistance.

Originally Posted by apple8

probably, in the US with a 2400 watt load current would be 20amps @ 120volt compared to our 10amps @ 240volts. It’s all about transmission losses in a cable called P=I^2R. In the case above for a cable resistance of one ohm is 20x20x1 or 400 watts loss in the US compared to our 10x10x1 or 100 watts loss . . . .

For a cable and connectors in good condition the voltage drop for a 1.5 mm^2 cable will be around 3 V which means a power loss of (voltage drop x current) or 3 x 10 = 30W, for a 2.5 mm^2 cable it will be 20W. This translates to a cable resistance R = (Voltage drop/current) of 0.3Ω for the 1.5 mm^2 cable and 0.2Ω for the 2.5mm^2 cable

If the connectors are in poor shape the cable resistance can indeed reach 1Ω and hence reach the power losses described above

A few years back I tested all my long extension cables (10 - 30m) and they were all up to spec except one which was well over spec. The connectors on this cable were highly oxidised and when I cleaned them up the resistance dropped, but it was still over spec plus it had a couple of nicks in the outer insulation so I binned it.

BTW this is not the only test that should be performed on cables. The cable and connector resistance might be fine but an insulation resistance test at high V should also be performed as over time the insulation breaks down and this test will pick up if the cable insulation has been damaged internally which can lead to a hot spot. This test is done with a special meter sometimes known as a Megger.

Why I asked is that my new compressor had a warning on the 3 pin plug and in the instruction manual.

My old 2.5hp compressor (cheap chinese) went for 16 years (motor is still fine, vessel rusted out) on the same set up so if the new 2.5hp one gives me 16 years I'll be happy.

I went through my box of instruction manuals and found the instructions for the old compressor and it stated as well that it should be plugged straight into a wall outlet and never an extension cord 16 years of bliss I had because I didn't read that.

Originally Posted by Jim Ferrous

Why I asked is that my new compressor had a warning on the 3 pin plug and in the instruction manual.

My old 2.5hp compressor (cheap chinese) went for 16 years (motor is still fine, vessel rusted out) on the same set up so if the new 2.5hp one gives me 16 years I'll be happy.

I went through my box of instruction manuals and found the instructions for the old compressor and it stated as well that it should be plugged straight into a wall outlet and never an extension cord 16 years of bliss I had because I didn't read that.

As far as the compressor goes, the compressor manufacture doesn't give a rats if your cable gets too hot and catches fire. However if an extension cable has too high a resistance the resulting V drop at the end of the cable means the compressor is operating at too low a V. This means the motor has to draw more current to produce the power needed to compress the air. The result is the motor will overheat beyond spec and cause long term damage. It may even get hot enough to catch fire.