Thread: 25 mtr extension cord and 2.5 hp compressor

"Heavy duty" doesn't mean much what you should look out for is the cross sectional area of the conductors in the cable - it should be marked on the cable itself.
For 2.5HP I'd be using at least a 1.5mm^2 cable - this is usually what Masters and Bunnings sell as HD cable.
If you wanted to be super sure then for a 25m long cable you may want to go to 2.5mm^2 cable.

I’m pretty sure it’s 1mm, so I won’t be using it.

I have other shorter, genuinely heavy duty leads - just less convenient.

Just out of interest, if you did use an underrated extension cord, what exactly would fail - the motor, the cord or the building circuits?

And does the length of the cord make a difference (provided it’s of adequate gauge) ?

Thanks for the reply
Arron

Yep, found it on the cable - 1mm.

So the 2.5hp is out.

Would you run a 2hp compressor off it?

Originally Posted by Arron

I’m pretty sure it’s 1mm, so I won’t be using it.
I have other shorter, genuinely heavy duty leads - just less convenient.
Just out of interest, if you did use an underrated extension cord, what exactly would fail - the motor, the cord or the building circuits?

Building circuits should look after themselves.
Longer/ thinner cable drops more volts cable heats up - short term it might not be that much of an issue but long term, insulation gets brittle and cracks and . . . .
Lower V at motor means it has to draw more current to achieve work rate (power) so more overheating of motors.


Here's a few calcs for a 25m long cable, 240V mains at GPO on a 2.5HP (1863W = 7.8A) load.

1mm^2 conductor will drop about 13V.
Power dissipated in cable = Volts x Amps = 13 x 7.8 so the cable is drawing 101W = that's a lot - think of an old incandescent 100W light globe - although it is spread along 25m of cable.

1.5mm^2 cable drops ~8.3V
Power is now 65W which is a bit better but still .

2.5mm^2 cable drops 4.5V
Power is now ~35W

This is BRAND NEW cable, older twisted cable with kinks and even lightly corroded plug and socket pins will have higher resistance loads.

Originally Posted by Arron

Yep, found it on the cable - 1mm.

So the 2.5hp is out.
.

Would you run a 2hp compressor off it?

Not really - 25mm x 1mm^2 is about a 1kW cable ie ~1.3HP

Just checked the regs which allow for a 5% V drop and the above combo is 3.5% so it is within spec.

You can find the V drop calculator here
Voltage drop calculator AS/NZS3008 | jCalc.NET

Ok, thanks for all that Bob.
Cheers
Arron

In my experience, "heavy duty" typically means that the insulation is thicker than normal, so the cable can withstand a little bit of physical abuse.

In the past Bunnings (or Mitre 10) sold a "15 Amp extension cable" which was fitted with 10 Amp plugs each end.

Originally Posted by ian

In my experience, "heavy duty" typically means that the insulation is thicker than normal, so the cable can withstand a little bit of physical abuse.

Or is coloured red, or red and yellow.

Looking at it more closely, I think the words ‘heavy duty’ refer to the cable reel, not the extension cord.

Seems a bit deceptive.

Originally Posted by BobL

Building circuits should look after themselves.
Longer/ thinner cable drops more volts cable heats up - short term it might not be that much of an issue but long term, insulation gets brittle and cracks and . . . .
Lower V at motor means it has to draw more current to achieve work rate (power) so more overheating of motors.


Here's a few calcs for a 25m long cable, 240V mains at GPO on a 2.5HP (1863W = 7.8A) load.

1mm^2 conductor will drop about 13V.
Power dissipated in cable = Volts x Amps = 13 x 7.8 so the cable is drawing 101W = that's a lot - think of an old incandescent 100W light globe - although it is spread along 25m of cable.

1.5mm^2 cable drops ~8.3V
Power is now 65W which is a bit better but still .

2.5mm^2 cable drops 4.5V
Power is now ~35W

This is BRAND NEW cable, older twisted cable with kinks and even lightly corroded plug and socket pins will have higher resistance loads.

Bob, those calculations are fine if the compressor is rated at input power, but in my experience compressors are usually rated by the power delivered by the motor, not that consumed by it. I would expect a 2.5HP motor to consume an estimated 12 Amps at full load. The only way to determine the correct number for the calculation is to either look at the motor rating plate or else meter the current.

Chas.

Originally Posted by Chas

Bob, those calculations are fine if the compressor is rated at input power, but in my experience compressors are usually rated by the power delivered by the motor, not that consumed by it. I would expect a 2.5HP motor to consume an estimated 12 Amps at full load. The only way to determine the correct number for the calculation is to either look at the motor rating plate or else meter the current.

Good point but a 12A draw is not what I usually see on a well maintained 2.5HP compressor.

Assuming the compressor motor is rated at max load, a 2.5HP (1.86kW) output power motor should be at least 80% efficient so should draw 2.235 kW or a current of 9.3A at 240V which is consistent with what I used to see on my 2.5HP that I used to permanently have a V/I meter on.

By way of comparison my 4HP (2,98kW) compressor shows it drawing a max of 3.65kW which works out to be 82% efficient.

A mate wanted to do up his front gate on a hobby farm. He borrowed my 2.1 cu ft cement mixer with a half horse power motor. The gate is more than one hundred meters from the nearest power point. He assured me his extension lead could reach OK. I told him to mix at the power point in his shed then transport the mixed barrow of concrete on the carry all on his Furgy. I dropped the mixer off but had to retrieve it weeks after the job was done (Grrrrr....pet hate I hate having to retrieve tools that I loan).
I put the mixer in its place under cover. About 9 months or so later I loaded up the mixer to do a job for a mate and guess what...... the motor died half way through the job. I am almost certain old mate with the long lead, used the mixer at the gate way instead of at the shed.
Lesson learned.....be nasty and don't lend tools.
I reckon the voltage drop over the 100 mts plus made the motor labour too much and was the cause of the motor's demise

Yeah I am turning into a grumpy old man. It was only money to fix the motor....just over $200. Made the loan expensive

If using a lead make sure it is fully uncoiled. If it has a cable reel, it must be fully off the reel. One of the signs of heat damage are melted reels or heat set where any twist become permanent in the lead.
Air compressors - short or no extension leads are best, use additional air hoses instead.