Thread: Can I run 2 hp motor off an extension cord

My choice of lead is the 15 amp lead with the 10 amp plugs

I strongly agree! But get an electrician to wire up the plugs for you.

You will be far better off with the heavier duty cable. Don't forget that for an induction motor, the starting current can be up to 6 times the normal full load current, which is why you often cannot start up more than one machine at a time.

You don't want to run the risk of a thin cable operating at its limits - the insulation can melt, the bare cable wiring shorts and you have a fire and a big bang that causes a lot of inconvenience, if not a risk to life, depending on how often your circuit breaker earth leakage protection is checked and tested.

Originally Posted by Dengue

Don't forget that for an induction motor, the starting current can be up to 6 times the normal full load current, which is why you often cannot start up more than one machine at a time.

Something else that I have to be mindful of, speaking about starting or running more than one machine at a time - I have a 3hp compressor so when I am running any other large machines, I make sure the compressor is turned off. I have three other 3hp machines, a tablesaw, bandsaw and a thicknesser. There would be a 2hp dusty running at the same time as any of these machines. The last thing I would want is the compressor to decide to charge up the tank when I am already running 5hp of gear plus fans, radio etc.

Just a little something to keep in the back of the mind; some machines like compressors can start by themselves and make an awful mess of your power consumption predictions.

Doug

Originally Posted by Karl Robbers

. . . . however I am not so sure that 1500W, (actually 2150), is not a lot of power - remember vacuum cleaners stipulate that their leads be unwound during use and they are a lot lower powered than we are talking here.

One significant reason that this is stated for vacuum cleaners is because the cord inside the vacuum cleaner is wound up inside the machine and has little chance of losing the heat. If the coil is covered up with anything that reduces heat loss even a small amount of heat may cause a problem but if the coil is exposed to open air the amount of heating is small for <10 A devices.

A few years ago I did an experiment on the 15 A extension cord that I use to power my welder. I rolled up the cord on a plastic bobbin so that the cord consiste of 3 layers (each of 10 turns) hanging about 500 mm from the roof of the shed. I placed a thermocouple in between two of the turns in the middle layer and welded 4, 3 mm rods more or less continuously, and paused briefly in between each rod to see what the temperature rise would peak at.
Recently I found the data and thought you might like to see it.,

Starting Temp was 19º
Turning on the welder and letting it warm up over 10 minutes - ie no arc, the temperature stabilized at 31º,
After welding with one rod, it was 35º;
after 2 it was 39º
after 3, 42º
after 4, 45º

The reason I stopped at 4 rods was the limited duty cycle of the welder meant I could not weld more than 4 rods before it stopped working.

Undoubtedly the temperature inside the wire would have been higher but there was no smoke.
I did a megger test on the cord before and after this treatment and there was no change in the spec

The temperature problem is exacerbated when
- its a hot day
- when the coil is covered up with something thermally insulating that prevents heat loss
- when the cord is just old or damaged increasing its resistance.
- when high currents are drawn for very long times.
- roll has many layers

I show this data because it is not quite as bad as it is sometimes made out to be - I still use that extension cord to power my welder - it's left with the same 3 roll layers hang from the roof.
Sometime I should re-megger it to see how it is going.

Of course I advocate unrolling long extension cords but I don't get too paranoid about leaving a couple of coils on a roll and I do make sure they are well ventilated.

Originally Posted by BobL

Of course I advocate unrolling long extension cords but I don't get too paranoid about leaving a couple of coils on a roll and I do make sure they are well ventilated.

I agree with Bob. I have a number of 15amp leads with 10amp plugs of various lengths so that I can use a fully unwound lead without having excess power lead lying around the workshop, but let common sense prevail.

Suppose I need to use a hand-held electric drill to drill a couple of holes and the only lead I had available was 30m and I was 2 metres from the power point and the drill had a 1.5m lead I would not unwind the 30 metre extention lead any more than necessary.

At the other extreme, if I needed to use a 30m extention lead to get me a metre further from the power point for one of my 3hp machines I would unwind it fully even if it was for just one cut. And it goes without saying that I would prefer not to use a lead that long on a machine that size if there is another way of doing it, eg shorter lead.

Doug