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

G'Day Arron,
I'm not a sparky but have worked as a sparkies off-sider & did some 120hours of electrical theory in my training.
I'd encourage you get the heavy duty 10amp leads, they have a heavier gauge wire & insulation and thus when running a 9amp machine the cable is going to be able to carry the load/heat, IMHO.
You could also go to your local electrical wholesaler to purchase a short length of the heavy duty 3core flex with plug & socket to assemble yourself,
though if you are not confident about this route, you could ask a local sparky to do it for you...
Cheers, crowie

Originally Posted by Arron

Hi. I've got a couple of woodworking machines with 2 hp motors. One has 9amp on the compliance plate, the other has no amperage rating. My question is can I safely run these machines off ordinary extension cords - maximum length contemplated being 2 or 3 metres. By 'ordinary' I mean the common domestic things you buy at KMart etc, which I guess are 10 amp. I've been told I need to use a 'heavy duty' lead, but it doesnt sound right to me and the only ones I've sourced have been annoyingly long and expensive.

cheers
Arron

Yep no problem. I have my 2HP BS on a short 10A elcheapo cord and I used to run my 2HP DC on about a 5 m long cord.

BTW just because it says 2HP/9A doesn't mean very much. I depends on what the machine is and how it is used as to how much power it develops.
Provided a DC is well designed and not throttled down should draw the max current close to its rating. All my DCs are/have been like that.
On the other hand a BS or TS will draw less than half the max rating when idling and can pull currents well in excess of the max rating if pushed really hard.
What the manufacturer banks on is that you don't do this for too long

I run nearly everything off extension cords at one time or another and I have not experienced any problems.

Your lead length is not very long, so you could get away with the lighter leads, personally I would go to Bunnies or A hardware store and buy a H/D 15amp lead with 10amp plug & socket, .
The reason behind this is that the heavier wire means less voltage loss, and the fact that even though it only lasts a few seconds the motor draws nearly double the amps when it originally starts up.

Jeff
vk4

The way I understand it a 10amp lead is a 10amp lead. A heavy duty 10amp lead just has a thicker plastic casing than a home duty lead to be better protected from damage.

A 15amp lead, e.g. a caravan lead, would have a 15amp plug with a bigger earth pin. I think you can buy 15amp leads with 10amp plugs, which is what the 'trade' leads probably are.

My choice of lead is the 15 amp lead with the 10 amp plugs. As has already been mentioned the startup power is often well above the manufacturers rating. even the domestic leads can withstand this because it is only momentary and does not have time to overheat the lead and cause damage as it would if it was carrying that much current continuously.

The downside is of course that these leads do tend to come in 25 or 30 metre lengths. It is not difficult to cut one of these longer leads into a number of short ones and it is actually cheaper to buy made-up leads than to buy the cable by the metre.

Bunnings sell the plugs and the packet has full instructions on how to fit them including a full-scale diagram on stripping the wires. A screwdriver, sidecutters and a wire stripper or even just a knife and a bit of common sense are all you need.

The plugs come with either grey or clear plastic covers. I prefer the clear ones for the workshop because you can see the connections inside and you can easily see if any of the connections have overheated or have been pulled out if someone tripped over the lead, for example.

Others have mentioned that you get less voltage drop over the 15 amp leads than over the 10 amp ones of the same length, but that is not as big an issue if you are talking about short leads, but should still be taken into consideration.

Another thing that I do if I feel that I am pushing the capacity of a 10amp GPO is to plug in an RCD between the lead and the GPO just to prevent any problems going beyond the RCD and possibly causing damage to the shed/house wiring.

Doug

Originally Posted by doug3030

The downside is of course that these leads do tend to come in 25 or 30 metre lengths. It is not difficult to cut one of these longer leads into a number of short ones and it is actually cheaper to buy made-up leads than to buy the cable by the metre.

When I rewire things that need a cord and a plug I usually just buy a short extension cord and cut off the socket as it is cheaper than buying cable and a plug.

Originally Posted by BobL

When I rewire things that need a cord and a plug I usually just buy a short extension cord and cut off the socket as it is cheaper than buying cable and a plug.

Another way of doing it: If I am throwing out an old electrical appliance, be it a refrigerator, vegie steamer, old hammer drill etc I always salvage anything that may be reused later including the cord, then if I need a new cord and plug for something, I go to my box of spare parts and pick out a suitable one if available. Its good for recycling and it has also saved me from having to stop mid-project and go to bunnings, thereby maximizing shed-time

Doug

Originally Posted by doug3030

Another way of doing it: If I am throwing out an old electrical appliance, be it a refrigerator, vegie steamer, old hammer drill etc I always salvage anything that may be reused later including the cord, then if I need a new cord and plug for something, I go to my box of spare parts and pick out a suitable one if available.

Sod's law means I usually only have 2 pin jobbies available when I need a 3 pin.

Before my shed was wired, I used to run my dust collector and saw off a 100 foot lead throught a 15amp site box. Dust collector and saw 3.5 hp. I used to start the dusty first, then the saw. When using all the other machines, the same principle applied. I never had any problems in that time.

Robert

Originally Posted by specialist

Before my shed was wired, I used to run my dust collector and saw off a 100 foot lead throught a 15amp site box. Dust collector and saw 3.5 hp. I used to start the dusty first, then the saw. When using all the other machines, the same principle applied. I never had any problems in that time.

Robert

Robert, that makes sense. I am guessing that it was a 15 amp lead, probably with 10 amp plugs? Its rare to see a lead that long that is only 10 map cable just because of the voltage drop issues.

(Long story short, Many years ago, when I didnt know as much as I know now, I acquired a 60m extention cord which had been made by an idiot out of 10amp cable. I had originally borrowed it to do some soldering work on radio equipment in my car. The voltage drop was such that solder would not melt. Over time I think I have reduced it to about 10 separate leads, the longest of which is about 15 metres. Even worse than the voltage drop - everyone kept wanting to borrow it, and none of them could ever wind it up to my satisfaction on its return. I honestly cant see why so many people think its ok to wind up a cord around their elbow; it totally ruins the cord unless they know how to put the quarter twist in for every coil, and if they Do understand that they they would not be winding it round their elbows in the first place. Don't even think about mentioning the Dutch Twist or you will really get me started.)

I am guessing a power board attached to the socket end with the dusty and saw plugged in to it?

Staggering the startup of the two machines is the way to go. You said the combined power was 3.5hp, what was the split between the two machines?

Doug

You're right, it was a 15 amp lead, but not a heavy one. It had 15 amp plugs on each end plugged directly into the powerbox. The site box was one of those orange clipsal ones with the rcd in the side, also 15amp. From there I had leads to the machines, a 2hp dc which I used to start first and 1.5hp saw. I should mention that I also had a 4' flouro plugged in and a radio . I used to run a 180 amp stick welder off the same lead, but not through the site box.

I didn't see a big difference in the welder settings from being plugged into the lead as compared to plugged directly into the shed.

Originally Posted by Arron

Hi. I've got a couple of woodworking machines with 2 hp motors. One has 9amp on the compliance plate, the other has no amperage rating. My question is can I safely run these machines off ordinary extension cords - maximum length contemplated being 2 or 3 metres. By 'ordinary' I mean the common domestic things you buy at KMart etc, which I guess are 10 amp. I've been told I need to use a 'heavy duty' lead, but it doesnt sound right to me and the only ones I've sourced have been annoyingly long and expensive.

cheers
Arron

A cheap extension cord may be 1mm2 or even 0.75mm2 (the cross-section of the conductors within it). I have a table somewhere showing voltage drop (in effect, volts turned to heat) of various conductor sizes vs. length at nominal 240v; but it's not worth pulling out.

Bottom line is that 2HP is only about 1500W. 10A at 240V is 2400W (I = W/V; Ohm's law).

Any standard 10A extension cord at 3M will have no problem with 1500W.

I'm not sure of the conductor size in a "heavy duty" extension lead; it can't be more than 1.5mm2 by eye - but I'd be happy to pull 10A/2400W through my ones at their maximum 25-odd meter length.

Don't pull lots of power through a coiled extension lead. It'll melt. (But 1500W isn't lots of power).

Amps is amps and start current is another thing.

Most importantly as stated above, if you decide a longer lead is the go, roll it out regardless of length. Inductance can make smoke if you pull a fair bit of current through a coiled up cable.

Always rollout your cable.

3RU