Electrical question: Running a motor with extension cord

K

K. Wilson

Joined
Oct 18, 2012
Messages
23
Bit of a technical question for any electrical savvy members on the board,
I'm looking for some assistance getting a properly rated extension cord to run a 3-speed 1- 1/2 HP Leeson motor on a KMG grinder. I'm running it off a standard outlet, not 220v.
Here are the specs on the motor..

http://www.leeson.com/leeson/search...ProductDetails&motorNo=110089.00&productType=

My shop is detached from my house and I need to run the grinder on a 100-ft extension cord. I know using an underrated cord can wear out the motor and be a safety issue. Can anyone tell me what I would need for this?
I was looking at this 14/3 13 amp 100-ft extension,

http://www.amazon.com/US-Wire-63100...qid=1462329142&sr=8-5&keywords=14/3+extension

But am unsure if that would be sufficient or not.
Thanks.
 
Last edited:
How would I run off 240volts? I'm a goof when it comes to electricity. My house is all off of a 110 main, I'm just trying to get my shop running temporarily as cheap as possible until I can run power to the shop which was quoted roughly $2500 :eek:
 
If you google "extension cord voltage drop calculator" there are a lot of tools available. Your motor draws almost 18 amps at full load using 120 volts and with a 100 foot run and a 10 gauge cable, one online calculator put the voltage drop at 4%. A rule of thumb is to keep voltage drop to less than 5%, so 10 gauge should work. Just don't run anything else off the cord at the same time as the grinder. 12 or 14 gauge won't work, or rather the motor will start, but the cord will heat up and the motor will be starved for power. It would end badly.
 
Lieblad said:
A 120 volt main in a house is extremely rare.
I dont think thats been done since 1920s & only very rural rudimentry electrification of the day.

If you have electric stove/oven, clothesdryer, waterheater of 40-ish gallon, its 99% sure you have 240volt service.
Alot of guys will plug into clothesdryer or stove receptacles to provide temporary 240volt power.
Click to expand...

How can I determine what service I have for sure? Someone told me if my main breakers in the house are 100amp then I don't have 240volt service, and my breakers are 100amp. If I do have 240v service then I could probably just rent a trencher and run the wiring myself from the shop and just have an electrician hook it up to the houses service.
I do have an electric stove, dryer, and waterheater - though if plugging into the dryer outlet I would need an additional 50ft of extension.

Mahoney,
Thank you for the reply. That answers my original question and if nothing else I may try the 10/3 cable.
 
You can't run a 120V motor off of 240 volts.
Just use the big extension cord. I'm sure you don't want to buy another motor.
 
If your main panel has two rows of breakers, then you have 240VAC available. The bigger breakers that take up two spaces are you 240VAC appliances (stove, dryer, water heater, well pump, etc...)

The bigger issue would be if you have "room" to add a shop full of circuits to a 100 amp main house panel. If it's just a small to medium sized shop, you'll likely be okay. Hopefully you have space in your existing panel to add one more double pole breaker to feed your sub.

Now you just need to figure out what your average house load is going to be on a heavy day (Air Conditioner, stove, pumps, etc... all running) and subtract that from the 100amps What you have left, give or take, is what you can comfortably run in your shop without upgrading your service to 200A. As for what you feed your sub with, that will just depend on how much you want to spend on wiring, and how much "head room" you'd like to have. It's not the worst idea to wire your sub feed for 100amp sub panel, just in case you do find you need to upgrade down the road, you won't have to pull a new run of cable to the shop. Granted, that will be a bit more money up front.

If you knew for a fact you'd never need more than, say, 60 amps service to your shop, you could save a little money on the copper, but again, if 10 years down the road you find you want to add a shop extension and start running a bunch of big machines all at once, you'll have to redo it all anyway.

Disclaimer: I'm not a licensed electrician, but I did stay at a Hoiday Inn once.
 
From Beaumonts website,
"Will run on 110 or 220V".
I thought these motors you could run either way?

Knife to a gunfight,
Taking the average load of a stove (50amps), dryer (30 amps), and air conditioner (10 amps) I'm already almost up to that 100amps you said to subtract from. And that isn't including our well pump, water heater or anything else. Am I doing that right? If so it seems like I'm already over that.
 
Last edited:
K. Wilson said:
From Beaumonts website,
"Will run on 110 or 220V".
I thought these motors you could run either way?
Click to expand...

They can, but you will have to re-wire it inside the pecker-head. There is usually a wiring diagram on the motor nameplate. Also make sure that you use a DPST (double pole single throw) switch for turning it on and off. You need to be able to isolate BOTH hot leads when operating 220V.
 
I've been running that same motor for 3 years now on my KMG. If you need to run an extension for it off 110V, use a 12/3. Trust me, you won't have any issues.
 
Adam Buttry said:
I've been running that same motor for 3 years now on my KMG. If you need to run an extension for it off 110V, use a 12/3. Trust me, you won't have any issues.
Click to expand...

Keep in mind that he's running off of a 100' extension cord too, with will pull more amps than running from a standard 25' cord.
 
knife to a gunfight said:
Keep in mind that he's running off of a 100' extension cord too, with will pull more amps than running from a standard 25' cord.
Click to expand...

I've ran that exact motor off of a 100' 12/3 many times with no issues at all. Just sayin'.
 
14 gauge stranded wire like what would be found in an extension cord has a resistance of about 2.5 ohms per thousand feet, 0.25 ohms per 100. But of course you have 100ft there and 100ft back for a total of 200 feet. That works out to a resistance of about 0.5 ohms. Drawing 20 amps of current through this cord will result in a voltage drop of 20amps times 0.5 ohms for a 10V drop. I don't thing your motor is going to like that. I used 20amps because that is the service factor current rating for that motor. In practice you probably won't draw that much current, but you'd likely get into the 15amp range easily which would still result in a 7.5Volt drop. Still too much.

Lets look at the case of using 12 gauge extension cord. This will have a total resistance of around 0.3ohms for 200 feet. This gives a voltage drop of 20amp times 0.3ohms for 6V, or the more practical scenario of 15amps times 0.3ohms for 4.5Volts. This would be the absolute smallest gauge that I'd be comfortable with. And even with that the power cord itself will dissipate just over 60watts of power...as much as a typical monofiliment light bulb! Of course the heat is not going to be as concentrated, so it will be ok.

Better would be a 10gauge extension cord, but this may be prohibitively expensive. This would have a total resistance of about 0.2ohms. At 20amps this yields a 4Volt drop and at the more practical current of 15amps just a 3V drop.
 
To summarize, don't use the 14gauge cord. Use an absolute minimum of a 12 gauge cord, and assuming you've got a decent voltage where you're at, dropping from 120VAC down to 115VAC should not be an issue. Also, as Lieblad stated, this should be run on a 20amp circuit which also utilizes 12 gauge wire to the outlet that you're plugging in to.
 
Take the guesswork out of it by measuring voltage at the outlet, and then at the end of the extension cord while under load. If the voltage drops below 110V at the end of the extension cord while under load then the cord is undersized. Use a larger gauge cord or upgrade the wiring or else damage will take place, or worse a fire.
 
I'm at work and don't have time to look at everything in this thread. That said, a breaker holds 80% of its rating for continuous load. 16 amps on a 20 amp breaker. They will hold more if new but will weaken overtime. 12 gauge wire is all you need on a 20 amp circuit. Voltage drop isn't really a factor until you get up around 200'. If the motor pulls over 16 amps I'd go up to a 30 amp circuit and use 10 gauge wire.

The chances of you not having a 240volt service is pretty much nonexistent.
 
There's a whole handful of isues.

Most of which have been addressed, but if you don't understand it, you're going to have trouble.


How about getting an electrician or two, or there in to give you a quote
call the nearest ten in the yellow pages


Quotes should be free.

Then put together what they have told you and see what common theme they have.



My position would be to 200 it from the dryer plug
but get help with that.
 
You must log in or register to reply here.
Back
Top