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Andrew W

Extension cords and belt grinders on 240–will I set my house on fire?

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Question for folks who know about electricity:

 

I want to run a 2hp VFD from my drier’s 240V outlet. But the outlet is 75’ from my shed.

 

How stupid would it be to plug a splitter into the socket and run a 75’ extension cord to my shed?

 

Some more info:

 

- The outlet is wired with the old style 3-prong 10-30 plugs (ungrounded)

 

- The VFD would draw just under 10A

 

- The drier’s circuit is 30A. I have no intention of running the grinder and the drier at the same time, I think both together might trip the breaker

 

- I’d use a splitter and extension cord rated to the full 30A of the 240 circuit

 

- Installing a 240 outlet in the shed isn’t an option (rental property)

 

Would this plan work, or am I going to set my house on fire? My backup plan is to run a 1.5hp motor off a 120 @ 20A circuit, but I’d rather use the 240 @ 30A if that’s safe.

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I just asked my home skillet who is a journeyman electrician. He said you can but, the appliance wont trip the breaker the way its designed to. 

 

 

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I figured I would mention it's not that hard to wire a breaker into your pannel. That's what we did with my power hammer. Then you can have the right breaker, and wire, and plug to run safely. I of course have to advise seeking professional advice. 

Best of luck! 

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Plus I think VFDs require a ground.  Not that that's hard to do, just thought I'd mention it.

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Posted (edited)

I have been using a 20' extension cord I made with 10 gage copper with the black rubber insulation for years for my belt grinder and 2 hp table saw. 
 

I have a 20 foot extension cord I use for my 50 amp welder. It is made of 6 gage copper wire. 
 

I would ask an electrician what wire size you would need for 75 feet, but if you don't mind spending the money, it should work fine. 
 

If it is a three prong plug, the third prong is a ground wire. 
 

the main consideration is how you plan to run it. If it is on the ground, there is a big safety consideration. If you plan to leave it in place you should run it through conduit. You should unplug it when not in use. 

Edited by Bill Kirkley

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Posted (edited)

Stop right there.

If you run that extension cord, size the wire for the breaker, not for the grinder. A lot of people will say, "The grinder only pulls 10 Amps so just use 12 or 14 ga wire".

That is a big NO.

Everything in the circuit should be sized for the breaker that feeds it. NO EXCEPTIONS ALLOWED.

There are various reasons for this, all of them are safety related.

Check the specs on your grinder motor, not just the VFD speed controller. 

ALWAYS provide an equipment ground. (or you may become the ground)

For a 30 Amp circuit running an additional 75 feet, you could use 10 ga wire and you should be fine. Upsize to 8 and you have a safety net against voltage drop.

Find some at the Home Depot that is already shielded in the heavy rubber sheath rated for exposed use. Do NOT skimp on this.

Forget the splitter. Just unplug the dryer and plug in the extension cord when you need it.

Edited by Joshua States
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13 hours ago, Alan Longmire said:

Plus I think VFDs require a ground.  Not that that's hard to do, just thought I'd mention it.

 

And my NEMA 10-30 outlet is nongrounded, is that right?

What would I need to do to ground the VFD running off that socket?

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I can't help with grounding the outlet (and with 240v a three-prong plug is hot-hot-neutral), but the machine ground Joshua mentioned is easy enough. Get some heavy (8 or 10 gauge) copper wire and attach one end to the grinder frame. Run the other end to a ground rod driven into, well, the ground. Ground rods are a 6-foot length of copper-plated 1/2 or 5/8 mild steel round bar.  This ensures that in the event of a short the current finds an easier way to ground than through you. 

Before I did a machine ground on my grinder it would zap me just from static buildup.

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I have a 240V extension cord from my dryer outlet to my shop which is about 50' long.  I don't recall the gauge of the wire (it came from a nuke plant which are rewired every few years just because) but it runs my rotary 3 phase converter made from a 5hp motor, a 60 gallon 6hp air compressor, a hypertherm plasma cutter and a Lincoln Idealarc 250 amp stick welder with no problem.  However there is only one outlet in the shop so only one machine is run at a time.  It has been this way for nearly 30 years with no problem.  From the rotary phase converter, I run a lathe, milling machine and a large belt grinder but again, only one machine at a time as there is only one 3 phase outlet.  I do have plans to upgrade the wiring and give the shop its own 100A panel which may actually happen this year.

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Update:

 

I flipped the breaker off and opened my 3-pronged dryer outlet. Turned out whoever built the house used a grounded cable for the circuit—they just didn’t connect the ground wire to anything. So I replaced the outlet with a 4-pronged 14-30 plug, checked everything with a multimeter, and now I have a grounded circuit to plug my grinder into (and my dryer’s safer too, an unexpected bonus).

 

I wired a 20A fused air conditioning disconnect box to a plug (for the dryer socket) and a length of SJOOW cord, so I can run only as much power as I need out the window to the grinder.

 

Just gotta hook up a vfd now.

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On 3/4/2020 at 5:29 AM, Alan Longmire said:

Before I did a machine ground on my grinder it would zap me just from static buildup.

That is exactly what fried my VFD controller.

 

burnt spot.jpg

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What "fried" it, Thomas, the static electricity or doing a machine ground?

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On 3/4/2020 at 2:29 PM, Alan Longmire said:

I can't help with grounding the outlet (and with 240v a three-prong plug is hot-hot-neutral), but the machine ground Joshua mentioned is easy enough. Get some heavy (8 or 10 gauge) copper wire and attach one end to the grinder frame. Run the other end to a ground rod driven into, well, the ground. Ground rods are a 6-foot length of copper-plated 1/2 or 5/8 mild steel round bar.  This ensures that in the event of a short the current finds an easier way to ground than through you. 

Before I did a machine ground on my grinder it would zap me just from static buildup.

Regular thing on my small belt grinders, but I've felt it on the 2x72's as well......this just became a priority, thanks!

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Posted (edited)

Now that I've replaced my outlet and it has a proper grounding wire, I can just splice a 10ga copper wire from my grinder frame onto the green grounding wire in the power cord--is that correct? No need for a separate grouding rod on the frame?

Edited by Andrew W

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You can, but a separate ground is far safer.  The ground in the cord grounds the motor, which is theoretically grounded to the frame anyway, but you're going to really want a separate small-gauge wire running from the frame to some other grounded object that is not potentially hooked to 240VAC.  This wire is just to dissipate static and act as a safety if the motor ground fails.  Mine is on the actual frame, grounded back to the conduit, which is grounded to a separate ground from the power cord.

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Success!

 

All up and running—and safely wired and properly grounded.

 

 

Thank you for your help! I’m really excited to put this through its paces now :)

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Congrats, man! May it serve you well. 

 

You might find a tool rest to be a handy attachment. I haven't quite gotten the hang of freehand grinding on a vertical platen just yet. It's funny, I gave away my 2x42, and find myself using files more often with the 2x72. 

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