Milling Machine Wiring Issues

Thanks Lyn,

Definitly want to ground it. I guess the real question is should I ground it to the machine using the white wire?

Ground it in the motor mounting box ?

etc etc

how many ways to skin this cat (I in no way condone I’ll treatnent of cats).

Thanks

jj

See #2⇓.

and color it green where visible, white could be construed as neutral, a current carrying conductor, for instance if there were some 120V loads for control equipment.

Then, as a rule-of-thumb, make the grounding/earthing conductor of no smaller diameter than the phase conductor(s). Its purpose is to operate the circuit protective device(s) in the event of a fault and it cannot do this if it melts before the upstream fuse blows or the upstream circuit breaker operates. To do otherwise invokes a risk of some unpopularity with the facility's fire insurance cover provider and a risk of a fatal electric shock.

Cable sizing is an art that is practised by qualified Electricians.

Correct again. I should have mentioned that detail.

Yes the green wire should be grounded on the machine and the motor and the box receptacle on the wall and the conduit and continuing to a ground rod outside...anything that is metal and can be touched should be grounded...green or bare wire, stranded or solid...

<...anything that is metal and can be touched should be grounded...>

Unless it is marked by the manufacturer with the square-within-a-square "double insulated" logo, which means it is not a requirement.

I assume that "east coast" refers to the United states. I also assume that this mill is to be installed at your home, since most industrial buildings have 3-phase wiring.

A common color code for 120/240 Volt "split phase" residential and small business wiring in the US is white for neutral and black for either phase of 120 V. 240 V wiring commonly has black for one phase and red for the other phase, again with white for neutral. That white neutral wire is likely connected to ground inside your breaker box, but for safety, a separate green or bare wire is usually used to insure grounding of the housing of any device having a metal exterior or other exposed metal parts.

I can't quite tell from the photos whether that plug has three or four terminals. If it has only three terminals (as I suspect), then you have no choice: the long terminal with the extra bend (and probably with a green coated screw inside) is neutral/ground, and it should be connected to the green screw inside the motor connection box with either a white or green wire.

Finally, I'm assuming that you have an existing, correctly wired, 240V outlet for it to plug in to. If not, then your best bet would be to rewire the motor for 120V operation, as specified on the motor nameplate. The motor should still be grounded. Note that the motor is rated at 21.4 Amps at 120V, so it needs to be connected to an outlet supplied by a 20 AMP breaker. Although 21.4 Amps should trip a 20 Amp breaker if left at that current for very long, the actual current used by the motor depends on the load, and with home use, you will likely never get even close to that current limit.

Note that this is NOT legal advice! It is advice from one who has done what you want to do.

It appears that you have a, " Twist lock plug ".

The blade that has the 90* angle is the ground.

Inside the plug will be 3 wire attachment screws, the ground goes to the aforementioned blade.

The white wire goes to the silver screw.

The black wire goes to the black or dark colored screw.

I would recommend using a new 12 ga. Jacketed 3 conductor cable.

And a new twist lock plug.

You notice that the motor name plate says 115/220.

This means it can be operated on either voltage.

Yes, but check out the amp draw for the 110V - at least a 25 or 30 amp circuit required. Probably why it was hooked up in 220V.

I would add to this that it is fine to tie the neutral(white) and Ground(green) to both the neutral lead on the motor/switchgear AND the body of the machine so long as this machine is the only one on this feed and breaker. In the appliance biz it is simply called a neutral tie. Some machines come with a strap between the neutral and ground on the machine already, whether they have a 3 wire 220 or 4 wire 220 plug.

If the milling machine is operating at 110 Vac, you definitely cannot bond neutral to ground. If the motor is wired to operate at 220 Vac then there is no need for a neutral conductor.

The variance allowing the neutral conductor to act as an equipment grounding conductor (or vise versa) is for specific listed appliances that draw most current as 220 Vac loads, but have an insignificant 110 Vac load such as a clock, control circuit or igniter. Such appliances do not have both neutral and ground run to them.

That would be correct IF he had a ground (bare or green), a white, and a black wire. In his photo, I see two blacks and a white.

To JJSeeker: The twist-lock plug shown in your photo appears to be of the kind normally used for 220V, but I can't read the text on the plug, and there does exist a twist-lock for 120V. I assume that you know from the previous owner that it is wired for 240V, but before you connect power, check to be sure that there is a single group of three connections joined together in the motor's connection box, as shown on the motor nameplate:

Also note that NONE of the motor wires connect to ground or neutral; only the motor case connects to ground/neutral.

Also, do measure the voltage at the outlet to be sure it is correct before inserting the plug. Don't worry about the motor nameplate indicating 230V, while you have ≈240V at the socket. It is actually better for the motor to have a voltage slightly higher than the specified voltage, than to have a voltage slightly lower.

I agree. He has a 4 wire twistlock with only 3 wires. He could add a ground and connect that to the machine body/case. He could also tie the neutral to the body as well. If the machine is a pump, then the tie would be the wrong way to do it and he would need GFCI breakers and a separate neutral and ground. However, he is running a milling machine with a single phase motor so the neutral/ground tie would give him a good clean connection and a robust current capacity on his wiring that would benefit a breaker trip should something go wrong with the motor.

After enhancing and enlarging his photo, I agree that it appears to be a 4 wire plug:

Now clearly the ground connection must go to the long terminal with the 90° bend. How the other three terminals should be connected depends on how the socket is wired. If the wiring of the plug does not agree with that of the socket, there will almost certainly a blown circuit breaker, and/or major fireworks.

Looking at this diagram I think T1 is one end of T2, T3 is one end of T4, and T8 is one end of T5.

T1-T2,T3-T4,T8-T5 are the three coils. So, in the lo voltage connection, all three coils are energized in phase across the two hot leads. That would be a higher current hookup in keeping with the rated wattage. In the Hi voltage connection the circuit is asymmetric. T1 is on Line 1, and T4 & 5 are on Line 2, with all three of their other ends, T2, and T3 and T8 tied and floating. (If we had one more Line we could have a 3 phase motor. ) This tie doubles the length of coil and so doubles the impedance and halves the current, sort of. Line 1 is carrying full circuit current, and Line 2 is split between T3-T4 and T8-T5. Those two coils would then be running less power at the applied voltage. Line 2 will be the same current as Line 1, but the motor will run differently than in the lo voltage hookup. Torque may not be much different.

T1-T2 is in series with T3-T4 & T8-T5 which are in parallel.

The existing plug and wiring is hopelessly messed up and must not be used as any kind of guide.

As I posted elsewhere, it is full of errors.

Furthermore, the plug may be either a 240 Vac without neutral or a 120 Vac with neutral and ground. The motor does not require both.

The only reason for supplying four conductors is if there are additional 120 Vac devices such as pumps or lamps on the milling machine, and motor is wired as a 220 Vac load.

Thanks everyone for the comments.

It is indeed a four prong plug, with only 3 wires attached. I will be wiring the outlet (4 prong)to match the plug. The white wire will be attached to the ground pin on the plug pictured and attached to the body of the machine.

The circuit will be wired 240v single phase and I’ll use a 30amp double pole breaker. With all wiring sized accordingly.

Ill use 3 wire conductor to run from the outlet to the panel with ground and a conductor from each pole.

thanks again to those offering helpful guidance

jj

Sounds good! Thanks for getting back to us.

If you are going to use a three conductor cable, then tie the neutral and ground pins together. Don't leave a floating leg.

Maybe I was wrong. I saw two black wires coming out of the motor junction box, I was thinking that L1 & L2 was attached to each black respectively. The white with the curl was attached to a screw as a ground, where white should of been common, and a separate green should be ground. Later after thinking about it, I thought the original installation was where neutral and ground are common from the breaker box, with metal emt carrying ground, rather than a separate ground to earth. I thought that if the winding shorted, and you touched it, you could get shocked since at that time, shoe soles were leather and current could follow to a concrete floor.

"I saw two black wires coming out of the motor junction box, I was thinking that L1 & L2 was attached to each black respectively."

That's what I saw, and think, too, at the junction box and pigtail.

But, seeing that short pigtail, my bet is that at the previous location (not necessarily the original), it was powered by a flexible cable (like or actually an extension cord), probably one with three conductors.

It could have been a 3 wire armored cable which provided the ground by the amour and used the 3rd (white or green) wire for neutral. That would have been ok.

Tie Neutral and Ground in your plug by wiring the same wire across the plug to both the Neutral and the Ground connections. That way you guarantee a good ground/neutral to the two hot leads. The neutral is floating in the motor coil circuit, but is attached to the body of the machine. Make sure both the ground and the neutral are the same terminals and your rig will be safe and able to carry any fault condition.

The reason for this is the plug. It is four wires and the circuit is only three. You need to account for the fourth leg in the plug because you don't know which of the two unconnected terminals are going to be connected through the four wire plug. By tieing the neutral and the ground in the plug, you are certain to have a tie back to neutral/ground and thus have a current source/sink should something go wrong in the motor. If for instance the outlet is only wired with three conductors and only the neutral is connected from the source, and your plug is wired only to the ground, then you have a floating ground condition and a shock hazzard. If however you have wired the plug tied neutral to ground, then it makes no difference to the circuit which terminal in the source is connected to which - neutral or ground...or both. The hookup is safe and has a proper current path back to the breaker box and the breakers.

"You need to account for the fourth leg in the plug because you don't know which of the two unconnected terminals are going to be connected through the four wire plug. "

He'd better know that before connecting. Over the years, I have seen several outlets that were not wired correctly. It is always wise to check before wiring or plugging in the device. Anyone who does not at least have a voltmeter, shouldn't be doing any wiring!

In case it wasn't known, this graphic from Wikimedia may be useful:

The correct plug should be the L14-20 or L14-30. In his photo, I think I see the tab on the ground lug extending outwards, indicating an L14-20.

Connecting the grounds only at the service panel also avoids ground loops, although that is not likely to be important for a manually operated milling machine.

Now, does that mean that the body of the machine should be grounded only by the neutral wire? Many of our machines are bolted to the concrete floor, and have neutral connected to machine ground. Those bolts constitute a low-grade ground, and if one of the bolts happens to be in contact with a rebar, it could be a fairly good ground.

Please clarify!

Dkwarner,

The OP stated it is a 240v machine with an open ground, as he received it. Therefore it has no neutral. The pigtail to the machine can be a 3-wire pigtail with two hot wires and a ground, but no neutral. The ground terminal on the cord end would plug into a receptacle in which the mating ground terminal is connected to the source's ground and not its neutral. This is a 2-pole, 3-wire, 250V, 20A, grounding receptacle configuration, which is NEMA line 6. Therefore, the proper receptacle (since it is a locking configuration) is NEMA L6-20.

To get a valid ground through anchor bolts is extremely unlikely. A rebar ground requires 20-feet of rebar in concrete in direct contact with the ground. I agree with you that a bolt (typically installed in a hole drilled into the concrete some time after concrete was poured) is not a reliable ground.

--JMM

"the proper receptacle (since it is a locking configuration) is NEMA L6-20."

Yes, but I was trying to help get a safe installation using the 4-prong plug currently installed, which appears to be an L14-20.

What would you do if his socket is supplied with only the two lines and neutral?

More than once (but not in recent years), I have felt a mild shock while touching a machine and the concrete floor beneath it. Obviously, those machines were not grounded. I assumed that the voltage I felt on the machines was due to some capacitive or inductive coupling between something inside the machine and its case. Would grounding such machines cause any problems?

(No, these weren't belt sanders. I'm quite well aware of belt sanders effectively being Van de Graff generators when not grounded.)

The voltage you felt was because the frame was tied to neutral or because the frame was not tied to equipment ground or because neutral is bonded to equipment ground beyond the main breaker panel. These practices are unacceptable.

You have implied that this practice is common in your experience. That is quite scary. The neutral conductor can have constant voltages of 3 to 4 Volts and frequent surges of 9 to 12 volts. During fault conditions it can reach 60 Vac.

I have a very long experience with electricity. The first I recall was being thrown across a small room by 120VAC while replacing the cover plates on switches and outlets in a room still covered everywhere with wet plaster. That was in 1947 or '48.

I said I had felt a mild shock while touching the frame of a machine. Actually now that you made me think a bit more about it, it may not have been a shock at all; What I remember feeling was a vibration as I moved my hand along the surface. It may well have been a painted surface, and my hand was acting as the second plate of a capacitor, which alternately attracted more and less. I did say I observed this more than once, which is true, but since I don't remember experiencing it in the last 30 or 40 years, I'd hardly call it common.

Finally, I have a very oily skin, and I believe that makes me feel shock much less than many, probably most other people. I haven't done it recently, but years ago I commonly compared my sensation of shock to that of others using a hand cranked telephone ringer generator, and invariably the others would let go before I considered it painful. Many times others would let go before I felt anything.

As I recall, my threshold of feeling in hands/fingers was around 60VAC, unless I had a cut or a sliver of metal (both of which were not unusual).

You are right of course about the codes, but he has a four wire plug and receptacle, so the only way he can do it without leaving a floating terminal is to tie.

Ranges and Dryers are tied at the factory and the tie is only removed if a four wire cord is installed. If the device is connected to a three wire outlet/cord then the tie remains in place. He clearly has to choose between the ground connection and the neutral on his setup, and so the best he can do is tie them or rewire his house.

Unless he has GFI or GFCI breakers on that line, in which case he would have to separate the two or the breaker won't break on a ground fault.

Codes are great, don't get me wrong, but they change with the times. Unfortunately the house wiring does not change with the code and we have to decide what we can do or not do in order to implement a safe connection.

Codes are not grandfathered in if they are inherently unsafe. They are grandfathered in to accommodate older wiring systems to newer appliances and their changes in common build practice or visa-versa. Here we have a proper four wire outlet, and proper breakers for it, but we also have an old machine with only 3 wires. What does the code say about that?

The proper "grandfather" way is to tie the neutral and ground together to insure a proper connection back to the box for any fault current that his ground wire sends to the plug. All the normal operating current in this setup is in the two hot conductors. All we have to do is make sure a fault current path exists and is robust enough to cause a trip and not a fire.

Double ovens are very often installed into homes with a three wire 220 feed yet they have a four wire lead into the machine. The Neutral and the Ground are tied at the factory for this reason. Only if the home has newer "up-to-code" four wire wiring does the tie get clipped and separated. If there were an old oven with only three leads and a modern home with a four wire lead, what would you do? Tie, that's what.

The only problems I have seen with this has had nothing to do with fault protection or safety. Instead, it has to do with the "cleanliness" of the ground connection and the effect that a dirty ground can have on the solid state electronic controls. Some ovens just don't want to work right if their four-wire system is tied. I've also seen this problem in two-wire house wiring and newer three-wire range hoods. They turn on but won't turn off because the pull-down on the MOSFETs can't get low enough due to current noise on the neutral. The only fix is to run a bare wire ground to the outlet box and bind it to the body of the hood.

"You said: Unless he has GFI or GFCI breakers on that line, in which case he would have to separate the two or the breaker won't break on a ground fault. No. If the two are tied together downstream from the GFI or GFCI breaker it will never reset. It is designed to trip whenever the resistance between neutral and ground downstream is less than 25-ohms. This typically occurs when you are dealing with an improperly weather-protected receptacle outside and it gets wet."

That is what I said about GFI and GFCI. No Tie and no argument from me brother!

"You said: If there were an old oven with only three leads and a modern home with a four wire lead, what would you do? Tie, that's what. No. You would replace the cord on the oven with a 4-wire cord and remove the tie that exists between the neutral and ground within the oven's terminal box."

Oven "cords" are shielded and factory installed. You cant replace them with a new cord as they hardwire into the service box behind them in the cabinet. A three wire Oven gets tied with both the neutral and the ground if the service into the box is four wire. And the nuts are greased if the service is aluminum.

"There is nothing in the OP that suggests he already has a circuit installed for his recently purchased milling machine."

True, but there is a reasonable assumption of a service connection that matches his plug because he only wants to know the best way to connect his ground back into his plug.

I strongly disagree with connecting neutral to equipment grounding conductors in feeders, branch circuits or appliances.

The only place they should be bonded is at the main panel and/or transformer.

Does it have a starter?

I would want a dedicated circuit, preferably #10 wire with the correct receptacle, cord, and plug, for a motor of that size. Or a regular disconnect and a piece of sealtite, if it is going to stay there for a while.

Either way, since it is a machine that moves/vibrates, I would want to use stranded wire for the supply connections, including the equipment grounding conductor.

Start over and do it right. That pigtail with the TW wire is not safe. Wire it in direct or put a correct piece of cord on it. 10 ga. if on 120 volt, install a 30 amp recepticle for it to plug into. Black, White, Green (For ground) If on 230 volt, 12 ga. wire. Black, Red, Green. -- JHF

Whoa whoa whoa

Nice work guys!

This is a great dialogue and has been a tremendous resource for me to further my own knowledge. Obviously I have to asses my situation, take the info that has been presented here and do my own further research to feel comfortable doing this in my own home.

To update. I do have a voltmeter, I will be using the L14-20 plug as that is what’s on the machine and it came with a nice long extension cord (Should come in handy as I’m building a mobile base for it, but that’s a topic for a different thread).

I will be using a 20 amp breaker instead of the 30 I indicated earlier. The two black conductors are 12 ga so I’ll stick with that.

Thanks again everyone!

jj

That photo is scary to me. I saw so very many errors in workmanship and Code violations. The wrong color of the equipment grounding conductor insulation was just one issue.

I notice that the visible grounding conductor is solid, not stranded - no good for plugs. These must be fine stranded conductors in a cord, not a cable.

The bare copper was poised next to a hole in the frame as if it was fastened there, but this was a painted surface - no good.

The grounding conductor must stay with the load conductors, not veer off this way. Look for a dedicated grounding post inside the junction box. (Never rely on a fastener that is doing any other task like holding a lid closed.)

The junction box had a cable clamp instead of a cord clamp. Never use cable clamps on cords.

Separate Insulated Conductors ran into a plug - no good. This plug needs a complete cord (with sheath) under the plug clamp. Never apply a clamp to individual insulated conductors.

The plug needs a mating cable connector, using the correct gauge cord, running to a junction box with the correct cord clamp. These are typically mounted above the milling machine so you can safely walk completely around it without interference.