Machine grounding wire question

When I bought my Bridgeport mill, the power supply wire to the machine mounted enclosure, with a 20 amp/3 phase CB (lockable) had the typical four wires, including the Green grounding wire and this Green wire was bonded to the machine casting. The wires to the E head motor (and the 2J head motor) however, were only 14/3 (White/Black/Green) with the Green connected to one of the phase windings in the E head motor and to a phase terminal in the enclosure. Also the switch on the 1/3 HP E head motor was a two pole, single throw switch, switching the other two phases to the other two motor windings. This switch looked original to the motor.
Question: Is this acceptable code or is it a violation. Does the grounding wire need to be brought all the way to the motor and bonded there? Can the green wire be used on anything other than for grounding in this manner? Joe

Machine grounding wire question Reply to Thread

Hi Joe,

On a 3- phase setup, there are normally 4 wires. Red, White, Black which each are hot wires that go to the motor. The green wire is the ground. The motor case on your milling machine should be in physical contact with the base of the machine. You can check this with your DVM (digital volt meter, or digital multi meter DMM) or with an analog multi-meter. Put it on the Ohm setting and check between the motor case and the machine casting. Be sure you are touching bare metal (not paint) with your probes. If you have a reading around 5-20 ohms or so, then you have a good electrical path between the motor base and the machine base.

The green wire serves 2 purposes. It acts as a safety wire to prevent electrical shock if one of the hot wires should come in contact with the metal case of the machine. The other purpose is to act as a neutral wire if one of the legs of the 3- phase power (red, white, or black) is also being used to supply power to a single-phase devise on your machine. An example of this would be the work light, DRO, or possibly the servo-power table feed (if so equipped).

A green colored wire is not normally used for power. But the fact that your machine does have a green wire used for this purpose probably just means that someone has used the wrong colors, not necessarily that your machine is wired incorrectly. Have you powered up and run the machine yet? If not, just do a continuity check of the wires (with no power applied) to make sure that the 3 phases are wired correctly.

David

From what I can tell, basically a violation.

Somebody cheaped out because they were unwilling to
install the correct conductors between the "machine mounted
enclosure" and the motors. Those should have four
conductors - three hots and a ground wire.

All three hot conductors should be switched as well, which
means a three pole switch should be used.

One nice way to do this is with half inch liquid-tite
conduit (it is flexible) so one can install whatever
conductors one wishes.

Are the wires to the motors flexible S or SJ cordage?
That is to say, flexible rubber covered cord?
If so that does not hold up terribly well in a machining
enviroment (especially the SJ cord) so changing it out
would be a good idea from that perspective no matter what.

But no matter what I would suggest a green wire personal
protection ground wire from the "machine mounted enclosure"
to each load motor on the machine.

Jim

"... The other purpose is to act as a neutral wire if one of the legs of the 3- phase power (red, white, or black) is also being used to supply power to a single-phase devise on your machine ..."

This purpose is strictly prohibited.

If you must have a neutral ... and most machinery is specifically designed to be used without a neutral ... than you must install a five-wire feeder: three phase conductors, the white current carrying neutral conductor (sometimes called the groundED conductor), and the green non-current-carrying equipment safety ground conductor (sometimes called the groundING conductor).

Joe reading back over your original post, you don't
specifically say this machine is powered from three
phase service. But my comments above are based on that
being true.

If the motors are single phase motors by all means say if
that is the case.

Jim

Whether it is single- or three-phase, the groundED and groundING conductors, and their purposes, remain the same.

If the machine has a neutral, a groundED conductor, then it must have a separate white conductor. This conductor carries any imbalance in the current carrying conductors.

If a machine has a safety ground, a groundING conductor, and now all machines must have such a conductor, then is must have a separate green conductor.

What remains is the current carrying conductors themselves, whether it is 120 single-phase, in which case there would be a single black conductor, 240 single-phase, in which case there would be a black conductor and a red conductor, or 240 three-phase, in which case there would be three conductors, usually black, red, and a third conductor color which may be any color except white, gray, green, green with a yellow stripe, or orange. Orange is especially reserved for indicating the "high leg" of a 120/240 three-phase system.

The entire point is the green conductor is mandatory, the white conductor is optional, but if it is present it must be installed in an approved manner (see the NEC for details), and any additional conductors are above-ground conductors, installed as required by the "utilization equipment".

OK, Thanks guys for the comments. The motors are 3 phase, and the E head cable was SO or SJ, I think. Rubber and fine stranded, anyway. The J head had a PVC type jacket. The cables was hard to read, dirty and had various colors of paint all over it.
I will replace the motor conductors with 4 wire, and bond the motors to the grounding wire. I will have to disassemble the E head motor again and move the wires around a little. Real tight in there already and if I can find a three pole-single throw switch, I'll replace the two pole one. There will be even less room with a three pole switch. I had to use a 240 to 120 volt transformer in my RPC for the control voltage because I did not have a grounded neutral in my 240 single phase building wiring run. Did find a nice 13' piece of SJOW - 10/4 at Lowes on the mis-cut rack at half price. Wired it from the CB enclosure to the RPC plug. Lowes guy said they don't stock it so someone must have special ordered a roll. Just wanted things right, in case the place burns down. Joe

One could *possibly* imagine a case where a numbskull wired in some -3, S cord, with
the colors all mixed up, that is the grounding conductor being not green, but rather
black or white by mistake. And the other two wired for 240 volt single phase, into a
two pole switch. Aside from the *ahem* color situation it woudl be a non-hazardous
situation.

But not what is being discussed, obviously.

Drum switches are a good way to do what you need to do, and I suggest again that
liquid-tite conduit is a great way to go if the 10-4 S cord winds up being troublesome.
I've always found that oversize (and I suspect number ten is, though I am not sure
of the current draw in the machine) conductors are a big pain in small wiring enclosures.
Seems like everything has to be wrassled into place.

Jim

Jim,
Yea, seems I always end up oversizing when I wire. I plan to get 14/4 for the motors. Tried to put the 10/4 to the 2J motor but the short piece from the motor to the drum switch has a sharp bent, and the drum switch is real tight too. 14/4 will do there I think. The E head draws 1.4 amps according to the motor plate, and the 2J head will draw 4.4 amps, I think. I picked up a couple of real nice 3 pole, 15 amp CBs at the junk yard for less than $5.00 (Cat. FDB3015L). They were mounted on a couple of large screw compressors, and if I can find a higher (deeper) enclosure I will change them out for the 20 amp(er). These things are $765.00 retail, each. $184.00 each refurbished, and look brand new. Joe

Ooh. Panama city! I spent some short time at the navy base there a while ago, at
the naval surface warfare center. Nice town, very pleasant folks.

I just never had very good luck using SJ cord around machine tools, especially from
drum switches to motors. Just never did last very long. The SO cord is nice from
the machine to a cord drop for power, but all the internal wiring I like to do in liquid
tite. Very rugged and easy to pull whatever conductors you want into it.

Jim