Direct wiring DRO to lathe?

Circuit breakers are more to protect against shots and with some breakers arcs, not necessarily to protect equipment. Breakers also come in a number of different trip curves (like B, C and D) which have different amperage vs. time trip curves, so when using electrical equipment with high start up current you often use D curve breaker but for a DRO you might use a B or C curve. Wire can sustain much higher current for short periods, so if you look at a 15A wall socket, often the cords you plug into it would be 16 or 18AWG wire. The electrical device itself will have a fuse/breaker that limits the power at/in the device. So in this case you are protecting the power wire from a dead short. As wire ages or becomes damaged, this is can happen. If you use a dual pole 3A supplemental breaker, then you could run power to the DRO or light with 18-20AWG wire cord. Also since it is 240VAC, you are pulling half the amps that you would at 120VAC. If you where to use a 15A breaker going to sockets, you would want to use 14-16 AWG wire.There are a number of factors that determine a wire/cord ampacity or current load over time.

Conductor amps.jpg

It is common practice in 240VAC (single and three phase) machines with a transformer to have a 110 or 120VAC tap off the transformer to power ancillary low current equipment. These are often fused or have a breaker sized to the load that it can deliver. The wiring after the fuse/breaker only needs to be sized based on the fuse/breaker size. In many machines there will be separate breakers to provide power to ancillary electrical devices. Below is a system build I did for a PM1236, power to the machine is a 30A dual breaker on the control board, a branch circuit goes to a dual 6A breaker which is used to power the coolant and other 240VAC low power electrical equipment. Wire to both breakers is 12AWG, wire from the 30A is 12AWG, wire from the 6A is 16AWG. A standard circuit breaker does not protect a motor from overload, there is often specific motor breakers, or thermal overload contator relay that is used. So if the motor locked up the thermal relay would trip before the motor insulation burned up and shorted.

PM1236 VFD Control System.jpg

So the 3A dual breaker you have chosen will work fine. I clip the plug off of the stock cord and connect the wires (white and black or brown and blue, green is ground) to the 3A breaker. Most DRO's have a separate grounding stud on the housing, which is suppose to be grounded to the machine. I normally do not use this is the DRO is hard mounted to the machine via metal to metal bond. Thecord ground may be an electrical ground relative to the electrical board and electronics, but a chassis ground may be separate and dissipate surface charge.
 
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Hi Guys
Just a 2 cents worth. That breaker on ebay for sale....................hmm i had one burn up in on position which then proceeded to burn wiring down the line, Get a good brand name do not skimp on safety.
 
If your power to the lathe is 220/240 VAC and the DRO can operate at that voltage, then go ahead and connect it to the lathe power in. Check the DRO and if it has a switch for selecting the voltage, be sure it is switched to the voltage you are connecting it to. The DRO in the picture has a power connector that is in general use and can be easily obtained if one did not come with the unit. Since you intend to wire it into the lathe power, turn off power, and open the wiring area and look to see if there is adequate room to connect the ground and two line and neutral connected wires. I would expect you have to cut the plug off the cord as you cannot use a 110VAC plug when connecting to 220/240VAC. The intent here is to wire it directly to lathe. So provided you have the room to wire, cut the cord plug off and take the cord with you to the store and find a cord grip fitting that fits the cord diameter. Use a connector for a 1/2" knockout sized hole.

A cord grip fitting is superior to a romex, bx style clamp for holding the cord. The screw connector clamps tend to crush the cable and eat through the insulation over time. A cord grip connector will hold the insulation without damage and keep dirt and crap out of the electrical box.

With the cord mechanically attached to the lathe power box, wire it. Connect the ground wire first. If you don't understand the colors of the wires look it up on the internet. It has stranded wire, use a ring terminal properly crimped (never soldered) if mounted on a grounding stud. If there is a grounding lug available you may use it. Don't share grounding lugs on barrier strips with multiple ground wires, let it have its own place.

For 220/240 VAC in the USA, you don't need a breaker or fuse on the DRO line. The DRO is already protected from cord connected power as shown by the UL or CE label. For safety, identify the two power leads with black tape so no one will expect a grounded neutral is in use on your lathe.

The wire used must be rated at 300V and selected for the environment it is used in. So if you plan to wash down the lathe wiring area with chemicals the wire insulation must be selected for it. If the wires will be subjected to extreme heat, insulation must be selected for temperature.
 
A few points to above, for 240V operation a neutral is usually not used in this type of equipment. Some DRO displays allow universal input voltage, so something like 100-240VAC is allowed (85-250V on the above DRO), since they usually have a detachable IEC socket at the DRO, one can change the cord for the appropriate voltage/current. The cord is only rated for a specified maximum current, and this is based on the wire gauge and plug that goes into the wall, the socket current rating is based on the breaker for that circuit. If you have a 30A breaker for your lathe, which is what I use, then the high voltage wiring in the machine has to be rated as such, unless you use a supplemental breaker/fuse for a branch circuit. This is suppose to protect the wiring, but as TO mentioned that buying a cheap breaker may not be the wisest choice. I use UL listed breaker's that are specific to the application, but I think the general message here is that safety is the first priority. On many machines that run 240VAC 3 phase, and some 240 single phase they use a separate transformer with a fused output to provide 110/120V single phase for subsystems, and this can be used for the DRO (which draws minimal current).
 
Haven't looked at the code recently as I have retired from the trade. In the old days on motor controllers voltage for the control circuit was limited to 120VAC thus the reason controls had the step down transformer. Circuit breakers are sized to the load and must be rated to be capable of breaking the maximum instantaneous current supplied. In most residential USA that was 10000 Amps. Wire sizes in the USA when the National Electrical Code (NEC) is accepted by the Authority having jurisdiction are selected from tables in the code book. So for example a 12 gauge copper conductor is capable of 30 amps but is derated based on temperature, type of insulation, how the conductors are cooled as in open air, conduit fill, and length. Exceptions to the rule used to be load sharing and the tap rule. Keep in mind that the NEC is the minimum safety requirement. Doing above and beyond is allowed.
 
Honestly I don't see the benefit of doing this other than a tad less clutter. If you have a 120v outlet reasonably close to the lathe I would use it and skip the integration with the lathe itself. Just my .02
 
The benefit for me is that I don't have to bother switching the power on/off for the DRO separately. Just a convenience aside from less clutter. When I turn the power on/off for my mill or lathe, the DRO does as well just like the lights mounted on both machines. I don't have very many 110v outlets in my garage either. My power feed on the mill is plugged into to 110v, I forget to turn it off all the time.

I agree with the points made in this thread. I asked my electrician about it. He told me not to worry about it. I understand just the power cord may not be protected but he said the chances of the power cord getting shorted is pretty rare how I have mine set up. My power cords are cut short to length & are tucked away. Perhaps I may add some inline fuses to them someday anyway.
 
Agree with Will, in fact on my my lathe I have the DRO as a direct power connection in the control box, but it is only 120V. When I get into the 240VAC range I get a bit more skittish, do not ask me how, but the wire can go up in flash quickly and violently. On my mill, I am bringing in 240VAC power with 10G wire, 32A fuses. I bring in 4 wire power and have two duplex respectable each going to 15A supplemental breakers. So different approaches, as long as the wire is in good shape, and terminated well, I do not think it is a big deal. I just like to erh on the conservative.
Mark
 
Watch your fuse holders for bad connections and fires, they are notorious for failures if the fuse holders are weak. Everyone should get the non contact temperature meter now that they can be bought for as little as $20 at HFT. Stand back about 2 feet and scan your electrical wiring, breakers, VFD, Motors, etcetera and note the temperatures. Bad connections will show spikes from ambient temperatures and other equipment will have nameplate operating temperatures. It is a great way to protect your investments.
 
One common practice: After you do your wiring / upgrade and some time has gone by, say a few weeks. Go check your terminal blocks and snug them up again. This is more important on higher power level joints, but still a good practice. Pop the cover off of your breaker panel ( POWER OFF and MEASURED) and check the lugs, you will see what I'm talking about.

Don't oversize wire sizes. This is common error that is done by electrical types in the business as well. Had a few arguments over this topic before. The bigger the wire is all good when everything is working, but when you have a failure....

Let's say your breakers, fuses etc don't work. what is next to go? If you got a 20 gauge wire compared to a 14 that has say 5 A running thru it. That larger wire will be able to run longer with more power before it burns up. I would rather have it burn fast breaking the circuit.

Example: I had a 14 AWG fuse link ( just a wire like 5 inches long. ) in my design. Wire was used to tap power off of a 300A , 3 phase busbar leg to a small breaker. Anyhow some E tech was in there adding an upgrade for something else and ended up running the wire from phase to phase. Anyhow on power up BOOM. The tech blamed my design and upgraded it to a 8 AWG, going around me to facility electrician post discussion. All his fix would have been was a louder bang and surge. I let it go and waited a while then had a different tech go back in and change it to 18 AWG. Design had a flaw I learned from this this event. I was 25 year old rookie engineer then and tech had some 30 year experience. He wasn't going to listen some punk kid. Years later I finally got him to see why. He was going to go change the wire, and I told him I had it done years ago without him knowing. LOL

DON'T GO CRAZY ON TINY WIRES EITHER. Practical aspects of them getting kicked around needs to be considered. I try to stay at 20 AWG or larger when possible.
 
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