CNC Lathe Build - Circuit Breaker Protection and Conductor size questions

[vtcnc]

Messy set of questions coming up. If you make it to the end, thanks for sticking with me on this, reading and providing some insight.

I'm gearing up to design and build a CNC lathe. The not so obvious reason for this is that I desire to have a fast, accurate gang tool style lathe that can handle C-axis style "plane switching". This means that the machine can mill using the spindle as an axis in combination with live tooling. If you are unclear on what I'm referring to, lookup Omniturn GT-75 lathes. Yes, it is ambitious and yes, I'm already in over my head. Jim Dawson has provided some great advice on planning and servo drive and motor selection.

I'm already in deep. I've purchased an XZ stage from a used Omniturn. Everything is NICE and solid, tight and in great condition. It was used to produce pool cues so very little abuse. I have 750W servos and DYN4 drives for the stage. I'm using the Centroid Acorn controller which supports C-axis control. I have Hardinge HC headstock with 5C spindle and the 7" dovetail bed. I've purchased all of my steel for the frame build. I'm committed.

OK, now my problem. Electrical worries. My shop has a 60A subpanel coming off the main 200A inside the house. For the sake of discussion, call it a 75ft. run through the basement, underground conduit and into the shop.

When I talked to the folks at DMM, they suggested the recommended sizes for circuit breakers is 20A for each of the XZ servo drives and 30A for the spindle servo drive. They also suggested that I could just combine it all into one breaker. That is a 70A breaker.

Time for me to panic?

Here are the rated currents for the DMM servo drives:



I have two of the H01 and will have one of the T01 for the 1.8kW spindle servo.

Here is the basic diagram with their recommendations on circuit protection and signal filters:



Here is the 1.8kW servo motor specs that gets paired up with the DYN4-T01:


And the specs for the 750W motors that get paired up with the DYN4-H01:



Hopefully you can see my confusion. I'm under the distinct impression that the motor protection is clearly for the servo drives. The drives must be doing some sort of inrush current management to the motors because the breakers are not sized to cover the max. rated current for the motors. It seems to me that the MCCB recommendations are overkill for the servo drives. I would think that a more appropriate size for the MCCB would be one 40A based on the rated current for those three drives. Mind you, this is a guess on my account. Not a calculated educated guess, but just a guess on loose experience. What say you?

Before you weigh in, now for my unpleasant surprise. As I was researching all of this, the question popped into my mind about what my current feeder conductor is. I stepped into the basement and located my run from the main panel making it's way over to the garage. It looks like I have a 60A run with 10AWG feeding the shop. While 10AWG can handle 60A based on what I researched, it also seems there is some question that it can't handle the 3% voltage drop tolerance for a run pushing 75-100ft. Do I already need to upgrade my service before I move forward?

Ideally, I'd like to run this machine off a 40A, 1Ph, 240V branch circuit coming out of the sub panel. I will likely have a coolant pump at some point and possibly other accessories like lighting but I'm not too worried about those. My biggest concern is safety of the main circuit. I don't want a fire and don't want to smoke a drive.

Any thoughts or questions that will help me resolve this? Am I worried about something I don't completely understand?

Thanks all.
Bryan

 

[ttabbal]

I'm not an expert, but from what I understand, line voltage is generally +/-10%. So a 3% drop over that line is not ideal, but still within spec. There's always some drop, based on current and resistance. I find it interesting that the recommendations are for larger breaker than the other parts in the circuit, but they might be considering inrush current and generally wanting the conductor and therefore the protection, to be higher than you expect to draw. The breakers are to protect the wire from overcurrent. The equipment is expected to provide protection for itself if it needs it.

My guess is that your intent to run the machine from a 40A circuit should be fine. Fire is an issue when the protection fails or allows more current than the wire can handle. So long as the wire is sized to handle the 40A the breaker will allow, I don't see any real risk there. The equipment attempting to draw more than the breaker will allow should not cause any damage. It will trip the breaker, but that's what it's for. So I don't think you can smoke a drive or other parts on the machine.

The only other thing I would suggest is to ensure you torque all the connections properly. Slightly loose connections will increase resistance and at 40A that could increase heat significantly. I used a torque wrench to tighten the panel connections when I did some work. I was a bit surprised at the amount of force called for by some of the connections.

 

[Reddinr]

I'm about to run off to work so I've only got a couple of minutes. I'm probably missing some things.

10 AWG won't likely burst into flames at 60 Amps but it will get very very warm. For the moment, I suggest that you put a 30A breaker on the house end of that wire for safety. For a 60A feed, at that distance I would use at least #6 but probably #4 to limit the voltage drop further. #6 has more than 2X the cross section of #10. If you ever want a good mid sized welder, you will be glad to have the extra beef.

It is unlikely that all of your servos will be fully loaded at once. You could have a main feed to the machine at some higher amperage and then have some supplementary breakers inside the machine's panel. Those breakers could be smaller to feed each servo. Is the DYN4 compatible with 240VAC? My guess is that 30A would even be sufficient for the main feed, but just an educated guess at the moment.

The max current of the motor can not be sustained except briefly and it is likely that no two motors will come close to that value at the same time. If you use circuit breakers, they are available with a time delay function that can ride through some of the peak currents. I assume there is some design margin in your motor choice and that you didn't design the machine to max out all motors. I think a better starting place for circuit breaker sizing is to use the rated current of the motors, not the max current.

 

[vtcnc]

Guys thanks for some immediate reassurance. I appreciate that and you have given me some practical things to consider.


Sent from my iPhone using Tapatalk

 

[vtcnc]

I'm not an expert, but from what I understand, line voltage is generally +/-10%. So a 3% drop over that line is not ideal, but still within spec. There's always some drop, based on current and resistance. I find it interesting that the recommendations are for larger breaker than the other parts in the circuit, but they might be considering inrush current and generally wanting the conductor and therefore the protection, to be higher than you expect to draw. The breakers are to protect the wire from overcurrent. The equipment is expected to provide protection for itself if it needs it.

My guess is that your intent to run the machine from a 40A circuit should be fine. Fire is an issue when the protection fails or allows more current than the wire can handle. So long as the wire is sized to handle the 40A the breaker will allow, I don't see any real risk there. The equipment attempting to draw more than the breaker will allow should not cause any damage. It will trip the breaker, but that's what it's for. So I don't think you can smoke a drive or other parts on the machine.

The only other thing I would suggest is to ensure you torque all the connections properly. Slightly loose connections will increase resistance and at 40A that could increase heat significantly. I used a torque wrench to tighten the panel connections when I did some work. I was a bit surprised at the amount of force called for by some of the connections.

Well, maybe if I take Randy's suggestion and upgrade the feeder conductor size, I can use the 10AWG I pull out of the conduit and use it on the branch circuit. This will be overkill for conductor size to power the lathe from the sub panel, but since the 10AWG is likely not adequate for the 60A run from the house to the shop (for what I am intending to use it for) I might as well use it for something worthwhile. This will also allow me to use the smaller breaker as well if I'm understanding this correctly.

I'm about to run off to work so I've only got a couple of minutes. I'm probably missing some things.

10 AWG won't likely burst into flames at 60 Amps but it will get very very warm. For the moment, I suggest that you put a 30A breaker on the house end of that wire for safety. For a 60A feed, at that distance I would use at least #6 but probably #4 to limit the voltage drop further. #6 has more than 2X the cross section of #10. If you ever want a good mid sized welder, you will be glad to have the extra beef.

It is unlikely that all of your servos will be fully loaded at once. You could have a main feed to the machine at some higher amperage and then have some supplementary breakers inside the machine's panel. Those breakers could be smaller to feed each servo. Is the DYN4 compatible with 240VAC? My guess is that 30A would even be sufficient for the main feed, but just an educated guess at the moment.

The max current of the motor can not be sustained except briefly and it is likely that no two motors will come close to that value at the same time. If you use circuit breakers, they are available with a time delay function that can ride through some of the peak currents. I assume there is some design margin in your motor choice and that you didn't design the machine to max out all motors. I think a better starting place for circuit breaker sizing is to use the rated current of the motors, not the max current.

I think I will have to upgrade the feeder conductor from the house. (sigh)

View attachment 357743

4 AWG will be more than adequate for the 60A sub panel. Kinda want to call the electrician and ask him what he was thinking...but this was 18 months ago now.

Yes, if you use the rated current for the motors, then we are looking at about 30A. This is where I was coming up with the 40A for as a practical guess at the size of the MCCB.

 

[JimDawson]

You have 2 two conditions here.

First the inrush current when firing up the drives, this can be crazy high, but only lasts for a few milliseconds. This is why the breakers are spec'd as high as they are, keeps them from tripping on startup. Also the wire size feeding your shop will limit this inrush current just due to the line resistance. I do power up the servo drives one second apart, under computer control, so as to not throw all of that inrush on the line at the same time.

The other condition is normal running. On average, the line will only see a fraction of the breaker rated current. The motors may have momentary spikes of current when accelerating, but this is very limited duration. The breaker ratings assume a continuous current draw at that rating.

Just to put this in perspective, My 10hp spindle CNC lathe loads the line at about 11 amps when idle, and the highest I have ever seen a continuous draw is about 20 amps. So that's a 10hp spindle motor, four 1.8KW servos, a 2hp hydraulic pump, and 2 coolant pumps, plus the 15hp rotary phase converter, all running off of a 50 amp breaker and #6 wire. The wiring doesn't even get warm.

As long as the the wire size is protected by the proper breaker rating, you'll be fine.

 

[vtcnc]

You have 2 two conditions here.

First the inrush current when firing up the drives, this can be crazy high, but only lasts for a few milliseconds. This is why the breakers are spec'd as high as they are, keeps them from tripping on startup. Also the wire size feeding your shop will limit this inrush current just due to the line resistance. I do power up the servo drives one second apart, under computer control, so as to not throw all of that inrush on the line at the same time.

The other condition is normal running. On average, the line will only see a fraction of the breaker rated current. The motors may have momentary spikes of current when accelerating, but this is very limited duration. The breaker ratings assume a continuous current draw at that rating.

Just to put this in perspective, My 10hp spindle CNC lathe loads the line at about 11 amps when idle, and the highest I have ever seen a continuous draw is about 20 amps. So that's a 10hp spindle motor, four 1.8KW servos, a 2hp hydraulic pump, and 2 coolant pumps, plus the 15hp rotary phase converter, all running off of a 50 amp breaker and #6 wire. The wiring doesn't even get warm.

As long as the the wire size is protected by the proper breaker rating, you'll be fine.

OK, so I size my wiring and breakers in the lathe panel according to their specifications, even if they exceed the subpanel feeder breaker of 60A. Adjust the timing of the startup of the machine servos to limit the inrush current.

Lastly, Jim, would you upgrade the 10AWG wiring from the main panel to the sub panel?

 

[vtcnc]

@JimDawson , do you think the line reactor that they recommend (as optional) is necessary? I think the answer is yes, if there is noise that needs to be removed, but I'm just curious what your experience is with these products and whether you have found them to be necessary and useful.

 

[JimDawson]

Lastly, Jim, would you upgrade the 10AWG wiring from the main panel to the sub panel?

NEC says #6 wire for a 60 amp circuit. The maximum on #10 is 40 amp with 90°C copper wire, that's way too hot for my taste.

https://www.wireandcabletips.com/what-is-ampacity/

 

[JimDawson]

@JimDawson , do you think the line reactor that they recommend (as optional) is necessary? I think the answer is yes, if there is noise that needs to be removed, but I'm just curious what your experience is with these products and whether you have found them to be necessary and useful.

I have not used nor needed line reactors. Normally only used on longer wire runs and higher power systems.