# Cable size for 10 hp lathe



## bigearl67 (Jan 29, 2014)

I have a 10 hp Mori-Seki CNC lathe I need to wire and would like to know what size cable I need to run. It will be about a 20 foot run from my RPC and I am hoping to be able to scrounge the cable from what I already have on hand, mostly 10 and 12 ga. Thanks, Earl


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## Ray C (Jan 29, 2014)

You're looking at 7500 Watts, divided by 220 Volts, is equal to 34 Amps.  Since you're running motors with potentially heavy start-up current, you need to double that at least -so, you're conservatively at 70 Amps.

If you look at this chart, it says 8 Ga will handle that load for chassis wiring purposes.  Since your distance is only 20 feet, you could probably go that route assuming this is a temporary setup.   If it were mine and if the setup was meant to be permanent, I would probably go with 6 Ga wire.

http://www.powerstream.com/Wire_Size.htm

Ray





bigearl67 said:


> I have a 10 hp Mori-Seki CNC lathe I need to wire and would like to know what size cable I need to run. It will be about a 20 foot run from my RPC and I am hoping to be able to scrounge the cable from what I already have on hand, mostly 10 and 12 ga. Thanks, Earl


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## AlanR (Jan 29, 2014)

Ray C said:


> You're looking at 7500 Watts, divided by 220 Volts, is equal to 34 Amps.  Since you're running motors with potentially heavy start-up current, you need to double that at least -so, you're conservatively at 70 Amps.



bigearl didn't directly say if the machine was three phase or not but he mentioned an RPC. If it is 3 phase you can divide your figures by three. I also think you're overdoing the start up current, it's short time enough that the wires won't overheat


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## Ray C (Jan 29, 2014)

AlanR said:


> bigearl didn't directly say if the machine was three phase or not but he mentioned an RPC. If it is 3 phase you can divide your figures by three. I also think you're overdoing the start up current, it's short time enough that the wires won't overheat



Duh, yeah... that sounds about right.  I'd be pretty comfortable with 8 Ga. then.


Ray


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## AlanR (Jan 29, 2014)

Ray C said:


> Duh, yeah... that sounds about right.  I'd be pretty comfortable with 8 Ga. then.
> 
> 
> Ray


Here's the spec http://www.engineeringtoolbox.com/three-phase-circuit-wiring-d_737.html

I think 8 gauge is plenty and I might even try the 10 since he's already got it and only needs 20ft. I'd lay it out on the floor and check it with a clamp on ammeter. Check the voltage drop along the wire with a voltmeter too.


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## Gary Max (Jan 29, 2014)

I ran 8 ga for my 7.5 hp compressor motor--- if my mind is still working--- I think it was $2.00 a foot.


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## AlanR (Jan 29, 2014)

Gary Max said:


> I ran 8 ga for my 7.5 hp compressor motor--- if my mind is still working--- I think it was $2.00 a foot.



It's not only the cost, it's also the difficulty of handling and pulling larger sizes than needed. Wrestling heavy gauge wire is not a lot of fun.


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## Walltoddj (Jan 29, 2014)

Even if his is 3 phase you have to feed with 220v 1 phase, my 5 hp draws 17 amps per leg running on 3 phase 220v it's on 10ga 4 wire copper. Have you looked to see at your local Lowe's or Home depot some time the have cuts at a discount I picked up some 6ga 4 wire real cheap. One thing I found I'll never do is skimp on the wire in the end it's better larger than small.

Todd


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## rdhem2 (Jan 30, 2014)

Ray C is usually pretty much up to date on such things.  In this case we are not concerned with wire heating. Although he failed to mention it I am sure voltage drop on start-up was in the back of his mind.  

Voltage drop is one of those critters that I believe the right term is exponential.  A small change up front is a increasingly big change on the backside.  In other words, a problem becomes a helluva pain in the ass in a real hurry.  #6 is not that big and still fits in a 3/4" pipe just like #8.

Don't go to these home improvement places.  Make a few calls and I'll bet a local electrical contractor will sell you some cheap.  Where do you think their beer money comes from?  Even a scrap yard may have some short pieces.  #6 is a pretty popular size.  Don't screw yourself to save a few pennies.


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## Ulma Doctor (Jan 30, 2014)

my 2 cents
the most critical wire will be from supply to the RPC, it will bear the amperage of running the system.
so long as that wiring is sufficient to the RPC, the lathe or any other ancillary equipment can safely be run on reduced sized wire in comparison to the single phase input to the RPC.
for example a short length of 10/4 would be sufficient to run a 30 amp circuit
8/4 is good up to 50 amps
6/4 is good to 65 amps


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## AlanR (Jan 30, 2014)

Ulma Doctor said:


> my 2 cents
> the most critical wire will be from supply to the RPC, it will bear the amperage of running the system.
> so long as that wiring is sufficient to the RPC, the lathe or any other ancillary equipment can safely be run on reduced sized wire in comparison to the single phase input to the RPC.



Agree, I was assuming the RPC was already wired to the supply.


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## Walltoddj (Jan 30, 2014)

I'd have to put an amp probe on it again but if I recall my 10 RPC was 15 amps and didn't vary when I use the mill and grinder together. What size RPC are you using?

Todd


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## Ray C (Jan 30, 2014)

rdhem2 said:


> Ray C is usually pretty much up to date on such things.  In this case we are not concerned with wire heating. Although he failed to mention it I am sure voltage drop on start-up was in the back of his mind.
> 
> Voltage drop is one of those critters that I believe the right term is exponential.  A small change up front is a increasingly big change on the backside.  In other words, a problem becomes a helluva pain in the ass in a real hurry.  #6 is not that big and still fits in a 3/4" pipe just like #8.
> 
> Don't go to these home improvement places.  Make a few calls and I'll bet a local electrical contractor will sell you some cheap.  Where do you think their beer money comes from?  Even a scrap yard may have some short pieces.  #6 is a pretty popular size.  Don't screw yourself to save a few pennies.



rdhem2,

-Thanks for the vote of confidence and yes, initial draw and the resulting voltage drop was foremost on my mind...  To be specific, two concerns are front-and-center:  1)  This is for a Mori-Seki CNC.  I do not know if they already have internal VFDs or some mechanism to intelligently manage current draw.  If they do not, and if they're gluttonous power hogs at all times then, my assumption was to at least double the theoretical draw at 7500 Watts (which does not even take power factor into consideration).   2)  The OP said he was powering it off an RPC.  In all my understandings of RPCs, they do not produce perfectly balanced 3 phase meaning that one or more legs is going to have a higher than expected current draw.  -So given this, I opted to err on the conservative side and recommend a slightly heavier gauge than what is theoretically required.-

And yes, I had a "senior moment" and forgot to divide the current by 3 because this is a Y-connected 3 phase system but, I still think that 8 Ga is the way to go...  Once a wire is undersized for the application, you won't know what crazy problems you'll have to troubleshoot until they start rearing their ugly heads...

The cost per foot and difficulty of managing 8 Ga vs 10 Ga is not burdensome.  When you start dealing with #2, 1 and 0 gauge... yes, that's a lot harder to work with.  8 vs 10...  not so much.


Ray


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## Tony Wells (Jan 30, 2014)

Is that the spindle motor? It is a continuously variable motor if it is as I recall on the SL and TL series. It's been a long time, but I have rebuilt a few of the motor drives on TL-5 and SL-3 and 4 Mori's.

Don't forget to fudge a little for the servo drives on the X AND Z axes.


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## Walltoddj (Jan 30, 2014)

Just a point of interest you say the initial draw on a 10 hp RHC if you use the start switch is 120 amps, if you dead start without the starter switch is 162 amps I know this because my Electrican and I did it just to see. I got this RPC cheap $300 because the owner moved and the new building had 3 ph power. I don't use it now but I'm wired on a 30 amp line and I never had a problem.

Todd


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## bigearl67 (Feb 2, 2014)

Thanks for all the replies, it sure does help. It is a 15 hp rpc that I have for my vmc but since Ipicked up the lathe I need to run them  both on the same rpc for now. I don’t want toinstall a “temporary” line, just  do itright the first time. Looks like I can run some 10 gauge SO to charge up  the batteries and see how the electricalrespond (first time I fired up this  machine). Then I Can put an amp meter on theline to see what the draw is and  go fromthere. Eight gauge isn’t a problem, just another expense I don’t really  want right now. Thanks again for all theinput. Earl 


- - - Updated - - -



Tony Wells said:


> Is that the spindle motor? It is a continuously variable motor if it is as I recall on the SL and TL series. It's been a long time, but I have rebuilt a few of the motor drives on TL-5 and SL-3 and 4 Mori's.
> 
> Don't forget to fudge a little for the servo drives on the X AND Z axes.



Yes, that's just the spindle motor. Thanks, Earl


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## Walltoddj (Feb 2, 2014)

bigearl67 said:


> Thanks for all the replies, it sure does help. It is a 15 hp rpc that I have for my vmc but since Ipicked up the lathe I need to run them  both on the same rpc for now. I don’t want toinstall a “temporary” line, just  do itright the first time. Looks like I can run some 10 gauge SO to charge up  the batteries and see how the electricalrespond (first time I fired up this  machine). Then I Can put an amp meter on theline to see what the draw is and  go fromthere. Eight gauge isn’t a problem, just another expense I don’t really  want right now. Thanks again for all theinput. Earl
> 
> 
> - - - Updated - - -
> ...



If you have a welder or dryer outlet close to your RPC I'd plug it in to see what the converter draws that will give you an idea if the 10 gauge is big enough, I run at 17 amps but I ran 10 gauge 4 wire though the whole shop with outlets at each machine so I will have no cords on the floor. As you said don't want to install temporary line it maybe better to wait till the funds or time are there to do it how you would really like to.

Todd


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## flutedchamber (Feb 5, 2014)

AlanR said:


> It's not only the cost, it's also the difficulty of handling and pulling larger sizes than needed. Wrestling heavy gauge wire is not a lot of fun.



When I ran 6 gauge wire to my compressor I first cut the conduit (plastic conduit) and installed the hangers and sweeps with no glue.  Once I was sure I had the lengths perfect I took the conduit down and ran the wires thru it, then glued and tightened the hangers with the wire inside.  A LOT easier than trying to run 4 #6 wires thru conduit that is already installed.

One word of caution though.  Leave an extra foot of wire sticking out of each end of the conduit.  Saves frustration later if the wire moves while you're installing the conduit.


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## Walltoddj (Feb 5, 2014)

flutedchamber said:


> When I ran 6 gauge wire to my compressor I first cut the conduit (plastic conduit) and installed the hangers and sweeps with no glue.  Once I was sure I had the lengths perfect I took the conduit down and ran the wires thru it, then glued and tightened the hangers with the wire inside.  A LOT easier than trying to run 4 #6 wires thru conduit that is already installed.
> 
> One word of caution though.  Leave an extra foot of wire sticking out of each end of the conduit.  Saves frustration later if the wire moves while you're installing the conduit.



Take a piece of rope tie a rag to it that will fit in the conduit and suck it though with a vacuum, now tie the rope to the wire and pull it though. Did 150' of 150amp 4 wire to feed my shop if it gets tight they have a compound you can use or a light soap and water mixture. As was said leave extra to tie in I have about a 2' loop to be sure I'll always be able to get it hooked up and it cheaper to do it when you pull it than to do it again because it was short.

Todd


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