Design calculation for newly arrived CNC MACHINE

Kyell

Member
HELLO
I hope this finds my friends on the forum well after hurricane Dorian. We are doing fine here in Brevard.
I do not post much instead I usually troll and read the post and LEARN. but now I need direction and confirmation that I am heading in the correct direction.

I have a "new" or gently used CNC machine from MAZAK that has arrived in my place of employment. Normally they are really easy to "hook up" the information is included however in this case NO SUCH LUCK.. the only information I have is the name plate of the machine and the 112.5 KW transformer included with it. It is of course 480/277v 3ph primary IT is easy to calculate the wire size for this. OCPD will equal 135amps at continous load 169amps and OCPD calculation of 175amps or 2/0 THHN. None of this a problem for me. Now on to the machine nameplate is all I HAVE FOR NOW manufacture has been contacted but no response yet.

88KW on machine at 208v I am seriously looking at the machine and 250 kcmil wire is the largest size that could have been previously running it. Since I have no other information to go on...
What am I missing about this where do I go from here?

Uncle Bob
 

electrofelon

Senior Member
HELLO
I hope this finds my friends on the forum well after hurricane Dorian. We are doing fine here in Brevard.
I do not post much instead I usually troll and read the post and LEARN. but now I need direction and confirmation that I am heading in the correct direction.

I have a "new" or gently used CNC machine from MAZAK that has arrived in my place of employment. Normally they are really easy to "hook up" the information is included however in this case NO SUCH LUCK.. the only information I have is the name plate of the machine and the 112.5 KW transformer included with it. It is of course 480/277v 3ph primary IT is easy to calculate the wire size for this. OCPD will equal 135amps at continous load 169amps and OCPD calculation of 175amps or 2/0 THHN.
Although I agree with your values, I dont agree on how you got there. I dont see your load as continuous, and your transformer is larger than your load, so you are really sizing the transformer primary OCPD to be be able to handle the transformer energizing current. 125% of FLA is a common value (see table 450.3(B). You could go 250%, if you have the appropriate secondary protection, but that gets you into 400 amp frame/class equipment and the conductors to match. I would go with 125%.

Now on to the machine nameplate is all I HAVE FOR NOW manufacture has been contacted but no response yet.

88KW on machine at 208v I am seriously looking at the machine and 250 kcmil wire is the largest size that could have been previously running it. Since I have no other information to go on...
What am I missing about this where do I go from here?

Uncle Bob
88Kw at 208 would mean 250A OCPD and conductors. Usually these machine nameplates are extreme worst case: everything loaded to absolute max with lowest acceptable input voltage. Just shooting from the hip, I would say go with 200 A and it will be fine. I have ran a CNC mill and I cant imagine what it would take to use the full horsepower available on most of these things for other than brief periods. How many HP is the main motor?
 

kwired

Electron manager
I agree with electofelon, transformer primary is only going to pull 106 amps if load is 88kW (at 100% power factor anyway). I'd bet 150 amp time delay fuses will hold during energizing of the transformer, maybe even 125's will hold. Longer the supply conductors the better that chance gets as the resistance of those conductors will limit inrush current to some degree.
 

Kyell

Member
Although I agree with your values, I dont agree on how you got there. I dont see your load as continuous, and your transformer is larger than your load, so you are really sizing the transformer primary OCPD to be be able to handle the transformer energizing current. 125% of FLA is a common value (see table 450.3(B). You could go 250%, if you have the appropriate secondary protection, but that gets you into 400 amp frame/class equipment and the conductors to match. I would go with 125%.



88Kw at 208 would mean 250A OCPD and conductors. Usually these machine nameplates are extreme worst case: everything loaded to absolute max with lowest acceptable input voltage. Just shooting from the hip, I would say go with 200 A and it will be fine. I have ran a CNC mill and I cant imagine what it would take to use the full horsepower available on most of these things for other than brief periods. How many HP is the main motor?

Dear electrofelon,


I wanted to thank you for your reply it helps to talk to another electrician, I (we) are often asked to install machines with very little or severely limited information and in this case the name plate is all the information we had to work with. Machine in question is a used reconditioned Mazak multi lathe CNC.

I also wanted to tank kwired your calculations are what I finally settled with and it works.. also the length of the primary conductors was 115ft and 150amp time delay fuses

you guys are great I REALLY DO APRICIATE YOUR HELP ON MY BEHALF ..... IT GETS LONELY OUT HERE.

UNCLE BOB
 
Top