80 KVA CNC Questions

[wirebender]

If I have a CNC machine rated at 80KVA, do I have to add 25% to the branch circuit size, or is that already calculated in?

The machines are 3phase 208v, so I plan to run a 480v feed to a transformer at each machine.
I am using 112.5KVA transformers and I intend to supply them from a 175amp breaker.
Does this sound right?

 

[petersonra]

If I have a CNC machine rated at 80KVA, do I have to add 25% to the branch circuit size, or is that already calculated in?

The machines are 3phase 208v, so I plan to run a 480v feed to a transformer at each machine.
I am using 112.5KVA transformers and I intend to supply them from a 175amp breaker.
Does this sound right?

the feeder ampacity has to be a minimum of 125% of the largest motor plus 100% of everything else. It can of course be larger. usually there is a nameplate that tells you what you need to know.

112.5 KVA @ 480V is 135 Amps. 175 A on the primary side is 129%. You are allowed to go up to 250% as long as you size the primary conductors appropriately.

 

[gar]

180619-1024 EDT

My comment has nothing to do with code, but rather with reality.

We have a total of 8 CNC machines all with advertized spindles of about 20 HP, plus some smaller motors. A 25 Hp belt sander, a manual lathe, a couple manual mills, some other equipment, air conditioning, and a substantial light load. This is all on a 240 V 200 A open delta supply at the end of 300 ft of copper that I think is 00.

The service calculates to about 80 kVA. Never a problem even though the CNC spindles alone might translate to 8*20 = 160 kVA. I have never looked at peak load, but for us it would never even approach 80 kVA.

From a reality perspective I would use a single central transformer with a rating much less than total rated load. That is what power companies do. Why waste power in transformer losses.

.

 

[wirebender]

the feeder ampacity has to be a minimum of 125% of the largest motor plus 100% of everything else. It can of course be larger. usually there is a nameplate that tells you what you need to know.

112.5 KVA @ 480V is 135 Amps. 175 A on the primary side is 129%. You are allowed to go up to 250% as long as you size the primary conductors appropriately.

180619-1024 EDT

My comment has nothing to do with code, but rather with reality.

We have a total of 8 CNC machines all with advertized spindles of about 20 HP, plus some smaller motors. A 25 Hp belt sander, a manual lathe, a couple manual mills, some other equipment, air conditioning, and a substantial light load. This is all on a 240 V 200 A open delta supply at the end of 300 ft of copper that I think is 00.

The service calculates to about 80 kVA. Never a problem even though the CNC spindles alone might translate to 8*20 = 160 kVA. I have never looked at peak load, but for us it would never even approach 80 kVA.

From a reality perspective I would use a single central transformer with a rating much less than total rated load. That is what power companies do. Why waste power in transformer losses.

.

The only info I can get about the machines is 80KVA. I have numerous request in. So, I assume that if the nameplate says 80KVA , I have to size per that?

I am trying to keep my feeders as small as possible, thus the 175amp OCPD and I know that 112.5kva tx is way overkill, so I don't think that will be a problem. Am i looking at that wrong?

I know that a 75kva tx would be plenty, if not a 45kva, but if something were to go wrong with a machine, they could come back at me, right? I'm not overly concerned about the code aspect, just the liability if the machine gets damaged.

Gar, the reason I was going with individual transformers is they are going to install 2 of these at this time but plan to add 2 more at a later date. This way I can just have 2 spaces in my 480 panel for them to use in the future, instead of having to install a 1000amp 208 panel to handle these machines.

Thanks for the help.

 

[wirebender]

What I have decided to do is feed an 800 amp panel with a 225KVA tx and use this panel to feed the two 80KVA CNC machines and all the smaller machines, lift and welders.
Although this calculates out to about 660 amps, I'm pretty sure that if they ever do add 2 more 80KVA machines, this panel will handle the load.
If not, I am leaving 2 large frame spaces in my 480V distribution panel and they can feed whatever they want. hmy:

This is my riser. Anyone see any problems?

 

[wirebender]

Well, I guess I have forgotten how to post an attachment