OCPD sizing for large laser cutter

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sokbok

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Howdy forum.
I have a 4000W (4kw) laser cutter. Its massive and used for cutting up to 1" thick steel.
No job on the laser will be continuous or over 3 hours.
The KVA of the laser is 69KVA @ 208V = 199.85 Amps.
So using the next size up fuse rule, I would use a 200A fuses in a 200A enclosure.

Where I am getting tripped up is on the 80% vs 100% rating of OCPD. Its confusing.
I so rarely size OCPD on non-continuous loads that I am usually covered because I just multiply all the continuous loads by 1.25 and
therefore the fuses/disconnect are appropriately oversized for the fact that they are designed for 80% of their rating.
I spoke with Eaton and no enclosures (disconnects) are rated for 100% of the rated fuse value, only certain circuit breakers.
So my question is, am I ok with 200A fuses in a 200A enclosure for this dedicated circuit or do I need to use 200A fuses in a 400A disconnect to
correctly acknowledge the disconnect is only rated to allow the fuses to be used at 80% of their ampacity.
Thanks in advance.
 
You can indeed use a "regular" fuse or circuit breaker at 100% of it rating if the load is non continuous. I think you are misinterpreting what Eaton said. That said, if the machine indeed draws 199.85 actual amps, I would not be providing a 200 amp ocpd, even if it was non-continuous. Perhaps an option to look into is something like a Siemens QR frame breaker with enclosure, that frame goes to 250A, perhaps a 225A would be a good choice? I don't know how they would compare cost wise to a 400A fused disconnect. Does the manufacturer say anything about OCPD size?
 
Where did you get the 199.85A number? Was it something you calculated? I would expect the equipment manufacturer to provide the OCPD size for a piece of equipment like this.
 
electrofelon said:
You can indeed use a "regular" fuse or circuit breaker at 100% of it rating if the load is non continuous. I think you are misinterpreting what Eaton said. That said, if the machine indeed draws 199.85 actual amps, I would not be providing a 200 amp ocpd, even if it was non-continuous. Perhaps an option to look into is something like a Siemens QR frame breaker with enclosure, that frame goes to 250A, perhaps a 225A would be a good choice? I don't know how they would compare cost wise to a 400A fused disconnect. Does the manufacturer say anything about OCPD size?
Click to expand...
Ok that is helpful and may illuminate some things.
Perhaps Eaton was unclear / vague when they told me that all of their fused disconnects are rated to allow 80% of a fuses rating. Perhaps their technician just assumes continuous for all things.
The manufacturer, Mitsubishi does not state anything about OCPD. That I can find. Its an older laser cutter from 2006. Its a beast.
The nameplate technically says 63KVA for the laser and 9KVA for the controller fed by the same power feed.
I was trying to get creative and run it on 200A fuses and a 200A disconnect, since it was non-continuous. From what I can gather from Eaton (without the continuous factor you mentioned in acknowledgment) then the correct fuses would need to be 250A in a 400A disconnect to properly get 200A worth of protection.
 
d0nut said:
Where did you get the 199.85A number? Was it something you calculated? I would expect the equipment manufacturer to provide the OCPD size for a piece of equipment like this.
Click to expand...
The nameplate says 63KVA for the laser and 9KVA for the controller at 208V on the side of the laser. It will very rarely run at this max ampacity, but thats what it says. I will search for a manual.
 
The laser is 4000Watts, which means very little really, the CNC assembly that guides the laser also contributes to the energy demand of 63KVA. The other huge draw is the chiller to cool the machine and thats a whole different story of sizing.
 
yes we are running a dedicated circuit from the service OCPD to this device, either 225 or 250A fuses in a 400A disconnect or a 225 A circuit breaker that can be adjusted.
 
sokbok said:
The nameplate says 63KVA for the laser and 9KVA for the controller at 208V on the side of the laser. It will very rarely run at this max ampacity, but thats what it says. I will search for a manual.
Click to expand...
You said 69 KVA in the OP (and just used it again last post) but looks like you used the correct number of 63+ 9 = 72 to calculate your 199.85A?
 
Ok so distilling all the discussion into one question:
-If a load is non-continuous, can a fused disconnect (regular or heavy duty) allow a fuse to be used at 100% of its rated ampacity?
therefore....is the 80% derating on a fuse only apply if the load is continuous? thx
 
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