- Occupation
- Licensed Electrician
Can't go too big is what that says to me. Similar language is what is in 430.
I went to the bathroom and you gave up before I had a chance to respond. Dang!
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Under sizing a breaker for CNC equipment
- Thread starter I^2R
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Can't go too big is what that says to me. Similar language is what is in 430.
I went to the bathroom and you gave up before I had a chance to respond. Dang!
- Location
- Massachusetts
- Occupation
- Licensed Electrician
You are right.
I still say it is a violation.
We need to add Instant Replay to the NEC.
I'm starting to think it's not an explicit violation, just a poor idea, but not something worth drawing a line in the sand over with a good customer. Of course all the pertinent disclaimers will be on the proposal.
The only potential looser I can see here is the customer when it trips out and he crashes a $600 tool into a $2000 aerospace part. on edit: at 2am during lights out production and makes no parts the rest of the night
The only potential looser I can see here is the customer when it trips out and he crashes a $600 tool into a $2000 aerospace part. on edit: at 2am during lights out production and makes no parts the rest of the night
JoeStillman
Senior Member
- Location
- West Chester, PA
You are right.
I still say it is a violation.
430.52 and 62 give maximums (maxima?) not minimums. Note the use of the phrase "not greater than."
This is a classic exercise in trying to prove that something is NOT in the code. Almost impossible, but the PDF version gives it up to the search tool sometimes.
electrofelon
Senior Member
- Location
- Cherry Valley NY, Seattle, WA
I dont know exactly who comes up with the nameplate data, but I believe they are based on several absolute worst case factors such as, every single actuator and component loaded to the max, spindle operating at max horsepower, and lowest acceptable input voltage. I also know that the guys at phase perfect phase converters will size a phase converter smaller than the nameplate. I have been a hobby machinist for years and ran a 15 HP VMC for 8 months - I cant imagine would it would take to load that machine to its full 15 HP for any significant amount of time. FWIW.
- Occupation
- Licensed Electrician
I'm starting to think it's not an explicit violation, just a poor idea, but not something worth drawing a line in the sand over with a good customer. Of course all the pertinent disclaimers will be on the proposal.
The only potential looser I can see here is the customer when it trips out and he crashes a $600 tool into a $2000 aerospace part. on edit: at 2am during lights out production and makes no parts the rest of the night
And if it does happen life will go on. And if it keeps happening someone will decide a change is necessary. And if it doesn't happen then nothing happens. Who's to say a power blip won't cause the same thing?
ptonsparky had it right. Do what your customer wants as long as it is safe, make clear that the breakers are undersized by nameplate and let them make the call.
- Location
- Massachusetts
Third person to point that out but thanks for busting my chops.
As far as my comment that it is still a violation that was referring to 110.3(B)
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- Location
- Connecticut
- Occupation
- Engineer
Sorry, I think I attributed the wrong Code section to Bob's thinking. He clearly meant that a smaller breaker would violate 220.18 based on the load calculated in 220.14(A).
JoeStillman
Senior Member
- Location
- West Chester, PA
Third person to point that out but thanks for busting my chops.
As far as my comment that it is still a violation that was referring to 110.3(B)
Sorry, I didn't mean to pile on.
I just wonder what actual safety risk there is for putting in a breaker that's smaller than allowed. I have a client with a whole fleet of CNC machines that are big enough to hog the transmission gears for a diesel train but they use them to cut little pockets in credit cards for the RFID chips. We did feed them at their nameplate rating but I sure didn't use that total load for my feeder sizing.
ptonsparky
Tom
- Occupation
- EC - retired
IMO, none. Unless of course they start punching out diesel train gears and the service tech can't figure out what the problem is and he goes poking around with a line up punch to find out which wires hot. Maybe then.
greg furey
Member
- Location
- old lyme ct
On an existing machine rated continuous duty do you have to multiply the machines ocpd by 125% to size wire and fusing? Thanks
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ptonsparky
Tom
- Occupation
- EC - retired
No.
There should be a nameplate or label of some sort that indicates the MCA. That will have included any calculations.
greg furey
Member
- Location
- old lyme ct
The machinery we are dealing with are rather old and the info we have to go on . Is usually looking at the breaker in the control enclosure and sizing to that. I was wondering if we have to multiply the breaker for the enclosure by 125% or is that already taken in account when the machine was built. These machines are used more than 3hours a day so would be continuous.
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125% pertains to sizing of conductors so it won't heat up if the load drawn is max at FLA. When you deal with breakers, you have to choose a breaker which does not trip on starting, holds the trip for the duration of the starting inrush and will protect the wires attached to its load terminals during overloads and faults.
The prohibition that you may have been confused with could be that breakers be loaded 80% of its rating (which is the same as saying 125%) when enclosed in a compartment--> less cooling air around!
- Location
- San Francisco Bay Area, CA, USA
- Occupation
- Electrical Engineer
One place that a MINIMUM size is called for in the NEC is that the DISCONNECTING DEVICE must be sized at a minimum of 115% of the motor FLA or combination loads.
So if that feeder breaker is going to be used as the main disconnect of the panel, it will need to be sized in accordance with 430.110. If the panel has it's own (properly sized) disconnect, then the breakers in the panel become just feeders and so long as the CIRCUIT (meaning conductors) is sized per the rules, there is no prohibition in using the smaller breakers. Nuisance tripping may be an issue, especially if the motors are across-the-line, but in a CNC machine they may not be, they are often servo motors fed through servo amplifiers which like VFDs, limit the inrush current substantially.
greg furey
Member
- Location
- old lyme ct
So my understanding is if a combination controller for motor and controls has a 50 amp breaker my fused disc and wire must be rated for 115% of the 50 amp breaker. To feed the combination controller.
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- Location
- San Francisco Bay Area, CA, USA
- Occupation
- Electrical Engineer
No. 115% of the LOAD current. The 50A is the OCPD used and has it's own sizing rules. Those values are related, but not the same.
However if it is truly a combination controller, the OCPD usually IS the disconnect device. So I don't see why you would add another one.
ptonsparky
Tom
- Occupation
- EC - retired
You add another So you can work inside that combination controller.
- Location
- San Francisco Bay Area, CA, USA
- Occupation
- Electrical Engineer
Yeah, I get it. Still not used to all the (relatively to me) new NFPA 70E rules about exposure to live terminals or conductors. I'm not complaining nor do I disagree with it all, I'm just saying that I'm still not used to thinking about it from a pre-installation system design standpoint. To me, the line side terminals of a combo starter breaker being hot was just a fact of life that all electricians understood. Now, it's considered an unacceptable hazard.
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