Calculation of machine isolation transformer

[electrofelon]

I'm partially convinced of the other interpretation.

You have 'unusual load types' and 'unusual duty cycles' listed explicitly, and 'and so forth' extending that list to other unusual factors that are similar to load types or duty cycles. The defining feature of this class is that they require oversized conductors or permit reduced sized conductors. This seems to make it clear to me that the 'marked full load current' can reasonably take into account the fact that in a machine most motors may not be running at full power most of the time.

Where I'm not convinced is: I don't believe that the NEC code tells us how to calculate the 'marked full load current', it simply requires the nameplate with the marked current so that the correct sized conductors can be chosen. I believe you would need to go to other standards or perhaps engineering practice (rather than code mandated standards) to figure out how to calculate the 'marked full load current'.

-Jonathan

Yeah it's really just wasted ink and more unnecessary ambiguous NEC word vomit!

 

[Ashu]

I found the codes for "marked full load current. Its NFPA 79, it didn't give any hope:
16.4.2 The full-load current shown on the nameplate shall
not be less than the full-load currents for all motors and other
equipment that can be in operation at the same time under
normal conditions of use. Where unusual loads or duty cycles
require oversized conductors, the required capacity shall be
included in the full-load current specified on the nameplate."

 

[tortuga]

Yeah it's really just wasted ink and more unnecessary ambiguous NEC word vomit!

Article 670 provides a way to have a custom complex machine or process equipment not have to be listed, and gives the engineer a wide berth to mark the machine with the rated current.
Say a factory builds this custom machine 'in house' but hires a outside EC to install the feeder and transformer, EC pulls a permit with the local jurisdiction. Everything goes smooth and the job is done and passed inspections.
Now say something happened like a fire and the conductors were blamed.
If I was the AHJ and my boss asked if the machine conductors were correctly sized I can cite 670 for accepting whatever @Ashu used in their engineering judgement to mark the machine's rated current.
Not that the AHJ would be liable anyway.
Now say the factory insurer wants to blame the electrical contractor for installing the wrong conductors, same thing the EC can use 670 in their defense. Now say the EE was a sub contractor the factory insurer will go after the EE's 'errors and omissions' insurance.

 

[don_resqcapt19]

Article 670 provides a way to have a custom complex machine or process equipment not have to be listed,

OSHA does not agree...equipment used in a workplace must be listed by a NRTL, or in the case of custom equipment, must have a field evaluation label by a NRTL or if it is a single custom machine that was designed, fabricated for, and intended for use by a particular customer, that has been determined to be safe for its intended use by its manufacturer on the basis of test data which the employer keeps and makes available for inspection.

 

[Ashu]

Thanks all.
I am still trying to read this right: "Machine Nameplate Data" Codes (NFPA 79)
"16.4.2 The full-load current shown on the nameplate shall
not be less than the full-load currents for all motors and other
equipment that can be in operation at the same time under
normal conditions of use. Where unusual loads or duty cycles
require oversized conductors, the required capacity shall be
included in the full-load current specified on the nameplate."

I am reading it that motor nameplate data should be used all the times for all the motors that run at the same time but if "unusual" loads require it to be higher than that, a higher value should be considered. Are there any disagreements with it?

 

[Ashu]

In my machine, the motors that don't run continuously are very less and they don't run when all other continuous duty motors are running. So, I haven't considered those in my calculations.