DE Rating FLA's on industrial machine. I'm going to get in trouble aren't I ?

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milmat1

milmat1

"It Can't Do That !"
Location
Siler City, NC USA
Occupation
Controls Engineer
This latest machine has 24 VFD's driving various size motors for different operations. The material is fed trhough the machine at a constant rate, though the operations along the way are sometimes "Stop n Go" operations. Material flow is constant.

Using my mormal method of adding up the current requirements Per the VFD manufacturers spec manuals, and considering the control voltage etc. and arriving at a full load rating of 379 amps. I have a 400 amp fused disconnect installed.

Now my boss (Company Owner) Has basically ordered me to derate this FLA to 250 Amps.
His reasoning is these motors will never run all at the same time and not at full load. "So we have to come up with a spreadsheet to show percentages of duty, and derate the machine accordingly"

Will it run on 250 Amps ? Probably ! But that doesn't give me any legal right to just "Make up" a number. with no data to support the Derate.


Is this done normally in determining FLA's ? Or is he going to get us all in a mess ?
 
texie said:
I think I would look at 430.7(D), 430.24 and 430.25 and 430.26 for a start.
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I think if you look closely, the NEC really does not state how the FLA of a machine should be calculated. The ampacity of the conductors is well covered (as in the sections you cited), but not how the FLA of a machine is to be calculated.

I would look to NFPA79 or UL508a for guidance on how the FLA of the machine as a whole is calculated.

UL508a says this:

49.2 The full-load ampere rating of the panel shall, at a minimum, include the sum of the ampere ratings
of all loads that are able to be operated simultaneously plus the primary ampere rating of all control
transformers connected to the input voltage.
Click to expand...

I do not have a current NFPA79 but the 2002 version says this:

17.4.2 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.
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There is nothing that prevents you from adding additional information to the nameplate that might indicate that the typical load is 250A, even if the calculated maximum load is much more.

It is also quite possible that there are loads that do not operate simultaneously that might allow you to legitimately reduce the nameplate rating.
 
petersonra said:
I do not have a current NFPA79 but the 2002 version says this:
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This is from NFPA 70, 2012. Chapter 17 became Chapter 16 after Chapter 11 was deleted in 2007.

16.4 Machine Nameplate Data.

16.4.1 Control equipment shall be legibly and durably marked
in a way that is plainly visible after the equipment is installed. A
nameplate giving the following information shall be attached to
the outside of the enclosure, or on the machine immediately
adjacent to the enclosure:

(1) Name or trademark of supplier
(2) Serial number, where applicable
(3)*Rated voltage, number of phases and frequency (if ac),
and full-load current for each supply
(4) Ampere rating of the largest motor or load
(5) Maximum ampere rating of the short-circuit and ground fault
protective device, where provided
(6) Short-circuit current rating of the control panel
(7) Electrical diagram number(s) or the number of the index
to the electrical drawings

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.
 
Note carefully that although there is a provision for not counting motors that cannot run at the same time, there is no allowance for reducing the rated current below the full load current just because the motor will not be fully loaded.
 
milmat1 said:
This latest machine has 24 VFD's driving various size motors for different operations. The material is fed trhough the machine at a constant rate, though the operations along the way are sometimes "Stop n Go" operations. Material flow is constant.

Using my mormal method of adding up the current requirements Per the VFD manufacturers spec manuals, and considering the control voltage etc. and arriving at a full load rating of 379 amps. I have a 400 amp fused disconnect installed.

Now my boss (Company Owner) Has basically ordered me to derate this FLA to 250 Amps.
His reasoning is these motors will never run all at the same time and not at full load. "So we have to come up with a spreadsheet to show percentages of duty, and derate the machine accordingly"

Will it run on 250 Amps ? Probably ! But that doesn't give me any legal right to just "Make up" a number. with no data to support the Derate.


Is this done normally in determining FLA's ? Or is he going to get us all in a mess ?
Click to expand...
I don't know about the NEC but can you apply a diversity factor?
 
Well I think you answered my question, Several different ways. Thank You.

Because my way of figuring the full load amps has always been by considering all the motors/VFDs by there nameplate and specs. Plus the control xformer etc...

However on this machine the motors will not be all under load at the same time I can rate it by developing a duty cycle for each motor as compared to the other motors. I know its confusing and Im not explaining it the best way....Sorry for that.

[17.4.2 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./QUOTE]
Click to expand...
 
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