Motor Nameplate vs NEC Table

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dvcraven0522

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St. Louis, Missouri
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Engineer
I have a compressor 60HP motor specification sheet that list 460/3/60 FLA = 87.2. NEC table 430.250 list this motor at 77. I understand in most cases NEC FLA current for amperages shall be used for branch-circuit short-circuit protection and ground fault protection, in lieu of nameplate data.

I know there are exceptions and I would thing this would be one. Although I could not find that exact wording in the NEC. Your thoughts are appreciated.

Dan Craven
Kaemmerlen Electric
 
So the specification sheet is higher than the table, is it marked only in amps? yeah I think this was clarified some in the 2023 NEC, when the motor is marked in amps not HP the rating I think you actually use the table backwayds and get a HP rating from the amps have a look at NEC 430.6
 
dvcraven0522 said:
I have a compressor 60HP motor specification sheet that list 460/3/60 FLA = 87.2. NEC table 430.250 list this motor at 77. I understand in most cases NEC FLA current for amperages shall be used for branch-circuit short-circuit protection and ground fault protection, in lieu of nameplate data.

I know there are exceptions and I would thing this would be one. Although I could not find that exact wording in the NEC. Your thoughts are appreciated.

Dan Craven
Kaemmerlen Electric
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Not sure I understand the exact question here? Does 430.6 (A)(1) Exception No. 2 and or Exception No. 3 help with your application?
 
DASKubiak said:
Not sure I understand the exact question here? Does 430.6 (A)(1) Exception No. 2 and or Exception No. 3 help with your application?
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I believe that's what would apply here. Most likely it's a low speed (high pole count) motor. They always show higher FLA than the charts and you would simply use the higher value. Either that, or that is not the MOTOR nameplate, it is the COMPRESSOR ASSEMBLY nameplate, and includes something like a dryer or lube pump in the FLA value, in which case you use Exception 3...
 
Without more actual details, all the responses so far seem plausible possibilities. One thing not mentioned yet is whether that 87.2 A is actual FLA or if it is service factor amps which sometimes will be on data plate/spec sheets.
 
The amp rating on the equipment is just a bit less than what you would get when you run the motor at its 1.15 service factor rating. I am aware of compressor manufacturer's that have done that.
 
don_resqcapt19 said:
The amp rating on the equipment is just a bit less than what you would get when you run the motor at its 1.15 service factor rating. I am aware of compressor manufacturer's that have done that.
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Every piece of equipment I've measured (largely residential, not industrial)
the nameplate is much higher than the actual measured value. I'm not sure why the vendors are so conservative, it's as if they round up to some much higher number just because.
 
brycenesbitt said:
the nameplate is much higher than the actual measured value.
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The name plate of the motor is when the motor is supplying the full rated horse power of the motor....that rarely happens on most equipment. Same with other equipment...the nameplate is worst case. Probably the equipment where the nameplate most closely matches the real world load is electric resistance heating.

The table full load currents are worst case currents to ensure if a motor is replaced the conductors will be of adequate size for the replacement motor. Given the changes in motor efficiency over the years, it is probably time for teh table full load currents to be revised.
 
don_resqcapt19 said:
Given the changes in motor efficiency over the years, it is probably time for teh table full load currents to be revised.
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Agreed.
And for another reason as well. We used to have motors with startup current, and the potential for a locked rotor current.
With today's electronically controlled motors neither is an issue. The motor will ramp up to speed under digital control, and shut down under digital control in a locked rotor condition --- at least if designed robustly.
 
brycenesbitt said:
Agreed.
And for another reason as well. We used to have motors with startup current, and the potential for a locked rotor current.
With today's electronically controlled motors neither is an issue. The motor will ramp up to speed under digital control, and shut down under digital control in a locked rotor condition --- at least if designed robustly.
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The vast majority of the motors in industrial occupancies in my area are started across the line with a stater.
 
the NEC is the bare minimum wiring. i'm a bit surprised that you have a supposed 60HP rated motor (60 * 746 / 460 / 1.73 = 56.25a) but the factory is stating that it's going to take 87.2a? holy smokes. that would be only 64.5% efficiency rating at full load. i'd recommend a more efficient motor.

anyhow, the NEC is the bare minimum requirements. you can always wire for larger. nothing prohibits such.

are you sure that 87.2a rating isn't the entire compressor unit that would include all of the controls and other peripherials to go with the compressor?
 
brycenesbitt said:
The motor will ramp up to speed under digital control, and shut down under digital control in a locked rotor condition --- at least if designed robustly.
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If motor load causes slow RPM starts & full-load current to run, max current ratings for VSD may require conductors sized to NEC table ratings.
 
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don_resqcapt19 said:
The table full load currents are worst case currents to ensure if a motor is replaced the conductors will be of adequate size for the replacement motor. Given the changes in motor efficiency over the years, it is probably time for teh table full load currents to be revised.
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Since wire length, or other I^2R losses are rarely zero, full-load current is expected to rise beyond motor nameplate

Beyond the table default near 1.0 Pf, NEC Tbl 430.250 legend notes the current rise adjustments * For 90 and 80 percent power factor

Inductive loads uniquely increase current with voltage drop, a design issue not factored by Nameplate, or Ohms law until Pf denominator is added.

Most Voltage-Drop calculators, including Southwire Re3, can adjust power factor, but fail to differentiate if its an inductive-load Pf for current rise.
 
ramsy said:
Since wire length, or other I^2R losses are rarely zero, full-load current is expected to rise beyond motor nameplate

Beyond the table default near 1.0 Pf, NEC Tbl 430.250 legend notes the current rise adjustments * For 90 and 80 percent power factor

Inductive loads uniquely increase current with voltage drop, a design issue not factored by Nameplate, or Ohms law until Pf denominator is added.

Most Voltage-Drop calculators, including Southwire Re3, can adjust power factor, but fail to differentiate if its an inductive-load Pf for current rise.
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Name plate voltages are generally less than actual input voltage which does bring things in line to some extent after voltage drop on the incoming line occurs. - For 480 volt nominal systems the motor nameplate is usually 460 volts, actual no load voltage of the supply might be almost 500 volts in many cases. Should you be in a situation where supply volts is only 468 before starting your motor, you may be more likely to have operating voltage of 460 or even lower and that might drive current at full output rating to more than nameplate current. Details will vary for each motor and it's set of circumstances but most cases it is kind of preferred to have the voltage a little high vs a little low

Beyond the table default near 1.0 Pf, NEC Tbl 430.250 legend notes the current rise adjustments * For 90 and 80 percent power factor
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That note applies to the columns on right side of table for synchronous type motors. The other values on the left are for squirrel cage type induction and wound rotor motors - all of which will have a power factor below 1.0. I believe they based on some the worst case power factor and efficiency motors out there, 2023 code it just says for motors running at speeds usual for belted motors and normal torque characteristics. Probably the typical 4 pole motor with a speed of ~1750 RPM. I've noticed 2 pole motors often still will end up in similar range as these tables but more than 4 poles will be low speed and high torque and may not have nameplate currents that align with this table.
 
kwired said:
Name plate voltages are generally less than actual input voltage which does bring things in line to some extent

That note applies to the columns on right side of table for synchronous type motors. The other values on the left are for ..power factor below 1.0
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Excellent, Thanks for sharing
 
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