Motor Calc.

[mstrlucky74]

If given the HP and FLA you would size conductor based on FLA X 125% and not HP and go to NEC? I understand it’s either continuous duty or duty cycle and not continuous load.

 

[beanland]

Ugly's

Ugly's

My Ugly's says a 125Hp 460V motor is 156A. So, would seem 125% is appropriate since there is no other data about the application.

 

[iceworm]

... I understand it’s either continuous duty or duty cycle and not continuous load.

I have no idea what this means. Maybe some context will help.

If given the HP and FLA you would size conductor based on FLA X 125% and not HP and go to NEC? ....

Start with 430.6A. If it this standard stuff, conductor ampacity, switch ratings, short circuit, ground fault, is per the tables.
Overload protection is per the nameplate FLA.

430.22 discusses standard stuff as well as other than continuous duty.

And I'm thinking you knew that. Maybe additional context will help.

 

[kwired]

Originally Posted by mstrlucky74
... I understand it’s either continuous duty or duty cycle and not continuous load.



I have no idea what this means. Maybe some context will help.

Isn't a continuous duty motor supposed to have conductors sized for 125% applicable table amps even if not used "continuously"?

Unlike many other loads you only add 25% if they are a continuous load. A few of them NEC tells us they are a continuous load particularly in certain 422-427 subsections.

 

[mstrlucky74]

On a side note this feed is from MCC to VFD then to motor. How would one know if neutral and/or EGG is required? RGS conduit. I would think just 3 phases and a separate ground. Thanks.

 

[kwired]

On a side note this feed is from MCC to VFD then to motor. How would one know if neutral and/or EGG is required? RGS conduit. I would think just 3 phases and a separate ground. Thanks.

Neutral not likely needed. May not even be present in the MCC.

EGC is nearly always required for anything over 50 volts. Metal raceways can be the EGC nearly all cases as well (outside of specifications exceeding code anyway).

 

[iceworm]

On a side note this feed is from MCC to VFD then to motor. How would one know if neutral and/or EGG is required? RGS conduit. I would think just 3 phases and a separate ground. Thanks.

Good to know. That is sort of important. Did you look at 430 Part X, Adjustable Speed Drive Systems?

430.122 changes the conductor sizing to 125% of the rated input current to the drive - not the table motor FLC.

 

[iceworm]

On a side note this feed is from MCC to VFD then to motor. How would one know if neutral and/or EGG is required? RGS conduit. I would think just 3 phases and a separate ground. Thanks.

Kwired is likely correct. I also recommend reading the mfg installation manual. Really - rtfm

 

[Jraef]

...Metal raceways can be the EGC nearly all cases as well (outside of specifications exceeding code anyway).

Although technically allowable per the NEC, you would NOT use only the conduit as the EGC for a VFD circuit unless you are a glutton for punishment. Even in metal conduit you always run a separate ground wire from the motor frame to the VFD ground terminal, not to a ground lug in the box or panel. Then the VFD is connected to ground as well using a different ground terminal, which is why VFDs will always have two ground terminals; one for the motor ground, one for the system ground.

NEC grounding requirements are for safety, VFD grounding requirements are for electrical noise abatement. Ignore at your own risk.

As said by iceworm, RTFM; Read the FACTORY Manual...

 

[Jraef]

Good to know. That is sort of important. Did you look at 430 Part X, Adjustable Speed Drive Systems?

430.122 changes the conductor sizing to 125% of the rated input current to the drive - not the table motor FLC.

For the input wiring. Output wire from the VFD to the motor is per the NEC tables.

 

[Dennis Alwon]

For conductor sizing you should use Tables 430.250 which happens to be the same as the nameplate

 

[mstrlucky74]

Neutral not likely needed. May not even be present in the MCC.

EGC is nearly always required for anything over 50 volts. Metal raceways can be the EGC nearly all cases as well (outside of specifications exceeding code anyway).

TY!!

 

[mstrlucky74]

For conductor sizing you should use Tables 430.250 which happens to be the same as the nameplate

TY!! I see that. So do you still add 125% to nameplate(FLA) if it's continuous?

 

[Dennis Alwon]

TY!! I see that. So do you still add 125% to nameplate(FLA) if it's continuous?

For conductor size you add 125%

430.22 Single Motor. Conductors that supply a single motor
used in a continuous duty application shall have an ampacity of
not less than 125 percent of the motor full-load current rating,
as determined by 430.6(A)(1), or not less than specified in
430.22(A) through (G).

 

[infinity]

As iceowrm stated you also need to know the input current of the VFD.

 

[kwired]

Although technically allowable per the NEC, you would NOT use only the conduit as the EGC for a VFD circuit unless you are a glutton for punishment. Even in metal conduit you always run a separate ground wire from the motor frame to the VFD ground terminal, not to a ground lug in the box or panel. Then the VFD is connected to ground as well using a different ground terminal, which is why VFDs will always have two ground terminals; one for the motor ground, one for the system ground.

NEC grounding requirements are for safety, VFD grounding requirements are for electrical noise abatement. Ignore at your own risk.

As said by iceworm, RTFM; Read the FACTORY Manual...

What is supposed to be proper with EGC to motor? NEC says you bond that EGC in any enclosure where you also have junctions in the current carrying conductors, something tells me the drive guys will say you run from terminal on the drive straight to the motor - no other connections.