# Harmonics and Wire Sizing for VFD



## swimmer (Mar 19, 2011)

VFDs must create lots of harmonics on the power line since their output is non-sinusoidal or distorted sinusoidal.

That being said, are supply wires for VFD pumps and motors typically larger than those for standard AC pumps and motors of the same horse power?


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## Wirenuting (Sep 12, 2010)

swimmer said:


> VFDs must create lots of harmonics on the power line since their output is non-sinusoidal or distorted sinusoidal.
> 
> That being said, are supply wires for VFD pumps and motors typically larger than those for standard AC pumps and motors of the same horse power?


Over a larger distance, yes. 
You get nuisance trips also. 
The formula is in an ABB install guide.


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## swimmer (Mar 19, 2011)

Wirenuting said:


> Over a larger distance, yes.
> You get nuisance trips also.
> The formula is in an ABB install guide.


Sounds more like a power company problem rather than a problem for the facility at which these pumps and motors are installed?


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## Wirenuting (Sep 12, 2010)

swimmer said:


> Sounds more like a power company problem rather than a problem for the facility at which these pumps and motors are installed?


I thought so to at first. Our summer time voltage drop wasn't to bad. We also had them on generator back up power.
These faults were summer/winter/anytime faults. No Rime or reason.
But the drives were about 200+ feet from the air handlers. The wires were sized for a normal install near by. The voltage never really dropped much and it would trip on motor over temp & sometimes over current. <as displayed on the drive and not when mechanically inspected.>
These were installed in 2000 and were problems from day one. A factory rep came out and told us they were to far. He also stated that our local motor disconnects would kill the drives when opened under load. 
Our maintenance guys would never turn the drives off. They just opened the disconnects. It was only last year that I finally got the boss to believe that installing interlocks on the disconnect were important. 
I guess that a $5000 drive replacement is cheaper then a $300 interlock... Or something like that... 

I finally moved the worst offending drive in that building to within 20' and the faults went away.


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## JRaef (Mar 23, 2009)

swimmer said:


> VFDs must create lots of harmonics on the power line since their output is non-sinusoidal or distorted sinusoidal.
> 
> That being said, are supply wires for VFD pumps and motors typically larger than those for standard AC pumps and motors of the same horse power?


The NEC requires that the line side conductors be no less than 125% of the Max Amp rating *of the VFD*, not the motor. But other than that and regular voltage drop issues, no there are no added requirements for over sizing conductors. the VFD is providing the exact same circuit protection for the motor leads as a regular motor starter will, no need to over do it.


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## Wirenuting (Sep 12, 2010)

JRaef said:


> The NEC requires that the line side conductors be no less than 125% of the Max Amp rating of the VFD, not the motor. But other than that and regular voltage drop issues, no there are no added requirements for over sizing conductors. the VFD is providing the exact same circuit protection for the motor leads as a regular motor starter will, no need to over do it.


When year was this added to the NEC?
Most installs here were done by outside contract and were retro fits of old across the line starters. 
They would normally just swap the starter for a new drive.


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## JRaef (Mar 23, 2009)

Wirenuting said:


> When year was this added to the NEC?
> Most installs here were done by outside contract and were retro fits of old across the line starters.
> They would normally just swap the starter for a new drive.


It was added in the 2005 NEC and modified again in the 2008.

430.122


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## Wirenuting (Sep 12, 2010)

JRaef said:


> It was added in the 2005 NEC and modified again in the 2008.
> 
> 430.122


Ok thanks.
I tossed my 05 last year. But that explains the 2000 install on the drives I mentioned.


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