VFD conductor sizing

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Davebones

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Have a VFD rated for 75 hp at 480 volt . According to 430.122 the conductors feeding the VFD shall be rated at 125 percent of the VFD rated input . The VFD input is rated at 112 amps . I figured the supply conductors would need to be rated at 140 amps . ( 1.25 X 112 = 140 amps ) . The VFD nameplate says use a supply fuse or breaker rated at 125 amps . My question is even though the supply fuse or breaker is rated at 125 amps I still need to pull conductors to the VFD rated for at least 140 amps ???
 
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Have a VFD rated for 75 hp at 480 volt . According to 430.122 the conductors feeding the VFD shall be rated at 125 percent of the VFD rated input . The VFD input is rated at 112 amps . I figured the supply conductors would need to be rated at 140 amps . ( 1.25 X 112 = 140 amps ) . The VFD nameplate says use a supply fuse or breaker rated at 125 amps . My question is even though the supply fuse or breaker is rated at 125 amps I still need to pull conductors to the VFD rated for at least 140 amps ???

That sounds low for the supply side fuse/breaker. What brand are you working with?
 

Jraef

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Conductor sizing is dictated by the NEC, many VFDs are made in and for places where the NEC is not valid and the rules are different, but WE still must follow the NEC when installing them.

But that said, I think maybe you missed something subtle here. 430.22 says that the conductors must be sized for the INPUT amps of the VFD, not the output amps. A 75HP motor should be 96A (usually slightly less). Your VFD is likely Asian, in which case the rating is based on the nearest kW rating at 380V 50Hz, so the amps are higher than they need be here. But regardless, the INPUT amps are almost always LOWER than the output amps because the VFD is correcting the power factor. So for example on a 75HP A-B VFD, the output is rated for 96A for the motor, but the INPUT current is only 90.5A. So you would size the conductors at 125% of 90.5A, 113A.

Also on A-B's chart, there is a MINIMUM size TD fuse of 125A, but a maximum of 200A, and a MAXIMUM breaker size of 275A. You can use anything in between. But on some drives, Asian mfrs in particular, they could only get them UL listed with SPECIFIC fuses or circuit breakers (both size and brand) in order to attain some reasonable level of SCCR, so you MUST follow whatever instructions they give you on that.
 

Besoeker

Senior Member
Location
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Have a VFD rated for 75 hp at 480 volt . According to 430.122 the conductors feeding the VFD shall be rated at 125 percent of the VFD rated input. The VFD input is rated at 112 amps
Are you sure that's the input current?
VFDs are usually rated in output terms. The 75 hp is output.
 

infinity

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Are you sure that's the input current?
VFDs are usually rated in output terms. The 75 hp is output.

According to the NEC the conductors to the drive are sized by the input current so that information should always be on the nameplate.
 

Besoeker

Senior Member
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According to the NEC the conductors to the drive are sized by the input current so that information should always be on the nameplate.
I'm sure you're right. I probably never looked for it. I based ratings on the output current which is normally higher than the input current
 

Ingenieur

Senior Member
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Have a VFD rated for 75 hp at 480 volt . According to 430.122 the conductors feeding the VFD shall be rated at 125 percent of the VFD rated input . The VFD input is rated at 112 amps . I figured the supply conductors would need to be rated at 140 amps . ( 1.25 X 112 = 140 amps ) . The VFD nameplate says use a supply fuse or breaker rated at 125 amps . My question is even though the supply fuse or breaker is rated at 125 amps I still need to pull conductors to the VFD rated for at least 140 amps ???
Sounds like you've interpreted correctly
the a 125 oc protection sounds low, perhaps 150 imo
but the mfg's spec is over-riding imo
 

Jraef

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So because that's an odd value for a 75HP VFD, I searched around and found that it's likely a Fuji drive, which was also sold as GE and Saftronics for a while. Unfortunately because if it being an Asian drive, they don't really care about our requirements and they don't list the input current for their drive, the 112A is the rated output current. So you may be stuck having to use the output current for lack of anything else you can prove.
 

Ingenieur

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Location
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So because that's an odd value for a 75HP VFD, I searched around and found that it's likely a Fuji drive, which was also sold as GE and Saftronics for a while. Unfortunately because if it being an Asian drive, they don't really care about our requirements and they don't list the input current for their drive, the 112A is the rated output current. So you may be stuck having to use the output current for lack of anything else you can prove.

he was very specific about the 112 being the input and referenced the code section that states 125% x input i
but who knows?
that is why a mfg/pn is critical when asking for specifics
 

Besoeker

Senior Member
Location
UK
So because that's an odd value for a 75HP VFD, I searched around and found that it's likely a Fuji drive, which was also sold as GE and Saftronics for a while. Unfortunately because if it being an Asian drive, they don't really care about our requirements and they don't list the input current for their drive, the 112A is the rated output current. So you may be stuck having to use the output current for lack of anything else you can prove.
Yes, it's why I asked the question.
But using output current rating should, at least, be a safe option from a technical point of view.
 

Besoeker

Senior Member
Location
UK
he was very specific about the 112 being the input
He was. And he may be correct. But do the calcs and it does seem a bit high.
As Jraef has posted, a 75 hp motor might be expected to draw about 96A or less - I agree with that.
And the input current is usually lower than the output current.
Perhaps the OP can clarify.
 

Ingenieur

Senior Member
Location
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He was. And he may be correct. But do the calcs and it does seem a bit high.
As Jraef has posted, a 75 hp motor might be expected to draw about 96A or less - I agree with that.
And the input current is usually lower than the output current.
Perhaps the OP can clarify.

I looked a few ABB drives
i out = i in = 96
IEC spec cb = 125 A

until the op gives a pn or clarification all idle speculation
He is a master electrician with alot of experience
I have to go with what he said until he confirms/corrects
 

Besoeker

Senior Member
Location
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I looked a few ABB drives
i out = i in = 96
Which is what Jraef came up with.
But not 112A.
We had a bit lower voltage here and for 55kW (about 75 hp) we'd design for 100A.


until the op gives a pn or clarification all idle speculation
Idle? Not really idle. We are given the power and the voltage and we calculate based on that. So not entirely speculation.
But, I agree. We need to see what the OP comes up with.
 

Ingenieur

Senior Member
Location
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Which is what Jraef came up with.
But not 112A.
We had a bit lower voltage here and for 55kW (about 75 hp) we'd design for 100A.

Idle? Not really idle. We are given the power and the voltage and we calculate based on that. So not entirely speculation.
But, I agree. We need to see what the OP comes up with.

we do not know if it is an abb
96 is the nec table rating for 75 hp

yes, idle speculation
how do you calculate with the input pf without making an ASSUMPTION?
 

Besoeker

Senior Member
Location
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we do not know if it is an abb
96 is the nec table rating for 75 hp

yes, idle speculation
how do you calculate with the input pf without making an ASSUMPTION?
If you have been involved to any great extent with VSDs you will have a pretty good idea that they are pretty much all the same configuration and that badge engineering is common practice.
 

Ingenieur

Senior Member
Location
Earth
If you have been involved to any great extent with VSDs you will have a pretty good idea that they are pretty much all the same configuration and that badge engineering is common practice.


Only for 30+ years
benshaw were the first I worked with

the salient points
he said 112 input
he ref code that sized conductor at input x 125%

until he clarifies idle speculation
 

Besoeker

Senior Member
Location
UK
Only for 30+ years
benshaw were the first I worked with

the salient points
he said 112 input
he ref code that sized conductor at input x 125%

until he clarifies idle speculation
Idle it is not but have it your way if that's what floats your boat.................:D
 

Ingenieur

Senior Member
Location
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Idle it is not but have it your way if that's what floats your boat.................:D

don't need to be a drive expert to read :lol:

he said input and referenced the code that quantifies 125% x input i
fact
sounds like he read the label or manual, re: 125 A ckt protection
everything else is idle speculation
not a bad thing, I guess that is what chat rooms/forums are for
But it gets us no closer to giving the op a definitive answer
only he can provide that information
 

Besoeker

Senior Member
Location
UK
don't need to be a drive expert to read :lol:

he said input and referenced the code that quantifies 125% x input i
fact
sounds like he read the label or manual, re: 125 A ckt protection
everything else is idle speculation
not a bad thing, I guess that is what chat rooms/forums are for
But it gets us no closer to giving the op a definitive answer
only he can provide that information
That you disagree with me is fine, fun, and expected.
 
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