# steel or aluminum conduit



## sparky970 (Mar 19, 2008)

It is my understanding that when feeding a motor from a VFD, you should use steel conduit. Does this still apply if you are using VFD type cable?


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## ponyboy (Nov 18, 2012)

One or the other. VFD cable inside a steel conduit is pointless IMO. I've never even used the VFD cable. All the motor runs I do need physical protection that VFD cable can't offer so it's going to be in grc or EMT anyway.


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## sparky970 (Mar 19, 2008)

ponyboy said:


> One or the other. VFD cable inside a steel conduit is pointless IMO. I've never even used the VFD cable. All the motor runs I do need physical protection that VFD cable can't offer so it's going to be in grc or EMT anyway.


It's not armored or installed in tray. It will be in conduit, steel or aluminum?


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## ponyboy (Nov 18, 2012)

sparky970 said:


> It's not armored or installed in tray. It will be in conduit, steel or aluminum?


Is the cable speced or do you need auxiliary contacts ran with the motor feeders? If not I'd just use THHN in steel conduit. But if I HAD to use the cable and I needed to protect it I'd put it in steel conduit simply because I don't keep aluminum around. It's a bit redundant but it won't hurt anything


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## sparky970 (Mar 19, 2008)

575V, 125hp motor. He specified inverter rated cable


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## JRaef (Mar 23, 2009)

sparky970 said:


> It's not armored or installed in tray. It will be in conduit, steel or aluminum?


Steel, you need the magnetic shielding it provides if you do not use shielded VFD cable, aluminum is non-magnetic so it might as well be PVC. Ground the conduit at BOTH ends.

But if you use steel conduit, the VFD cable is a little redundant.

Most VFD mfrs will now tell you NOT to use THHN by the way, the PVC insulation used to make it can have voids (bubbles) in it that compromise the insulation in the presence of what are called "standing waves" of higher voltage pulses that can be created by cable capacitance between the conductors as a result of the high speed DC switching that makes up the PWM output of a VFD. The recommended wire type, if NOT using VFD cable, is XLPE (most VFD cable uses XLPE too). The insulation thickness is more uniform and there are no voids in the plastic.



> 575V, 125hp motor. He specified inverter rated cable


The other advantage of VFD cable then is that it is typically rated for 1000V. On a 575V drive, your DC bus voltage, and thereby the voltage of the pulses going to the motor, is 810V. 600V cable is generally considered OK for 480V drives, but you get a little more dicey at 575V if there are any standing waves going on.


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## ponyboy (Nov 18, 2012)

JRaef said:


> Steel, you need the magnetic shielding it provides if you do not use shielded VFD cable, aluminum is non-magnetic so it might as well be PVC. Ground the conduit at BOTH ends. But if you use steel conduit, the VFD cable is a little redundant. Most VFD mfrs will now tell you NOT to use THHN by the way, the PVC insulation used to make it can have voids (bubbles) in it that compromise the insulation in the presence of what are called "standing waves" of higher voltage pulses that can be created by cable capacitance between the conductors as a result of the high speed DC switching that makes up the PWM output of a VFD. The recommended wire type, if NOT using VFD cable, is XLPE (most VFD cable uses XLPE too). The insulation thickness is more uniform and there are no voids in the plastic.


Good to know about the xlpe. So what's your opinion on the millions of installs done with conduit and THHN? Still good?


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## JRaef (Mar 23, 2009)

ponyboy said:


> Good to know about the xlpe. So what's your opinion on the millions of installs done with conduit and THHN? Still good?


No point in changing it if it hasn't failed yet. But if it does, change it with XLPE. The entire issue came up BECAUSE of a high_*er*_ number of cable failures being reported to major cable mfrs. It's not a high number, but it is significantly high_*er*_ than it was before (or without) VFDs.


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## sparky970 (Mar 19, 2008)

JRaef said:


> Steel, you need the magnetic shielding it provides if you do not use shielded VFD cable, aluminum is non-magnetic so it might as well be PVC. Ground the conduit at BOTH ends.
> 
> But if you use steel conduit, the VFD cable is a little redundant.
> 
> ...



I was hoping you would give some input. Before there wasn't much "vfd" cable, it was always in steel conduit, never aluminum. I've been looking but can't find anything that backs up, the type of conduit.


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## JRaef (Mar 23, 2009)

sparky970 said:


> I was hoping you would give some input. Before there wasn't much "vfd" cable, it was always in steel conduit, never aluminum. I've been looking but can't find anything that backs up, the type of conduit.


Regardless of what brand of drive you use, this is the best document I've ever found on VFD installation. It originated with the old Reliance drives group when AB bought them, but has been continuously updated over the years. When I worked for ABB and again for Siemens later, I used to tear the covers off of this and give it to my customers...

The Bible of VFD installation...

Pay particular attention to Chapter 5


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## sparky970 (Mar 19, 2008)

JRaef said:


> Regardless of what brand of drive you use, this is the best document I've ever found on VFD installation. It originated with the old Reliance drives group when AB bought them, but has been continuously updated over the years. When I worked for ABB and again for Siemens later, I used to tear the covers off of this and give it to my customers...
> 
> The Bible of VFD installation...
> 
> Pay particular attention to Chapter 5


Thanks, I had just downloaded that pdf but hadn't read it yet.


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## oliquir (Jan 13, 2011)

here we use 1kv teck cable for 600v vfd and never had problem, teck cable have xlpe wiring inside


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## don_resqcapt19 (Jul 18, 2010)

JRaef said:


> ... The recommended wire type, if NOT using VFD cable, is XLPE (most VFD cable uses XLPE too)...


What exactly is XLPE? It is not an insulation type that is permitted for single conductors installed under the rules of the NEC. Does it go by another name in Table 310.104?


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## sparky970 (Mar 19, 2008)

don_resqcapt19 said:


> what exactly is xlpe? It is not an insulation type that is permitted for single conductors installed under the rules of the nec. Does it go by another name in table 310.104?


RHH/RHW-2 and XHHW are showing up as alternatives to THHN in acidic environments for two reasons: They both use cross-linked polyethylene (XLPE) insulation rather than PVC. Second, they both offer more insulation thickness than THHN. XHHW offers a little more thickness, RHH/RHW offers quite a bit more. In general, in the demanding waste-water treatment environment, the thicker the insulation, the better off you are.

Here’s an example: A single-conductor 350 kcmil THHN cable uses 60 mils of PVC. An XHHW conductor delivers 65 mils of XLPE. RHH-RHW gives you 95 mils of XLPE. The table below summarizes the constructions.


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## AllClear (Dec 31, 2013)

Drop a High powered magnet down an Aluminum or copper. The Magnet will slowly hover down the center. You tube "aluminum tube magnet" and watch, Eddy currents slow it down, It's neat.
Based on that, I don't think it matters if you use Steel or AL Conduit.
Doesn't VFD and many other cables use Aluminum coated mylar or plastic and Copper braid.


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## Big John (May 23, 2010)

AllClear said:


> Drop a High powered magnet down an Aluminum or copper. The Magnet will slowly hover down the center. You tube "aluminum tube magnet" and watch, Eddy currents slow it down, It's neat....


 Same thing is true with copper pipe. I agree it's slick.


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## Going_Commando (Oct 1, 2011)

JRaef said:


> Regardless of what brand of drive you use, this is the best document I've ever found on VFD installation. It originated with the old Reliance drives group when AB bought them, but has been continuously updated over the years. When I worked for ABB and again for Siemens later, I used to tear the covers off of this and give it to my customers...
> 
> The Bible of VFD installation...
> 
> Pay particular attention to Chapter 5


That's awesome info man. I don't get to play with VFDs and whatnot anywhere near as much as I would like, but that is something I never would have thought of. Thanks! :thumbup:


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## MTW (Aug 28, 2013)

JRaef said:


> Most VFD mfrs will now tell you NOT to use THHN by the way, the PVC insulation used to make it can have voids (bubbles) in it that compromise the insulation in the presence of what are called "standing waves" of higher voltage pulses that can be created by cable capacitance between the conductors as a result of the high speed DC switching that makes up the PWM output of a VFD. The recommended wire type, if NOT using VFD cable, is XLPE (most VFD cable uses XLPE too). The insulation thickness is more uniform and there are no voids in the plastic.


To my knowledge, FANUC has no such requirement. We regularly use THHN/THWN for VFD circuits, and the ready made VFD cable from FANUC for larger motors is nothing more than rubber cord.


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## chewy (May 9, 2010)

AllClear said:


> Drop a High powered magnet down an Aluminum or copper. The Magnet will slowly hover down the center. You tube "aluminum tube magnet" and watch, Eddy currents slow it down, It's neat.
> Based on that, I don't think it matters if you use Steel or AL Conduit.
> Doesn't VFD and many other cables use Aluminum coated mylar or plastic and Copper braid.


Aha! Our instructor did that to us and we couldnt figure it out.


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## greenman (Apr 20, 2012)

oliquir said:


> here we use 1kv teck cable for 600v vfd and never had problem, teck cable have xlpe wiring inside


 Tec is the best.:thumbsup:


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## greenman (Apr 20, 2012)

And Al is better becaue it dose not have magnetic properties, thats what i think,:thumbup:


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