Current thoughts on VFD conductor types

Electromatic

Electromatic

Senior Member
Location
Virginia
Occupation
Master Electrician
I'm reviewing plans for an upfit of a wastewater pump station that will include three 150HP motors run from VFDs. The distance from the drives to the motors is around 100'. What (if any) is the current consensus on conductor types on VFD outputs? I think VFD cable is probably overkill and would be difficult to install in the existing/re-used conduits. I am, though, considering recommending or specifying that XHHW/XLPE be used.
 
S

Strathead

Senior Member
Location
Ocala, Florida, USA
Occupation
Electrician/Estimator/Project Manager/Superintendent
In my amateur, but somewhat read opinion, When in conduit and always separate from other VFD load side branch circuits, VFD cable is not necessary. The point (as I understand it) of VFD cable is to prevent induction and overheating when two different VFD wires are run in close proximity to each other.
 
G

garbo

Senior Member
I retired from an old large hospital/research buildings that had over 500 VFD'S. Some of the motors were plain Jane NEMA motors from the 1960's. Are were feed by THWN wire with a copper ground wire to each motor. I talked to drive techs from three different companies and all said for well over 99% of the time the expensive shielded drive cables are a waste of money. Most of these drives had a line reactor. Last few drives that were installed had load side fancy reactor sleigh capacitors & 1 or 2 contactors. Would spend the extra 15 to maybe 20% extra and go with WHHW wire. In the newest 12 story building they went with 18 pulse drives for every motor 40HP and larger. Every drive tech that I asked thier option on 18 pulse drives thought they were waste of money and create more problems then the 6 pulse drives.
 
Electromatic

Electromatic

Senior Member
Location
Virginia
Occupation
Master Electrician
The conductors from each drive to each motor will be in separate raceways. I definitely didn't want to suggest VFD cable but did wonder what people's thoughts were on specifying XHHW vs. THHN.
I also see on the plans calling for a "line terminator" (Allen-Bradley 1204). I'm not familiar with these. They seem to be like a reactor but installed close to the motor to prevent "reflected wave spikes" from harming the motor. Even with this--as it's on the motor end of the run--I'm considering XHHW.
 
infinity

infinity

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Staff member
Location
New Jersey
Occupation
Journeyman Electrician
Electromatic said:
The conductors from each drive to each motor will be in separate raceways. I definitely didn't want to suggest VFD cable but did wonder what people's thoughts were on specifying XHHW vs. THHN.
I also see on the plans calling for a "line terminator" (Allen-Bradley 1204). I'm not familiar with these. They seem to be like a reactor but installed close to the motor to prevent "reflected wave spikes" from harming the motor. Even with this--as it's on the motor end of the run--I'm considering XHHW.
Click to expand...
Unless the raceway is in a wet location IMO there is nothing wrong with THHN/THWN. So unless XHHW has the same availability and price I wouldn't want to use it.
 
don_resqcapt19

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
Electromatic said:
The conductors from each drive to each motor will be in separate raceways. I definitely didn't want to suggest VFD cable but did wonder what people's thoughts were on specifying XHHW vs. THHN.
I also see on the plans calling for a "line terminator" (Allen-Bradley 1204). I'm not familiar with these. They seem to be like a reactor but installed close to the motor to prevent "reflected wave spikes" from harming the motor. Even with this--as it's on the motor end of the run--I'm considering XHHW.
Click to expand...
I started specifying XHHW for all conductors between a VFD and a motor and for all conductors in wet locations about 15 years ago.. I have seen a lot of wet location failures with THWN, and XHHW it is the type of insulation used on the conductors in the VFD cables. XHHW is a much more robust insulation than THHN/THWN.

In the smaller sizes XHHW has a larger diameter than THHN/THWN so watch your conduit fill.

Note, that if you are not using metallic conduit, you need to use VFD cables to shield the interference that will exist without shielding. Metal conduit provides that shielding.
 
Jraef

Jraef

Moderator, OTD
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
Think of the output conductors of a VFD as being a powerful local FM (Frequency Modulation) radio transmitting antenna for a really boring station. It is bleeding EMI and RFI into the surrounding area. You need shielding in some form, so if you use PVC conduit, there is zero shielding and you need the shielded VFD cable and the shield needs to be grounded at BOTH ends. The same would be true of any non-metallic raceway (or in the case of cable tray, no raceway).

If the conduit is METAL** and you have ONE VFD output per conduit, then IMHO VFD cable is not as necessary. The reason why all of the installation manuals say to use VFD cable is because outside of North America, metal conduit is rarely used. So it’s a safer default to just use it.

But yes, if you use metal conduit on one VFD output, you want XLPE insulation, which is what you get with XHHW (or RHH/RHW). THHN/WN uses PVC insulation which can have microscopic bubbles that allow standing wave phase-to-phase voltage spikes to break through. I’ve seen it multiple times. It doesn’t happen in every situation, but you don’t know when it will.

**Properly GROUNDED metal conduit.
 
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