OT: VFD Cable in Rigid Conduit

We are working on a new control system install for a machine and the management here wants us to use VFD cable on all new jobs as we have had a couple of THHN failures in the past couple years.

On this install we have to run rigid conduit between the control / drive cabinet and the motors and the runs are from 70 to 100 feet.

Most VFD cable that I am finding is armored or like this http://www.lappusa.com/Spec_Template.asp?nGroupID=20305 and must be grounded using this.

We need to run rigid conduit with liquid tight flex to the motor connector box but we need to use VFD cable.

Anyone versed in how to do this?

We use a lot of Belden VFD cable, and they recommend terminating the shields at both ends. I suppose you could use a grounding bushing on your sealtite connector for the shield in the peckerhead. The cable is pretty stiff and doesn't take kindly to bending, I doubt you'll get it through a 90 degree liquid tight fitting, or even a 45 might be 'iffy"

we used lapp cable

regards,
james

Tim Ganz said:

We are working on a new control system install for a machine and the management here wants us to use VFD cable on all new jobs as we have had a couple of THHN failures in the past couple years.

On this install we have to run rigid conduit between the control / drive cabinet and the motors and the runs are from 70 to 100 feet.

Most VFD cable that I am finding is armored or like this http://www.lappusa.com/Spec_Template.asp?nGroupID=20305 and must be grounded using this.

We need to run rigid conduit with liquid tight flex to the motor connector box but we need to use VFD cable.

Anyone versed in how to do this?

Click to expand...

208v motors or 480v? THHN is probably fine for the lower voltage, as the P-P voltage doesn't exceed the 600v insulation rating. 480v is another story. The P-P voltage is high enough to damage THHN insulation and its use should be avoided. At that distance, line reactors might be a good idea, regardless of the voltage. The conduit will serve as a good shield. Use metallic flex at the motor, and pull a ground wire as well.

I assume the requirement for rigid is for prevention of physical damage? Perhaps finding a VFD cable that is TC, or "tray" rated might be a solution; run the TC cable in rigid for protection, and then make the hop to the peckerhead with just the cable. It's NEC rated for exposed running without a raceway. Just use the proper glands to terminate the cable.

http://www.southwire.com/products/type-tc-vfd-power-cable.htm


-rpoet

Is rigid conduit connected to environment metal constructions?

Motor Cable must be symmetrical. All current leaves from DC circuit of VFD and all current must came back to DC circuit inside then cable, not between the steel constructions.

As for routing through the sealtite, we have sometimes had to punch out the motor j-box to a larger size to make cable threading more feasible. Especially in wash-down environments, I would rather see it run in the open air (without sealtite) for the last couple of feet through cord grip bushings. It is shielded anyway and every time I have seen this type of cable get damaged, a ground fault occurs right away. This may be in violation of some rules or code, but putting it in sealtite with water is a violation of common sense. Just my opinion.

seppoalanen said:

Is rigid conduit connected to environment metal constructions?

Motor Cable must be symmetrical. All current leaves from DC circuit of VFD and all current must came back to DC circuit inside then cable, not between the steel constructions.

Click to expand...

Can you elaborate? Do you mean run the VFD cable in the rigid just don't bond it to the rigid in any way?

Two things: first, take a look at Okonite VFD cable. It's comparatively flexible. Second, I suggest using the cable grounds to tie the motor ground to the VFD ground as a bond wire only. Do not tie it to the rigid conduit directly. Let the conduit ground connect as usual to building steel, motor and drive cabinet frames, etc.

And be sure to ground both ends of the cable grounds as a bond wire. These cables are designed to tolerate current and having the motor at the exact same potential as the VFD frame is a good way to avoid false ground faults.