Conduit crosstalk

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Tower2821

Member
We recently completed a data center project in Missouri as a specialty contractor. Part of our work was hiring a local, electrical contractor to connect motors (40 HP) and VFD's. After the fact we discovered that the contractor included the motor's thermostat wires (115v) in the same conduit as the high voltage (460v) conductors causing significant crosstalk resulting in VFD faults.

My question is: Does the NEC permit this? and if not, shouldn't the electrical contractor have complied with the code since it is clearly referenced in the project specs?
 

raider1

Senior Member
Staff member
Location
Logan, Utah
Welcome to the forum.:)

300.3(C)(1) permits 120 volt and 480 volt conductors to be installed in the same raceway or cable provided that the insulation of the conductors is rated for the highest voltage in the raceway.

This section does not apply to mixing Class 2 or 3 circuits with power circuits.

Chris
 

Tower2821

Member
Just so I understand ....

The code does not specifically prohibit the inclusion of these types of conductors together in the same conduit? Or is this a grey area?

If it makes a difference, we are talking about a 200' run of metal conduit, 1''.
 

raider1

Senior Member
Staff member
Location
Logan, Utah
Just so I understand ....

The code does not specifically prohibit the inclusion of these types of conductors together in the same conduit? Or is this a grey area?

If it makes a difference, we are talking about a 200' run of metal conduit, 1''.

300.3(C)(1) does permit 120 volt circuits and 480 volt circuits to be installed in the same raceway. (Again unless we are talking about a Class 2 or 3 circuit)

The length and type of raceway don't matter in regards 300.3(C).

Chris
 

tom baker

First Chief Moderator
Staff member
The solution may be as simple as using 600 V sheilded tray cable.
But first look at the VFD and find out what type of circuit the wiring to the motor stat is. What you are looking for is "Class 1" "Class 2" or "Class 3". Class 1 can be run with the motor circuit condutors if its functionally related. If there is no class it may be considered a power and lighting circuit. A call to the VFD mfg may be in order.
 

glene77is

Senior Member
Location
Memphis, TN
We recently completed a data center project in Missouri as a specialty contractor. Part of our work was hiring a local, electrical contractor to connect motors (40 HP) and VFD's. After the fact we discovered that the contractor included the motor's thermostat wires (115v) in the same conduit as the high voltage (460v) conductors causing significant crosstalk resulting in VFD faults.

My question is: Does the NEC permit this? and if not, shouldn't the electrical contractor have complied with the code since it is clearly referenced in the project specs?

Tower,
I worked around these for a few years.
At first, I questioned it,
but 300.3 allows combining 120 and 480 in the same conduit.

However,
The VFD requires a pristine signal,
and cross talk between cables can be a faulting problem.
This is from experience. Design issues are critical with a VFD.
:)
 

benaround

Senior Member
Location
Arizona
Tower2821,

Why would 120v t-stat cause 'cross talk' anyway ? The way the EC ran the wires is done

every day, and has been for decades.

Are you sure that this is the problem ?
 

Cold Fusion

Senior Member
Location
way north
Tower -
Look in the mfg installation specs. If the NEC allows these conductors to be run in the power conduit and the MFG doesn't want you to because it will cause trouble, they will say that or show that in the installation literature. And, as others have suggested, call the mfg. You really need to do those first. None of us know what either of those say or will say. None of us have access to the connection diagrams or specs. You do.

I'm assuming you didn't hire an engineer familiar with these types of installation. Rather you gave the drive and the books at the EC and asked them to install. If you wish to know if the EC did the installation correctly then, "Read the Book". Compare the installation with the installation instructions.

Just curious: Who commissioned the drive? Was the mfg commissioning procedure followed? You will be able to tell by looking at the filled out commissioning forms that you were give at closeout.

Maybe Thermistor leads, not really 'thermostat' wiring.

Bob has an excellent idea, could easily be an RTD. If so, it will be three (or four) wires. Generally RTD don't come out in the motor power connection box. They use a separate connection box. And they won't be 115V, they will be millivolts.

So if it is two wires, and it is 115V, and it is connected to the correct terminals in the VFD, and as benaround suggested, you have a method to test that interference on these two leads is actually the cause of the drive faulting - then I suggest you .... .... .... .... ... Call the MFG.

cf
 
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