NEC 2020 table 310.6 or article 400 for a 3-conductor VFD cable? 2

[PaulKraemer]

Hi,

I am working on a project in which I will be using a VFD to drive an AC motor. For the motor power cable from the VFD to the motor, I am planning to use a cable with a braided shield that is specifically intended for use with VFD's. The cable will have three current carrying conductors and one ground conductor. Table 310.16 in the 2020 NEC lists the ampacities of of "insulated conductors with not more than three current carrying conductors in a raceway, cable, or earth." As my VFD cable will have three current carrying conductors, this makes me believe table 310.6 *might be* where I should look to determine what wire size (AWG) I should get for the cable conductors.

What confuses me is that article 400 of the NEC covers the ampacities of "flexible cords and cables". It is unclear to me for my application whether I should be consulting table 310.16 or if I should be consulting Article 400. The VFD cable I will be using is "flexible" in that I am able to bend it (specifications list the minimum bend radius as 7.5 x cable diameter). In my application, however, it will not be required to flex or bend during use. The cable will be routed from the VFD to the motor as necessary and secured in place before the machine is put into service.

With this being the case, for the purpose of determining the required size (AWG) for my conductors, should I be looking at table 310.16 or Article 400? Any advice will be greatly appreciated.

Thanks and best regards,
Paul

 

[mparenteau]

Paul,

Doesn't the VFD cable have a cut-sheet, giving you the ampacity, etc.?

Mike

 

[waross]

It would be a great help if you posted the designation of the cable.

"flexible cords and cables"

in the code refers to a specific class of cable, not the bendability.
If you can not find the designation of the table in Article 400 then it is not a flexible cable barring special permission.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[PaulKraemer]

Hi Mike and Bill,

Thank you for your responses. I have attached the datasheet for the Olflex VFD Slim cable I have in mind. I don't see anywhere on this datasheet that lists the ampacity. As far as the designation, I am not sure what I should be looking for on this datasheet to determine whether or not it would be categorized as a "flexible cord or cable" as covered by article 400. If either of you can let me know if this datasheet includes enough information to make this determination, I would greatly appreciate it.

In the meantime, I will email the manufacturer and see if they can provide the ampacity information.

Thanks and best regards,
Paul

 

[waross]

HP?
Length of run?
Rated FLA of motor?
Added cost to use the more restrictive rating?

BUT:
From the cut sheet:
TC-ER = Tray Cable-Exposed Run.
Not a Flexible Cord per NEC.
Link
Use the 75 Degree column in the table for three conductors in a raceway or cable.
(My CEC numbering does not match the NEC numbering)

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[PaulKraemer]

Hi Bill,

Thank you for your response.

My largest drive / motor pair is a 2 HP rated VFD which I plan to use with a 1-1/2 HP motor. The drive has a continuous output rating of 7.8 amps, but the motor nameplate current is only 4.2 amps. Cable length will be no more than 20 feet. It is good to know that TC-ER means that it does not necessarily have to be enclosed. That was a helpful link.

I will have the drive mounted inside an Electrical Control Enclosure. Inside this enclosure, my VFD cable will be routed from the VFD through a (Panduit) wire duct to the side of the enclosure, where it will exit the enclosure and pass through about 1-2 feet of open space before entering the section of the machine where the motor is located. Inside this machine section, the VFD cable will be routed to the motor as necessary and secured to the machine framework using cable ties.

If you still recommend using the 75 degree column in the table for three conductors in a raceway, cable, or earth, at least in my copy of NEC 2020, this is table 310.16 (I have attached a screenshot). In the 75 degree column, the smallest wire gage for which an ampacity is listed is 14 AWG. In this 75 degree column, the rows for 16 and 18 AWG have a dash ("-----") listed for ampacity. This is something I have always wondered about. Does this mean that it is never ok use less than 14 AWG copper wire unless it is rated for 90 deg C? Or is there somewhere else I should look for the ampacities of smaller gauge wires?

Also, I am not sure I understand your question regarding the "added cost to use the more restrictive rating". I am not really concerned with the cost of my cable - I am more concerned about the diameter. The VFD Slim cable I am hoping to use has a fairly large outer diameter. The 16 AWG version has an OD of 0.465 inches and the 14 AWG version has an OD of 0.514 inches. If 16 AWG is enough to satisfy applicable codes, my preference is to use the smaller diameter cable because it will make routing the cable a little easier and keep my wire duct inside my enclosure a little less full. If applicable codes require 14 AWG, then that is what I will use.

I really appreciate your help with this.

Thanks again,
Paul

 

[waross]

Not sure about the NEC but in the Canadian code, #14 has been for years the minimum size allowed for field wiring of power circuits.
There may be some exceptions in recent changes to the code but I don't know.
If the column has a --- then the cable is most likely not available in those sizes.
Use #14 cable and get a good night's sleep.
75 degree table; In the CEC the cable connecting to a motor must use the 75 degree temperature ampacity rating.
If this is a completed machine then panel standards rather than wiring standards may apply.
If that is the case, the NEC may be over ruled by other standards.
--------------------
"Why not the best?"
Jimmy Carter

 

[PRS2000]

Check NFPA 79 -12.6 Conductor Sizing and you'll see where 16 or 18 AWG can be used if certain conditions are met.
16 AWG is under 12.6.1.1 and 18 AWG is under 12.6.1.2

 

[PaulKraemer]

Thank you Bill and PRS2000,

I purchased and downloaded NFPA 12.6.1.1 says that16 AWG shall be permitted if part of a jacketed multiconductor cable assembly or flexible cord for motor power circuits supplying a motor having a full-load current rating of 8 amperes or less, provided (a) Circuit is protected in accordance with Chapter 7, (b) Circuit is provided with Class 10 overload protection, and (c) proper overcurrent protection is provided.

In my case, I am good with (a), (b), and (c), and my largest motor has a full-load current of 4.2 amps. The 75 deg C column in the ampacity table lists the ampacity for 16 AWG wire as 10 amps. This makes me think I would most likely be ok if I were to just have a single motor. In actuality, however, I will have four drives side by side in my electrical enclosure, and I will most likely be routing the motor power cables from these through a (panduit) wire duct to the side of the enclosure and then to the machine section where the four motors are located. The full load currents for the four motors are 4.2 + 3.0 + 1.6 + 1.6 = 10.4 amps. If routing these four cables together forces me to derate by 50%, this makes me think that 16 AWG would be undersized. Am I interpreting this correctly?

I am going to go with #14 AWG as Bill suggested. I am just trying to get a better understanding of this stuff. I appreciate both of your help.

Best regards,
Paul

 

[waross]

It sounds as if your research has answered your own question.
When your 10 Amp rated cable is derated 50%, the new rating is 5 Amps.
If you use a cable for each motor you should be good to go with three of the motors, as three of the motors are under the 5 Amp rating of the cable.
The 4.2 Amp motor will require a conductor ampacity of 4.2A x 1.25 = 5.25 Amps. Use #14 AWG for this one.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[waross]

A warning:
If this panel is installed under a wiring permit, and does not have a panel builders certification you may have a problem.
In Canada we can submit a request to have non approved equipment field inspected and approved on a one of basis.
This possibility and the standard that the UHJ will inspect to are matters to discus with the AHJ.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[PaulKraemer]

Thank you Bill,

This panel will be installed as part of a fairly large machine that is about 20 feet long, 5 feet deep, and 8 feet high. It has associated furnace equipment (and a secondary control panel) that will be located on a mezzanine above the machine. Installation involves not only running wires/cables between the control panels and the different sections of the machine, but also running ductwork to support the equipment air handling requirements. This is the third of this type of machine that we will be supplying to this same customer (in New Jersey). On the previous two machines, I was not involved in the process of obtaining whatever permits were required for installation, but I do know that we didn't have any issue with electrical inspections. With this being the case, I don't expect that we will have any issues with this one, but I am curious to find out who was the AHJ (state of New Jersey perhaps) and to what standard they inspected to. I have a pretty good relationship with the customer's engineers, so I will ask them.

I really appreciate your help with this.

best regards,
Paul