What type of wire for 690VDC?

[wireman]

This topic may be belong in the Engineering and Calculations section of the forum but I'll start in the NEC section.

What type of wire should be used in where the voltage on it is 690VDC output from a VFD? Is 600VAC rated insulation good for 690VDC?
This 690VDC comes from the (output bus?) of a 480VAC VFD made by ABB.
Here are the basic details:

This system and has been engineered by others so a single 480VAC VFD feeds not only a 125HP/480VAC motor and also goes to the line side of a VFD that powers a 50HP/480VAC motor.
ABB says the voltage on the wires between the drives is 690VDC. We have to connect the two drives together and I am not sure what kind of wire to order.

The ABB guy is coming in next week and I'm sure we can get some info from him but I'm just trying to better understand things. First time I've run into this type of thing.

The main cabinet is UL listed and rated/designed for this connection to the second VFD so I don't want to turn this into a big discussion about ratings.

Thanks in advance for any responses.

 

[jumper]

As far as voltage is concerned, Table 310.15(B)(16) makes no distinction between VDC and VAC.

 

[Smart $]

As far as voltage is concerned, Table 310.15(B)(16) makes no distinction between VDC and VAC.

Yeah, but it states right in the title its applicable to conductors rated up to and including 2000V.

I think the OP's question here is whether 600V-rated cable is suitable. Considering 600V AC has ~850V peaks, it should be able to handle 690V DC... but not a recommendation for vfd-drive conductors.

 

[petersonra]

The output of the drive can never be more than 480VAC RMS, even though it is made of 680 VDC pulses.

The DC bus however really is 680 VDC, and arguably should use wire rated for that voltage. I will point out that UL508a allows for this wiring to be rated 600V though.

29.1.2 All internal wiring shall have insulation rated for the voltage involved.
Exception: The voltage rating of conductors connected to a dc circuit, such as a dc bus or dc motor
circuits supplied from power conversion equipment, shall be the peak equivalence of the rms voltage
(rated voltage of the conductor multiplied by the square root of 2) marked on the conductor.

 

[wireman]

The output of the drive can never be more than 480VAC RMS, even though it is made of 680 VDC pulses.

The DC bus however really is 680 VDC, and arguably should use wire rated for that voltage. I will point out that UL508a allows for this wiring to be rated 600V though.

I believe your reply begins to address my real question, which is would standard 600V rated MTW or THHN/THWN that we use all the time on 480VAC circuits be acceptable for use on this 690VDC (680?) bus circuit? The factory guy is coming in tomorrow so hopefully he will be able to clear it up.

 

[Carultch]

This topic may be belong in the Engineering and Calculations section of the forum but I'll start in the NEC section.

What type of wire should be used in where the voltage on it is 690VDC output from a VFD? Is 600VAC rated insulation good for 690VDC?
This 690VDC comes from the (output bus?) of a 480VAC VFD made by ABB.
Here are the basic details:

This system and has been engineered by others so a single 480VAC VFD feeds not only a 125HP/480VAC motor and also goes to the line side of a VFD that powers a 50HP/480VAC motor.
ABB says the voltage on the wires between the drives is 690VDC. We have to connect the two drives together and I am not sure what kind of wire to order.

The ABB guy is coming in next week and I'm sure we can get some info from him but I'm just trying to better understand things. First time I've run into this type of thing.

The main cabinet is UL listed and rated/designed for this connection to the second VFD so I don't want to turn this into a big discussion about ratings.

Thanks in advance for any responses.

If you really want to play it safe with a conservative design to avoid rejection, I recommend 1000V PV wire. The main disadvantages would be cost and increased conduit size, but the advantage is that it has a voltage rating in excess of what your system is nominally.
https://www.encorewire.com/wp-content/uploads/EncoreWire-Photovoltaic.pdf

As others have said, standard 600V wire is capable of withstanding the instantaneous peaks of a nominal 600Vac system. And given that nominal AC voltage is the RMS rating, this means for a standard sinusoidal waveform, the peak voltage is 850V.

Is there a difference between how wire insulation performs for 850V instantaneous with an "average" equivalent to 600V, compared to the alternative of a steady DC voltage of 850V? I'm not familiar with how the testing is done, but if you find someone who is, maybe they can provide some additional advice on your selection of cable insulation.

 

[sii]

This system and has been engineered by others so a single 480VAC VFD feeds not only a 125HP/480VAC motor and also goes to the line side of a VFD that powers a 50HP/480VAC motor.

Is this the way you intended to type this? You cannot feed one vfd with another. Did you mean one BREAKER feeds two motors?

 

[wireman]

Is this the way you intended to type this? You cannot feed one vfd with another. Did you mean one BREAKER feeds two motors?

Yes, is the simple answer. Wires are attached to the DC bus of VFD #1 and feed terminals on VFD #2 that are NOT the 480VAC line side terminals. I don't have the one-line diagram in front of me to tell you the exact internal component on VFD #2 that it attaches to but that's how it works.

I may not be able to explain it clearly but the idea is if VFD #1 loses power they don't want VFD #2 to stop immediately. By feeding VFD #2 with this DC bus voltage it somehow allows its motor to run briefly. Supposedly this is a standard configuration for this centrifuge system.

 

[jim dungar]

You cannot feed one vfd with another.

It sounds like the OP is describing a set of VFDs that share a common DC bus. This type 'tying together' is very common on machines with multiple interrelated motors especially ones with large braking needs. These are often called sectional drives, they are common in the paper and printing industry. Having the DC busses tied together allows the energy from one decelerating motor to be used by a different motor rather than getting dumped into a resistor or back into the power grid.

 

[sii]

It sounds like the OP is describing a set of VFDs that share a common DC bus. This type 'tying together' is very common on machines with multiple interrelated motors especially ones with large braking needs. These are often called sectional drives, they are common in the paper and printing industry. Having the DC busses tied together allows the energy from one decelerating motor to be used by a different motor rather than getting dumped into a resistor or back into the power grid.

Oh I see. Never heard of that before. Thanks.

 

[MasterTheNEC]

If you really want to play it safe with a conservative design to avoid rejection, I recommend 1000V PV wire. The main disadvantages would be cost and increased conduit size, but the advantage is that it has a voltage rating in excess of what your system is nominally.
https://www.encorewire.com/wp-content/uploads/EncoreWire-Photovoltaic.pdf

As others have said, standard 600V wire is capable of withstanding the instantaneous peaks of a nominal 600Vac system. And given that nominal AC voltage is the RMS rating, this means for a standard sinusoidal waveform, the peak voltage is 850V.

Is there a difference between how wire insulation performs for 850V instantaneous with an "average" equivalent to 600V, compared to the alternative of a steady DC voltage of 850V? I'm not familiar with how the testing is done, but if you find someone who is, maybe they can provide some additional advice on your selection of cable insulation.

The portion of the installation that will be exposed to over 600V (regardless of peak to peak or RMS) would need to use a wire that has been evaluated under UL 83 or 44. If the installation of the VFD portion does exceed those values then my advice would be to look for RHH/RHW-2 that has been evaluated for 2000V since 1000V is not within those standards. Now, the use of PV Wire is fine as well as in most cases it has multiple ratings...but as it stands now their is no Thermoplastic that can handle 1000V and of the thermoset type insulation the only variety currently evaluated for 1000V is...None.....they can be evaluated for 2000V in some Thermoset (which the PV Wire is) which means they clearly are acceptable for a 1000V application. However, manufacturers of Wire and Cable are still working with UL to develop guidance on getting thermosets (and possibly Thermoplastics with a thicker insulation, not conforming to any NEC Tables) investigated and listed specifically to meet the 1000V.

Hope this helps.....but as of the PEAK TO PEAK.....Exceeding the 600V in that aspect is acceptable under the short duration of the events that result in such current.

 

[don_resqcapt19]

When I had to hook up some 1500 hp DC motors with 700 volt armatures, I used DLO/RHH conductors. That solved the voltage issue and made the terminations so much easier because of the flexibility of the DLO cable.

 

[GoldDigger]

When I had to hook up some 1500 hp DC motors with 700 volt armatures, I used DLO/RHH conductors. That solved the voltage issue and made the terminations so much easier because of the flexibility of the DLO cable.

Out of curiosity, how did you end up terminating the extra fine stranding of the DLO?

 

[Carultch]

The portion of the installation that will be exposed to over 600V (regardless of peak to peak or RMS) would need to use a wire that has been evaluated under UL 83 or 44. If the installation of the VFD portion does exceed those values then my advice would be to look for RHH/RHW-2 that has been evaluated for 2000V since 1000V is not within those standards. Now, the use of PV Wire is fine as well as in most cases it has multiple ratings...but as it stands now their is no Thermoplastic that can handle 1000V and of the thermoset type insulation the only variety currently evaluated for 1000V is...None.....they can be evaluated for 2000V in some Thermoset (which the PV Wire is) which means they clearly are acceptable for a 1000V application. However, manufacturers of Wire and Cable are still working with UL to develop guidance on getting thermosets (and possibly Thermoplastics with a thicker insulation, not conforming to any NEC Tables) investigated and listed specifically to meet the 1000V.

Hope this helps.....but as of the PEAK TO PEAK.....Exceeding the 600V in that aspect is acceptable under the short duration of the events that result in such current.

I think the main issue I was trying to get at, is whether or not you are able to EXTRAPOLATE the peak-to-peak instantaneous voltage that the wire is inherently rated to withstand, when the nominal voltage rating is applied for AC systems, and apply them to DC applications. It doesn't make sense to me that you would be able to take credit for multiplying a component's voltage rating by sqrt(2), when applying it to DC.

I have lightning arresters that were sold as 1000V rated, and both 1000V and 600V are indicated on the print legend. The wire seems to have THWN markings as well, and resembles THWN-2 a lot more than any other kind of wire. Could this be a UL vs IEC situation, where UL has stricter standards than IEC?

And perhaps because it is part of a UL listed manufactured product, listed for 1000V, it is OK that it is built with wire rated at 1000V IEC?
I've seen PV modules in a similar situation, where the product is listed for 1000V, but uses 600V wire.

 

[don_resqcapt19]

Out of curiosity, how did you end up terminating the extra fine stranding of the DLO?

Same way that I would have terminated standard stranded wire....compression lugs. I used the ones that are specifically listed for fine stranding on those motors.