300V conductors in 480V system

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wallydog85

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I followed a thread which unfortunately was closed before I could comment.
The discussion was on 300V wire in a 480V contactor.
A senior member wrote;

"Section 300.3(C)(1) makes it clear that the maximum circuit voltage in the raceway, not the maximum insulation voltage rating of the conductors in the raceway, is what determines the minimum voltage rating required for the insulation of conductors for systems of 600 volts or less.
The conductors of a 3-phase, 4-wire, 208Y/120-volt ac circuit; a 3-phase, 4-wire, 480Y/277-volt ac circuit; and a 3-wire, 120/240-volt dc circuit may occupy the same equipment wiring enclosure, cable, or raceway if all the conductors are insulated for the maximum circuit voltage of any conductor. In that case, the maximum circuit voltage would be 480 volts, and 600-volt insulation would be suitable for all the conductors.
If a 2-wire, 120-volt circuit is included in the same raceway with a 3-wire, 120/240-volt circuit having 600-volt conductors, the 2-wire, 120-volt circuit conductors could use 300-volt insulation because the maximum circuit voltage is only 240 volts.

Raceway. An enclosed channel of metal or nonmetallic materials designed expressly for holding wires, cables, or busbars, with additional functions as permitted in this Code. Raceways include, but are not limited to, rigid metal conduit, rigid nonmetallic conduit, intermediate metal conduit, liquidtight flexible conduit, flexible metallic tubing, flexible metal conduit, electrical nonmetallic tubing, electrical metallic tubing, underfloor raceways, cellular concrete floor raceways, cellular metal floor raceways, surface raceways, wireways, and busways.

Though it is not precisely defined I've considered J boxes, pull boxes, gutters, & panels part of the raceway system."

Everyone seems to miss the important wording in NEC 300.3(C)(1):
All conductors shall have an insulation rating equal to at least the maximum circuit voltage APPLIED TO ANY CONDUCTOR within the enclosure, cable, or raceway.

Yes there is 480V present within the enclosure, however each conductor is supplying 277V.
At no time would you have more then 277V on any one conductor unless there is a problem where the neutral and/or ground reference point have been compromised in which case you will have bigger problems to contend with anyway.
So the way I interpret this code section, it IS permissible to have 300V rated wire in a 480V system, enclosure, multi-conductor cable, or raceway.

Discussion please???
 
petersonra said:
I think you are missing that while there is 277V to ground, there is 480V line to line.
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In the standards for making and listing wire, the insulation voltage potential on the wire is defined as the maximum between phases. If the phase to ground is lower, that's fine, but irrelevant. I don't have my NEC with me, but look at the definitions section in the first part of the book.

This has been debated many times, the interpretation always ends up the same. If there is any voltage present that exceeds 300V, regardless of how it is measured, the insulation must be rated 600V.
 
Jraef said:
In the standards for making and listing wire, the insulation voltage potential on the wire is defined as the maximum between phases. If the phase to ground is lower, that's fine, but irrelevant. I don't have my NEC with me, but look at the definitions section in the first part of the book.

This has been debated many times, the interpretation always ends up the same. If there is any voltage present that exceeds 300V, regardless of how it is measured, the insulation must be rated 60oV.
Click to expand...

How about the DC bus voltage on a VFD? On a 480V drive it exceeds 600V.

UL508a provides an exception to allow 600V insulation conductors for this but I don't see it in the NEC anywhere.
 
If insulation is rated 600V RMS AC, which is a sine wave that peaks at 850V, does that automatically mean it can handle 850V DC?

Cheers, Wayne
 
Jraef said:
In the standards for making and listing wire, the insulation voltage potential on the wire is defined as the maximum between phases. If the phase to ground is lower, that's fine, but irrelevant. I don't have my NEC with me, but look at the definitions section in the first part of the book.

This has been debated many times, the interpretation always ends up the same. If there is any voltage present that exceeds 300V, regardless of how it is measured, the insulation must be rated 600V.
Click to expand...

I think thats a hold over from the old ungrounded delta days where historically most industrial systems automatically had to consider a phase to phase rating in relation to ground.
 
From the NEC article 100 definitions:

Voltage (of a circuit). The greatest root-mean-square (rms) (effective) difference of potential between any two conductors of the circuit concerned.
Informational Note: Some systems, such as 3-phase 4-wire, single-phase 3-wire, and 3-wire direct current, may have various circuits of various voltages.

Voltage, Nominal. A nominal value assigned to a circuit or system for the purpose of conveniently designating its voltage class (e.g., 120/240 volts, 480Y/277 volts, 600 volts).
Informational Note No. 1: The actual voltage at which a circuit operates can vary from the nominal within a range that permits satisfactory operation of equipment.
Informational Note No. 2: See ANSI C84.1-2006, Voltage Ratings for Electric Power Systems and Equipment (60 Hz).

Voltage to Ground. For grounded circuits, the voltage between the given conductor and that point or conductor of the circuit that is grounded; for ungrounded circuits, the greatest voltage between the given conductor and any other conductor of the circuit.
Click to expand...
 
wwhitney said:
If insulation is rated 600V RMS AC, which is a sine wave that peaks at 850V, does that automatically mean it can handle 850V DC?
Click to expand...
So, I briefly looked up part of NEMA WC 70-2009 "Power Cables Rated 2000 Volts Or Less For The Distribution Of Electrical Energy". It is a document that I'm not familiar with, so perhaps I am reading it wrong. Section 3.3 on "Insulation Thickness" contains the following subsection:

NEMA WC 70-2009 Section 3.3.1.4 said:
Cables for use on direct current systems, up to and including 2000 volts, use the same insulation thickness as for three-phase ac systems, in accordance with 3.3.1.1.
Click to expand...

The insulation thickness tables referenced in 3.3 only list two different voltage ranges, 0-600 and 601-2000. So it would appear that a cable rated 600VAC (RMS) is only rated for 600VDC.

Cheers, Wayne
 
wwhitney said:
So, I briefly looked up part of NEMA WC 70-2009 "Power Cables Rated 2000 Volts Or Less For The Distribution Of Electrical Energy". It is a document that I'm not familiar with, so perhaps I am reading it wrong. Section 3.3 on "Insulation Thickness" contains the following subsection:



The insulation thickness tables referenced in 3.3 only list two different voltage ranges, 0-600 and 601-2000. So it would appear that a cable rated 600VAC (RMS) is only rated for 600VDC.

Cheers, Wayne
Click to expand...
Well, I just read somewhere else that the insulation thickness has to to with the dielectric strength, which is calculated based on the PEAK voltage for AC, not the RMS. they also spoke of the heating effects on the insulation, and that the heating effects of DC is the same as that of the RMS AC voltage. They went on to say that the DC voltage rating of wire insulation rated for an AC RMS voltage will be 1.41x higher, and specifically stated that 600V wire insulation is good for 846VDC. So it looks like the debate continues.

I've been taking a safer route lately, in that on anything having to do with the output of a VFD, I've been using and suggesting RHW or XHHW insulation rated 1000V. In most that I've priced, the difference between the 1000V rated and the 600V rated is not enough to worry about, and in fact a lot of suppliers don't even bother with the 600V rated version. I use that because there are other issues with using THHN on the load side of VFDs, mainly due to voids (bubbles) in the PVC, cold flow under compression and the hygroscopic nature of PVC, all leading to potential long term problems with corona discharge effects. I started that after pulling some cables out of an installation that had been in place 6 years and failed. When we laid them out on the ground, you could see the burn marks where the standing waves punched through the insulation. In fact the wave length was evident by the uniform distances between the burn marks repeated the entire length of the run. I read up on the issue and found that this is not unknown, albeit not terribly common. But since then I've noticed it popping up in trade journals and in many of the VFD installation manuals now.
 
Jraef said:
Well, I just read somewhere else that the insulation thickness has to to with the dielectric strength, which is calculated based on the PEAK voltage for AC, not the RMS. they also spoke of the heating effects on the insulation, and that the heating effects of DC is the same as that of the RMS AC voltage. They went on to say that the DC voltage rating of wire insulation rated for an AC RMS voltage will be 1.41x higher, and specifically stated that 600V wire insulation is good for 846VDC. So it looks like the debate continues.

I've been taking a safer route lately, in that on anything having to do with the output of a VFD, I've been using and suggesting RHW or XHHW insulation rated 1000V. In most that I've priced, the difference between the 1000V rated and the 600V rated is not enough to worry about, and in fact a lot of suppliers don't even bother with the 600V rated version. I use that because there are other issues with using THHN on the load side of VFDs, mainly due to voids (bubbles) in the PVC, cold flow under compression and the hygroscopic nature of PVC, all leading to potential long term problems with corona discharge effects. I started that after pulling some cables out of an installation that had been in place 6 years and failed. When we laid them out on the ground, you could see the burn marks where the standing waves punched through the insulation. In fact the wave length was evident by the uniform distances between the burn marks repeated the entire length of the run. I read up on the issue and found that this is not unknown, albeit not terribly common. But since then I've noticed it popping up in trade journals and in many of the VFD installation manuals now.
Click to expand...

Just out of curiosity, what was the overall length of the failed conductors? Were they long enough to raise red flags about standing waves in the original design?
 
GoldDigger said:
Just out of curiosity, what was the overall length of the failed conductors? Were they long enough to raise red flags about standing waves in the original design?
Click to expand...
Yes, a little over 150ft drive to motor, 3 x 250HP motors and VFDs, no filters or even reactors on the outputs. The drives kept tripping on Over Current (not Over Load) so the user kept replacing the DRIVES (3 times!) and blaming them before I got called in to see why "the drives kept failing". Turned out, of course, that they were just doing their job. They didn't like hearing it, because it meant the warranty claims all became invalid. But we replaced the conductors with XHHW and poof!, the "drives fixed themselves".
 
Utility wants to drop support for our 230 ungrounded delta

Utility wants to drop support for our 230 ungrounded delta

We have a plant in the northwest that has three 500 KVA "pole" type transformers tied together to form a 1500 KVA ungrounded delta system. The load side is 240 volt. The utility is telling us that they don't want to support our 240 Vac system anymore with transformers and we need to find a solution (IMO to their problem) as in convert to a 480 vac. Between the VFDs, re-wire motors and control power transformers etc., it would cost us a small fortune. The plant was built in the 1950s.

One item that is not often considered is the motor wire. Most of the newer wire (80's and younger) is probably 600V but I don't know that nor do I know how to find out. I realize that newer wire is identified on the wire but the older stuff- I don't know.

PS The utility says the transformers that form a ungrounded delta are different that ones that form a grounded. I thought that grounding was all external to the transformer and that no extra lugs were required to convert between the two? Does anybody know?
 
Dan Kissel said:
We have a plant in the northwest that has three 500 KVA "pole" type transformers tied together to form a 1500 KVA ungrounded delta system. The load side is 240 volt. The utility is telling us that they don't want to support our 240 Vac system anymore with transformers and we need to find a solution (IMO to their problem) as in convert to a 480 vac. Between the VFDs, re-wire motors and control power transformers etc., it would cost us a small fortune. The plant was built in the 1950s.

One item that is not often considered is the motor wire. Most of the newer wire (80's and younger) is probably 600V but I don't know that nor do I know how to find out. I realize that newer wire is identified on the wire but the older stuff- I don't know.

PS The utility says the transformers that form a ungrounded delta are different that ones that form a grounded. I thought that grounding was all external to the transformer and that no extra lugs were required to convert between the two? Does anybody know?
Click to expand...

It's a guess, but it might be what they are getting at is that the only MV to 240V transformers THEY are going to stock any more are the ones used for residential single phase service, meaning those with a center tap. Technically you COULD still connect 3 of those as an ungrounded delta, but they are generally smaller transformers and they may not have anything that large. In other words, they are looking to standardize and simplify their system to keep costs down by paring down their inventory of transformers. That's not uncommon in the utility world, and you can't blame them. In order for them to be able to justify carrying a lot of inventory for emergency replacement, there must be some "critical mass" of installations, otherwise it just sits there tying up money. 1500kVA 240V delta is likely now so rare, they can't support you any more. Think about it; if your transformer blows and has to be replaced, but they don't have one in stock, are you going to be willing to wait 26 weeks for it to be made? No, you are not, plus they have PUC rules saying they have to PAY whatever it takes to get you back on line. So the only control they have over this is to set the requirements YOU must adhere to.

By the way, there is nothing stopping you from taking their 480V service and buying your own 240V delta transformer bank, rather than change everything down stream. Or better yet, buy "bulk" power at whatever their MV transmission line is and install your own service transformers. Your rate will be significantly lower if you didn't know that, but of course you now add the cost of maintaining your own service equipment. Lots of industrisls do that though. You may be able to pick up a "bargain price" used transformer package from a local utility who is looking to get out of the 240V delta service business!
 
Jraef said:
It's a guess, but it might be what they are getting at is that the only MV to 240V transformers THEY are going to stock any more are the ones used for residential single phase service, meaning those with a center tap. Technically you COULD still connect 3 of those as an ungrounded delta, but they are generally smaller transformers and they may not have anything that large. In other words, they are looking to standardize and simplify their system to keep costs down by paring down their inventory of transformers. That's not uncommon in the utility world, and you can't blame them. In order for them to be able to justify carrying a lot of inventory for emergency replacement, there must be some "critical mass" of installations, otherwise it just sits there tying up money. 1500kVA 240V delta is likely now so rare, they can't support you any more. Think about it; if your transformer blows and has to be replaced, but they don't have one in stock, are you going to be willing to wait 26 weeks for it to be made? No, you are not, plus they have PUC rules saying they have to PAY whatever it takes to get you back on line. So the only control they have over this is to set the requirements YOU must adhere to.

By the way, there is nothing stopping you from taking their 480V service and buying your own 240V delta transformer bank, rather than change everything down stream. Or better yet, buy "bulk" power at whatever their MV transmission line is and install your own service transformers. Your rate will be significantly lower if you didn't know that, but of course you now add the cost of maintaining your own service equipment. Lots of industrisls do that though. You may be able to pick up a "bargain price" used transformer package from a local utility who is looking to get out of the 240V delta service business!
Click to expand...

I'd say its not even that, as they would be stock 150kva 240 volt units had they not axed delta prior. Most POCOs want to ditch anything primary delta or wye ungrounded due to ferrorsonance. Grounded wye delta solves this problem, but it will result nuisance tripping of the transformer protection devices any time a short circuit occurs out on the line.


I do agree though, either buy power in bulk from a sub-transmission or transmission line (which will result in high reliability in most cases) or have the POCO set down a classic service and then step it down to delta.
 
mbrooke said:
I'd say its not even that, as they would be stock 150kva 240 volt units had they not axed delta prior....
Click to expand...
I think you dropped a digit, it's 1500kVA, not 150...

Didn't think about the ferroresonance issue though. I've only associated that with being an issue with URD systems and since I haven't done residential distribution, I paid little notice. But per your comment I did some more snooping and found some interesting stuff. Thanks for that. And yes, that makes more sense as to why the utility would want to get rid of it.
 
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