Insulation on MTW or THHN/THWN Wires

[GoldDigger]

I thought what rbalex was talking about was whether a 277 volt circuit would be okay-- that wasn't discussed-- I took it that way. Anyway I am glad this is resolved.

I did try and look up in the white book but had no luck. I don't have the nema stuff and never used it.

In a 480Y/277 circuit, rbalex's point tells us that 300V insulation would not be OK for a three or four wire circuit, or even a two wire line to line circuit. And I think it would be interpreted to prohibit use of 300V wire even for a derived two wire line to neutral circuit

 

[Dennis Alwon]

In a 480Y/277 circuit, rbalex's point tells us that 300V insulation would not be OK for a three or four wire circuit, or even a two wire line to line circuit. And I think it would be interpreted to prohibit use of 300V wire even for a derived two wire line to neutral circuit

I agree but I thought he stated that they did not discuss that.

 

[GoldDigger]

I agree but I thought he stated that they did not discuss that.

They did not refer to neutral or ground in a three phase system, which indirectly tells us that the insulation used for any combination of wires from a four wire 480Y/277 system would have to be rated for 480 or greater. And that tells us that the insulation just exceeding the line to ground voltage is not good enough.

 

[Dennis Alwon]

Yes, I understand that part... So a 300 v wire would be good for a - lets say 300/150v system (probably doesn't exist but just for argument sake) where the line to line is 300V but not good for 277V to ground. I guess I don't get the why.

 

[rbalex]

I thought what rbalex was talking about was whether a 277 volt circuit would be okay-- that wasn't discussed-- I took it that way. Anyway I am glad this is resolved.

I did try and look up in the white book but had no luck. I don't have the nema stuff and never used it.

Actually, I was just responding to the OP:

If my wire is rated for 600V, would this be considered its line to neutral rating? Or would it be the line to line rating?

Neither the old nor current insulated conductor standards mention a "line to ground" or a "line to neutral" voltage rating. This is unlike many NEMA 600V equipment standards.

GoldDigger has been doing a fine job reviewing several other conductor and voltage system issues.

While the actual insulated conductors are essentially the same between old and new standards, the current NEMA standards generally emphasize the manufacturing and material requirements rather than the test standards although they still exist too.

What isn't immediately obvious is that, for most 600V building wires, insulation thickness is actually a mechanical rather than dielectric issue. This isn't usually a problem; however, for use on ungrounded or high impedance grounded systems where the insulation thickness is 20 mils or less, the insulation will be overstressed under an uncleared ground fault. This is why I don't recommend THHN/THWN smaller than #8 for those applications.

 

[rbalex]

I should have said, "... for use on 480V, 3-phase ungrounded or high impedance grounded systems ..."

 

[hurk27]

Yes, I understand that part... So a 300 v wire would be good for a - lets say 300/150v system (probably doesn't exist but just for argument sake) where the line to line is 300V but not good for 277V to ground. I guess I don't get the why.

I would guess that even though it is only connected to one phase and the neutral it will be in very close proximity to other phase conductors in the panel and or in race ways leaving the panel, so you could most likely have two conductors touching each other with 480 volts between them, in the case of an 480v ungrounded system if a phase faults to ground the conductor would be subject to the higher voltage just to ground, I have also seen installations where insulated conductors cross over bussbars and sometimes make contact with them so it kind of makes sense to require the conductor rating to equal or exceed the line to line rating or the max voltage it will be subject to between any two conductors.

Heres another example, have a multi-gain box with switches switching 277 volt loads each on a different phase, it would not be too uncommon to see one of the conductors up against one of the screws of an adjacent switch except that the code would require dividers between them.

 

[Dennis Alwon]

So in reality a 300v conductor is basically rated 300V but it would never seen anywhere near it unless you have a phase to phase system that is at 300V . Let me be clear I totally get it but it seems like a very odd way to rate wire. If the wire only sees 300V on one phase it should work regardless of the other conductor otherwise rate it differently.

Some have mentioned the other wires next to it. So If I install 300V wire, of course I never would since 600V is pretty standard, then what happens if there is a 480v pair with 600V insulation in the same conduit as a 240V @ 300V or say a higher voltage then 240 but less than 300V. Perhaps a rhetorical question but you see where I am going.

The bigger question is why I am up at 2:50 am typing this instead of sleeping?

 

[MasterTheNEC]

I am not convinced but I do believe that may be the intent. I have an email in to dave mercier from southwire. Let's see if he responds to this

hmmm....lol

 

[MasterTheNEC]

Actually you might be interested to know that for the testing that takes place in UL44 and 83 you would see the following:

5.6.3.1 The 90?C rated insulation of the individual conductors without covering shall have insulation
resistance in air at 97 ?1?C that is not less than indicated in Table 24 at any time during an extended
period in an acceptable full-draft circulating-air oven under the following conditions. The period in the oven
shall be 12 weeks or more, to a maximum of 36 weeks, if the insulation resistance throughout the last 6
weeks of the period is 3 GW?m (10 MW?1000 ft) or higher. If the insulation resistance is less than 3 GW?m
(10 MW?1000 ft), but equals or exceeds the values in Table 24, the period in the oven shall be 24 weeks
or more to a maximum of 36 weeks. A 600 V rms voltage shall be applied at all times other than while
reading the insulation resistance.


But long story short, all manufacturers of wire and cable mark their products for specific uses (power limited or not) in accordance with the UL Marking and Application Guide which explains which products must have 300V or 600V or BOTH ratings or even higher ratings for MV and so on.

For example THHN/THWN-2 you would be hard pressed to find anything in 300V so the question is really moot. I encourage a search online for the UL Marking and Application Guide that is used by wire and cable manufacturers to ensure compliance to the marking requirements. The other factor is that most of the wiring and cable methods that permit 300V ratings are for the majority power limited in some nature.

Hope that was helpful. Honestly, get the free guide...or wait I will post it (until it is removed)

http://ul.com/global/documents/offe...ctrical/newsletters/W&CMG_April2007_Final.pdf