Insulation on MTW or THHN/THWN Wires
- Thread starter fifty60
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jumper
Senior Member
- Location
- 3 Hr 2 Min from Winged Horses
Both. It the maximum in either case.
- Location
- Chapel Hill, NC
- Occupation
- Retired Electrical Contractor
The conductor alone can handle 600V so theoretically if you had a 1200v single phase load then each conductor could carry 600V and you would be fine.
- Location
- Illinois
- Occupation
- retired electrician
Would that mean that I could use a 300 volt rated conductor on a 480Y/277 volt system?
- Location
- Placerville, CA, USA
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- Retired PV System Designer
Except when the single wire insulation is required by code to be rated for the maximum line to line voltage, as in 600V and under.
- Location
- Chapel Hill, NC
- Occupation
- Retired Electrical Contractor
That is what I think. Each conductor is only seeing 277 volts so why not if it is rated 300 volts. It doesn't say rated for a 300V system. Maybe I am wrong but that is what I always thought.
big john
Senior Member
- Location
- Portland, ME
I don't disagree that it should technically work, but it would be violation of 110.4.
fmtjfw
Senior Member
- Location
- Fairmont, WV, USA
It would seem to me that the voltage rating of conductor insulation is the amount of voltage difference it can withstand between the outside o the insulation and the the voltage on the conductor.
But that would mean 300V insulated cable could been used on grounded 277Y/480, which sound a little odd.
But that would mean 300V insulated cable could been used on grounded 277Y/480, which sound a little odd.
- Location
- Chapel Hill, NC
- Occupation
- Retired Electrical Contractor
So I could use it for 277 but not 480? That seems odd to me. If the insulation is rated 300V then it should be able to handle 480V Obviously something else I can't understand.
electricman2
Senior Member
- Location
- North Carolina
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- Retired Electrical Contractor
I believe it means conductors with 600 volt insulation cannot be used where the voltage between conductors or between any conductor and ground exceeds 600 volts
hurk27
Senior Member
- Location
- Portage, Indiana NEC: 2008
I think the problem would be is that before even leaving the main panel the conductor would subject to other conductors on other phases that would subject the conductor to a higher voltage then it is rated for, so it would have to have a rating of the highest voltage it would be subject to which would be the line to line voltage.
fmtjfw
Senior Member
- Location
- Fairmont, WV, USA
Aha! I don't think the problem would be in contact with other phase insulated conductors, rather in contact with other uninsulated conductors of other phases. such as terminations or busbars.
big john
Senior Member
- Location
- Portland, ME
No, I think we agree that it works technically speaking: It should withstand the phase-to-ground voltage because that is only 277V. And when you have two insulated conductors, their insulation value sums, so they should be able to withstand 480V between phase-to-phase.So I could use it for 277 but not 480? That seems odd to me. If the insulation is rated 300V then it should be able to handle 480V Obviously something else I can't understand.
That said: 110.4 says
So the installed equipment must match the voltage of the circuit which is defined as:
Between the two of these this reads to me that any equipment must be rated for the phase-to-phase voltage. 300V wiring on a 480V system would not meet that criteria even if I believe it would work.
PetrosA
Senior Member
- Location
- Lancaster, Pennsylvania
I may be wrong but this is how I understand it:
If you had a car that claimed it could protect the driver in collisions up to 50 MPH, it would be rational to assume that you would be OK hitting a brick wall at 50 MPH but it would NOT be rational to assume you would be safe if you had a head on collision with another vehicle and both vehicles were traveling at 50 MPH. That would be the equivalent of hitting a brick wall at 100 MPH.
Aside from the voltage you see on each conductor, you also have the phases to consider which is why you can't use 300V rated cable on 480V circuits. It's the same effect as the head-on collision.
If you had a car that claimed it could protect the driver in collisions up to 50 MPH, it would be rational to assume that you would be OK hitting a brick wall at 50 MPH but it would NOT be rational to assume you would be safe if you had a head on collision with another vehicle and both vehicles were traveling at 50 MPH. That would be the equivalent of hitting a brick wall at 100 MPH.
Aside from the voltage you see on each conductor, you also have the phases to consider which is why you can't use 300V rated cable on 480V circuits. It's the same effect as the head-on collision.
- Location
- Placerville, CA, USA
- Occupation
- Retired PV System Designer
You are wrong.
A car going 50 mph hitting a stationary car of equal weight and crush characteristics is like hitting a brick wall at 25 mph.
Two identical cars hitting head on at 50 mph each is like two cars each hitting a brick wall at 50 mph.
Imagine the two cars hitting opposite sides of the same brick wall. Now remove the wall and nothing changes.
PetrosA
Senior Member
- Location
- Lancaster, Pennsylvania
- Location
- Chapel Hill, NC
- Occupation
- Retired Electrical Contractor
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
- Location
- Mission Viejo, CA
- Occupation
- Professional Electrical Engineer
I don't know why I stumbled on this thread and I'm slightly surprised the NEMA WC 70 standard hasn't been referenced. The old ICEA S-95-658 standard was clearer when it was independent of NEMA but its remnants are still reflected in several NEMA WC 70 tables; e.g., Table 3.1.
Essentially the voltage rating of non-shielded cables/conductors rated 2000V or less is the phase to phase voltage.
The old IPCEA/ICEA standards said it was specifically the phase to phase voltage between conductors of a three phase system. Ground and/or neutral didn't enter the discussion directly.
Essentially the voltage rating of non-shielded cables/conductors rated 2000V or less is the phase to phase voltage.
The old IPCEA/ICEA standards said it was specifically the phase to phase voltage between conductors of a three phase system. Ground and/or neutral didn't enter the discussion directly.
- Location
- Placerville, CA, USA
- Occupation
- Retired PV System Designer
It is not impossible, but it would be a very odd system in which the line to ground voltage is greater than the line to line voltage, so we are OK on that.
There are some places in NEC where line to ground voltage is important, and for those purposes the line to ground voltage in an ungrounded system is considered to be equal to the maximum line to line voltage.
There are some places in NEC where line to ground voltage is important, and for those purposes the line to ground voltage in an ungrounded system is considered to be equal to the maximum line to line voltage.
- Location
- Chapel Hill, NC
- Occupation
- Retired Electrical Contractor
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.
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