Cable Insulation Requirements

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ashelton

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I am presently designing the installation of a transformer with a 600VAC, three-phase, wye secondary. Can I use 600V rated cable, or am I required to increase the rating to a minimum of 125% of the system voltage (i.e. 480V systems use 600V rated cable).? Please provide reason for any answer.
 
As far as I know, if your voltage is 600V or less phase to phase, 600V rated insulation is the min that is required. I do not see anything that tells us differently in NEC.
 
So, why is the insulation rating for cable (600V) 125% of the system voltage (480V)? Is this just a coincidence? Will there be potential problems if the voltage fluctuates +/- 5%?
 
ashelton said:
So, why is the insulation rating for cable (600V) 125% of the system voltage (480V)? Is this just a coincidence?
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It is a coincidence. 600V insulation allows the same conductors to be sold in the US and Canada.
Also it is a breakpoint for UL listed equipment (notice how most insulation is listed as 300V or 600V).
 
All equipment voltage ratings are RMS unless otherwise specified.

Unless you're running a grounded ∆ you're not putting full system voltage on each conductor, anyway: Low voltage wire rating isn't like MV cable, if it says 600V it means it will withstand that full value to ground.

As far as how much official wiggle room their is, I don't know. I asked this same question when I first began working on 600V systems because it was common to find them running 5%+ over the voltage rating of the wire.
 
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Precision and definition

Precision and definition

For 2017 I put in a PC (Public Comment) that was pretty massive. I suggested that voltage numbers in the Code be identified as "actual" or defaulted to nominal. I thought a standard ought to say that 600VAC rating for wire was nominal, based on its usability in a 600V rated circuit. I also suggested that 50VAC as the limit under which you could use white as a non-grounded conductor was actual (50V meant 50V, not sort of 50V). There are not quite a zillion places voltage is referenced in Code.

I don't know how that went over. I've not bothered to look since the initial submissions, not liking the "improved" submission method.
 
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big john said:
...if it says 600V it means it will withstand that full value to ground.
...
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Not true. Wire rated 600V is rated for up to 600V Line to Line on AC or DC systems. You cannot use it on a system that has a nominal line to line voltage over 600V, such as on a 600V Line to ground system and having more than one line of different phase or polarity at 600V to ground.

That statement is TTBOMK without actually verifying, so someone please correct me if that is in error. :angel:
 
TTBOMK your interpretation is wrong. But I have not found a relevant code citation either.
The closest that I have come is that in a single phase center tapped system or three phase wye system *operated ungrounded* the nominal line to ground voltage, for insulation purposes, is taken to be the maximum line to line voltage present.
 
Smart $ said:
Not true. Wire rated 600V is rated for up to 600V Line to Line on AC or DC systems.
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I think you're mistaken. It must be able to withstand the full line voltage of the system it is used on. For example, for a THHN conductor, per UL 83 it must pass insulation-resistance tests after being immersed in water with 600VRMS applied to the conductor for the duration of the test.
You cannot use it on a system that has a nominal line to line voltage over 600V
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I never said you could.
Such as on a 600V Line to ground system and having more than one line of different phase or polarity at 600V to ground.
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Eh? On a 600V grounded delta, all nominal voltages are 600V or less.
 
big john said:
I think you're mistaken. It must be able to withstand the full line voltage of the system it is used on. For example, for a THHN conductor, per UL 83 it must pass insulation-resistance tests after being immersed in water with 600VRMS applied to the conductor for the duration of the test. I never said you could. Eh? On a 600V grounded delta, all nominal voltages are 600V or less.
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We're probably on the same page using different words to express the same concept.

When you say a conductor rated 600V is rated for 600V line to ground, I automatically assume the worst case scenario in that 600V line to ground can be associated with 1039Y/600V 3Ø 4W system. Yes, it is definitely an uncommon configuration... but so is a 600V 3Ø 3W Grounded-B Phase system. Perhaps more uncommon, but in the big picture both are quite uncommon, at least on U.S. soil.

Simply put, you cannot use 600V wire on any system which has greater than 600V potential across any two system circuit conductors.
 
ashelton said:
So, why is the insulation rating for cable (600V) 125% of the system voltage (480V)? Is this just a coincidence? Will there be potential problems if the voltage fluctuates +/- 5%?
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Most "building wire" for 600V and below usually utilizes 600 volt conductors. (class 2 conductors/cables is an exception).

NM cable - which the majority of what is out there is used for 120 volt circuits, yet it is listed as a 600 volt assembly, don't even know if they make NM cable in other then 600 volt versions.

Amount of 300 volt conductors/cables you do find is fairly limited, almost makes you wonder why they even make a 300 volt conductor.
 
ron said:
In Canada, where the nominal voltage is 600V, they use standard building wire rated at 600V
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I am agree with ron, it would be sufficient for you as i know. You just need to choose one of good company product which will solve your rest of the problem too.
 
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