PV wire, USE-2, what's the diff?

ggunn
ggunn Solar Expert Posts: 1,973 ✭✭✭
in PV Installers Forum - NEC, Wiring, Installation #1
I once thought (erroneously, it now appears) that PV wire was rated to 1000V while USE-2 was 600V wire. Per the 2011 NEC, ungrounded systems require the use of PV wire and not USE-2 for exposed wiring. Why is that?
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  • solarix
    solarix Solar Expert Posts: 713 ✭✭
    #2
    Re: PV wire, USE-2, what's the diff?

    I've been trying to figure this out for some time as well. I can tell you that when you order "PV wire" from different vendors, you can get different things. My understanding is that the correct PV wire is double insulated, that is there are two layers of insulation in order to meet the UL-4703 requirements for ungrounded systems. That is not to say it is rated for 1000Vdc. Personally, I find that the plastic used in these two layers is not as tough as the USE-2 stuff. Also, good luck finding it in colors other than black. You can get white, which of course is not to be used in ungrounded systems though. I'm still looking for red, however the story I've heard is that the red fades badly in the sun (turns to white) so what's the use. Just buy black and tape the ends red I'm told. USE wire is originally intended for Underground Service Entry use. It is about time the solar industry has its own classification for the unique application we have. Like I tell customers, solar is expensive because every last bit of it is specially made for this use. Virtually none of the components and parts for a quality installation are available in your hardware store or home center.
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  • ggunn
    ggunn Solar Expert Posts: 1,973 ✭✭✭
    #3
    Re: PV wire, USE-2, what's the diff?
    solarix wrote: »
    I've been trying to figure this out for some time as well. I can tell you that when you order "PV wire" from different vendors, you can get different things. My understanding is that the correct PV wire is double insulated, that is there are two layers of insulation in order to meet the UL-4703 requirements for ungrounded systems. That is not to say it is rated for 1000Vdc. Personally, I find that the plastic used in these two layers is not as tough as the USE-2 stuff. Also, good luck finding it in colors other than black. You can get white, which of course is not to be used in ungrounded systems though. I'm still looking for red, however the story I've heard is that the red fades badly in the sun (turns to white) so what's the use. Just buy black and tape the ends red I'm told. USE wire is originally intended for Underground Service Entry use. It is about time the solar industry has its own classification for the unique application we have. Like I tell customers, solar is expensive because every last bit of it is specially made for this use. Virtually none of the components and parts for a quality installation are available in your hardware store or home center.
    John Wiles says to only use black for exposed wiring. Apparently the bleaching process that eventually fades the colors also compromises the flexibility and insulating qualities of the plastic. We use all black with white marking tape for negative on grounded systems and all black with red marking tape for positive on ungrounded systems. The electrical supply house we use locally carries 600V PV wire, which surprised me when they told me that because I thought PV wire was all 1000V. This begs the question of why the NEC specifies PV wire for ungrounded systems and not for grounded systems if it's not for a higher voltage rating.
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  • solarix
    solarix Solar Expert Posts: 713 ✭✭
    #4
    Re: PV wire, USE-2, what's the diff?

    The history of grounding has the concept that ungrounded systems are more dangerous an thus need extra - or double grounding to ensure safety. If the outer layer cracks, there is still another layer of protection. This is why appliances with all plastic housings can just have 2-prong cords. However, as long as your wiring has a safety ground, like being installed in grounded conduit, the ungrounded inverter systems are actually safer because the potential problem of a grounded conductor having a fault to egc and thus providing a path around the ground fault protection is eliminated.
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #5
    Re: PV wire, USE-2, what's the diff?

    There's another explanation, but I'm too much of a gentleman to say it. :p
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,639 admin
    #6
    Re: PV wire, USE-2, what's the diff?

    The standard is 600 VAC for "house" wiring in North America... And the wire is "highpot" tested to something like 1,800 VAC (as I recall--too many years).

    Also, remember you the the sqrt of 2 in here for DC ratings: 600 VAC * sqrt(2) = 848.5 VDC ... Which bumps the rating up a bit too.

    The "1,000 volt" rating seems to be coming from Europe... And I am not sure if it is AC or DC voltage.

    To add confusion, UL/NRTL documents will talk about about voltages (125 volt rated fuses) -- Is that nominal, never exceed, VDC or VAC, etc... When working with the inspectors, they would usually not answer and just allow us do what we thought was "safe".

    On occasion, they would do something a bit difficult to justify--For example, I had "floated" the DC power system of a computer and used 1,000,000 ohm resistors to bleed static electricity (was a hot plug card pack system) and 50 VDC rated capacitors between digital and frame ground (to reduce radio emissions).

    UL had put in a rule that required separate DC systems have 500 VDC highpot to ground for design verifications--Could not talk them out of it... So, I had to clip all the resistors and capacitors from a dozen+ card packs to pass the test (500 VDC is not that hard to pass--any surface gap >~0.010 inches can pass). No factory built system would pass the test--But that did not matter to UL. :confused: They later killed that requirement.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #7
    Re: PV wire, USE-2, what's the diff?
    BB. wrote: »
    No factory built system would pass the test--But that did not matter to UL. :confused: They later killed that requirement.

    -Bill

    That was a much more eloquent way of saying what I was thinking. :roll:
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  • jaggedben
    jaggedben Solar Expert Posts: 230 ✭✭
    #8
    Re: PV wire, USE-2, what's the diff?

    UL has different standards for USE and PV wire.

    A google search turned up this:

    http://www.ul.com/global/documents/offerings/industries/wireandcable/techcorner/PVWire.pdf

    "The UL Standards used for investigating USE-2 wire (TYLZ) and PV wire (ZKLA) are UL 854 Service-Entrance Cables and UL Subject 4703 Outline for Photovoltaic Wire, respectively."

    "In summary, when compared to USE-2 wire, PV wire has superior sunlight resistance and low-temperature flexibility in addition to a thicker insulation or jacket and a proven level of flame resistance."

    For what it's worth...
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