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Thread: PV WIRE AMPACITY

  1. #1
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    PV WIRE AMPACITY

    Do I use the 90 degree chart or the 75 chart to begin derating of PV Wire? I am thinking specifically of application of the "number of current carrying conductors in a raceway" factor and the ambient temperature derate factor.

    Does this change for THHN/THWN-2?

    Assume the PV wire (or THWN-2) runs into inverter terminals. Are these terminals typically 75 or 90 degree rated?

    Do i just stick to the good ole rule of "assume most terminals and boxes and switches etc are 75 degree rated, so just stick to the 75 degree chart for sizing the wire also, as it connects to it"?

    Bonus: is cable tray a raceway, (wire mesh/snake tray) and would I apply a 50% derate for 20 PV wires in it?

  2. #2
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    Quote Originally Posted by Zee View Post
    Do I use the 90 degree chart or the 75 chart to begin derating of PV Wire? I am thinking specifically of application of the "number of current carrying conductors in a raceway" factor and the ambient temperature derate factor.

    Does this change for THHN/THWN-2?

    Assume the PV wire (or THWN-2) runs into inverter terminals. Are these terminals typically 75 or 90 degree rated?

    Do i just stick to the good ole rule of "assume most terminals and boxes and switches etc are 75 degree rated, so just stick to the 75 degree chart for sizing the wire also, as it connects to it"?

    Bonus: is cable tray a raceway, (wire mesh/snake tray) and would I apply a 50% derate for 20 PV wires in it?
    Terminals are generally rated for 75C, although for 100A and less, you have a burden of proof to check, as they'd be rated 60C by default.

    Derating applies to wires, and not to terminals. If you use 90C wire, you use the 90C rating as the starting point for both your temperature and bundling derates, even if you have 60 or 75C terminals. This is the main advantage of 90C wire.

    Other than for 1/0 & larger wires and tray rated cable that the NEC has allowed in a cable tray for several cycles, small PV Wires in a cable tray is uncharted territory. NEC2014 allows listed PV wire in all sizes, which had previously not been allowed by omission. For the most reliable way to pass inspection without it being questioned, assume it is a raceway and apply the same calculation.

  3. #3
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    Quote Originally Posted by Carultch View Post
    Terminals are generally rated for 75C, although for 100A and less, you have a burden of proof to check, as they'd be rated 60C by default.

    Derating applies to wires, and not to terminals. If you use 90C wire, you use the 90C rating as the starting point for both your temperature and bundling derates, even if you have 60 or 75C terminals. This is the main advantage of 90C wire.

    Other than for 1/0 & larger wires and tray rated cable that the NEC has allowed in a cable tray for several cycles, small PV Wires in a cable tray is uncharted territory. NEC2014 allows listed PV wire in all sizes, which had previously not been allowed by omission. For the most reliable way to pass inspection without it being questioned, assume it is a raceway and apply the same calculation.
    Excellent, thanks.

    I want to believe i can use the 90C chart.
    I have been told that sizing wire with the 90C chart will allow the wire to heat up to 90C and when connected to lesser rated (75C or 60C) terminals the HEAT itself will pass into the terminal that is not rated for such high heat. Opinion?

  4. #4
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    Quote Originally Posted by Zee View Post
    Excellent, thanks.

    I want to believe i can use the 90C chart.
    I have been told that sizing wire with the 90C chart will allow the wire to heat up to 90C and when connected to lesser rated (75C or 60C) terminals the HEAT itself will pass into the terminal that is not rated for such high heat. Opinion?
    That is the basic rationale for the rule, yes.

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  5. #5
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    Quote Originally Posted by Zee View Post
    ...
    I want to believe i can use the 90C chart.
    I have been told that sizing wire with the 90C chart will allow the wire to heat up to 90C and when connected to lesser rated (75C or 60C) terminals the HEAT itself will pass into the terminal that is not rated for such high heat. Opinion?
    Quote Originally Posted by GoldDigger View Post
    That is the basic rationale for the rule, yes.
    He can use the 90°C column for the 'field' ampacity... but he has to use the 75°C column for 'in the box', i.e. termination temperature limitation, minimum size, maximum circuit ampacity. Derating for ambient temperature and proximity applies to the former, while 125% padding for continuous loads applies to the latter. The lesser of the two is the circuit ampacity, which must be considered protected by the OCPD per Article 240.

    Cable tray is not a raceway. It is a wiring method which is considered only as a support system. See 392.2.
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  6. #6
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    Quote Originally Posted by Zee View Post
    Excellent, thanks.

    I want to believe i can use the 90C chart.
    I have been told that sizing wire with the 90C chart will allow the wire to heat up to 90C and when connected to lesser rated (75C or 60C) terminals the HEAT itself will pass into the terminal that is not rated for such high heat. Opinion?
    If you are using 90 degree wire, derate it at 90 degrees for conditions of use. From the 75 degree column for the same gauge wire, derate it (multiply by 0.8) for continuous use. The first derate is to protect the conductor insulation and the second is to protect the terminals. Both derated ampacities must be greater than the nameplate maximum output current of your inverter(s).

  7. #7
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    Quote Originally Posted by ggunn View Post
    Both derated ampacities must be greater than the nameplate maximum output current of your inverter(s).

    And the wire must also be protected by the OCPD, where required. Note that 240.4(B) often applies here, so in general, this means that the wire ampacity must exceed the previous OCPD and "round up" to the OCPD you actually are installing. So the 75C ampacity directly (or 60C if applicable), and the derated 90C ampacity for conditions of use, need to be between the previous OCPD size and the actual OCPD size.

    One application where this might be the driving factor, is in a group of DC feeders to a central inverter, with the inverter manufacturer including a group of breakers for the DC subcombiner. Suppose you have a "runt of the litter" combiner output that is supplied by a source that is significantly smaller than the smallest breaker available in the inverter. It is possible that using a larger than default size OCPD could drive the wire size. For instance, if 1.56*Total Isc is only 149A, but the smallest breaker that fits the subcombiner is 225A. Note that most of the other combiners are connecting to 400A breakers, so the previously mentioned combiner deviates from the project norm, due to being sourced from an abnormally smaller array. A "runt of the litter" as I like to call it.

  8. #8
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    Quote Originally Posted by Carultch View Post
    .... So the 75C ampacity directly (or 60C if applicable), and the derated 90C ampacity for conditions of use, need to be between [greater than] the previous OCPD size and the actual OCPD size.
    ...
    Just greater than the next lesser standard OCPD rating. The ampacity (or should I say 'ampacities'?) can equal or exceed the actual OCPD rating. 'Between' is a practice in economics, not NEC compliance.
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  9. #9
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    Quote Originally Posted by Carultch View Post
    And the wire must also be protected by the OCPD, where required. Note that 240.4(B) often applies here, so in general, this means that the wire ampacity must exceed the previous OCPD and "round up" to the OCPD you actually are installing. So the 75C ampacity directly (or 60C if applicable), and the derated 90C ampacity for conditions of use, need to be between the previous OCPD size and the actual OCPD size.
    Only the conditions of use ampacity needs to be greater than the next size down OCPD.

  10. #10
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    Quote Originally Posted by ggunn View Post
    Only the conditions of use ampacity needs to be greater than the next size down OCPD.
    Not if you are using an oversized OCPD. The basic termination requirement is based on the load, but it also assumes one is going to use the minimum standard OCPD rating (determined using noncontinuous plus 125% continuous, same as the minimum wire size for termination). Step that OCPD rating up one size and you are looking at the potential to overheat the termination if the minimum wire size for termination is not also upsized. This is why 110.14(C) says "selected and coordinated".
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