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Thread: Ampacity Adjustments for Conditions of Use on PV Source Circuit

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    Ampacity Adjustments for Conditions of Use on PV Source Circuit

    Hi,

    I have a question regarding ampacity and conditions of use on a PV source circuit. Ultimately I am trying to determine wire size -

    Say I have a string of modules. The way I generally size my PV source circuit wire is this - Isc x 1.25 (for increased irradiance) x 1.25 (for 80% ampacity rule); we will call this "min ampacity". I then do a second calculation to find "adjusted ampacity" using conditions of use calculations - Isc x 1.25(for increased irradiance) x temp. derate factor x conduit fill derate factor.

    I then compare "min ampacity" value to "adjusted ampacity" and use the larger of the two as my ampacity to size the wire.

    However, I have been wondering why the 80% ampacity rule is not taken into account when derating for conditions of us... in essence, conditions of use are changing the ampacity. So why would the 80% rule not apply after derating ampacity for conditions of use?

    Thank you for any help!

    Jack

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    Quote Originally Posted by jackerdose View Post
    Hi,

    I have a question regarding ampacity and conditions of use on a PV source circuit. Ultimately I am trying to determine wire size -

    Say I have a string of modules. The way I generally size my PV source circuit wire is this - Isc x 1.25 (for increased irradiance) x 1.25 (for 80% ampacity rule); we will call this "min ampacity". I then do a second calculation to find "adjusted ampacity" using conditions of use calculations - Isc x 1.25(for increased irradiance) x temp. derate factor x conduit fill derate factor.

    I then compare "min ampacity" value to "adjusted ampacity" and use the larger of the two as my ampacity to size the wire.

    However, I have been wondering why the 80% ampacity rule is not taken into account when derating for conditions of us... in essence, conditions of use are changing the ampacity. So why would the 80% rule not apply after derating ampacity for conditions of use?

    Thank you for any help!

    Jack
    Assuming 90 degree wire there are two calculations: 90 degree ampacity derated for conditions of use and 75 degree ampacity derated for continuous use. Whichever is less is your "real" ampacity. The 0.8 multiplier for continuous use does not apply to the derating of the 90 degree ampacity; the COU90 calculation is for the protection of the conductor insulation and the CU75 calculation is for the protection of the terminals.

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    Quote Originally Posted by jackerdose View Post
    Hi,

    I have a question regarding ampacity and conditions of use on a PV source circuit. Ultimately I am trying to determine wire size -

    Say I have a string of modules. The way I generally size my PV source circuit wire is this - Isc x 1.25 (for increased irradiance) x 1.25 (for 80% ampacity rule); we will call this "min ampacity". I then do a second calculation to find "adjusted ampacity" using conditions of use calculations - Isc x 1.25(for increased irradiance) x temp. derate factor x conduit fill derate factor.

    I then compare "min ampacity" value to "adjusted ampacity" and use the larger of the two as my ampacity to size the wire.

    However, I have been wondering why the 80% ampacity rule is not taken into account when derating for conditions of us... in essence, conditions of use are changing the ampacity. So why would the 80% rule not apply after derating ampacity for conditions of use?

    Thank you for any help!

    Jack
    The same reasoning applies for continuous loads in general, and not just PV source/output circuits in specific. For uncontrolled PV source and output circuits, the continuous load is defined to be 125% of the short circuit current (Isc) at standard test conditions, accounting for the possibility of a "super sun" of irradiance. "Uncontrolled" means that the current is coming directly from the modules, and is not processed by any optimizers or inverters.

    When you use the terminal ampacity, you are really accommodating the rating of the equipment that hosts the terminals. Unless otherwise rated for continuous duty, you have to correct for continuous duty with the 125% safety factor. So we apply 125% of the continuous load to determine the termination ampacity, which is then selected from the column that applies for the equipment's termination temperature rating. Usually 75C.

    Wire in general, once you are out of the piece of equipment, is rated for continuous duty by design. Therefore, only the continuous load and conditions of use factors need apply to the calculation to determine the size.

    There are four checks to do when sizing a continuous load circuit for ampacity:
    1. Size terminations for 125% of continuous load. Select ampacity from termination temperature column, usually 75C.
    2. Size wire for 100% of continuous load, and conditions of use factors (bundling, ambient temp, elevated ambient temp). Select ampacity from wire's temperature rating column, usually 90C with most wire manufactured today.
    3. The OCPD, where required, shall "protect" the wire at its termination ampacity.
    4. The OCPD, where required, shall "protect" the wire at its ampacity corrected for conditions of use

    What "protect" means in the context of 3 and 4, is that the ampacities must "round up" to the size of the OCPD you are using, as 240.4(B) specifies. Some cases require ampacity to completely meet or exceed the OCPD, such as if OCPD > 800A, or if OCPD is not a standard size.

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