Re: [RE-wrenches] DC wire sizing

[Kent Osterberg]

Erika, You should refer to NEC 690.8 this information.  There are two issues that contribute to the 1.56 factor.  First is that the maximum current from the PV array is considered the short circuit current multiplied by 1.25 to account for higher than standard irradiance such as cloud edge effect, high altitude, and high operating temperature.  The second multiplier of 1.25 is because PV current is considered to be continuous.  All conductors (and breakers) operating continuously (over three hours I believe) that are subject to NEC rules are limited to 80% of the ampacity in Table 310.16.  The two 1.25 factors equals 1.56. For normal operation, wires selected under this criteria are conservatively rated.  But it is also the objective of the NEC to design power systems in a manner that they will remain safe under abnormal circumstances. Kent Osterberg Blue Mountain Solar, Inc. Erika M. Weliczko wrote: To my understanding the 156% on PV source and output circuits is related to the ability of PV to deliver more than rated and be continuous. Therefore, the wire has to be able to carry this current, so now the temperature and fill corrections are applied to find the wire capable of the 156%.   I am in a debate where the question is why correct for temp and fill on 156% of ISC and spend all that extra money when the normal operating is at Imp. Or why correct the 156% but why not correct the Isc or Imp.   I am going to stick to the fact that the circuit has to carry the 156% under all conditions… Thoughts?   Erika

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