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20.20.027 (B) Power Transformers. Over cur- rent protection for a transformer with a source(s) on each side shall be pro- vided in accordance with 450.3 by considering first one side of the transformer, then the other side of the transformer, as the primary. Exception: A power transformer with a current rating on the side connected toward the photovoltaic power source not less than the short-circuit output current rating of the inverter shall be permitted without over current protection from that source. (C) Photovoltaic Source Circuits. Branch-circuit or supplementary-type over current devices shall be permit- ted to provide over current protection in photovoltaic source circuits. The over current devices shall be accessi- ble but shall not be required to be readily accessible. Standard values of supplementary over current devices allowed by this section shall be in one-ampere size increments starting at one ampere up to and including 15 amperes. Higher standard values above 15 amperes for supplementary over current devices shall be based on the standard sizes provided in 240.6(A). (D) Direct-Current Rating. Over cur- rent devices, either fuses or circuit breakers, used in any do portion of a photovoltaic power system shall be listed for use in do circuits and shall have the appropriate voltage, current, and interrupt ratings. (E) Series Over current Protection. In series-connected strings of two or more modules, a single over current protection device shall be permitted. 690.10 Stand-Alone Systems. The premises wiring system shall be adequate to meet the requirements of this Code for a similar installation connected to a service. The wiring on the supply side of the building or structure disconnecting means shall comply with this Code except as modified by 690.10(A), (B), and (C). (A) Inverter Output. The ac inverter output from a stand-alone system shall be permitted to supply ac power to the building or structure discon- necting means at current levels below the rating of that disconnecting means. (B) Sizing and Protection. The cir- cuit conductors between the inverter output and the building or structure disconnecting means shall be sized based on the output rating of the inverter. These conductors shall be protected from over currents in ac- cordance with Article 240. The over current protection shall be located at the output of the inverter. (C) Single 120-Volt Supply. The inverter output of a stand-alone solar photovoltaic system shall be permit- ted to supply 120 volts to single- phase, 3-wire, 120/240-volt service equipment or distribution panels where there are no 240-volt outlets and where there are no multi-wire 710-21 (Pleasanton April 2003)