WIRE AND DISCONNECT SIZING
The MX60 has a 60 amp current output limit (default) and is listed to operate continuously at 60 amps depending on the nominal PV array voltage and the nominal battery voltage. There is no 80% de-rating as required by the NEC for fuses, conductors, and most circuit breakers.
The MX60 is a buck type converter and cannot boost the output current when the PV array peak power point voltage is at or below the battery voltage as may happen on hot days in 24 VDC PV and a 24 VDC battery system or a 48 VDC PV and a 48 VDC battery system.
To meet minimum NEC requirements (NEC 310.15, 690.8, 9), the output conductor should have an ampacity of 75 amps after any temperature and conduit fi ll corrections. This would normally indicate that the output conductors be 4 AWG, but a larger size may be required if there are temperature and/or conduit fi ll corrections required. With an output conductor rated at 75 amps (1.25 X the continuous output current), the OutBack OBDC-60 breaker—rated for continuous 100% duty at 60 amps —can be used to provide the code-required disconnect and output circuit over current protection.
The PV array output connected to the MX60 input may be as high 60 amps, but at this current level, there is very little (if any) current boosting or maximum power-point tracking due to the
60-amp output current limit. Additionally, the input current may exceed 60 amps on bright sunny days and any excess power would be lost. The size and ampacity of the input conductors must be selected to handle 1.56 times the short-circuit current of the PV array. Any disconnect or circuit breaker connected to the input conductors must also be rated at 1.56* times the short-circuit current for the PV array unless the breaker is rated for 100% duty in its enclosure. If that is the case, the circuit breaker may be rated at 1.25 times the PV array short-circuit current. OutBack OBDC-XX breakers are 100% duty rated breakers.
In terms of NEC compliance and the MX60’s 60-amp output rating, the largest PV array it can con- nect to should have a rated short-circuit current of 48 amps. This meets NEC requirements and allows the MX60 to perform maximum power-point tracking functions. The following charts show maximum distance in feet of various gauge two-conductor copper wire from the PV array to the MX60 with a 1.5% maximum voltage drop. Temperature and conduit fi ll corrections may
be required.
*UL requires Isc to be multiplied by 125% for the conductor rating