Properly sizing fuses for photovoltaic (PV) systems is critical for the safe, reliable and long-term operation of this renewable power source. Unlike typical electrical power distribution and control applications, fuses in photovoltaic systems are subject to unique conditions. Prolonged exposure to elements of the environment can produce abnormal ambient temperatures, which in turn affects fuse performance, conductor selection and sizing. Also, unlike traditional circuits which are normally sized based on continuous loads, PV modules produce continuous currents, leading to additional considerationswhen sizing fuses. Taking these conditions into account, a unique method for sizing fuses in PV systems is necessary. This white paper will help you determine when fusing is required and outlines a five step process for sizing fuse ampere ratings for photovoltaic applications according to the 2011 National Electrical Code (NEC).