SMA SB 5000-US - Manual (Page 87)

SMA America, LLC Maintenance
SB50US-80US-IA-en-37 SB50US-80US-IA-en-37 87
PV String Fuse Sizing
If the fuses are designed too small, they may open too early and cause interference. If fuses are
designed too large, they do not provide the required protection as they open too late.
For PV plants, the minimum and maximum size of a fuse connected in series is specified by the
electrical nominal sizes of a PV module and by UL and the National Electrical Code. Contact the
manufacturer of the PV modules in order to obtain the corresponding nominal sizes of the PV string
fuses.
The minimum sizes of fuses and cables are calculated using the Short Circuit Current Rating (Isc) of
the PV module. The National Electrical Code stipulates that all fuses and cables are designed to be
at least 1.56x as large as the Isc of the PV module being used in the plant.
The correct size of the PV string fuse is specified by calculating 1.56x the Isc (of the PV module) and
then rounding this up to the nearest standard size for fuses.
DC Disconnect Requirements
National Electrical Code 690.15-18 enables the use of fuse holders as a suitable medium for
disconnecting PV arrays for maintenance. Additional DC disconnects external to the inverter may be
required by the local authority having jurisdiction.
PV String Fuses
The DC disconnect is shipped with 15 A, 600 V DC fuses in the fuse holders. The maximum nominal
size for string fuses in the DC disconnect is 20 A at 600 V DC.
For example, if the Isc of the PV module is 6.9 Adc, then the size of the PV string fuses is
determined as follows: 1.56 x 6.9 = 10.76
The nearest standard size for fuses would be a 12 A and 600 Vdc fuse.
The size of a string fuse may not be greater than the maximum nominal value of a PV
module fuse. Observe the data sheet of the PV module. If no maximum size for fuses is
stated, contact the manufacturer of the PV module.
WARNING
Electric arc and electric shock when removing the fuses under load.
Risk of death or serious injury.
• Do not remove any fuses when the inverter is under load.
When dimensioning the fuses, observe the National Electrical Code
®
690.8 and 690.9.