SMA SBCBTL6 - Manual (Page 21)

SMA America, LLC Electrical connection
Installation Guide SBCBTLUS-IUS094510 21
6.2 Fuse sizing
Fuses are used to protect wiring and equipment from excessive currents that can cause damage,
heating or in extreme cases even fire. If the fuse rating is too small the fuses can trip during normal
operation. If the fuse rating is too large, the fuses cannot provide adequate protection. In PV systems,
the minimum and maximum size of the standard fuses are determined by the electrical ratings of the
PV module as well as National Electrical Code
®
requirements. Be sure to consult your PV module
manufacturer for appropriate fuse ratings.
The minimum size of fuses and wiring is calculated by using the Short Circuit Current Rating (Isc) of
the PV module. The National Electrical Code
®
requires that all fuses and wiring must be sized for a
minimum of 1.56 times of the Isc of the PV module used in the system.
• To determine the proper size of PV string fuses calculate 1.56 x Isc of the PV module and round
up to the next standard fuse size.
PV string fuses
1. Determine the required number and type of fuses.
– Use the same number of fuses for negative and positive string input.
2. Determine the required fuse size referring to your system calculation.
– Place fuses only in positions with corresponding numbers.
– Insert all fuse holders provided with the Sunny Boy Combiner Box.
The table below shows the string current per number of strings:
Sunny Boy Combiner Box requirements
NEC 690.15-18 allows the use of fuse holders as a suitable means of disconnecting PV arrays for
servicing. Additional external DC disconnects at the inverter may be required by the local authority
having jurisdiction.
The maximum string fuse rating for the Sunny Boy Combiner Box is 20 A.
If the Isc of the PV module equals 6.9 Adc, then the fuse size is determined by
1.56 x 6.9 = 10.76. The next standard fuse size would be a 12 A, 600 V DC fuse.
Number of strings Maximum string current Maximum continuous string current
3 18.70 A 12.00 A
4 14.00 A 9.00 A
5 11.20 A 7.20 A
6 9.40 A 6.00 A