Sizing the DC Disconnect for Solar PV Systems

A solar PV system typically has two safety disconnects. The first is the PV disconnect (or Array DC Disconnect). The PV disconnect allows the DC current between the modules (source) to be interrupted before reaching the inverter.

The second disconnect is the AC Disconnect. The AC Disconnect is used to separate the inverter from the electrical grid. In a solar PV system the AC Disconnect is usually mounted to the wall between the inverter and utility meter. The AC disconnect may be a breaker on a service panel or it may be a stand-alone switch. The AC disconnect is sized based on the output current of the inverter and will be looked at in depth in a different article.

How do I size an AC or DC Disconnect?

In general, sizing refers to equipment, components, and connectivity (wiring) throughout a solar PV system as it relates to NEC requirements. The following terms are used to determine component output:

a. Voltage

b. Circuit Load

c. Amps/Beaker Size

d. Wiring/Cables

Sizing and Protection of the AC disconnect

NEC 690.10 stipulates, “The circuit 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 accordance with Article 240. The over current protection shall be located at the output of the inverter.”



Sizing of Module Interconnection Conductors and DC Over Current Protection

NEC 690.80, “Where a single over current device is used to protect a set of two or more parallel-connected module circuits, the ampacity of each of the module interconnection conductors shall not be less than the sum of the rating of the single fuse plus 125 percent of the short-circuit current from the other parallel-connected modules.”

A PDF file for 2011 NEC (4.5 MB) requirements may be reviewed for free at the National Fire Protection Agency website or at NEC PLUS*.

*NEC Guidelines are available for viewing free of charge for 24 hours; paid subscribers are provided unlimited access.

Disconnect Switches Applications in Photovoltaic Systems – Sizing Example

Assume that a disconnect switch must be chosen to provide means for disconnecting an inverter from its source. The supplying solar PV array consists of 20 parallel-connected PV-strings. Each string consists of 30 series-connected PV-modules, each of them having a maximum Voc of 28.4 VDC and an Isc rating of 7.92 A. The highest inverter power output is obtained at the maximum power point, which occurs with approximately

146 A (IMPP) at the inverter input.

The Voc determines the minimum voltage rating of the disconnect switch:

30 × 28.4 V = 852 V.

Selecting a disconnect switch with a Vi and Ve of 1000 V DC would give a safety margin greater than 15%.

The sum of ISC parallel-connected strings determines the current-capability requirements for the switch. The sum of ISC gives:

20 × 7.92 A = 158.4 A.

At a minimum NEC 690.8 requires this value to increase by 125% (or 158.4 x 1.25 = 198A) to address increased currents during solar noon.

If the ambient temperature at the installation site may rise, e.g., up to 60 °C, a temperature-derating factor must be taken into account. For 60 °C the factor is 0.80, calculated as described earlier. Applying the factor by dividing the maximum power-point current by the factor tells us how the disconnect switch should be rated under normal conditions: 146 A / 0.80 = 182.5 A. The calculations have now given us a picture of the requirements for the disconnect switch and can be used to properly select a disconnect switch for a given PV application.

Resources

• Homesolar - Solar Electricity Basics

• ABB Disconnect Switches - Applications in photovoltaic systems