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 overcurrents in accordance with Article 240. The overcurrent 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 overcurrent 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.”

Standard Array Sizing Specifications
Rating Type	Rating
Maximum System Voltage	600 VDC
Range of Operating DC Voltage	230 - 600 VDC
Maximum Operating Current - DC	9.5 Amps
Maximum Array Short Circuit Current - DC	10 Amps
Maximum Utility Back Feed Current - DC	0.075 Amps
Operating Voltage Range - AC	106 - 132 VAC
Operating Frequency Range	59.3 - 60.5 Hz
Nominal Output Voltage - AC	120 VAC
Nominal Output Frequency	60 Hz
Maximum Continuous Output Current	15.0 Amps
Power Factor	>0.99
Maximum Continuous Output Power - AC	1800 Watts
Maximum Output Fault Current - AC	15 Amps
Maximum Output Over-Current Protection	15 Amps
Efficiency	96.5%
Total Harmonic Distortion	<5%

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

Comments

Great article, thanks. How do you know if the disconnect needs to be fused or not fused? Also, doesn't the disconnect need another 125% safety factor applied on top of the circuit current, which is 125% of the Module or Array Isc?

The disconnect has to be fused if the available Circuit Current from the array can exceed the rated current of the shorted circuit or module. So usually, that means if you have more than two parallel modules or strings, then you need OCPD combiners (or fuses).
The disconnects sold by Greentech Renewables are rated for continuous use at 100% of rated current. So the second 125% multiplier isn't necessary for the disconnect assembly.

What fusible disconnects does Greentech Renewables sell that are rated for continuous operation? I called Square D about the D222NRB 60A fusible disconnect and they told me they are only rated for 80% of the nameplate rating (continuous use). Are the Midnight Solar disconnects rated for continuous operation?

When does the disconnect need 2-pole switch? At the roof? at the inverter?
(Negatively grounded at the charge controller or inverter)
thanks. B

I assume you are asking about the DC disconnect - the number of poles is determined by the number of circuits you need to interrupt at one time.

Or, possibly, the total current you need to interrupt in one circuit.

In the case of an ungrounded DC side (non isolated inverter), you have to interrupt both the positive and negative leads of each circuit.

Here's a sample disconnect with some specs:

Thanks. I guess I was asking about best practices on neg ground arrays.
On the roof, you might want to interrupt even the negative (grounded) leg for service as allowed in the NEC, but generally you'd not want/need to break a grounded conductor.

Great article.. I want to know about,

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%.

Are you sure about this is safety.

Pass4sure 642-554

Why we need AC Disconnect in circuit, wether we use a backfeed breaker for disconnect? Please resolve my issue.

AC Disconnects are a means of facilitating code compliance, most specifically for rapid shutdown initiation in most cases, and are usually a requirement of an Utility and/or Emergency Services to have an easily identifiable and workable means to disconnect solar generation.

Thank you for your comment.