2014 NEC 705.12(D)(2) - Understanding PV Interconnections

2014 National Electrical Code

Published on August 22, 2014 by JADE Learning

Having trouble applying the new PV interconnection rules in NEC 705.12(D)(2)? For the last few code cycles we have had just one basic rule when it comes to the interconnection of the PV system to busbars or conductors, "the sum of the ampere ratings of overcurrent devices in circuits supplying power to a busbar or conductor shall not exceed 120 percent of the rating of the busbar or conductor". The 2014 NEC has replaced those 31 words in 705.12(D)(2) with 477 words that are easy to read but might be difficult to apply without some real world examples.

Let's start out with the first basic requirement in 705.12(D)(2): 125% of the inverter output circuit current must be used for the ampacity calculations for most of the interconnection methods used. Whether interconnecting the Inverter Output Circuit through a breaker at the service panel or directly to a feeder, it is important to first know the inverter output circuit current. This can be found on the nameplate of the inverter but most inspectors or installers will simply divide the wattage of the inverter by the applied voltage to find the answer. This method may be more accurate since many inverters can be used in multiple voltage configurations and the value listed on the inverter nameplate is usually based on the lowest voltage.

For interconnection to a busbar, there are 3 new options that will be commonly encountered:

• Option one, 705.12(D)(2)(3)(a) (Image 1): 125% of the inverter output circuit current plus the rating of the OCPD protecting the busbar cannot be greater than the ampacity of the busbar. When using this option, there is no restriction on where the backfed PV breaker must be located in the panelboard.

  

• Option two, 705.12(D)(2)(3)(b) (Image 2): 125% of the inverter output circuit current plus the rating of the OCPD protecting the busbar cannot be greater than 120% of the ampacity of the busbar. This method can only be used where a service main breaker is at one end of the busbar and the PV breaker is at the opposite end.

  

• Option three, 705.12(D)(2)(3)(c) (Image 3): First, the sum of the ampere ratings of all OCPD's on the panelboard, both load and supply devices, excluding the rating of the OCPD protecting the busbar, shall not exceed the ampacity of the busbar. In this method, we are simply adding the face value of the OCPD's in the panelboard excluding the main. Second, the rating of the OCPD protecting the busbar shall not exceed the rating of the busbar. If this method is used then there is no restriction on where the backfed PV breaker must be located in the panelboard.

  

Remember, in a utility interactive system, there is no need to have an additional fastener to lock down a backfed plug on type PV breaker. Since the utility interactive inverter stops the current flow when utility power is not present, 705.12(D)(5) allows the additional fastener required by 408.36(D) to be omitted.

Don't forget the additional labeling that must comply with 110.21(B). If using option 2 from above, then the following labeling is required at the electric panel adjacent to the backfed breaker:

WARNING:
INVERTER OUTPUT CONNECTION;
DO NOT RELOCATE THIS OVERCURRENT DEVICE.

If using option 3 from above, the following label is required at the distribution equipment:

WARNING:
THIS EQUIPMENT FED BY MULTIPLE SOURCES.
TOTAL RATING OF ALL OVERCURRENT DEVICES,
EXCLUDING MAIN OVERCURRENT DEVICE,
SHALL NOT EXCEED AMPACITY OF BUSBAR.

A new rule in 705.31 requires that if connecting the PV system on the line side of a service disconnect, then the OCPD protecting the inverter output circuit conductors must be located within 10 feet of the connection to the service (Image 4).

In the next post we will cover the remaining new interconnection rules pertaining to feeders and tap conductors.

Learn more in the 2014 NEC Changes courses at https://www.jadelearning.com.