Mike Dolt Enterprises
Mike Dolt Enterprises
User: earshavewalls, Location: Massachusetts
Asked on: Monday, April 11, 2011 at 2:48 pm EDT
Answered on: Tuesday, May 14, 2024 at 4:03 am EDT
Question: I have an ongoing discussion with a PV designer concerning bus ratings on the AC side of the inverters. (2010 California Elect. Code, based on 2008 NEC).Setting aside 690.54(A)&(B), 705.12(D)(2) contains similar verbiage concerning not exceeding 120% of the rating of the busbar based on the ratings of the overcurrent devices in circuits supplying power to the busbar. The argument from the designer is that you can exceed the rating of the busbar as long as no branch circuits (loads) are expressed in the panel. In other words, they feel that they may use load centers as AC combiners for the output of the inverter(s) and do not need to take into account the power from the utility at the busbar(s).I have trouble going along with this, but at the same time see what they are trying to say.....they are not adding loads to these panels, they are only passing power through them, for the most part in one direction (except for the inverter power from the utility), and they suggested signage on each of the AC load centers stating that no loads are to be installed on these panels (that option is not in the code, either).I am leaning toward interpreting that they must consider the feeds from BOTH directions (Utility and the PV output) for each AC busbar that are connected to each power supply, even if they add signage dissallowing any loads on these panels.Any input out there? Give me something that helps their case, if you can......otherwise, I've been right all this time.....and we can't have that!
Arguments for Exceeding Busbar Rating
NEC 705.12(D)(2) Intent
The intent of this section is to prevent overloading of busbars due to simultaneous high demand from multiple loads. In the case of AC combiners for PV systems, there are typically no loads connected to the panel.
Single Direction of Power Flow
The power flow through the AC combiner is predominantly unidirectional (from the PV system to the grid). This reduces the risk of overloading the busbar compared to panels with loads that draw power in both directions.
Signage Disallowing Loads
Clearly marking the AC load centers as "No Loads" helps prevent accidental or intentional connection of loads, further mitigating the risk of overloading.
Arguments Against Exceeding Busbar Rating
Code Wording
NEC 705.12(D)(2) explicitly states that the sum of overcurrent device ratings supplying power to the busbar shall not exceed 120% of the busbar rating. It does not make an exception for panels without loads.
Potential for Future Loads
Even if there are no loads currently connected to the AC combiners, it is possible that loads may be added in the future. This could create an overloading condition, especially if the inverter is supplying power from the grid.
Safety Considerations
Exceeding the busbar rating can compromise its ability to safely handle the current flowing through it. This could lead to overheating, arcing, or other hazardous conditions.
Conclusion:
While there are some arguments to support exceeding the busbar rating in AC combiners for PV systems, the code language is clear that it is not permitted. The potential risks associated with exceeding the rating outweigh the benefits. Therefore, it is recommended to design the AC combiners to meet the requirements of NEC 705.12(D)(2) and consider both the PV output and utility feed when determining the busbar rating. Flag for review