Ask away, but please do not focus on the fuse sizing in the PV AC disconnect; I should have left that detail out because it is a separate issue. My only question is whether 705.12(B)(3) needs to be complied with in an MLO inverter AC combiner panel connected to service conductors through a fused disconnect. I say yes, and so far everyone else has agreed.
Thank you. Before I comment further I want to get a full picture.
What further information do you need? My question is pretty simple; see the bolded text in your post. There's a fused PV AC disconnect between an AC inverter combiner panel and a set of service conductors on the line side of an MDP. Does 705.12(B)(3) apply to the busbars in the combiner panel or not? I am convinced that it does, but I told the electrician (he disagrees) on the project that I would do my due diligence on the question.
The intent of the 705.12 rules is meant for mixed use panels, at and after the point of interconnection. The underlying reason for rules such as the 120% rule, and its 100% counterpart for same side feeding, is to avoid the blindspot created when a panelboard filled with a lot more loads than its rating (as is routine) is fed from two sources. An adversarial user could deliberately turn everything on and create an unprotected overload. This situation doesn't happen for a dedicated AC combiner, even if it hosts a few insignificant loads.
The 120% rule and the 100% rule , which are the 2020 NEC articles 705.12(B)(3)(2) and 705.12(B)(3)(1), respectively, both require the OCPD protecting the busbar to be considered in the calculations; this severely limits the size of a PV system that can be connected in a given panel. In 2014 what is in the 2020 NEC as 705.12(B)(3)(3) was brought in specifically to enable the use of AC combiner panels, and it is the only option under 705.12(B)(3) which allows the OCPD protecting the busbar to be omitted from the calculation. It allows for loads to be connected in the same panel as breakers backfed from PV inverters, but it does not require them, i.e. the panel does not have to be mixed use.
It really comes down to me, since I will be asked to affix my PE seal on the design. So far, I am not convinced that the ambiguity in the 2020 NEC supports the electrician's interpretation on this point.
(More a response to everyone else than to you directly) While the aforementioned ambiguity exists within 705 (as well as some of the definitions in Article 100), the argument from post #14 is unrebutted.
That depends on whether the panel is supplied under 230.40 Exception #2 or Exception #5.
It's not relevant because the AHJ in question is enforcing the 2020 NEC, although every AHJ I have dealt with would balk at adding five more service disconnects for a PV system. They typically require a single handle to disconnect all the PV.
FWIW, 2020 NEC 230.71(B) clearly does allow 2-6 PV disconnects: "Two to six service disconnects shall be permitted for each service permitted by
Local interconnection guides are not bound by the NEC; they are free to amend them with more stringent requirements and they frequently do. Also, many AHJs require the metering of PV production, and all that I have dealt with require a single PV meter and a single disconnect. The AHJ in question is one of those.
Ah, I was interpreting AHJ as building department, but you are using it to include the POCO as well. IIRC you deal with some jurisdictions where the POCO is municipal and this distinction is rather blurred?
Yes, I do, and this is one of those, but to me the AHJ is whatever authority has jurisdiction over the design and installation of a PV system I am working on. Sometimes there are more than one.
