What is the truth about using tap rule to tie in photovoltaic systems?

[MJHaleakala]

I was taught that when tying PV onto the buss of a meter combo the maximum tie in is limited to 120% of the bus rating (705.12(b)(2)). I was also told that this 120% rule applies to tapping on to the load side of the main service disconnect with the wire ampacity of the wire as the limiting factor. The reason I was given for this is that if you tap 30A of PV on to a 100A conductor, then it is possible for that feeder to pull 130A without tripping the main over-current protection. This makes sense if the house were to draw 130A while the PV was at full production, but this seems to be a bit of a stretch from any real world situations.

I have not been able to find anything in code to support this. It seems to me that by code we could tap 100A of PV on a 100A main.

What is the truth about using tap rule to tie in PV? And Why?

 

[jaggedben]

The relevant code section in the 2011 NEC is 705.12(D)(2). In the 2008 and earlier codes it is in 690.64.

705.12(D)(2):

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.

Emphasis is mine.

This makes sense if the house were to draw 130A while the PV was at full production, but this seems to be a bit of a stretch from any real world situations.

First, note that the 120% rule is already giving you some additional allowance beyond what the conductor is rated for. Second, just suppose it does happen? You can have burning wires. This is not the same kind of situation as applying a load factor to the size of a service, because if the unusual event happens in this case, no breakers will trip in the case of an overcurrent.

It seems to me that by code we could tap 100A of PV on a 100A main

Definitely not by code. Now, as to whether doing so would be safe in most situations, you could have up to 200A flowing on a 100A conductor in the situation you describe. That doesn't strike me as safe. The code might be too conservative in some cases, but not by that much.