Solar Expansion & NEC Code 690.64(B)(2) Issues

It's been a while since my last post. Enjoying the new house immensely. We decided this past year to increase our solar system size on the roof in preparation for an electric car. I thought it would be beneficial to share our experience.

The previous system is a 36 panel (5.8kW) with 6000 Watt Sunny Boy inverter.

The new system is a 10 panel (2.2 kW) with 4000 Watt Solectria inverter. It has some room to grow to fully use up the 4000W inverter.

(I was quite amazed at how the panel costs have fallen to half of what they were in 2006! All of the new system was purchased from an outfit called Solar Depot in Petaluma. Although California's incentives are almost nothing now, the Federal 35% tax credit incentive is sweet to bring costs down even further).

When we connected the two systems to the 200A panel that we have, I was made aware of NEC code 690.64(B)(2) which is a fire code. The panel must have a main breaker that can carry the sum of both inverts and meet this NEC code. The code as it pertains to a residential installation says:

"For a dwelling unit, the sum of the ampere ratings of the overcurrent devices shall not exceed 120 percent of the rating of the busbar or conductor."

Here were our calculations of the circuit break requirements for our two inverters:

6000W/240V = 25, then 25 x 1.25 = 31.25A so a 35A breaker is used.

4000W/240V = 16.7, then 16.7 x 1.25 = 20.8A so a 25A breaker is used.

The current panel is 200A so 120% x 200A = 240A. If we just connected both solar systems into the panel with 35A + 25A breakers = 60A, then 200A + 60A exceeds the 240A maximum.

We struggled with issue for many months trying to find a solution that avoided us having to scrap the new 200A panel we put in when we built our house, which would require breaking stucco and rewiring the whole thing, or adding a new panel and again costing a lot to install it etc.

We were unable to get answers from the solar reseller, our utility PG&E, the City of San Mateo and various electrical distributors for the panels. It appears that runing up against this code issue is rare, but I suspect it will become increasingly problematic for those of you who put up large solar systems.

The solar reseller said "do a line side tap" to avoid the panel bus - how the heck do you safely splice together a solar system wire with the wire on the meter side of the panel and meet PG&E requirements? We looked into special splicing lugs, but none could be mounted safely within the PG&E side of the meter. The PG&E inspector had never seen or heard of anyone doing that and was not thrilled by the lug solution sitting on their side of the panel.

The City of San Mateo had no idea - they had not run into this before and had insufficient training.

The panel distributors could not recommend any parts to do a line side tap, no one had asked for this before at the couple of shops I called.

So I turned to an expert on this subject - John Wiles at University of New Mexico, who proved quite helpful. John published a series of papers that ultimately helped us identify a solution. Here is one called Supply Side PV Utility Connections and another called Making the Utility Connection.

What we determined was that, although not ideal, if we could downrate our main house circuit breaker, we could meet the 120% requirement without having to install a new panel and break stucco.

We did a load analysis of the house to show that the house only needed a 175A main breaker. So by switching out the 200A main breaker to 175A, the load would be:

175A (house) + 35A (solar 1)+ 25A (solar 2) = 235A which is < 240A and meets the NEC code.

Our electrician used a simple load analysis spreadsheet available from the Inspection Bureau (IBI) here to do the calculations, going room by room and listing appliances etc. Our house load ended up being about 150A.

We then got the City to sign off, which they did and were very appreciative of my assistance in helping them understand this important safety issue. PG&E signed off too since we did not have to do anything on their side of the meter.

My advice for any of you building a new home: plan in advance, if you expect to put up a large solar system, be sure to provision for a large panel size - my recommendation being 400A - so that you can easily connect the solar system to the panel and have room to grow, rather than deal with the complexities of a line side tap or switching out your panel in the future.