I designed a 12.8kWdc rooftop PV system for a client. Brand new house. Two Sunny Boy 6.0. The outputs of the inverters were wired via 8 AWG THHN/THWN-2 into an Eaton 100 A subpanel, with Eaton 40A 2-pole breakers, and the sub-panel fed the main panel via 6 AWG THHN/THWN-2. The breaker on the main panel was 100A.
This all took place in late 2016.
We installed the SMA a Sunny Portal so the client could monitor his production. All was well until a few months ago: he began getting messages that "production is down by more than 20%" three days in a row. This was back in Dec. 2021 when we got a lot of rain. Then several sunny days in a row, and the inverters showed no production, but no error messages. He went on line and the Sunny Portal showed no production. At one point, one of the inverter displayed a 1203 error code -- no grid.
He went to the sub-panel and both breakers appeared to be in the "on" position. He moved one of the breakers, but not forcing it into the "off" position, and the breaker appear to fall apart behind the deadman plate. He removed the deadman plate and the breaker in question fell apart in several pieces. He moved the second breaker a bit and it also fell apart.
At this point he called me and, after making sure that the subpanel was de-energized, I directed him check how well the 8AWG conductors were torqued to the terminals and to put to put wire nuts on the l1/l2 leads of both inverter conductors. He said the conductors appeared to have been well torqued down to the terminals.
I picked up the old breakers, and ordered two new Eaton brand 40A 2-pole breakers. I examined the old breakers and the area of the breaker near the jaws were severely burnt to the point where the plastic was in pieces and the jaws themselves has come apart in pieces. No similar heat damage was found at the wire terminals.
I told the client that "The definition of insanity is doing the same thing over and over and expecting different results" (c.f. Albert Einstein) and I did not recommend replacing with new Eaton brand breakers, until I got more input from professionals. He has decided to clean the tangs on the buss bars, replace the breakers, torque the wire terminals to spec, and to monitor the vicinity of the breakers with an IR thermometer on a regular basis.
The location of the sub-panel is in a garage with no A/C. The ambient temperature can reach 110 degF in the summer, and so I am going to assume that the subpanel, which is located inside the garage on a north-facing wall, could get as hot as 130 deg F or 54 degC. The temperature de-rating for THHN/THWN-2 Cu (0.76), and there are two inverter outputs in a single conduit, 4 current carrying conductors (0.80).
So, the temperature-corrected/conductors-in-conduit-corrected ampacity is 55A*(0.76)*(0.80)=33.4A. The maximum output current for a SB6.0 at 240 Vac is 25.0 A. So I believe that, to first order we are okay on ampacity. But I forget how to calculate the actual temperature (above ambient) of 8AWG THHN/THWN-2 at an RMS current of 25A, and I don't know what the panelboard temperature rating is (I'll find out).
I have pictures of the old breakers and the sub-panel for those who are interested troubleshooting our problem, provide me with an email address.
- Peter
Peter T. Parrish, Ph.D.
President, California Solar Engineering
820 Cynthia Ave.
Los Angeles, CA 90065
(323) 839-6108
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