Limitations of a Derated Electrical Panel with Solar Grid-Tie

[Zeigh]

Hello,


I need a little help understanding what a derated electrical panel for a grid-tie solar system means in day-to-day functionality...

An 18.46 kW Solaria solar panel and Enphase microinverter system has been designed to interface with my 200 amp utility service. The electrical panel will be derated to a 150 amp main breaker, while the backfeed solar electricity input breaker will be at 80 amps. Initially, this didn’t raise any concerns with my electrical usage history. However, I am now considering the installation of a 20 kW electric heater into my garage and I am finally focusing in on the numbers with some concern.

If I have that garage heater fired up at 20000 watts (83 amps), then what electricity do I have left for the remainder of the home? 16000 watts (67 amps)? OR am I only able to safely access 80% of that 150 amps, leaving about 9000 watts (38 amps) for the rest of the home? Of course this is all based on just utility power and I am under the assumption that whatever the solar systems pumps into the grid doesn’t really make any difference in the actual available usage, just the billing credits.

Maybe there are some variables that I am missing or knowing me, I am completely wrong here, but I may need to re-evaluate the whole project and look for other design options. Please tell me if my calculations are close and if so, what I might be able to do without decreasing the size of my solar array pending installation or upgrading to a 400 amp utility service.

BTW, I have a twist to this whole configuration that may or may not make a difference. My electrical panel (attached photo) is what I have been told is a "farm panel" or split bus panel. This means that there are two separate 240 volt bus bars directly from the utility meter and one of those bus bars cannot be de-energized by the customer. Would connecting the solar array breaker to the top bus bar omit the need to derate my utility main breaker since it is backfeeding upstream of the main house side feed?


Peace,
Dr. Z.


IMG_6448.jpg

[hardwired]

Don't run it thru the panel and keep it as is.

Talk to your electrician about a Line Side connection for your grid-tie array. You probably want 100amp capability.

I don't even think the code allows for 80A thru your panel. Maybe 60

[Zeigh]

Update: I have been able to research more and find that the NEC 120% Rule may be sidestepped with a supply line tap. There are a couple of web sites by electrical engineers that directly illustrate an example similar to what I have presented. However, it is all dependent on how the local jurisdiction interprets the code or how much money one chooses to spend on an electrical engineer to override the decision. The NEC doesn't address every scenario (especially on the growing sizes of residential solar electric installations) or if it does, it is done with ambiguous text that each side uses as proof. Logically and for all practical purposes, a supply line tap is safe in this situation, as stated by an electrical engineer friend of mine who would sleep well at night should such a configuration be installed upon his home.

As for the status of my project, my installer has acknowledged the new plan and is making an addendum with my local utility company. (Yes, an 80 amp backed breaker is to code as long as the panel such as mine is derated or bypassed as indicated.) Thankfully I purchased a home that had an uncommon split bus bar utility panel that makes my overall solar installation slightly easier and less expensive for the design.


Peace,
Dr. Z.

[JSchnee21]

Why do you want a 20kW Electric space heater? Other heating options would likely be much more economical (natural gas). Is this for a residence or a business? I assume residence since you have a 200amp service.

Is the solar already installed? Do you live in California? Certainly a line side tap would be the best approach for your solar (that's what I have for my 12.2kW system in NJ). BUT, for some reason in California (and some other jurisdictions) get a line side tapped approved by the AHJ/PoCo seems to be a real issue.

[Zeigh]

Why I want a 20 kW electric heater is outside the scope of this post, but I would be happy to answer with three words, "enclosed space heating". 20 kW is only the max, I will just need the heat a few hours a week during comparatively mild winters in Northern Arizona, it is portable for more efficient heat direction in a 1900 square foot insulated garage/workshop, the heat adjustment will rarely be set above 60F, and no carbon monoxide to worry about. With my cost calculations and intended use, electric verses gas were almost the same cost in the long run.

As for my solar electric installation, the final hardwiring is pending and I happened to catch this detail just in time.


Peace,
Dr. Z.

[JSchnee21]

Fine by me. Though I'm not sure I agree on the operational costs if you have other resources available (natural gas in particular or propane) for gas fired forced hot air or baseboard hot water (from a boiler).

Wide spectrum halogen lamps provide heat as well as light, wink, wink.

Anywhoo, line side tap is your best bet to avoid derating. Or, get a separate meter and panel run to the garage for your hobby.