pv system connected to sub-panel

[Jest Waitn]

Yes. Microinverters operate at line voltage (240 Vac), while string inverters can operate up to 500 Vdc, depending on panel selection and array design. The same power at higher voltage means less current. Less current means less loss.

if there is an 'issue' with over-production, it should show on the 3kw system. i will then have to re-configure it as a string system. the distance to transformer won't change. i won't run dc beyond the array site.

[Jest Waitn]

That generator could have a dual winding output where it provides both 208Y and 240 Delta. The only issue would be how well the two output voltages are isolated so you do not make the mistake of plugging in the wrong load. My guess would be a different receptacle configuration (4 wire for the 208v and 3 wire for the 240v) for 3 phase loads.

the MEP-002A 5KW military generator produces ~10kw at sea level. 5kw rating is for highest point on earth (thinnest air). i had this one tested at 9kw resistive load before buying.

[Jest Waitn]

A three phase generator huh? Sounds cool. I'm not familiar with generators but I've been studying transformers and the many three phase configurations. If your saying it's a 240v and 208 at the t
same time that definitely sounds like a high leg. So that means you'd get 240v from any leg to leg. But one phase is split just like your standard residential service. So that will give you 120v from leg to neutral for two of the legs. But the third leg to neutral give you 208v but it's not very stable which is why it's called a wild leg service. And if your not careful and you accidentally wire a 120v circuit with the high leg to neutral and fry your electronics with 208 or more volts, then you'll quickly realize why it's also called a stinger, or bastard leg.

you connect the output to your system based on the configuration. so it won't produce 208 and 240 simultaneously at load. one or the other. it also has a 120v output.

[McMac]

690.8(B)(1)(a) disagrees with you.

Thank you for that. I noticed the exception for devices listed for continuous operation @ 100% of it's rating, which makes me wonder how many if any breakers out there are rated for this.

[SunEagle]

A three phase generator huh? Sounds cool. I'm not familiar with generators but I've been studying transformers and the many three phase configurations. If your saying it's a 240v and 208 at the t
same time that definitely sounds like a high leg. So that means you'd get 240v from any leg to leg. But one phase is split just like your standard residential service. So that will give you 120v from leg to neutral for two of the legs. But the third leg to neutral give you 208v but it's not very stable which is why it's called a wild leg service. And if your not careful and you accidentally wire a 120v circuit with the high leg to neutral and fry your electronics with 208 or more volts, then you'll quickly realize why it's also called a stinger, or bastard leg.

That generator could have a dual winding output where it provides both 208Y and 240 Delta. The only issue would be how well the two output voltages are isolated so you do not make the mistake of plugging in the wrong load. My guess would be a different receptacle configuration (4 wire for the 208v and 3 wire for the 240v) for 3 phase loads.

[McMac]

i have a military generator that outputs 208vac & 240vac. it's real loud. might even wake the dead. will run the entire house, but haven't had to use it.

A three phase generator huh? Sounds cool. I'm not familiar with generators but I've been studying transformers and the many three phase configurations. If your saying it's a 240v and 208 at the t
same time that definitely sounds like a high leg. So that means you'd get 240v from any leg to leg. But one phase is split just like your standard residential service. So that will give you 120v from leg to neutral for two of the legs. But the third leg to neutral give you 208v but it's not very stable which is why it's called a wild leg service. And if your not careful and you accidentally wire a 120v circuit with the high leg to neutral and fry your electronics with 208 or more volts, then you'll quickly realize why it's also called a stinger, or bastard leg.

[sensij]

regarding this 'line loss', are string inverters 'better' at handling long runs than micro-inverters?

Yes. Microinverters operate at line voltage (240 Vac), while string inverters can operate up to 500 Vdc, depending on panel selection and array design. The same power at higher voltage means less current. Less current means less loss.

[Jest Waitn]

Be aware of "Line Loss" with these long runs from the solar, to main panel to utility pole or transformer. Any line loss will reflect a an increase in grid voltage, and at some point, the inverters start to shut down from overvoltage in your long line runs. Solution, shorter runs, heaver cable (or aluminum feed conductors)

regarding this 'line loss', are string inverters 'better' at handling long runs than micro-inverters?

[Jest Waitn]

One thing with SAM, if you use it, I'd suggest doing your own POCO tariff rates, or at least confirm the one's SAM uses. I've found SAM's rates for my POCO to be incorrect and incomplete after several years of use. Pass that on to your millennial coworker and tell him it's from a long retired P.E. prick.

had to look up this S.A.M. to find out what was being discussed. like 2 things i've already seen in the SDK: linux 64-bit, and python!

[J.P.M.]

J.P.M. Knows whats up. Get your design appropriate for your usage or planned future usage. I feel ancient now cause I have this millennial in the office that snubs PV watts cause he uses S.A.M. either way, you gotta design for smart economics. I don't deal much with usage numbers anymore, but I am responsible for catching inefficient designs. but dont design to profit, i know this. you want a short ROI and then its all profit from your investment after that.

Well, maybe. Opinions vary.

It's been written (Duffie & Beckman, chap. 11, "Solar Process Economics") that: "Solar processes are generally characterized by high first cost and low operating/fuel costs. This leads to a comparison of an initial known investment (in the PV equipment), with estimated future operating costs/potential fuel cost savings.............
The objective of the economic analysis can be viewed as the determination of the least cost method of meeting the energy need, considering both solar and non solar alternatives. For solar energy processes, the problem is to determine the size of the solar energy system that gives the lowest cost combination of solar and aux. energy. systems."

Reducing an energy bill can be thought of as a way to make a profit, but it's not usually treated that way.

There are any number of sources of information on process economics and life cycle costing. Duffie & Beckman is one. There are lots of others. It ain't rocket science, but it does take some knowledge of the time value of money, and life cycle costing. In any case, before spending perhaps ten's of thousands of dollars on a PV system, and/or DIY labor, a cost analysis that's a bit more sophisticated than the moron method of initial cost/annual savings and calling that "payback" may be warranted. PV systems are costly and can last a long time. It's thought by at least the smart money that spending time to learn and do some economics based on a long view, the time value of money and consideration/estimation of other factors such as discount rates, taxes, etc, can pay big dividends.

SAM uses a method known as the LCOE (Levelized Cost of Energy) method. It's basically process economics on a cost per kWh basis. See the references in SAM for particulars. Or see the NREL website for more information.

Whatever method chosen, if the lowest long term cost of meeting an energy need is one objective of getting solar, as is usually the case, some number crunching that's more than initial cost/initial savings is necessary. Often, such an analysis, if it uses realistic assumptions and time frames will show that solar energy is not the most cost effective way to meet an energy need, and may well be cost ineffective. What often happens then, even when folks understand the concepts (which is rare), is they wind up using unrealistic assumptions, like unrealistic time frames (say 30 yrs)., or impossible discount rates or very high salvage (resale) values, or etc. to justify a foregone decision based on lemming behavior and/or emotion, and back into a B.S. justification and call it sound economics, mostly (it would seem) because they used a lot of fancy formulas.

As for SAM vs. PVWatts, SAM is pretty much PVWatts on steroids and can be useful if you know what you're doing and understand the engineering involved. FWIW, and IMO only, for most folks, using SAM can be like giving a loaded pistol to a 2 yr. old, with more than a few users not knowing as much as the diff. between a kW and a kWh. Maybe your millennial coworker is not one of those people. Anyway, for most initial sizing (and, for better or worse, usually final sizing as well), PVWatts is fit for purpose.

One thing with SAM, if you use it, I'd suggest doing your own POCO tariff rates, or at least confirm the one's SAM uses. I've found SAM's rates for my POCO to be incorrect and incomplete after several years of use. Pass that on to your millennial coworker and tell him it's from a long retired P.E. prick.

[Jest Waitn]

I get busy so be patient. I just wrapped up my largest and most complicated design yet. only a 208kw. 830 panels. 1000vDC to 480v wye stepped down to 240 delta high leg before the interconnection. two different services. part of it hits a closed delta high leg and the other hits an open delta high leg. Gotta love rural power! lol. I asked many engineers for help in transformer selection but I figured out all the options before they could, but I am more familiar with the solar application. And wild leg set-ups just confused those city folk. lol

i have a military generator that outputs 208vac & 240vac. it's real loud. might even wake the dead. will run the entire house, but haven't had to use it.

[Jest Waitn]

I design a lot of rural systems and when an easier more efficient way is possible I will find it, but only if it is safe and compliant. It sounds like you have much more to consider then I than I thought. hard do give proper design advise through text when it is not a straight forward situation. I don't know why I couldn't view your drawing last night but I'd love to see it and redline it or redraw for you if needed. I've always been a teacher at heart so I signed in to this forum to satisfy my crave to teach people that want to learn. (but I do know enough to know what I don't know). Maybe there is something I did wrong or maybe we can link by email. I am new to forums when it comes to contributing. So there is one thing I don't know. lol

the design was in sketchup and exported to a .pdf; guess it didn't do well. i'll try again.

this is rural residence; originally done before 1990. 200W main panel on pole. 50A breaker feeds 125A sub-panel via 3-wire service cable (ground not carried to sub-panel; bonded neutral/ground carried). bought the 125A sub because it supports four 220V circuits. distance between sub-panel & main is about 120'. distance between main panel & transformer is about 150'.

planned for three arrays maximum contingent on performance of each implemented. just about to finish work on the first array. bought 250' of 12/3 wire to connect the 3 arrays to the sub-panel. about 50' remained after the wire was pulled. so the arrays are not far (~50') from the sub-panel.

[Jest Waitn]

FPL is my POCO.

i just learned that FPL does NOT support virtual net metering. so the 9kw option probably just left the table...

[McMac]

A respectful suggestion or two before you go further:

1.) Check out if net metering is possible w/ your POCO and if so, find out what the deal is, including reimbursement for over generation.

2.) Your sizing estimates leave a bit to be desired. If you're in a sunny climate, 5 kWh/day/m^2 (which is the technically correct version of "sun hours" and avoids a lot of neophyte confusion) is probably a decent est., but it's relatively easy to get a bit harder est. If you have not done so already, check out something called PVWatts on the net. Read the help screens and use a 10 % system loss parameter rather than the 14 % default rating. Get the azimuth and tilt as close as you can measure or estimate.

If that 5 kWh/day/m^2 average daily irradiance does turn out to be a reasonable representation of reality, and unless you have well defined and certain plans for a rather large increase in usage, a well designed 9 kW system will produce a lot more electricity than you're likely to use. If your POCO is like most, the meager amount they'll pay you for overproduction on a NEM arrangement is peanuts and nowhere near what you're charged for power, making overproduction a losing proposition in most cases.

Overproduction may feel good and suit yourself. I'm only suggesting that you're looking way oversized for your stated usage based on what I see, and IMO only, as a lack of information and familiarity w/ PV on your part. Kind of like oversizing HVAC equipment, except the penalty from oversizing PV is a lot more costly than oversizing conventional HVAC equipment.

Take what you want of the above. Scrap the rest.

J.P.M. Knows whats up. Get your design appropriate for your usage or planned future usage. I feel ancient now cause I have this millennial in the office that snubs PV watts cause he uses S.A.M. either way, you gotta design for smart economics. I don't deal much with usage numbers anymore, but I am responsible for catching inefficient designs. but dont design to profit, i know this. you want a short ROI and then its all profit from your investment after that.

[sensij]

sizing to 125% of inverter output applies to the ampacity of your conductor. once a conductor is chosen, then you size your over current protect for the conductor, not the inverter output.

690.8(B)(1)(a) disagrees with you.