pv system connected to sub-panel

[J.P.M.]

zip: 32025, longitude ~30 degrees, tilt ~28 degrees, azimuth unknown, believed ~195 degrees.

the 5 hours is an estimate based on observation of sun in the area where the pv array is located.

Thank you.

SWAG ~~ 1,400 - 1,500 kWh/yr. per kW of array, +/- maybe 4-5% or so. A well designed 6 kW array ought to meet most of current usage.

[sensij]

Florida is still on 2011 NEC. That helps with shutdown and grounding requirements, but hurts the sub-panel calculations.

[J.P.M.]

i don't understand what you mean by this exactly. i have shopped around and purchased materials at the lowest cost i could find. given the 30% tax credit on the system, this helps decrease the ROI time. i am doing the work myself whenever possible and when parity in the PV electrical vs. grid usage is achieved then the system will start to pay for itself. the 3kw is a 60% system; the 6kw is a 120% system. this should pay for itself in a few short years. so the 8/9kw will be ~ a 180% system, which will reduce the ROI time even more. given tax credits and system cost, i expect ROI to be ~ 6 years.

I was referring to your goal of "free" electricity. Unless all that's done to replace POCO power is acquired at no cost, it has a cost and is therefore not free.

As for oversizing and how that may affect the financial viability or cost effectiveness of a PV system, tax credits/rebates/etc. or not, if I have an oversized system, the degree to which it is oversized decreases the cost effectiveness of the system, which is the same as saying it either decreases the ROI or increases the payback time. I'm not sure I understand the sense in which you use the term parity. If by parity you mean a system sized so that it will replace all the power purchased from the POCO, then I'd need to disagree with your logic for most situations.

[Jest Waitn]

Thank you.

SWAG ~~ 1,400 - 1,500 kWh/yr. per kW of array, +/- maybe 4-5% or so. A well designed 6 kW array ought to meet most of current usage.

right. 6kw should be 'over the top'.

[Jest Waitn]

I was referring to your goal of "free" electricity. Unless all that's done to replace POCO power is acquired at no cost, it has a cost and is therefore not free.

As for oversizing and how that may affect the financial viability or cost effectiveness of a PV system, tax credits/rebates/etc. or not, if I have an oversized system, the degree to which it is oversized decreases the cost effectiveness of the system, which is the same as saying it either decreases the ROI or increases the payback time. I'm not sure I understand the sense in which you use the term parity. If by parity you mean a system sized so that it will replace all the power purchased from the POCO, then I'd need to disagree with your logic for most situations.

if the monthly bill is $0 <=, then parity is achieved and then some. basically the system will cost about $1/watt with everything added in. is't not just the panels and the inverters.

[McMac]

McMac, how would you justify using a 40 breaker under 2011 NEC? Don't you still need 1.25 x inverter output current, 36 A x 1.25 = 45 A in this case?

The real difference between 2011 and 2014 here is how the backfeed currents are summed. In 2011, if each of the 3 strings is landed in the sub with its own 20 A breaker, 60 A is the number to use for the 120% rule calculation. In 2014, you just use 1.25 x the inverter output circuit for the 120% rule, or 45 A if there are 3 strings with 12 M250s each.

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.

So...
if 45A is your design current then you will most likely use a #8 thwn-2 and that is a perfect fit for a 40A breaker. But there are derate factors for ambient temp. and number of current carrying conductors in conduit that will lower the ampacity and step you to a #6 real quick.

You need to get another little 60A sub-panel to use as your 'AC Combiner'. Labels will state that it is PV AC Combiner and no loads can be added. This is because, without loads on a bus bar, you dont have to worry about the main supply breaker to the bus. I generally size my AC combiners so the design ampacity (125% inverter output) is no more than 100% of bus ampacity. If there is a rule here I have missed it but I'm sure I'm under if not on the mark. So you take the combined feed on you're #8's out of the AC combiner and feed that into your sub-panel for the shed through a 40A breaker at the opposite end from the main OCPD. So 40A is still your adder for back feed calculation. Make sense? I have no problem breaking it down further. This is my passion. I started installing and now I design and engineer.

[McMac]

Florida is still on 2011 NEC. That helps with shutdown and grounding requirements, but hurts the sub-panel calculations.

I don't know any AHJ in my are that has adopted 2014. and since rapid shutdown changed from 2014 to 2017 I would they jump to 2017 as that was the way I really interpreted it all along. Either way, this tech is rapidly changing and we can't wait on the AHJ's to adopt the latest safety standards. It is our responcibilty and due diligence to try to adhere to the most current code if we are talking important safety standards like rapid shutdown. don't wait. just do it. If you are installing micros or DCOs then you are ahead of the game

[McMac]

Unless you get the grid alternate means of supplying electricity for nothing in mat. and labor including the value of sweat equity, you will not achieve # 1.

On making a few bucks, if your POCO is like most that allow net metering, you'll probably find they pay a relatively small amount for overproduction vs. what they charge. Also, net metering is, in general, becoming less "net" than it once was.

Just a thought: Are you under the impression that a 9 kW system will only produce 9,000 kWh/yr.? If so, a suggestion: Do yourself a favor and educate yourself about how much a PV system will produce at your location. If you know, and don't care, forget I mentioned it.

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

Oh yeah. don't ever think PV is for profiting... at least for now. Is your true up monthly or annual? If coop then they aren't even required to net meter. We used to tell people that based a 25year investment that your solar power costs you around $.06 KWH. that changes from market and over time. I don't have my head in the dollars and cents of the company anymore so I'm not sure where its at now. but the point is that you receive an 'avoided' cost for overproduction which was $.03 at the time. So in a sense, you loose money to overproduce. But depending on the type of net metering will determine how we size and design systems.

[McMac]

the average KWH usage/day for 15+ years is 23.2KWH. the average/month is 696KWH. the high/day is 45.5KWH; the low/day is 9.6KWH. so this is modest consumption of electricity.

the main panel is a pole mounted 200A 240VAC square d outdoor unit. on the main panel, the well has a 20A breaker. the A/C has a 30A breaker, but it is not in use presently. window units are used. the house has 100A breaker and the sub-panel is fed by a 50A breaker. the total breaker values = 200A, but the effective load is nowhere near this value. the PV array is connected to a 20A breaker at the sub-panel. there is 60A switched cut off (not breaker) at each PV array for convenience. the cutoff is not required by the power company.

i haven't yet done the breakdown of the panels as above, but the ampacity should be fine due to the modest usage.

i believe option 1 is doable, if it needs to be done.

the z brackets arrive tomorrow and the PV panels will be mounted on the PV framework. then all the 'good stuff' happens.

thanks for the support.

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

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

[McMac]

Truthfully I don't mind offering my work email to one person at a time if the forum will allow it. All i do is make drawings and work scopes

[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.

[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.

[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...

[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]

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.