pv system connected to sub-panel

[Jest Waitn]

Yes. If you stick with less than 32 A total, on two strings of 16 panels, I think your existing sub-panel, sub-panel feeder, and main panel can be left unchanged. If you increase over that value, you'll have to derate your main breaker for sure, need to work through the sub-panel calculations appropriate for your specific NEC code cycle to figure out if the feeder size and OCPD are correct.

thanks, this is my belief. i haven't gone through these calculations in detail (just ball-parked them) yet, but will. FWIW, the max # inverters/circuit/array is 14 per enphase. i pulled wire for 3 circuits/arrays. the plan is to build an array each year for 3 years, if warranted.

[sensij]

the gcl panels produce 290wdc and produce 250wac @ 1a through the m250 micro inverter. 12 are daisy chained, so this gives 12*250 = 3kw @ 1A/ea = 12A max output, ~13.2A peak.

Yes. If you stick with less than 32 A total, on two strings of 16 panels, I think your existing sub-panel, sub-panel feeder, and main panel can be left unchanged. If you increase over that value, you'll have to derate your main breaker for sure, need to work through the sub-panel calculations appropriate for your specific NEC code cycle to figure out if the feeder size and OCPD are correct.

[J.P.M.]

thanks for the input.

i don't see much difference between an 8kw & a 9kw system; total difference is about $1k. the framework for the 8kw system is only slightly less than the 9kw and the panels are dirt cheap now. bought mine for 25 cents/watt (gcl class a from sunelectric). the micro inverters are $100/ea.

but if there is a problem with over production as noted above then unless the virtual net metering is supported, i'll stick with the smaller system. i have no problem with this.

i expect to achieve ~5 hours average sun time daily * 365 = 1825 * 9kwh = 16,425kwh annually. with 2 arrays (6kw): 365*5*6 = 10,950kwh annually. so with 1 array (3kw): 365*5*3 = 5,475kwh annually. my belief is that the 3kw system is about a 60% system. it might work out that the 6kw system is too much given the distance and some of the inverters need to be disconnected from the end of the chain. time will tell.

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.

[SunEagle]

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.

It really isn't how quickly your ROI or payback can be that determines if you will ever get "Free" electricity. All it means is that you can generate power at a lower cost then purchasing it from your POCO. But during all the times you can't generate your own power will mean you will be purchasing the electricity so not "Free" but maybe cost avoidance depending on the Net metering contract you have.

[Jest Waitn]

9 kW is killing your design. Stick with 32 M250's or smaller, much more compatible with your existing electrical service. Please consider J.P.M.'s question carefully... how are you estimating the kWh output of your system?

the gcl panels produce 290wdc and produce 250wac @ 1a through the m250 micro inverter. 12 are daisy chained, so this gives 12*250 = 3kw @ 1A/ea = 12A max output, ~13.2A peak.

[Jest Waitn]

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.

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.

[Jest Waitn]

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.

thanks for the input.

i don't see much difference between an 8kw & a 9kw system; total difference is about $1k. the framework for the 8kw system is only slightly less than the 9kw and the panels are dirt cheap now. bought mine for 25 cents/watt (gcl class a from sunelectric). the micro inverters are $100/ea.

but if there is a problem with over production as noted above then unless the virtual net metering is supported, i'll stick with the smaller system. i have no problem with this.

i expect to achieve ~5 hours average sun time daily * 365 = 1825 * 9kwh = 16,425kwh annually. with 2 arrays (6kw): 365*5*6 = 10,950kwh annually. so with 1 array (3kw): 365*5*3 = 5,475kwh annually. my belief is that the 3kw system is about a 60% system. it might work out that the 6kw system is too much given the distance and some of the inverters need to be disconnected from the end of the chain. time will tell.

[J.P.M.]

#1) want FREE electricity. 90% of tier 1 grid-tied pv system = ~9kw. #2) make a little $ off of it.

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.

[sensij]

9 kW is killing your design. Stick with 32 M250's or smaller, much more compatible with your existing electrical service. Please consider J.P.M.'s question carefully... how are you estimating the kWh output of your system?

[Jest Waitn]

OP: NOMB or concern, but a 9kW system for an 8,400 - 8,500 kWh/yr. load seems maybe a bit oversized. How did you determine the array size ? Where are you located ?

#1) want FREE electricity. 90% of tier 1 grid-tied pv system = ~9kw. #2) make a little $ off of it.

[J.P.M.]

OP: NOMB or concern, but a 9kW system for an 8,400 - 8,500 kWh/yr. load seems maybe a bit oversized. How did you determine the array size ? Where are you located ?

[Jest Waitn]

However, you are going to run into a problem with the 120 rule, 705.12(D). 2011 said sum of breaker ratings of the back feed and main OCD can be over 120% of the busbar rating and only IF (now this is a big 'IF') the back feed OCD is at the furthest position from the main OCD. but then 2014 said that you need to use 125% of the inverter output plus the main OCD rating to get your sum that can go 120% over the the bus ampacity still with the same big 'IF'. So what does that mean for you? Thought youd never ask .

...2011...
back feed breaker + main breaker cant equal more than 120% of bus.

...at the sub
125A busbar x 1.20 = 150A Allowable. So our sum cant be more than 150A here.

36A would need 40A breaker. assuming 125A main breaker thats a sum of 165A
Option 1. Downsize main breaker to 100A then you're golden here but maybe not at the main, we'll see when we get there get there. Don't forget to apply the appropriate label.
Option 2. Line Tap. I just use polaris blocks to tap the feeders right above the main breaker. Done it a lot and never had a single AHJ question it. But you need to make there is still an OCPD for the wire. now that can come from your AC combiner being a main breaker if it is less than 4ft from lug to lug (don't quote me on that distance) but best practice is to just use a fused disconnect and size the fuses for your wire. This puts your OCP even closer to the interconnection.

...at the main
200A x 1.20 = 240A Allowable
Well for all intensive purposes we have turned the 100 amp supply breaker for the sub-panel into a back feed breaker. If AHJ is on 2011 but has a head on his shoulders he will use the 40A that you probably use for OCP to get a sum of 240A and then your fine. And the same should apply for the line tap too.


...2014...
inverter current x 1.25 + main breaker cant equal more than 120% of bus.

...at the sub
36 x 1.25 = 45A So this is still the same option set. You can downsize the main breaker in the sub to 100A or line tap to make this work. However, this is gonna bite us at the main...

...at the main
200A bus x 1.2 = 240A Allowable
at 125% of inverter current we are looking at 45A. now add the main breaker and we are 245A. No Go.

Option 1. Downsize Main Breaker to 150A. you'll probably be fine with that but I don't know your loads. You could try to find a breaker in between but good luck. It wont be easy if you do.
Option 2. Remove the subpanel feeders from the main panel into a junction box where you can splice then reroute them outside to a 100A fused disco w/ 100A fuse and then carry them back inside and into the main panel and line side tap for the interconnection. don't splice in the panel and go out to the disco cause then you've used your load center as a raceway and that is not allowed. once a wire is in the load center it must be terminated. so you can splice to make wires longer to reach a breaker but not to carry the conductor out of the load center cause then it would just be passing through.
Option 3. Trench 120 feet to the house, hit the disco, then pop inside and make the interconnection.

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.

[Jest Waitn]

I couldn't view your drawing so let me just make you aware of a couple things based on what I've gathered here.

off the cuff, 36A should be fine in terms of voltage drop. even if its aluminum you are probably around a 2Vac drop and just over 100ft. thats still under 1%. Industry standard is 1% DC Loss and 2% AC loss. but definitely dont want to be over 5% cause you start to make the inverter want to crank up the voltage to overcome the added impedance in order to back feed the grid. The inverter has to stay at 240V + or - 10% and therefore it shuts down above 264V.

this will be three different circuits, each on a 20A breaker at the sub-panel. 12A max current, ~13.2A peak. no real load on the 125A sub-panel, except as PV supply.

this 9kw system might not work connected to the sub-panel for the reasons you state. 9kw production will exceed consumption by a good margin. the 3kw system might work fine, since the distance to the main panel is ~120', the electricity consumed by the home might not result in the over-voltage condition.

so the 9kw plan might NOT be implementable unless virtual net metering is allowed. i will deal with this issue after becoming a net metering customer.

[Jest Waitn]

Having a meter to send power through and one the use power is often refered to as virtual net metering. but I havn't found a utility or coop in our area that will allow it. Tried to do it to save trenching a 70kw over 600ft but they wouldn't have it.

ok. virtual net metering sounds good. what i plan to do is to first become a net metering customer and then try this avenue since it will mean a much shorter distance to the transformer. eventually i plan this to be a 9kw system. this year it will be 3kw with 3kw added each year unless there are problems due to the distance.

70kw over 600' - wow, that's a real system. might run DC the distance, but put fencing along the trench. i'm not a fan of running DC over a distance due to the potential lethality.

[sensij]

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.