Help Understanding 120% Rule

[jmoles]

I’d like to put up a 10 - 12Kwh panel array. I’m trying to figure out what the cost effective path is for me to do my grid tie - derating my main breaker, upgrading my breaker, etc.

How did you do your grid tie and what did you find to be the right solution for your install?

My current panel has a 200 amp Main breaker and 40 slots. While I don’t have any open slots I have about 10 slots with 15 amp breakers so I could open up slots by adding in double pole breakers.

From what little I know, right now it seems the cheapest and easiest option would be derating my breaker.

 

[MisterSandals]

I’d like to put up a 10 - 12Kwh panel array.

Is this an array that produces 10-12kWh per day or a 10-12kW array?

12kWh / 5 hours solar = 2400W array.

How did you do your grid tie and what did you find to be the right solution for your install?

You need a grid tie inverter or micro inverters for each panel (i have these and like them a lot as they connect to 15A breakers in my electrical panel with 12-3 wiring).

 

[Bkord]

How many kw can your inverter backfeed to the grid? Your pv array dose not determine that number.

 

[jmoles]

Thanks for the questions…

I’m aiming for a system size between 10 - 12 kw. So for example, 30 x 400watt solar panels on the roof. I’d be using Microinverters.

I’m looking at Hoymiles, AP Power DS-3, or Enphase.

 

[kennyb]

I’d like to put up a 10 - 12Kwh panel array. I’m trying to figure out what the cost effective path is for me to do my grid tie - derating my main breaker, upgrading my breaker, etc.

How did you do your grid tie and what did you find to be the right solution for your install?

My current panel has a 200 amp Main breaker and 40 slots. While I don’t have any open slots I have about 10 slots with 15 amp breakers so I could open up slots by adding in double pole breakers.

From what little I know, right now it seems the cheapest and easiest option would be derating my breaker.

What size is your load centre busbar? Is it 200A or 225A?

200 with a 200 main is 40A breaker or 32A continuous , 225 with 200 main is 70A breaker or 56A continuous

Busbar*1.2 -(main breaker size) = back feed breaker max.

Whatever you decide on, it'll probably be easiest to land all the Ac couple strings into an appropriate sized subpanel, then use the lugs to back feed a breaker in your main load centre.

 

[BentleyJ]

While I don’t have any open slots I have about 10 slots with 15 amp breakers so I could open up slots by adding in double pole breakers.

They are actually called "Tandem" breakers. Sometimes the manufacturer will publish the number of tandem breakers allowed in a particular panel based on its model number.

You can derate your 200A main to 175A if one is available for that panel. However, In addition to the 120% rule, there also a load calculation that determines the minimum panel/breaker amperage capacity. If you live in a large, all electric home with more than one A/C unit for example, it's possible the main breaker cannot be derated.

 

[jmoles]

What size is your load centre busbar? Is it 200A or 225A?

200 with a 200 main is 40A breaker or 32A continuous , 225 with 200 main is 70A breaker or 56A continuous

Busbar*1.2 -(main breaker size) = back feed breaker max.

Whatever you decide on, it'll probably be easiest to land all the Ac couple strings into an appropriate sized subpanel, then use the lugs to back feed a breaker in your main load centre.

My main panel breaker is 200A with 40 slots.

So do I add up the total max amperage of my microinverters to determine my system’s max amp output?

If I went with 29 Enphase IQ8M it’d look like: 29 x 1.35a = 39.15a So I could use a 40 amp breaker? Or am I missing something…..

 

[kennyb]

My main panel breaker is 200A with 40 slots.

So do I add up the total max amperage of my microinverters to determine my system’s max amp output?

If I went with 29 Enphase IQ8M it’d look like: 29 x 1.35a = 39.15a So I could use a 40 amp breaker? Or am I missing something…..

No you're over the continuous rating so you'll need to derate. And go up a breaker and wire size.

Are you sure the bus is 200 and not 225?

 

[jmoles]

No you're over the continuous rating so you'll need to derate. And go up a breaker and wire size.

Are you sure the bus is 200 and not 225?

Very helpful, thank you...so the feeder breaker needs to be sized at 125% of the total amperage.

My panel has a 200a Main breaker. I’m not sure about the bus bar. The inside panel reads:

“QOM2FP- CLOSES 150-225 AMP. MAIN BREAKER OPENING.”

 

[zanydroid]

200 with a 200 main is 40A breaker or 32A continuous , 225 with 200 main is 70A breaker or 56A continuous

To be precise, solar isn't really rated as continuous and I don't think continuous is the right way to think about this.

Solar is separately given a 125% scaling factor (and it is 156% in some cases, but 125% for microinverter). So you can't say "my solar is not going to be at peak output because of reason X", you have to do 125% of AC nameplate UNLESS you have some approved way of limiting export via configuration.

I’m looking at Hoymiles, AP Power DS-3, or Enphase.

If I went with 29 Enphase IQ8M it’d look like: 29 x 1.35a = 39.15a So I could use a 40 amp breaker? Or am I missing something…..

Enphase has a bad reputation on this forum currently for poor AC coupling compatibility with hybrid inverter systems that don't also come from Enphase. And Enphase hybrid inverter/batteries kind of suck.

Enphase has a 20A branch circuit limit so you need two 20A 2 pole breakers and split the IQ8's accordingly

Hoymiles has a 30A branch circuit limit which gives more wiring flexibility.

I think Hoymiles might be slightly more DIY friendly, there are two distributors I know working with DIY. One of them (ncsolarelectric) is on this forum, and is who I bought my Hoymiles system from. Price out HMS1600-4T versus 4 IQ8Ms, the HMS1600 should be a fair chunk cheaper while also clipping less. Looking at the IQ8M, they can also clip due to 12A input current which is typically slightly under the STC output 54 cell panels (quite common these days), vs something 14A for HMS1600s which is squarely above the STC output of 54 cell panels.

You would need 7 HMS1600-4T and 1 HMS800-2T for your goal of hitting 30 panels.

IMO the main downside of Hoymiles vs Enphase is that it is unlikely for Enphase to disappear from US market, and it's much easier to buy Enphase accessories. There's also a good possibility that the handful of distributors for Hoymiles that do DIY will retire at some point. BUT if you save crap a ton of money up front, maybe you don't care about that.

Personally I think the bad IQ8 interoperability is a much worse problem.

You should investigate whether AP Power is actively anti-DIY.

Contrary to another post above, I don't think you need a separate combiner panel for all microinverters. For Enphase you should install one b/c you need one to hold some Enphase magic to comply with their standard installation.

For my Hoymiles install I used tandem breakers, but you can also achieve 40A with two single breakers at the bottom of your panel.

(i have these and like them a lot as they connect to 15A breakers in my electrical panel with 12-3 wiring).

Most microinverters these days are connected with L-L-G, rather than L-L-N-G.

“QOM2FP- CLOSES 150-225 AMP. MAIN BREAKER OPENING.”

Much simpler if post a picture of the whole wiring label and we'll hunt for the busbar rating.

There's a QOM2175VH 175A breaker.

 

[kennyb]

While technically correct you are limited to a 20A circuit, Enphase limits you to 11 micros. Or in this case 15A, which is just shy of 80% of the breaker rating - which you are limited too. 12 would be over at ~16.5A and not allowed.

So they'd need 3 circuits for the iq8m

With the hoymiles yes technically correct 30A, but you're limited to 4 per string for the 1600, or 24A, which is conveniently 80% of breaker capacity.

So they'd need 2 circuits for the hoymiles.

A sub is the way to go IMO especially if they end up with a AIO battery inverter down the line. Much easier to just feed the sub directly to the AIO than to have to rewire since you can't backfeed the load of an AIO apparently.

Enphase has a 20A branch circuit limit so you need two 20A 2 pole breakers and split the IQ8's accordingly

Hoymiles has a 30A branch circuit limit which gives more wiring flexibility.

 

[zanydroid]

With the hoymiles yes technically correct 30A, but you're limited to 4 per string for the 1600, or 24A, which is conveniently 80% of breaker capacity.

Sure, I previously mentioned that PV AC output circuits need the 80%/125% adjustent factor in the first paragraph of #10. And my point was that you can get more onto a branch circuit of 30A than one of 20A.

A sub is the way to go IMO especially if they end up with a AIO battery inverter down the line. Much easier to just feed the sub directly to the AIO than to have to rewire since you can't backfeed the load of an AIO apparently.

If you go to a hybrid inverter a lot of rewiring is needed anyway, so it's not super necessary IMO to plan ahead. Unless installing a hybrid inverter. In that case, the physical location of the hybrid inverter needs to be planned out in advance to satisfy UL9540 and residential ESS code requirements, which are quite restrictive. As well, solar AC wiring is very relaxed compared to other kinds of AC and ESS wiring.

I haven't looked extensively into the AIO situation. For some AIO that support AC coupling (well, there's only a handful right now with well documented AC coupling, Schneider SW is one, compared to hybrids/on-grid inverter chargers) the AC coupled solar has to traverse through the AIO as a middleman before it goes to the grid, so the AIO can detect excess AC and absorb it in the charger. Some AC batteries are not capable of grid forming for grid tie inverters, only to absorb their excess power, this means there's much less need for the GTIs to be connected directly to the battery (IE to the critical loads port or generator port).

 

[jmoles]

Very helpful, thank you...so the feeder breaker needs to be sized at 125% of the total amperage.

My panel has a 200a Main breaker. I’m not sure about the bus bar. The inside panel reads:

“QOM2FP- CLOSES 150-225 AMP. MAIN BREAKER OPENING.”

Well as luck would have it, my panel does not have a simple label saying bus bar rating. The first picture is of the sticker from the panel cover. The other pictures are of the sticker on the inside of the panel.

 

[zanydroid]

I'm guessing it's 200 since 225 looks like a separate part in the catalog. Check page 6 in the catalog for how to decode the part number.

https://www.se.com/us/en/product/QO140M200P/load-center-qo-1-phase-40-spaces-60-circuits-200a-convertible-main-breaker-pon-nema1-ul/

https://download.schneider-electric.com/files?p_enDocType=Catalog&p_File_Name=1100CT0501.pdf&p_Doc_Ref=1100CT0501

I think the QOM2FP is just the Filler Plate (FP)

 

[jmoles]

I think Hoymiles might be slightly more DIY friendly, there are two distributors I know working with DIY. One of them (ncsolarelectric) is on this forum, and is who I bought my Hoymiles system from. Price out HMS1600-4T versus 4 IQ8Ms, the HMS1600 should be a fair chunk cheaper while also clipping less. Looking at the IQ8M, they can also clip due to 12A input current which is typically slightly under the STC output 54 cell panels (quite common these days), vs something 14A for HMS1600s which is squarely above the STC output of 54 cell panels.

You would need 7 HMS1600-4T and 1 HMS800-2T for your goal of hitting 30 panels.

IMO the main downside of Hoymiles vs Enphase is that it is unlikely for Enphase to disappear from US market, and it's much easier to buy Enphase accessories. There's also a good possibility that the handful of distributors for Hoymiles that do DIY will retire at some point. BUT if you save crap a ton of money up front, maybe you don't care about that.

I'm interested in the cheapest solution... based on my limited searching it looked like Enphase was way more than Hoymiles or AP, without much good reason for the extra $$. So I guess next I'd need to compare the DSW to the HMS1600-4T to see which would be the better option.

Sounds like you think the Hoymiles is a bit easier to work with. Is that because I'd only need to install 1 for every 4 panels?

Ap Systems DS3 (QTY:15) = $2560

Enphase IQ8+ (QTY: 30) = $4620

Hoymiles (QTY: 7) = $2695 (this system would only take 29 panels)

 

[zanydroid]

I'm interested in the cheapest solution... based on my limited searching it looked like Enphase was way more than Hoymiles or AP, without much good reason for the extra $$. So I guess next I'd need to compare the DSW to the HMS1600-4T to see which would be the better option.

Sounds like you think the Hoymiles is a bit easier to work with. Is that because I'd only need to install 1 for every 4 panels?

You may want to add in the balance of system components - IQ Combiner etc etc for Enphase (I think there's a minimum amount of hardware you need to get full warranty coverage) and DTU-S for Hoymiles. DTU-S is optional but I sort of want to buy everything up front for something that is less widely distributed.

I don't have experience with AP systems. Whether on the installation side, support, or planning... Maybe search the forum for owners and tag them with questions.

Ap Systems DS3 (QTY:15) = $2560

Enphase IQ8+ (QTY: 30) = $4620

Hoymiles (QTY: 7) = $2695 (this system would only take 29 panels)

You're going to need to add a one-port micro from Hoymiles to get to 29, which is probably like $180. Also you linked the 2000W version, which may force you to increase the AC size of your wiring / get you over some regulation threshold with AHJ, among other liability like making AC coupling potentially more complex down the line. The 1600W is not really that much cheaper but simplifies these matters.

Of course it would be even cheaper to go with a DC coupled system (50% of Hoymiles), and the savings likely get even bigger if you want to go to hybrid/battery down the line. The tradeoff is more up-front design complexity with DC and probably more work to wire it up properly / to code.

(granted, AC has extra design complexity down the line if you want to go hybrid/battery, however this is more of getting it to work reliably at a good price vs melting down or not being able to get a permit / pass inspection)

 

[Mike 134]

I’d like to put up a 10 - 12Kwh panel array. I’m trying to figure out what the cost effective path is for me to do my grid tie - derating my main breaker, upgrading my breaker, etc.

How did you do your grid tie and what did you find to be the right solution for your install?

My current panel has a 200 amp Main breaker and 40 slots. While I don’t have any open slots I have about 10 slots with 15 amp breakers so I could open up slots by adding in double pole breakers.

From what little I know, right now it seems the cheapest and easiest option would be derating my breaker.

/\ /\ /\ This change to a 150-amp breaker then you can add up to 90amps on a 2-pole breaker from the solar equipment and not exceed the 120% rule. 200A buss @120% equals 240A 240-150= 90.
As to which inverter folks have mentioned is best, I have no idea, I used a string inverter w/optimizers for my setup.

 

[zanydroid]

I would go for 175A main breaker, it is available for this panel. You are less likely to get in trouble with load calculations / run out of service with 175A derate than 150A derate.

Changing the main breaker is a reversible decision but you need to coordinate power shutdown with POCO and probably want to hire an electrician too to guarantee you can do it quickly enough. They probably have a better torque wrench than you would have for the conductor size involved here. So that adds up to $$$ and inconvenience.

 

[Mike 134]

Wouldn't a real electrician just pull the meter? No reason to involve a power company to change the main breaker.

One caveat to the OP make sure the lugs on the new breaker are CU/AL rated I see you have aluminum service conductors.

 

[zanydroid]

Wouldn't a real electrician just pull the meter? No reason to involve a power company to change the main breaker.

Depends on local regulations. That's not allowed here and has lead to some Kafka-esque horror stories proving that it was not done with intend to defraud (IE breaking the tamper-evident seal and creating a gap in the smart meter telemetry)