innersurffer
Member
i sent in the pictured system below for certification by a state of Florida Solar governing body.
Hold down the control (Ctrl) and Scroll at the same time to make this picture larger.
Anyway my question is the breaker sizing and the 120% rule related.
Please answer from my responses to the AHJ below.
"Do I have to go with the max ampacity for the the SMA SB5000TL-US of 30 amps?
If the string size in permitting, and including over-irradiance, can I go to 25 amps like my design calculates?
(Please look at my 3-line drawing and the multiple email comments below)
From his writings (that I have to comply with-he?s like an AHJ) I think that he has good theory and book sense, but has no field experience.
He?s killing me on this one amp.
This will be the 4th revision.
Please help me out !
AHJ COMMENTS:
"SMA doesn't state that the max AC side breaker is 22A- it says that's the maximum current. Looking at the manual in more detail, see page 79:
http://files.sma.de/dl/18726/SB3-5TLUS22-IA-en-12.pdf
Usually for a 5kW 240V inverter we'd calculate the max continuous output as 5000W/240V = 20.8A (690.8(A)(3)). Inverters are usually designed so that they can put out slightly more power than their rating to ensure that they can produce at their rated power, but we don't need to consider that-- just the published rating of the inverter isfine (an Enphase 215 can do 224W, for example, and according to the gosolarcalifornia website the SMA here can actually do 5070W at 240V). The SMA docs show that it can do 22A max, which appears to correspond to its 208V rating-- 4550W / 208V = 21.9A. Since p79 of the manual shows that the max 240V rating is 5000W, 5000W/240V = 20.8A. You then apply the 125% that you're used to applying on the ac side for continuous load and you get a minimum breaker size of 26A. Per the inverter manual, the largest breaker you can use is a 30A. So, given equipment availability and the general desire of the industry to not fabricate custom UL listed 26A breakers, you're stuck here with 30A breakers. 25A breakers just aren't big enough. The practical impact is that if you operate a 25A breaker at 21A continuous, "officially" you could have nuisance tripping when you just have a really nice day and maximum production. In reality, it's unlikely this would ever happen. However, NEC requires a 30A breaker here and since it's easily accommodated in the panels thanks to the 120% rule and the 25A and 30A breakers are the same price anyway, there's no reason to use a 25A breaker here.
Your "efficient will never be one to one" comment is valid in one way- you'll never have an inverter that is greater than 100% efficiency. However, don't forget that inverters not only invert but also convert. If you had your array producing 16Adc @ 330Vdc, that's 5280Wdc. At 96% conversion efficiency the 240V inverter would be putting out its maximum 5070W or 21.1A. 16A -> 21A doesn't mean we're breaking the first law of thermodynamics."
:weeping:
ALL OVER 1 AMP.
Someone help me out.
Hold down the control (Ctrl) and Scroll at the same time to make this picture larger.
Anyway my question is the breaker sizing and the 120% rule related.
Please answer from my responses to the AHJ below.
"Do I have to go with the max ampacity for the the SMA SB5000TL-US of 30 amps?
If the string size in permitting, and including over-irradiance, can I go to 25 amps like my design calculates?
(Please look at my 3-line drawing and the multiple email comments below)
From his writings (that I have to comply with-he?s like an AHJ) I think that he has good theory and book sense, but has no field experience.
He?s killing me on this one amp.
This will be the 4th revision.
Please help me out !
AHJ COMMENTS:
"SMA doesn't state that the max AC side breaker is 22A- it says that's the maximum current. Looking at the manual in more detail, see page 79:
http://files.sma.de/dl/18726/SB3-5TLUS22-IA-en-12.pdf
Usually for a 5kW 240V inverter we'd calculate the max continuous output as 5000W/240V = 20.8A (690.8(A)(3)). Inverters are usually designed so that they can put out slightly more power than their rating to ensure that they can produce at their rated power, but we don't need to consider that-- just the published rating of the inverter isfine (an Enphase 215 can do 224W, for example, and according to the gosolarcalifornia website the SMA here can actually do 5070W at 240V). The SMA docs show that it can do 22A max, which appears to correspond to its 208V rating-- 4550W / 208V = 21.9A. Since p79 of the manual shows that the max 240V rating is 5000W, 5000W/240V = 20.8A. You then apply the 125% that you're used to applying on the ac side for continuous load and you get a minimum breaker size of 26A. Per the inverter manual, the largest breaker you can use is a 30A. So, given equipment availability and the general desire of the industry to not fabricate custom UL listed 26A breakers, you're stuck here with 30A breakers. 25A breakers just aren't big enough. The practical impact is that if you operate a 25A breaker at 21A continuous, "officially" you could have nuisance tripping when you just have a really nice day and maximum production. In reality, it's unlikely this would ever happen. However, NEC requires a 30A breaker here and since it's easily accommodated in the panels thanks to the 120% rule and the 25A and 30A breakers are the same price anyway, there's no reason to use a 25A breaker here.
Your "efficient will never be one to one" comment is valid in one way- you'll never have an inverter that is greater than 100% efficiency. However, don't forget that inverters not only invert but also convert. If you had your array producing 16Adc @ 330Vdc, that's 5280Wdc. At 96% conversion efficiency the 240V inverter would be putting out its maximum 5070W or 21.1A. 16A -> 21A doesn't mean we're breaking the first law of thermodynamics."
:weeping:
ALL OVER 1 AMP.
Someone help me out.
