Does Open circuit voltage multiply by number of panels?

[Bugout1]

My growatt inverter manual says that open circuit voltage my not exceed 145 volts DC.
So for a renogy 300 watt, 24 volt panel, the open circuit voltage is 38.80. Since I'm running a 48 volt system, I would need 3 of them to get to 72 volts, above the 60 volt minimum of the MPPT voltage range.
My question is, does that 38.8 volts Open circuit also triple with three panels in series? while that only comes to 116 volts if it does, It would be nice to know if it would be a problem should I grow my system.

 

[Bob B]

Yes ... panels in series add voltage but current stays the same.

Panels in parallel add current .... but voltage stays the same. If you add on in the future you may be able to do a series / parallel combination.

 

[CJMainer]

my understanding is if you run 3 panels in series and it comes out to 116v and you cant exceed 145v then you should be ok. I may be wrong though as im fairly new to looking at this stuff. I plan on getting a 48v Growatt and running 3 arrays of 3 panels in series at 120ish v with 9.68 amps each array. I plan on using a combiner box so im not sure if that changes the equation or not. Im very bad at math so i could be horribly wrong.

 

[upnorthandpersonal]

Panels in parallel don't add current

Typo?

 

[Bob B]

Typo?

Yes Thanks, ..... removed the don't

 

[Dzl]

Yes every panel you add in series will adds to the Voc. You also need to account for increased VOC below 25*C / 77*F.

Voc x 1.25 is a conservative way to ballpark it, or you can do the math for your specific panels based on the datasheet or use a table like this:

 

[Dzl]

For your system, once you get to 3 in series, the simplest way to expand will be to add parallel strings. Once you get to 3 or more parallel strings you need to add individual fuses for each string. At 2 or less you just need to make sure the wire of each string can handle the current of both strings

 

[Bugout1]

Yes every panel you add in series will adds to the Voc. You also need to account for increased VOC below 25*C / 77*F.

Voc x 1.25 is a conservative way to ballpark it, or you can do the math for your specific panels based on the datasheet or use a table like this:

Thanks, however being in the south, we rarely go below 40 degrees. usually over 70. Should I allow for the twice a year freeze in my calculations?

 

[MichaelK]

Thanks, however being in the south, we rarely go below 40 degrees. usually over 70. Should I allow for the twice a year freeze in my calculations?

Only if you don't want you controller fried twice a year.

 

[Dzl]

Thanks, however being in the south, we rarely go below 40 degrees. usually over 70. Should I allow for the twice a year freeze in my calculations?

Yes, since its a safety limit you want to use the absolute minimum temperature for your area, not the typical low or average low. Some people advise to look up the record low for your area and use that.

Though in your case you don't need to pay it much thought, since 3 of your panels in series is safe down to -40*F and 4 of your panels in series could be too much even at 100*F.

 

[Hedges]

At 2 or less you just need to make sure the wire of each string can handle the current of both strings

Nah, I'd say the wire of each string only has to handle the current of the string. With margin, I think that's size for 1.56 x Isc.
Neither wire is every going to carry the current of both strings.

Only after two strings combine and current of both make a home run in a single wire does it need to be sized to handle both.
So that ought to be 2 x 1.56 x Isc.

(I forget if a higher ampacity is required for un-fused wires. 1.56 x is the recommended minimum fuse/breaker size for PV, so obviously wire needs to be sized for at least that much regardless.)

 

[Dzl]

Nah, I'd say the wire of each string only has to handle the current of the string. With margin, I think that's size for 1.56 x Isc.
Neither wire is every going to carry the current of both strings.

I am not 100% sure, but I think I took the 2x thing from the NEC, let me see if I can find the reference. I may be mistaken.

(I forget if a higher ampacity is required for un-fused wires.)

This is what I am referring to, with the 2x recommendation. Unfused. I am not well versed in any of this so I may be misunderstanding something I read.

Only after two strings combine and current of both make a home run in a single wire does it need to be sized to handle both.
So that ought to be 2 x 1.56 x Isc.

1.56 x is the recommended minimum fuse/breaker size for PV, so obviously wire needs to be sized for at least that much regardless

The 1.56 refers to the main OCP for the whole array?

 

[Hedges]

So it was 2x Isc without fuse?
Nothing to do with 2 strings then, just extra safety margin.
And of course that would become the fuse size to be used with the panel. That tends to be 2x Isc or higher.

Normally we size fuses/breakers 1.25 x the continuous current, to avoid nuisance tripping. Fuses and thermal/magnetic breakers are affected by ambient temperature, trip at less current in hot environments. Which is good for wire ampacity reduction.

PV panels in direct sun with extra illumination reflected or glowing from clouds could exceeds Isc.
That added a 1.25 margin.

Multiply them together and we get 1.56
So that would be the minimum margin in fuse size you want for each string, also for the entire array.

Midnight has magnetic/hydraulic breakers which they say can be used at full current without derating. Maybe that takes away one 1.25 multiple.
But I wouldn't like having no margin. Unless their trip curves actually show they take 1.10 continuously.

 

[Dzl]

I could use another pair or two of eyes on this, but I found the section I was thinking of, its more complicated than I remembered, and actually might be more conservative than I thought.

690.8 Circuit Sizing and Current

(A) Calculation of Maximum Circuit Current.
The maximum current for the specific circuit shall be calculated in accordance with 690.8(A)(1) through (A)(6).

Informational Note: Where the requirements of 690.8(A)(1)and (B)(1) are both applied, the resulting multiplication factoris 156 percent.

(1) Photovoltaic Source Circuit Currents.
The maximum current shall be calculated by one of the following methods:
-- (1)The sum of parallel-connected PV module–rated short-circuit currents multiplied by 125 percent
-- (2)For PV systems with a generating capacity of 100 kW or greater...

skipping down to B

(B) Conductor Ampacity.
PV system currents shall be considered to be continuous. Circuit conductors shall be sized to carry not less than the larger of 690.8(B)(1) or (B)(2) or where protected by a listed adjustable electronic overcurrent protective device in accordance 690.9(B)(3), not less than the currenti n 690.8(B)(3).

(1) Before Application of Adjustment and Correction Factors. One hundred twenty-five percent of the maximum currentscalculated in 690.8(A) before the application of adjustmentand correction factors.

Exception: Circuits containing an assembly, together with its overcurrent device(s), that is listed for continuous operation at 100 percent of its rating shall be permitted to be used at 100 percent of its rating

skipping down to D

(D) Sizing of Module Interconnection Conductors. Where a single overcurrent device is used to protect a set of two or more parallel-connected module circuits, the ampacity of each of themodule interconnection conductors shall not be less than thesum of the rating of the single overcurrent device plus125 percent of the short-circuit current from the other parallel-connected modules.

And finally to 690.9

690.9 Overcurrent Protection.

(A) Circuits and Equipment. PV system dc circuit and inverter output conductors and equipment shall be protected against overcurrent. Overcurrent protective devices shall not be required for circuits with sufficient ampacity for the highest available current. Circuits connected to current limited supplies (e.g., PV modules, dc-to-dc converters, interactive inverter output circuits) and also connected to sources having higher current availability (e.g., parallel strings of modules,utility power) shall be protected at the higher current source connection.

Exception: An overcurrent device shall not be required for PV modules or PV source circuit or dc-to-dc converters source circuit conductors sized in accordance with 690.8(B) where one of the following applies:

-- (1) There are no external sources such as parallel-connected source circuits, batteries, or backfeed from inverters.
-- (2) The short-circuit currents from all sources do not exceed the ampacity of the conductors and the maximum overcurrent protective device size rating specified for the PV module or dc-to-dc converter.

Informational Note: Photovoltaic system dc circuits are current limited circuits that only need overcurrent protection when connected in parallel to higher current sources. The overcurrent device is often installed at the higher current source end of the circuit.

 

[Dzl]

So it was 2x Isc without fuse?
Nothing to do with 2 strings then, just extra safety margin.

If you go back and reread my earlier comment (or if you want to go to the source 690.8 and 690.9) again in context it may make more sense:

For your system, once you get to 3 in series, the simplest way to expand will be to add parallel strings. Once you get to 3 or more parallel strings you need to add individual fuses for each string. At 2 or less you just need to make sure the wire of each string can handle the current of both strings.

And of course that would become the fuse size to be used with the panel. That tends to be 2x Isc or higher.

Normally we size fuses/breakers 1.25 x the continuous current, to avoid nuisance tripping. Fuses and thermal/magnetic breakers are affected by ambient temperature, trip at less current in hot environments. Which is good for wire ampacity reduction.

PV panels in direct sun with extra illumination reflected or glowing from clouds could exceeds Isc.
That added a 1.25 margin.

Multiply them together and we get 1.56
So that would be the minimum margin in fuse size you want for each string, also for the entire array.

Midnight has magnetic/hydraulic breakers which they say can be used at full current without derating. Maybe that takes away one 1.25 multiple.
But I wouldn't like having no margin. Unless their trip curves actually show they take 1.10 continuously.

I think we are talking about different things here. My statement on 2 x Isc (now I am wondering if it is 2 x Isc x some safety factor) dictates the conductor size for unfused strings (At least that is my current shaky understanding). What you are saying might well be correct and accurate for individually fused strings (which is necessary with 3 or more strings in most or all cases).

 

[Hedges]

I saw:
Isc x 1.25 x 1.25 = Isc x 1.56 as OCP size.

If several PV strings are paralleled without individual OCP and fed through a single OCP, each one needs to be able to handle sum of Isc from the other strings plus the OCP (which could be backfeeding)

I'm still not sure what gauge is required if no OCP.

It does suggest electronics might backfeed the PV string. How can we know it wouldn't do that in case of a fault?
My earlier SWR2500U had 15A AC breaker, so that was presumably the most it could backfeed into my PV string rated to handle 20A.
My larger inverters have larger breakers.

 

[Dzl]

I'm still not sure what gauge is required if no OCP.

If I'm honest I have a fighting chance at interpreting the ABYC (marine) code, but reading the NEC code makes my eyes glaze over and my mind go into sleep mode.

I think the section that relates to unfused arrays is the exception in 690.9(A), specifically the bolded parts:

Exception: An overcurrent device shall not be required for PV modules or PV source circuit or dc-to-dc converters source circuit conductors sized in accordance with 690.8(B) where one of the following applies:

(1) There are no external sources such as parallel-connected source circuits, batteries, or backfeed from inverters.

(2) The short-circuit currents from all sources do not exceed the ampacity of the conductors and the maximum overcurrent protective device size rating specified for the PV module or dc-to-dc converter.

I interpret this to mean:
You can avoid individual fuses if both of the conditions below are true:
1. The ampacity of the conductor (determined in accordance with 690.8) is greater than Isc x number of strings
2. The 'series fuse rating' of the panel or string is greater than Isc x number of strings.

Then considering 690.8, we may need to multiply by 1.56x? I'm less sure about this.

 

[Bugout1]

For your system, once you get to 3 in series, the simplest way to expand will be to add parallel strings. Once you get to 3 or more parallel strings you need to add individual fuses for each string. At 2 or less you just need to make sure the wire of each string can handle the current of both strings

I've been looking at panels, and found the Panasonic HIT 330W Solar Panel 96 Cell VBHN330SA17
$334.26 per item . These produce 69.7 VOC per panel, so I would just connect 6 of them in parallel to get my wattage requirements. Am I thinking correct? Is there a downside to using a panel with a high voltage output, other than cost? I also like using less roof space.

 

[Hedges]

Voltages just need to be compatible with charge controller MPPT range and Voc.

$1/watt?
Check out SanTan Solar. We pay $0.12 to $0.50/watt depending on new/used/damaged. Truck shipping of pallets $100 to $400 spread over several panels makes a modest size order cost-competitive. I drive my pickup to the freight dock, save the "last mile" cost.

 

[Dzl]

I've been looking at panels, and found the Panasonic HIT 330W Solar Panel 96 Cell VBHN330SA17
$334.26 per item . These produce 69.7 VOC per panel, so I would just connect 6 of them in parallel to get my wattage requirements. Am I thinking correct? Is there a downside to using a panel with a high voltage output, other than cost? I also like using less roof space.

That seems somewhat expensive in $/Watt, but yes, in that situation you would wire all 6 in parallel as 2 in series would exceed what your controller can handle in cold weather.

I second Hedges mention of santansolar.com (or other similar outfits depending on where in the country you live)