solar panel voc

[GlennWest]

If a controller is rated at 450 vdc and my array is 454 voc and 411 vmp, would I have a problem? Know I will never see full output from panels, 4000 watt like 3700ish watts. Is voc and vmp less also?. I could run two strings but would like one. The Growatt units have built in controller. I am contemplating 2 5k units. The 12k  inverter weight (170ish) is just too much for me. Now I would have two controllers so splitting array I could feed each. I know most here don't like that high a voltage but it is common in residentials. I will be running residential panels. With these voltages I can use #8 wire at 2% loss. I actually have #10 prerun from roof by Teton. Could tie to it and pull larger.

Edited May 1, 2021 by GlennWest

[jcussen]

I am running 2 strings of six 90 cell panels [45 volt] in series at 270 VMP, through two 250 volt solar controllers. Have seen controllers shut down a couple of times in the morning when voltage was 250+ volts, but they started charging again when voltage went below 250 volts. I run 8  gauge from each string.

Edited May 1, 2021 by jcussen

[GlennWest]

So the VOC does make a difference. Interesting. Know inverter states vdc and we have VOC and VMP on panels. VOC being highest.

[Darryl&Rita]

VOC is voltage open circuit, and will be higher. VMP is voltage at maximum power. The point where highest voltage and current cross over. The controller will be looking at VMP more than VOC.

ETA: Whether the controller shuts down temporarily, or flambé is dependent on the manufacturer, and the quality of their products.

Edited May 1, 2021 by Darryl&Rita

[GlennWest]

Actually done some more research they are referring to VMP with VDC. VOC can be right much higher. I may just run two lines down splitting my array. Will have two mppt controllers anyway. Might as well use them.

[oldjohnt]

Since Voc is with no load and applying a load typically drops the voltage and the highest you will see is Vmp, that's the number Id be concerned with when sizing the controller. Ive NEVER gotten the power a panel is rated at since bright sunlight is seldom direct overhead (flat roof panels) . If you run higher voltage up top (via series or series parallel connections) that's less current and less possible line voltage drop (subject to current, wire size and distance)  so I like higher net array voltages provided the controller handles it..........

John T

[GlennWest]

So you saying you never see the rated vmp from your panels. See that is what I was wondering. So if I am just a few volts over???

[noteven]

16 hours ago, Darryl&Rita said:

VOC is voltage open circuit, and will be higher. VMP is voltage at maximum power. The point where highest voltage and current cross over. The controller will be looking at VMP more than VOC.

ETA: Whether the controller shuts down temporarily, or flambé is dependent on the manufacturer, and the quality of their products.

flam bay shut down ahahahahahaaaaa

[GlennWest]

I will have 2 units so 2 mppt controllers. Split panels and carry less than 40 amps each. That will be 230ish volts so #10 wire work. 2 of them down vent pipe work easy. 

Edited May 2, 2021 by GlennWest

[Darryl&Rita]

5 hours ago, noteven said:

flam bay shut down ahahahahahaaaaa

Est' cie, flambeau!

[Randyretired]

8 hours ago, GlennWest said:

I will have 2 units so 2 mppt controllers. Split panels and carry less than 40 amps each. That will be 230ish volts so #10 wire work. 2 of them down vent pipe work easy. 

Just a thought for the wire you choose.   The NEC is very conservative for sizing panel array wire.  It states that the maximum amperage be used as a starting point.  Then it states to multiply that by 125%.  Then size the wire by multiplying that by 125% again.  Then for roof mounted panels there are charts to further reduce the wire capacity due to heat.  I know most RV arrays don't meet that but the NEC wire sizing recommedation suggest that one should proceed with caution.

[Bill w/bus]

Running four panels two in series, MPPT controller max is 120VDC. Four panels in series is max at 130VDC at 70F. A max observed   voltage is 58+ VDC , that is two panels in series. Probably would have OK with four in series. But, the flambe part was a concern. Panels did well today in South Texas while waiting on the tire fixer, about 5.5 KWH in four hours. Was still in the campground but it plugged in.

Bill

[GlennWest]

My figures was on 4000 watts of solar. Run 2k to each unit. But Growatt also has a 8k unit and checking it out. So this may change. 

[oldjohnt]

23 hours ago, GlennWest said:

So you saying you never see the rated vmp from your panels. See that is what I was wondering. So if I am just a few volts over???

While MY observations are based strictly on the time of year (suns location in the sky) and sunlight intensity and angle to panels NO I haven't seen full rated Vmp, but I have no doubt under better conditions it could indeed reach that..

 Once you size the wiring from panels down to your solar charge controller and arrive at the absolute minimum THE NEXT STEP is to compute the line voltage drop (its based on Current, Wire Size and Distance) and if needed increase wire size even bigger to further reduce voltage drop

 John T

[Randyretired]

There are times when solar panels will actually exceed the stated power.  One is during certain partly cloudy conditions when the sun can be reflected to intensify the sunlight.  This reflected sunlight could also happen from a building or whatever.  The second thing to consider is the temperature.  Solar panels are rated at a specific temperature, (I think it is 25c?) but at lower temperatures the power can increase.  This is why the wire calculations for panel arrays are what they are.  

[GlennWest]

Temperature calculator is fine for residential. We travel so I see no way to figure that in.

[oldjohnt]

 

2 hours ago, GlennWest said:

Temperature calculator is fine for residential. We travel so I see no way to figure that in.

Glenn, you will most likely be fine. The thing is if you consider a wire exposed to sunlight (not in a house etc) in bright sunny high temp conditions, such affects its heat dissipation which is the theory behind the 125% type rules.

John T 

[Randyretired]

I don't know what will happen if a charge controller encounters voltage higher than it is rated for.  Unlike amps which many manufactures specify limits well above the rated capacity I can't find any info on it.  There is a lot of chatter on the internet that higher voltage will quickly fry a charge controller but  nothing that I have found confirms that.  Obviously the NEC suggests 125% for head room plus another 125%+ for wire sizing.  My guess is that is conservative as one would expect.  For controllers I pulled a number out of space that I will leave 10 to 15% headroom.  I don't have any data to back that up and that is just what I have decided.  You and others may feel differently.  As for wire sizing I usually step up a little to handle voltage loss so I think that covers it.  But again that is just me.  Of course if it needs to be inspected then the NEC rules.

[oldjohnt]

16 hours ago, Randyretired said:

But again that is just me.

Hey Randy, that makes TWO of us lol When I practiced power distribution I never liked to operate near the edge and sized conductors accordingly IE over safe and conservative AND THATS ME ALSO..As you but some non sparkies may not realize is, ampacity limits (and the type rules you spoke of) are in part to protect the integrity of the insulation such that it doesn't overheat and degrade....The ampacity of single conductors in free air is typically higher then if enclosed in say conduit (how many and what size etc) which is related to the ability to dissipate heat... A conductor (say from a solar panel up on an RV roof) out in open sunlight on a 100 degree day is different then inside a home..

As always fun sparky chatting with you, hope we don't bore others lol

John T Also retired

Edited May 4, 2021 by oldjohnt