wire size for 10kW PV system

[Mikesolar]

Assuming a 250w panel and the slightly older M215 inverters, there is a max of .91A x 60 panels = 54.6A x 1.25 (safety) = 68.25A.

So you would need a #4 wire as #6 is only approved up to 60A. Note that you are on the low side of the capacity for a #4 (at a minimum) so without doing much more calculation, I suspect you would be quite good for voltage drop too. #2 is more typical for a 100A service (if you use a 90C wire).

[pinballlooking]

If you buy Enphase M215 you will use .9 amps for the current but if you end up using the new M250 you will use 1 amps for the current. (use these number in your calc just like in the doc @ 240 Volts)
M215 actually max out at 225 watts where the M250 max out at 250 watts.

So the 250 watt panels go well with the M215.

Here is a good sizing doc's

http://enphase.com/global/files/Enph...ightsizing.pdf

Enphase has put out some really good documentation
http://enphase.com/global/resources/geo_us/
They have module Compatibility list and much more.


If you are comparing solar modules go here California actually test the modules to show how they perform.

Incentive Eligible Photovoltaic Modules in Compliance with SB1 Guidelines - Go Solar California

My Canadian Solar modules are 230 watts and they tested out at 211 watts
My Light way 240 watts tested out 215 watts.

This helps you see what you are paying for really works. I have on occasion max out the M215 @ 225watts not very often at all 250 watt is a better match.

When I called around all the sales people told me the M215 maxed out @215 watts this would have made the 230-240 a good match. They we giving out bad info that is why I am bring this up now.

I would not use M250 with 250 watt modules because you would not make more power and it would put you using 1 amp in the calc. this would just make you spend more money on wiring with no real additional power being made.

I am using M215 with 54 panels 54 X .9 = 48.6 X 1.25 = 60.75 amps
If I was using the M250 54 X 1 = 54 X 1.25 = 67.5 amps
I would make no more power but have to buy bigger wire.

Now if I would have bought 270 watt panels that would be better for the M250. It seems it is always the smaller wattage panels that go on sale. if space is a issue M250 might be better I don't think that is the case for you.

[Mikesolar]

The reason I stated using the M215 was, as you stated. I have installed lots of systems like this because the 250w panel was the best price point and the M215 has max performance that is seldom seen in a 250w panel. You may lose a few watts here and there but very few. As you say, if there was a 260w+ panel, that is different.

[stevehull]

Originally Posted by pinballlooking

http://enphase.com/wp-uploads/enphas...tions_M215.pdf

Here is an Enphase doc that shows the voltage drops calculations.
Enphase microinverters do not like that big of voltage drop.

Luckily I was about 130’ away not as far as you going.

If there is too much voltage drop the micro inverters will think the grid is unstable and shut off wait 6 min and check again if the grid is back.

This is very interesting as my main service panel is actually even farther away than the 200 feet I stated. The 200 feet length brings me back to a sub-panel where I have a larger diameter wire (4G) going to the main point of contact.

I may have to reconsider putting the PV panels on the shop and use the big livestock barn instead. The main service panel is locate in that barn.

I had no idea that the Enphase inverters were so sensitive to voltage drop (or increase depending which way current is flowing). A 1% voltage drop is a very small amount.

So my question: Is the stated 1% voltage drop an actual necessity or is it something that is really not needed?

I can understand a too small wire size causing a voltage increase given that the PV panel will backfeed across the wires and this current multiplied by the wire resistance will cause an inappropriate increase in voltage at the Enphase end of things (Ormond's comment).

But 1%?

Nowhere in the Enphase documentation does it say why 1% is needed.

I bet most main wiring for Enphase installations do not meet this rigor.

Steve

[Mikesolar]

IIRC, the code is quite lax and may even be 3 or 5%. I think that they are saying 1% so the most power gets out of the system. Remember that power is transmitted based on voltage differential between the grid and the inverter and to some extent, the higher the voltage the grid sees, the more transfer will take place.

[pinballlooking]

I really think it is the voltage rise that becomes a issue.


They are back feeding the grid so they monitor the voltage. The grid voltage goes up they need to go higher to back feed the grid. If the grid has loose tolerances and you have too much voltage rise you can get out of the window that the micro inverter can go. They can adjust these tolerances some the window they call grid profile some. I had them adjust it on my array. They told me any more adjust needed I would have to call my power company to do a line study and get their power more in speck. I am one of the last two houses on this line a new power company provides just two house away.


"
Utility voltage: The utility strives to maintain voltage at the PCC within +/- 5% of nominal. The protective functions of the microinverters are set to +10%/-12% by default. The high voltage end of the tolerance is of most concern because the inverters are a SOURCE and not a LOAD. If the utility is consistently 5% high, that leaves less than 5% for all wiring and interconnection losses and inverter measurement accuracy. If you are concerned about the utility’s voltage, you may request that your utility place a data logger at the PCC and make a record of the voltages available to you at the site. “

This is the doc I used for my calculations.
http://enphase.com/wp-uploads/enphas...Vdrop_M215.pdf
page 4 talks about line volatage rise. I center feed all my runs and ran bigger wire because of my distance.

I found this to be a very useful doc.

Just call Enphase support they were really good with pre sale support. I called a couple times before I purchased my system. As long as it is not a NEC code question they are good at answering questions.
Every time I have called support they knew what they were doing or very quickly got me to someone that knew the answer. I have not found this in most other companies support. Usually first level cant tie their shoes.


Keep in mind that I am going by the documents Mike has lots of installs under his belt. Book smart does not ever take the place of actual field experience.

The other thing most people just install on their roof none of this stuff applies because the cable runs are pretty short. 95% of the install never have to even think about these voltage rise issues. It is just when you add distance that makes the difference then it matters.

Maybe Mike has done some installs with long distances and can share his experience.

The video I watched with one install with distance similar to mine they center taped and use bigger wire just like my install.