New Member, Microinverter wiring question

[Tim Godshall]

I'm planning to install a 3 kw PV system on my roof in Harrisonburg, VA. I have twelve 255-watt panels and am using Enphase M215 microinverters. Everything seems pretty straightforward so far except for selecting the correct wiring between the roof junction box and the panel. The wire run will be between 60-75 feet, with a utility disconnect within 10 feet of the end of the run at the panel. The instructions say that one 2-pole 20-amp breaker is sufficient for up to 17 panels, so that is what I'm planning on. I will be wiring the panels in 240 VAC, not three phase.

Can anyone help me to choose the correct size wire for this run? Is it true that the bigger the wire you use, the less power is wasted? If so, I assume there is a point of diminishing returns on this point.

Also: the M215 microinverters I am using have their own integral grounding, but I will need to ground the racking to the wire that connects to the panel. Should this ground wire be the same gauge as the wire I'm using for run between the roof and the panel?

Thanks!
Tim

[sensij]

You might want to see if there are any design firms in your area that would, for a few hundred dollars, help you with your design and permit documentation. The answers you could get here, while well intentioned and maybe even right, will not help your cause if you need to explain your design to an AHJ.

[DanKegel]

3kw ... The wire run will be between 60-75 feet... 20-amp ...240 VAC...
Can anyone help me to choose the correct size wire for this run? Is it true that the bigger the wire you use, the less power is wasted?

3kw and 208 volts is about 15 amps.

http://www.csgnetwork.com/wiresizecalc.html says 14 gauge is ok, but:

http://www.powerstream.com/Wire_Size.htm conservatively suggests 10 gauge.

Yes, the bigger the wire (the smaller the gauge), the less power is wasted as heat.

The suggestion to hire an engineer is good, but 10 gauge copper wire should be safe.

I are not an electrical engineer, but my dad is.

[sdold]

I'm planning to install a 3 kw PV system on my roof in Harrisonburg, VA. I have twelve 255-watt panels and am using Enphase M215 microinverters.

The most you should see running at full inverter saturation is a little over 11 amps (225W x 12 = 2,700 Watts total, / 245V = 11A). Somewhere I remember Enphase recommending keeping the voltage drop (actually rise) to less than 2%, and I had this in mind when I designed my system using 16 M215s. #10 put me under that, and was well within Ampacity, including temp and fill corrections. I suggest #10 for that reason, but I would reverse-engineer it using the NEC so make sure I could justify it with the inspector. Your run is the same length as mine (140 feet), just a little lower current.

[Tim Godshall]

Thanks for the guidance on my question. I was able to talk to an Enphase representative who said that 12 gauge wire would be fine. However, I think I'll go with 10-gauge just to be on the safe side (and it's only $10 more for the length of wire I need to get 10-gauge.)

[Amy@altE]

I did a video on wire sizing that includes conduit fill and temperature derations. It references DC a lot, but also applies to AC. It may be helpful for your permitting to show the math and NEC tables used. Go to YouTube and search for "Wiring your solar electric system".

[sensij]

I might be misinterpreting what has been written, but since the OP has not referred to code at all, and is taking wire sizing advice from Enphase, it sort of sounds like the plan is for this system to be installed without permitting or an agreement from the power company (which usually requires permitting). If I've misunderstood, the rest of the post is unnecessary.

An interconnect agreement of some kind with the power company is necessary to get the most benefit from the system. Without an agreement, the following outcomes are possible, assuming that the system has been installed safely:

1) Worst case - Power company detects the system and issues a cease and desist notice. Ignoring this notice would get you cut off from the grid. Some poco's are more aggressive with this than others.

2) Your meter may not be able to detect the direction of current flow. Any power that you feed into the grid may be counted as power consumed, and show up on your electric bill.

3) Your meter may not recognize reverse current flow at all. You would benefit from your system when the sun is out, but any excess produced would just be fed into the grid without any credit to you. It is unlikely that the value of the energy cost avoided just while the sun is out will ever justify the cost of your system.

Without an agreement, you will definitely not be able to benefit from any sort of net metering.

Again, please forgive my misunderstanding if the intention is in fact to have this designed and permitted properly. With respect to design practice, you might already be aware that wire sizing requires at least two calculations... one to consider voltage drop, and the other to meet code. Not enough information has been provided to adequately address either of those calculations. Voltage drop is easier to figure, but determining where the breakeven point is between the cost of larger wire relative to the value of the energy that is not lost depends, to some extent, on the price of electricity and the expected cost of the wire.

[SunEagle]

I might be misinterpreting what has been written, but since the OP has not referred to code at all, and is taking wire sizing advice from Enphase, it sort of sounds like the plan is for this system to be installed without permitting or an agreement from the power company (which usually requires permitting). If I've misunderstood, the rest of the post is unnecessary.

An interconnect agreement of some kind with the power company is necessary to get the most benefit from the system. Without an agreement, the following outcomes are possible, assuming that the system has been installed safely:

1) Worst case - Power company detects the system and issues a cease and desist notice. Ignoring this notice would get you cut off from the grid. Some poco's are more aggressive with this than others.

2) Your meter may not be able to detect the direction of current flow. Any power that you feed into the grid may be counted as power consumed, and show up on your electric bill.

3) Your meter may not recognize reverse current flow at all. You would benefit from your system when the sun is out, but any excess produced would just be fed into the grid without any credit to you. It is unlikely that the value of the energy cost avoided just while the sun is out will ever justify the cost of your system.

Without an agreement, you will definitely not be able to benefit from any sort of net metering.

Again, please forgive my misunderstanding if the intention is in fact to have this designed and permitted properly. With respect to design practice, you might already be aware that wire sizing requires at least two calculations... one to consider voltage drop, and the other to meet code. Not enough information has been provided to adequately address either of those calculations. Voltage drop is easier to figure, but determining where the breakeven point is between the cost of larger wire relative to the value of the energy that is not lost depends, to some extent, on the price of electricity and the expected cost of the wire.

Good information and great words of caution to anyone looking to perform a DIY PV install without the co-operation or approval from their POCO.

[Tim Godshall]

I might be misinterpreting what has been written, but since the OP has not referred to code at all, and is taking wire sizing advice from Enphase, it sort of sounds like the plan is for this system to be installed without permitting or an agreement from the power company (which usually requires permitting). If I've misunderstood, the rest of the post is unnecessary.

An interconnect agreement of some kind with the power company is necessary to get the most benefit from the system. Without an agreement, the following outcomes are possible, assuming that the system has been installed safely:

1) Worst case - Power company detects the system and issues a cease and desist notice. Ignoring this notice would get you cut off from the grid. Some poco's are more aggressive with this than others.

2) Your meter may not be able to detect the direction of current flow. Any power that you feed into the grid may be counted as power consumed, and show up on your electric bill.

3) Your meter may not recognize reverse current flow at all. You would benefit from your system when the sun is out, but any excess produced would just be fed into the grid without any credit to you. It is unlikely that the value of the energy cost avoided just while the sun is out will ever justify the cost of your system.

Without an agreement, you will definitely not be able to benefit from any sort of net metering.

Again, please forgive my misunderstanding if the intention is in fact to have this designed and permitted properly. With respect to design practice, you might already be aware that wire sizing requires at least two calculations... one to consider voltage drop, and the other to meet code. Not enough information has been provided to adequately address either of those calculations. Voltage drop is easier to figure, but determining where the breakeven point is between the cost of larger wire relative to the value of the energy that is not lost depends, to some extent, on the price of electricity and the expected cost of the wire.

I have an electrical permit in hand and a power purchase agreement document from our utility. Inspections are pretty lax around here, so I'm just trying to make sure things operate safely and efficiently. I'm willing to pay an extra $10 for the ten-gauge wire even if it doesn't pay off in terms of energy saved.

[sensij]

I have an electrical permit in hand and a power purchase agreement document from our utility. Inspections are pretty lax around here, so I'm just trying to make sure things operate safely and efficiently. I'm willing to pay an extra $10 for the ten-gauge wire even if it doesn't pay off in terms of energy saved.

Just make sure the insulation rating of the wire you buy matches what is in your permit.