Solar install turning out really bad - looking for some help/advice please.

[Ben25]

Perfect. Glad it was done right!

[HX_Guy]

Yeah, wondering that too. The idiot probably had the DVM in resistance mode and popped the meter's fuse.

His voltmeter didn't die..he thought the utility kwh meter died because it wouldn't turn on. Problem was that it wasn't getting power from the disconnect.

[HX_Guy]

All my hot and neutral wiring in and out of the meter and disconnect box are #6 wires, and the ground is 8 awg. My whole system is rated at 11 KW. If your system is at comparable size, I don't think at that point #10 wire is big enough even for ground. My current rating is 46 A for 11 KW at 240V. It goes to a 60A breaker.

My permit actually shows #10 ground between the inverter and through the disconnect to the main service panel, though the installer put in #8.
For the hot and natural, he put in #6, though the permit says #4.

There is also a #6 bare ground wire running from the inverter to the existing grounding electrode (but it doesn't pass through any conduit or other meter/disconnect boxes).

My rating is 47.5A for a 12.4kW system going to a 60A breaker.

[Volusiano]

My permit actually shows #10 ground between the inverter and through the disconnect to the main service panel, though the installer put in #8.
For the hot and natural, he put in #6, though the permit says #4.

There is also a #6 bare ground wire running from the inverter to the existing grounding electrode (but it doesn't pass through any conduit or other meter/disconnect boxes).

My rating is 47.5A for a 12.4kW system going to a 60A breaker.

Although my rating is 46A, multiply by 240V = 11.04KW AC in rating, my system size of 11KW DC STC (44 panels x 250W per panel) is actually only 9.632kw AC. So my rating of 46A @ 240V = 11.04kw AC is probably a little over rated for the actual 9.632kw AC.

Do you know if your 12.4kw system rating is for DC STC, or is that actually 12.4kw AC?

If it's actually 12.4kw AC, then your current is going to be 12.4kw/240V=51.67A. But that's a continuous load which can only be at 80% of wire capacity, so your wire must be 51.67A/0.8=64.6A -> or 4 awg like they spec in the permit. That would also mean that the 60A circuit breaker is not adequate for 12.4kw AC.

BUT if your system rating is 12.4kw DC STC, then your actual system AC rating will be smaller than this. As long as your system's AC rating (not DC rating) is 11.52kw AC or less then you can use 6 awg and 60A breaker).

I also have a #6 bare ground wire from my inverters to the facility ground exposed outside and not in any conduit like yours.

[HX_Guy]

Thanks Volusiano, the electrical stuff is so over my head, I love when it's explained well like that so thanks for that.

The 12,400 is DC STC (40 x 310w panels). For AC the permit says 11.4kW AC though that seems like the maximum amount, not for my specific system.
I say that because looking at the SolarEdge spec sheet, it shows a max DC STC of 15,350 with a nominal AC output of 11,400.

http://www.solaredge.com/files/pdfs/products/inverters/se-single-phase-us-inverter-datasheet.pdf

Going by what you said, 11.4kw (AC) / 240V = 47.5A...with the 80% rule, that's 59.4A...but again that's the max the inverter will put out if I maxed out the DC input I assume, right?

So how do I figure out the AC system rating for my actual system if the 11.4kW seems to just be something they plugged in from the spec sheet?

[HX_Guy]

Maybe I should have looked it up before I asked, but found some info online which says:

"In a solar power system, the inverter is responsible for converting a DC electrical current into an AC electrical current, the type of electrical current that normal home appliances use. The AC wattage is figured when you take a solar panel’s listed PTC measurement and multiply it by the inverter efficiency"

The specs on my panel show a PTC of 285W and the inverter has a max efficiency of 98%, so 285 X .98 = 279.3 X 40 panels = 11,172W AC...sound right?

http://www.civicsolar.com/product/canadian-solar-maxpower-cs6x-310p-310w-poly-slvwht-1000v-solar-panel

Going back to your math, 11,172W/240V = 46.55A...with the 80% rule that's 58.19A so the 60A breaker and 6AWG wire is good to go, right?

[sensij]

You guys are going down the wrong path. With respect to the breaker, 2011 NEC 690.8(A)(3) says:

(3) Inverter Output Circuit Current.
The maximum current shall be the inverter continuous output current rating.

and continues on in 690.8(B)

(B) Ampacity and Overcurrent Device Ratings.
Photo-voltaic system currents shall be considered to be continuous.

(1) Overcurrent Devices.
Overcurrent devices, where required, shall be rated as required by 690.8(B)(1)(a) through (1)(d).
(a) To carry not less than 125 percent of the maximum currents calculated in 690.8(A).

In other words, the number of panels has no effect on the breaker, only the continuous AC rating of the inverter. The SE11400A has a 47.5 A rating, and 125% of that is just under 60 A. Once the breaker is set at 60 A, the wire size follows.

[HX_Guy]

You guys are going down the wrong path. With respect to the breaker, 2011 NEC 690.8(A)(3) says:



and continues on in 690.8(B)



In other words, the number of panels has no effect on the breaker, only the continuous AC rating of the inverter. The SE11400A has a 47.5 A rating, and 125% of that is just under 60 A. Once the breaker is set at 60 A, the wire size follows.

Correct...I think we were just trying to figure out based on the system we actually have, what would the real world needed breaker size be (not what NEC would require).

Either way I like that the NEC goes by the max the inverter will put out, it means I can max out the inverter (add another 2,950W worth of panels) and I don't have to make any changes to the wiring or breaker.

[sensij]

Correct...I think we were just trying to figure out based on the system we actually have, what would the real world needed breaker size be (not what NEC would require).

If you say so. I would think that "real world" calculation would also take into account the various angles your panels are at and the potential for shading, since they will all not be at peak power at the same time. NEC is real world enough for me.

[foo1bar]

it means I can max out the inverter (add another 2,950W worth of panels)

So it's a 11400W inverter.
And you just calculated:
"279.3 X 40 panels = 11,172W AC"

11400W - 11172W = 228W

So I think you can add ~228W worth (~250W DC nameplate) of panels.
And possibly you'd be starting to clip your peak production.
(but probably not because you have multiple angles and shading)


In any case - IMO sensij's right - "real world" is what the inverter can possibly put out - which is at most 47.5A - which means a 60A breaker, and appropriate wire size for that (although there's no reason you can't go with larger wire - it would even give you some very very tiny increase in production realized because of lower resistance - probably not enough to be economically worthwhile.)

If you really wanted to ignore NEC and really go by "real world" - you could probably go smaller since there's some pessimism in the code by necessity - heck even 12AWG would probably last quite a while before it melted, shorted and caught fire. (lets see - 20' of 12AWG would be ~0.03ohms; 47A would be ~1.5W) So only 1.5W more than what it gets from sunlight on the conduit... Might last a real long while. Of course if it does fail you're screwed since the insurance company will quite possibly not pay.

[HX_Guy]

So it's a 11400W inverter.
And you just calculated:
"279.3 X 40 panels = 11,172W AC"

11400W - 11172W = 228W

So I think you can add ~228W worth (~250W DC nameplate) of panels.
And possibly you'd be starting to clip your peak production.
(but probably not because you have multiple angles and shading)

Yeah I don't think I'll ever see close to that with the multiple angles, much less with the shading. For example today it peaked at just under 7,100W AC. Looking at other SolarEdge accounts from the Phoenix area, it seems May is the best month for production and I won't have any shading problems so Ill have to see what it peaks at then, and subtract that from the 11,400 and see what I could add. As an example, there's a 13.42kW DC system that peaked at 11.54kW AC on May 10th and a 16.0kW DC that peaked at 13.476kW AC on May 13th. Seems to be about 85% average DC to AC production between those two systems...applying that to my system would mean 10.54kW AC or an additional 860W AC of panels (3 more Canadian Solar 310w DC would probably just about take care of it).

Good thinking though, I hadn't thought of looking at it that way...was only going by what the max DC input specified on the spec sheet.

[Volusiano]

Thanks Volusiano, the electrical stuff is so over my head, I love when it's explained well like that so thanks for that.

The 12,400 is DC STC (40 x 310w panels). For AC the permit says 11.4kW AC though that seems like the maximum amount, not for my specific system.
I say that because looking at the SolarEdge spec sheet, it shows a max DC STC of 15,350 with a nominal AC output of 11,400.

http://www.solaredge.com/files/pdfs/products/inverters/se-single-phase-us-inverter-datasheet.pdf

Going by what you said, 11.4kw (AC) / 240V = 47.5A...with the 80% rule, that's 59.4A...but again that's the max the inverter will put out if I maxed out the DC input I assume, right?

So how do I figure out the AC system rating for my actual system if the 11.4kW seems to just be something they plugged in from the spec sheet?

The spec sheet shows max continuous current of 47.5A (I assume you have the SE11400A-US), so I think your installer is correct to go by that spec for the wiring rating and sizing.

It looks like your installer did not just conveniently spec that rating into the permit to fit within the 6 awg 60A breaker system. They properly followed the inverter's spec. It's actually more like the manufacturer who intentionally limit their design to within that range as to not require more than 6 awg/60A breaker combo, or else it may be harder to sell the inverter if it exceeds that size combo. Even the name of the model SE11400A-US implies that it's designed for 11.4KW AC.

Although the spec sheet shows a max power output of 12kw AC and max voltage output of 260V AC, those 2 numbers are irrelevant and only the max current of 47.5A is relevant when it comes to sizing the wire and breaker. Also, when connected to the main panel, you're limited to the 240V on the grid anyway, so your inverter can never get to 12KW AC or 260V AC under that situation anyway. The only way it can hit that power or voltage level is if it's not tied to the grid but is used to drive a non-grid load instead.

In reality, those are all max rating numbers anyway and you probably won't hit those limits in real life because that inverter's max input DC (STC) power rating is at 15350, and you're only at 12,400 DC STC for your panels.

So I think it's all good. The only thing weird is why the permit calls for 4 awg wiring when only 6 awg is needed due to the 47.5A rating. Maybe somebody got confused between the DC STC power rating and the AC power rating so they over calculated the wiring size thinking incorrectly that it's 12.4kw AC.

[HX_Guy]

The only thing weird is why the permit calls for 4 awg wiring when only 6 awg is needed due to the 47.5A rating. Maybe somebody got confused between the DC STC power rating and the AC power rating so they over calculated the wiring size thinking incorrectly that it's 12.4kw AC.

Per several online wiring size calculators, 4AWG is what is called for, though the Southwire spec sheet says 6AWG is good for up to 65A @ 75º which is what the system is spec'd for. So there might have been some confusion there from the designer that designed the wiring diagrams.

What's even more confusing though is that for the DC run from the combiner box to the inverter (50' run), they show 2AWG wire! Now that's a little nuts because the inverter won't even take that big of a wire, the largest it will allow is 6AWG which is what we used.

[HX_Guy]

Oh...and what the heck is with this overcast weather today! Booooo!

[Living Large]

Per several online wiring size calculators, 4AWG is what is called for, though the Southwire spec sheet says 6AWG is good for up to 65A @ 75º which is what the system is spec'd for. So there might have been some confusion there from the designer that designed the wiring diagrams.

What's even more confusing though is that for the DC run from the combiner box to the inverter (50' run), they show 2AWG wire! Now that's a little nuts because the inverter won't even take that big of a wire, the largest it will allow is 6AWG which is what we used.

Yes, but there are adapters which will allow you to make that connection. The problem is the voltage drop across a length of 50' at the maximum current. That is a long run if you are pushing a fair amount of current (say 60A), and you could lose 2% or more of your voltage with 6AWG. I believe you should be more like 0.5 to 1%. So you may need a larger wire not to carry more current, but to lose less voltage across the 50'. In other words, the inverter manufacture doesn't plan for everyone to require oversized wire, so they provide a termination capability most can use without an adapter. In my case, to achieve what Amy at AltE calls the Dance of Joy, which loosely translates to meeting the NEC requirements, I also had to go to 2AWG, and plan for a terminal adapter. AltE has an excellent YouTube that guides the viewer through the determination of the combiner to load center wire size.