Newbie ready to get started

System · September 2007

Hi All,

I've spent the last year reading up on solar here and at the solar-guppy board and I'm ready to start my project. I have received the commitment letter from the state (took almost a year) and have 9 months to complete the project. I am going with a ground mounted array between 9500 and 10kw. I have a large yard and will locate it about 200' from the service entrance. I submitted a layout with my town and am waiting for approval before I break ground. I have a few questions about conduit and wire size that I hope someone can answer. I am planning to run a 2.5" sch 40 conduit in a trench from the array to the house where I will bring the wire into the basement and connect to the main panel. With a 10kw system (using either 2 or 3 inverters), what size cable would I need to pull through the conduit and how many wires should I use? I want to get the trenching and pulling done before the ground starts to freeze and can finish the rest of the installation in the spring if I run out of time. Should I also pull some CAT5 or similar through the conduit for inverter monitoring, etc?

Thanks in advance!

BB. · September 2007

Re: Newbie ready to get started

You need to check the NEC first. I don't think you should (and NEC may not allow--I don't have a a code book handy) pull signal wires with power cables. Besides not having the same electrical insulation ratings, the smaller signal cables can easily be broken when pulling with the larger cables.

For voltage drop calculation, have you tried the excel spread sheet on this site? Once you know the gauge and number of runs for your cable--the NEC will help you decide on the type of insulation and size of the conduit.

Personally, while you have the trench open, just put in one (or a couple) 4" (or whatever size you like) diameter ABS or PVC pipe to pull miscellaneous signal wires, and bury a water line too--so that you can easily wash/rinse the panels. You may wish to put a manhole 1/2 way down the run in case the pulls become difficult.

In any case, you should be working with an licensed electrical contractor on all of this--they will have to present all of this to the local inspector(s) for sign off... You can make requirements (like minimum diameter of of cable--require copper wire--etc.), but they should be responsible that the installation is to code.

Be careful that you don't get in the situation where the contractor is trying to please you and does things that they normally would not (too long of cable pulls, too small of conduit for the size of the cable, materials that they may have not used before, etc.) where you are left with an empty hole in the ground wonder what to do next...

-Bill

System2 · September 2007

Re: Newbie ready to get started

gecko555 wrote: »

Should I also pull some CAT5 or similar through the conduit for inverter monitoring, etc?

Are you planning on installing your inverter(s) at the array?

Cheers,

Bad Apple

System2 · September 2007

Re: Newbie ready to get started

Yes I was planning to mount the inverters on the array so I can convert the DC to AC for the 200' run. I would rather have them near the main AC panel, but wouldn't a 200' run of DC have some major voltage drop?

System2 · September 2007

Re: Newbie ready to get started

gecko555 wrote: »

Yes I was planning to mount the inverters on the array so I can convert the DC to AC for the 200' run. I would rather have them near the main AC panel, but wouldn't a 200' run of DC have some major voltage drop?

It depends on what DC voltage you're planning for your system and that depends on what type of inverter you're going to use. The most familiar battery backed systems have an ~150 Voc DC limit, the large battery-less ones run much higher DC voltages, higher voltage = lower loss with the same wire gauge.

Cheers,

Bad Apple

BB. · September 2007

Re: Newbie ready to get started

From the point of voltage drop... Say you have 240 VAC service for you home... Some Grid Tied inverters operate from 200-~550 VAC... So, if your panels operate at ~240 VDC, the losses are roughly the same whether you run DC back to your home or AC back to your home... If you run the panels at 400-500 VDC, then you would actually have less voltage drop with a DC run because of the lower currents.

The down side is that the inverters become slightly less efficient as the DC voltage goes up from 200 VDC to 550 VDC... But, if you can reduce the amount of copper by 1/2 with the higher DC voltages, it may be worth the money.

There may be some other issues regarding why you would want to install the inverters at the panels... For example, you would run just one big 240 AC circuit to your panels, vs a bunch of smaller 400 VDC circuits back to your home and mounting the inverters there...

Solar Guppy has some big arrays and a test lab at his house... He can probably give you some good pro and con arguments about which would be best to do.

-Bill

PS: And as Mr. Apples says, it also depends on what type of system you are going to install--pure grid tie, or battery backed--Only the pure grid tied systems have the higher voltages (as far as I am aware)...

If you are going to do an Off-Grid system, then placing the battery/inverter shed at the array and running AC back is probably the better way to go...

System2 · September 2007

Re: Newbie ready to get started

BB. wrote: »

If you are going to do an Off-Grid system, then placing the battery/inverter shed at the array and running AC back is probably the better way to go...

I'm not saying that they don't exist, but I have never seen an off-grid system that is elgible for taxpayer subsidies so I'm assuming that this is a grid-tied system.

Cheers,

Bad Apple

BB. · September 2007

Re: Newbie ready to get started

Mr. Apple,

From what I have seen, I think that even the hybrid Grid Tied / Off Grid systems also limit their DC input voltage to about 150 VDC (even the new Xantrex system uses something like 150 VDC max for the battery charging side of the system).

We have had numerous discussions with Solar Guppy, and it always comes down to the fact that switching high voltage DC down to typical battery voltages has more losses than people are, apparently, willing to put up with... Which is a shame for those of us with Grid Tied systems and would be willing to put up with a lower efficiency Panel to Battery system that could take the 200-600 VDC that a Xantrex GT series inverter can...

And even the Grid Tied inverters also have higher losses when operating at the upper end of the DC input voltage range... I have not seen the actual numbers (that I recall) from Solar Guppy--but I would wonder how it trades off between cost of copper vs the (I assume) couple of percentage points loss caused by operating at higher input voltages...

-Bill

System2 · September 2007

Re: Newbie ready to get started

BB. wrote: »

Mr. Apple,

From what I have seen, I think that even the hybrid Grid Tied / Off Grid systems also limit their DC input voltage to about 150 VDC (even the new Xantrex system uses something like 150 VDC max for the battery charging side of the system).

-Bill

That's why I said in a previous post:

The most familiar battery backed systems have an ~150 Voc DC limit,...

Anyway, Gekko, you need to specify an inverter or inquire about the pros and cons of each type. Here http://www.solar-electric.com/solar_inverters/inverters_for_solar_electric.htm is the URL to NAWS inverter FAQ, it has lots of good info to help you decide.

Cheers,

Bad Apple

System2 · September 2007

Re: Newbie ready to get started

Thanks so much for the info/suggestions. It's going to be strictly a grid-tied system without any batteries. I am still up in the air on the inverters -I was told to split the array (10kw) into 2 and get 2 Fronious 5100 inverters.

Solar Guppy · September 2007

Re: Newbie ready to get started

There are lots of options on GT inverters, check out

http://www.consumerenergycenter.org:80/cgi-bin/eligible_inverters.cgi

I suggest to people the Xantrex GT5.0, its a sealed unit ( no fans ), best in class efficency ( CEC rated ) at 95.5% , ten year warranty and comes with data communications standard.

The Fronius is a fan cooled design and is limited to a maximum of 500Vdc, CEC rated at 94.5%. Most inverters of this class are 600V and this makes sizing the array more restricitve limiting to 500V. Street prices on the Fronius is about 250 more than the Xantrex GT5.0

Shop around and educate yourself to whats available, an installer is probably steering you toward equipment based on there stock or wholesalers partnerships, nots what best for you equipment wise

System2 · September 2007

Re: Newbie ready to get started

OK. If I go with 2 Xantrex GT5.0 units and mount them in the house (I like this idea for the monitoring capability), the spec sheet says the inverters will support 235-600V. With 50 panels (25 per inverter), how would I get the voltage down so I won't have a large DC loss over the 200' wire run? I assume with 25 24V panels per side of the array, I would be looking at 600V (24v * 25 panels)? Would I have to split the array into 4 and have 4 "runs" of 300V?

mike95490 · September 2007

Re: Newbie ready to get started

gecko555 wrote: »

OK. If I go with 2 Xantrex GT5.0 units and mount them in the house (I like this idea for the monitoring capability), the spec sheet says the inverters will support 235-600V. With 50 panels (25 per inverter), how would I get the voltage down so I won't have a large DC loss over the 200' wire run? I assume with 25 24V panels per side of the array, I would be looking at 600V (24v * 25 panels)? Would I have to split the array into 4 and have 4 "runs" of 300V?

Use the String Sizing Tool at the Xantrex website:
http://www.xantrex.com/support/gtsizing/index.asp (you have to click on a disclaimer to get to the actual caculator) You select your panel brand/size, and local weather conditions (cold days = maximum voltage)

BB. · September 2007

Re: Newbie ready to get started

In terms of wiring losses, you want to get the DC voltage higher so that the current is lower (P=V*I--double voltage/half current equals same power delivered). And power loss goes with the square of the current (P=(I^2)*R--1/2 the current, 1/4 the losses due to wire heating)...

The inverters are slightly less efficient at high voltage--but it probably saves you more in copper losses to keep the voltage high.

Check Mike's link to the xantrex sizing tool--but basically, you want a hot day under full load to be above the minimum operating voltage of 235 VDC and on a cold day, open circuit the Voc to be less than 600 VDC (to prevent damage) and you want Vmp on a cold day to be less than 550 VDC (above this voltage, the inverter starts to cut back on power generated).

The issue is that a (for example) BP 4175 (175 watt, 35 volt panel), actually will go from Vmp=28.5 volts (or lower) on a 95F day, and up to Voc=49.6 volts on a 14F day... If you miss these numbers, either you will not make power on a hot day, or damage your system on a cold day (especially if you have a utility power cut--such as an ice storm--which lets the Vopencircuit voltage rise above Vmaxpower--if that makes sense to you)...

For my example, the GT 5.0 inverter will support 9-12 panels per string... To get 5,000 watts per inverter, you would need a maximum of 3 strings of 11 panels or 4 strings of 9 panels... Other combinations will work, but with fewer panels (less power), or on cool/clear days, the inverter may limit maximum power for awhile during the day...

-Bill

Newbie ready to get started

Comments

Categories