Basic Grid Tie Wire Size Question

LHawes

This is my first post and I have searched a bit for what is probably a simple answer to a simple question but I can't seem to find a definitive answer.

We want to install 4 strings of eight 216 watt panels and need to size the wire from the panels to the inverter. There are (2) runs from 2 locations of 180 ft. and 95 ft. and we'd like to know what size wire to run.

I've read that today's higher voltage systems don't require as large a wire as the older lower voltage systems but I'd like to be sure of the correct wire sizes.

Thank you

mike95490

Re: Basic Grid Tie Wire Size Question

to size the wire, we need to know the anticipated amps the panels will put out.

Have you chosen your inverter (s) yet? Did you use the PV array sizer/selector to stay within the voltage and wattage limits ?

You can go the the thread that has voltage drop calculator - download it, and calculate your own.
http://forum.solar-electric.com/showthread.php?t=29

If you keep your Array Voltage low, you will have more current (amps), needing heavier wire. Higher voltage = less amps, but more internal inverter losses. You have some long runs, but you only buy wire once.
Personally, I'd set the array voltage about 50V over the min inverter start up (on hot day) [320V?] and enjoy the extra few % of inverter efficiency. And nothing says you can't use a larger wire, till it won't fit the terminals.

If your two arrays make matching the panels tough, you might consider 2 separate inverters. That's a little extra cost, but lessens the chance both could die at same time.

LHawes

Re: Basic Grid Tie Wire Size Question

Thanks very much for the reply.

Pretty sure we'll use 2 PV Powered PVP 3000 inverters and their string calculator shows 7.6 amps per 8 module string.

And no I didn't use the PV array sizer/selector to stay within the voltage and wattage limits? Did I mention I'm pretty new to this? Is there a link?

Again thanks for the reply.

Larry

BB.

Re: Basic Grid Tie Wire Size Question

Sizing of the arrays to the PV inverter... Generally, you cannot do the install yourself and need a licensed electrical contractor or solar installer. (building permits, and in many states, a licensed solar installer to get your local rebates).

But, basically, the Grid Tied inverter has a minimum voltage (say 200 VDC), and a maximum voltage (say 600 VDC) (you need to check the specifications to be sure).

Solar panels, while they are the equivalent of "solar powered batteries" do not supply very stable "voltage" like you would expect from batteries.

Basically, the two extremes you need to look at... Open circuit voltage--when the panels are "unloaded" and cold (this would be the maximum Voc "open circuit" voltage). If you exceed the Voc voltage on the inverter--you will violate the warranty and possibly damage the inverter (plus it will turn off until the voltage drops).

The second would be the maximum loaded current/power of the solar panels on a very hot and sunny day (when panels are hot, voltage drops, plus you have the voltage drop from the load of the inverter). This would be the Vmp (maximum power voltage when hot)... On very hot/windless days, to low of Vmp will cause the inverter to shut down until the voltage rises again (panels cool down).

The ratio (I have seen for temperate climates--hot summers cold winters) between Voc Hot and Vmp Cold is almost 2:1... So, if you size the array for Voc=500 volts cold, you would end up with Vmp hot of 250 volts... A reasonable compromise.

There have been some errors in the "array sizing tools" from at least one vendor--If you want to better understand, I found the Xantrex sizing tool (pick something like the Xantrex GT 3.0 inverter to get results) to be very handy--very clear on the ratings and voltages (based on vendor solar panel specs., and min/max string voltages). You can take the numbers and fit/comfirm them against your brand of grid tied inverter.

-Bill

LHawes

Re: Basic Grid Tie Wire Size Question

I downloaded the calculator as one of the first steps but I could find no way to use it my situation. The panel volts seem to be set (protected and password protected) at 60 volts and there is no indication of wire size nor a calculation for wire size that I could find.

And I do plan to use 2 inverters - the PVPowered PVP 3000's.

mike90045 wrote: »

to size the wire, we need to know the anticipated amps the panels will put out.

Have you chosen your inverter (s) yet? Did you use the PV array sizer/selector to stay within the voltage and wattage limits ?

You can go the the thread that has voltage drop calculator - download it, and calculate your own.
http://forum.solar-electric.com/showthread.php?t=29

If you keep your Array Voltage low, you will have more current (amps), needing heavier wire. Higher voltage = less amps, but more internal inverter losses. You have some long runs, but you only buy wire once.
Personally, I'd set the array voltage about 50V over the min inverter start up (on hot day) [320V?] and enjoy the extra few % of inverter efficiency. And nothing says you can't use a larger wire, till it won't fit the terminals.

If your two arrays make matching the panels tough, you might consider 2 separate inverters. That's a little extra cost, but lessens the chance both could die at same time.

LHawes

Re: Basic Grid Tie Wire Size Question

Thanks Bill,

BB. wrote: »

Sizing of the arrays to the PV inverter... Generally, you cannot do the install yourself and need a licensed electrical contractor or solar installer. (building permits, and in many states, a licensed solar installer to get your local rebates).

We are licensed and certified but this our first install other than my own home so we want to get things right.

But, basically, the Grid Tied inverter has a minimum voltage (say 200 VDC), and a maximum voltage (say 600 VDC) (you need to check the specifications to be sure).

Solar panels, while they are the equivalent of "solar powered batteries" do not supply very stable "voltage" like you would expect from batteries.

Basically, the two extremes you need to look at... Open circuit voltage--when the panels are "unloaded" and cold (this would be the maximum Voc "open circuit" voltage). If you exceed the Voc voltage on the inverter--you will violate the warranty and possibly damage the inverter (plus it will turn off until the voltage drops).

The second would be the maximum loaded current/power of the solar panels on a very hot and sunny day (when panels are hot, voltage drops, plus you have the voltage drop from the load of the inverter). This would be the Vmp (maximum power voltage when hot)... On very hot/windless days, to low of Vmp will cause the inverter to shut down until the voltage rises again (panels cool down).

The ratio (I have seen for temperate climates--hot summers cold winters) between Voc Hot and Vmp Cold is almost 2:1... So, if you size the array for Voc=500 volts cold, you would end up with Vmp hot of 250 volts... A reasonable compromise.

PVPowered calculator give us 182 vdc warm weather (VMP) and 333 vdc coldest day (VOC)

There have been some errors in the "array sizing tools" from at least one vendor--If you want to better understand, I found the Xantrex sizing tool (pick something like the Xantrex GT 3.0 inverter to get results) to be very handy--very clear on the ratings and voltages (based on vendor solar panel specs., and min/max string voltages). You can take the numbers and fit/comfirm them against your brand of grid tied inverter.

-Bill

I think I'm getting the same numbers from the PVPowered calculator but still no answer on wire size or how I might figure that?

mike95490

Re: Basic Grid Tie Wire Size Question

> The panel volts seem to be set

Modify from cell I 4, and then all the PV's will update

LHawes

Re: Basic Grid Tie Wire Size Question

mike90045 wrote: »

> The panel volts seem to be set

Modify from cell I 4, and then all the PV's will update

VERY helpful and it seems that #10 wire should give a very low voltage drop of .412% which should be fine. Did I miss something?

mike95490

Re: Basic Grid Tie Wire Size Question

There are a lot of details, like 190F, wet rated wires, size of conduit, breakers, grounding, inspectors, etc...

I'll question your
> 182 vdc warm weather (VMP)
number, as your inverter is likely to shut down at that low of voltage. (not knowing the specs of your inverter) as it's easy for the inverters to DOWN convert 300V to 240VAC, but they can't usually UP convert too.

LHawes

Re: Basic Grid Tie Wire Size Question

Thanks very much for all the info Mike but I think the 182 vdc, which comes from the PVPowered calcs should be OK in. light of the inverter's specs that follow.

"Flexible System Configurations
• Standard (150-500VDC) or Extended (115-500VDC) DC operating ranges"

and I'll keep in mind all the details you mentioned.

Thanks again

morrisrv

Re: Basic Grid Tie Wire Size Question

For Lhawes:
In responce to your voltage drop inquiry. As an old electrician I'v been using for years the following equation:

CM = k x l x I divide these by the "system design" voltage drop give you your min. wire size.

CM= circular mill area of wire Table 8 NEC
k = resistivity of one mil foot copper wire. use 12.6 ohms
l = total legnth of wire, there and back
I = expected amps on wire (Imp)

You have 120 volt 10 amp load located 125 feet from panel. what size wire is need to keep voltage drop to no more than 3%

12.6 x 250 x 10= 31,500 divide by 3.6 volts = CM of 8750 need a #10 at 10,380
hope this helps,
voorhees

niel

Re: Basic Grid Tie Wire Size Question

you don't need to do the calculations as i've got that covered in the voltage drop calculator here. it is on an exel (xls) format, but any general type of program resembling it will work such as the free open office i use. this will take the percentages for 120v as well as account for differing temperatures the wires are in.
http://forum.solar-electric.com/showthread.php?t=29

Dave Sweeney

Re: Basic Grid Tie Wire Size Question

Hi I have a 500w 12v wind turbine 3 phase connected to a 500w GTI its about 35 feet away, what wire size is recommended from the turbine to the inverter to maximise the volts from the turbine

ggunn

Re: Basic Grid Tie Wire Size Question

Dave Sweeney wrote: »

Hi I have a 500w 12v wind turbine 3 phase connected to a 500w GTI its about 35 feet away, what wire size is recommended from the turbine to the inverter to maximise the volts from the turbine

There is no maximum voltage (i.e., minimum voltage drop) to achieve, only a point of diminishing returns. That point varies on how much you are willing to spend on wire.