wire size for 10kW PV system
 

I am in the process of building a shop upon which I plan to eventually place 10 kW of grid tie PV panels. The choice of 10 kW reflects a lot of things, but this could increase to 15 kW, but not more.

Given that this is the output (10 kW), the maximum single phase current with full sun should be about 41-42 amps. From this shop, back to the house, is about 200 feet. I am guessing number 6 copper inside PVC underground would be sufficient for less than a 5% voltage drop.

I went to one of the handy dandy online voltage drop calculators and found out that my guestimate of 6G wire was correct (only 3.4% drop).

Voltage Drop Calculator - for single and 3 phase ac systems and dc systems

Now, I need to put in footings TOMORROW and am thinking of the PVC size to place said wiring in. You "can" place three 6G wires in 1"conduit, but the 1.25 inch is only a tiny fraction of money more. And a LOT easier to pull cable through.

If I went up to 15 kW, then 6G may not be large enough . . . .

Am thinking of putting in a 4" sleeve under the footing to take almost anything.

Now here is where I am ignorant. I am an engineer NOT an electrician and would like to hear from the electrician end as to recommendations.

The shop will have very little amp demands, no 3 phase welders, no high current stuff. Just lights, a fan and maybe a small AC unit (total of maybe 20 amps). So any wire that backfeeds power at 40 amps (or more) will certainly be large enough for any loads in the shop.

Recommendations before I make a fool of myself (and certainly not the first time!!)?

Thanks in advance,

Steve

The panels are not going to be making 10kW all day, in fact, it only gets there every once in a while. Try using a smaller solar panel and some sort of data logger to get an idea of actual production. You still need to size the wire to safely carry the maximum current, but you can use typical production when oversizing for efficiency reasons, maybe even not have to oversize at all.

Wire is rated in amps for safety reasons. As long as you stay under this limit, the sizing is all dependent on voltage and length.

The same wire drops the same volts per amp per length.

A piece of wire that drops 1.2v at 10A at 120v is perfectly fine.
It wouldn't be appropriate for 12v 10A, because it will still drop 1.2v. (10%)

You didn't say DC side or AC side.
DC side is more dangerous btw.

This is the AC output (10 kW, 42 amps at 240 VAC). Will be using Enphase inverters.

The peak current issue is what I am concerned about with wire sizing, not the average output over 24 hours.

Steve

What kind of A/C? Size and tonnage?

Code permitting, I would go for doubly fed setup if your run is that long. The wire numbers are completely made up, but the setup on right is more desirable:

That inverter is a modern high frequency full conversion gutless power electronics type inverter and have pretty much nothing when it comes to surge power. The starting current will be sourced from your energy provider service. If you were to try to start the air conditioner off grid, it probably won't be able to start it blow up and bellow out the semiconductor smoke.

http://i.imgur.com/8FUiA0m.png

Even if left set is fine as far as safety and capacity, a run that long means your lights will flicker every time the compressor starts and can get VERY annoying. It depends on the compessor's LRA and wire length.

The setup on right would significantly isolate the lighting circuit from flicker. The voltage sag will be there at every start, but it just avoids it from affecting lights.

Probably stating the obvious here but check your mains voltage at the time of day you expect maximum solar output.
Add this to the maximum voltage drop you calculated and check it,s well under the maximum voltage output of the intended inverters.
If it,s close use bigger cables to reduce the voltage drop, the last thing you want is to have the inverters constantly shutting down due to over voltage.

At 240v and you size according to code (NEC in the US) for safety and voltage drop for the given current level, it should be fine.

ANSI standards allow +/- 5% steady state voltage at the service entry, so a non-sucky inverter shouldn't have any problem pushing the power into steady state of 228 to 252v.

Ormstron

I did check the voltage and voltage drop at the input of the system - by the meter. I did this with a load test and put about a 300 amp load on and only got a 2 v drop.

This is the result of putting in large G copper (not aluminum) with 400 A total service. 100 A to two homes. 100 to large livestock barn (including office) and 100 for future shop (now under construction.

So by backfeeding to grid (from PV panels) I don't have to worry about a large resistance causing a significant voltage rise on mains. Excellent point that I did not think of.

I know ANSI speaks of code being 5% drop. Code is the MINIMUM standard and I would prefer to be better than that. The issue is how much better. I think a 3% voltage drop is about half that and is OK. I can reduce it to a 2.5% voltage drop with 5G wire, but with a lot more money.

Engineering is about getting the most for your $, but an electrician will tell you that it is all about safety as well. Trying to balance both.

Steve
ps - only will need a 12,000 btu AC unit - essentially nothing for load

An A/C that small is not going to be any problem. It would be very unlikely for a wire run rated for at least 42A to cause starting issues, especially if you get a 240V mini split and even more so if it's inverter drive. (In the unlikely event it does, add a hard start kit!)

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.