wireing questions

72puma

i presently have 18 sharp 235 watt panels with enphase m190 inverters.two strings of 9 to two 15 amp breakers in an outdoor circuit box at the base of a ground mount array.from ther to utility main circuit box is a run of 120' of 12ga. wire put in by original installer.
now i am adding another 20 220 watt panels with enphase m215 inverters to the original array.i will have to replace that original 120' of 12 ga.wire since it will be carrying around double the current. will 8 ga. be large enough or should i go to 6 ga.?a concern is the 6ga. is pretty big for my present conduit also.
second question.the new array will be 40' away from the first.i plan on running two strings of that 12ga. wire from the new panels to two 20 amp breakers added to the original outdoor box.is this ok?
third question.will the new continuious ground from the new array,40' away have to be run all the way to the buss bar on the original outdoor circuit box and then to present ground rod connecting the to arrays together?or can i just put a new ground rod at the base of the new array?

BB.

Re: wireing questions

Lots of questions.

With Micro Inverters, one issue is if you have high voltage (i.e., ~255 VAC instead of 240 VAC nominal), higher voltage drop in the wiring can cause the micro inverters to shut down (i.e., 255 VAC + 10 volt "rise" = 265 VAC at inverter, which will trip >~260-264 VAC).

Another question is your main panel, you are normally limited to 20% over panel rating for solar circuits. Example:

• 200 Amp panel * 20% = 40 amp maximum breaker(s) for solar

But, you can also derate the main breaker to give you some more headroom:

• 200 amp panel - 175 amp main breaker + (200 amp panel * 20%) = 25 amps + 40 amps = 65 amps of solar

Some inspectors will allow you to put multiple solar circuits in the main panel (solar breakers must be installed at opposite end of bus bar from main breaker). Others will require a dedicated sub panel for all the solar breakers, then one solar breaker in the main panel.

Grounding. You probably should place a new ground rod at the new solar array. However, you need to think about running a ground wire from that array back to the main home ground rod too... I am not sure how code would handle that issue (I am not a code expert by any means). But, normally, I would want all of the safety grounds connected back to your main ground rod(s) at the home. You want any AC short circuit current to be routed back to the home safety ground so they will trip the breaker back at the panel. If you don't, it can energize the local earth ground/metalwork without tripping the circuit breaker (in my humble opinion).

If you have lightning in your area, you need to take grounding very seriously and do some more reading/planning.

-Bill

72puma

Re: wireing questions

sorry i don't understand your first comment.i still need to know what gauge wire i need to run from that 120' from those two arrays with a total of 38 enphase inverters to meter?
to clear one thing up all solar power goes through its own meter directly to the grid.i have 2 meters .one reads all power i use in the house and the other all solar power produced.no solar power goes to my main house breaker box. solar power gets credited toward power used on my bill.

BB.

Re: wireing questions

I was just trying to say--If you have very high line voltage from your utility, voltage "Rise" can be an issue. I.e., if you have 255 VAC at the street and 265 volts at the Inverters, they will shut down until the voltage falls below ~260-264 VAC for five minutes.

To answer your question, say you have 4x 15 amp 240 VAC circuits. 120 Feet, and you want 3% maximum voltage drop. Using a generic voltage drop calculator:

• Maximum circuit current is 4x15 amps = 60 Amps @ 240 VAC

Technically, the maximum current you can run through a 60 amp circuit is:

• 60 amps * 0.80 NEC derating = 48 amps

Maximum voltage drop of 3%:

• 240 VAC * 3% = 7.2 VAC

From the voltage drop calculator:

• 120' one way run, 48 amps, 7.2 volts maximum drop => 120' of 6 awg wire @ 48 amps will give you 5.5 VAC drop

Note that your losses (wire heating) will be roughly:

• Power = V*I = 48 amps * 5.5 VAC = 264 watts

The maximum cost effective drop is ~1%, so same as above but:

• 120' one way run, 48 amps, 2.4 volts maximum drop => 120' of 2 awg wire @ 48 amps will give you 2.2 VAC drop

So you can figure out if the losses of 3% drop are worth the heavier gauge wire for 2 or 1% drop to you.

The NEC chart says your minimum wire gauge would be around 4-6 AWG depending on insulation, conduit size/fill, buried, etc...

Obviously, you could also use Aluminum wire, of larger gauge, and with correct terminations and transitions to copper.

I am not the code expert--But you will have to find out what version of the Electric Code your local city/county use and review their requirements for buried cable, etc....

-Bill