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Old 01-13-14, 05:55 PM   #1
Exeric
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Default Cable type and size

Hi all,
I'm in the midst of a whole house renovation and am about to put up ceiling drywall right below a future PV panel installation on the roof. Now is the time to make the provisions for wiring from the panels (microinverter installation) and the main CB panel below before the access is limited or I'd have to trample on insulation in the attic. I plan on tacking the wire in place in the attic and just leave about 10 ft coming out of a new riser in the roof to attach to the later solar panel installation.

Naturally it will be 120 volts since its for a microinverter install. It will be about 3000 watts so I figure 3000/120 = 25 amp rated cable for about 75ft. Am I overrating this since it will be 3 wire cable? I'm a bit worried about the neutral wire carrying too much current if I downrate it just for the hot wires. No future expansion of PV is anticipated.

Also, exactly what specification of cable do I need? I figure it needs to be
outdoor and UV rated even though there will only be a few feet exposed.
Thanks in advance.

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Old 01-13-14, 06:57 PM   #2
nokiasixteth
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What kind model micro inverter are you usin ? 3k micro is pretty big micro.
By cb your talkin about your power main where the power come into your house right im guessin ?

If you have a 220 power main you will come out cheaper by using 220 volt from the inverter to the main.
More things to consider how much voltage drop is acceptable here in your install and how much volts does the inver put out ?
But for 120 volts at 75feet in conduit copper wire 3% voltage drop your goin to need some pretty pricey wire . 8 awg. Thatll keep you at around give or take 3% voltage drop
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Old 01-13-14, 07:02 PM   #3
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When I said "microinverter" I didn't mean just a small inverter. Microinverter is the proper name of a type of inverter attached to each solar panel. The total of the power for all the solar panels will be about 3000 watts. Each microinverter on each panel is around 200 or so watts.
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Old 01-13-14, 07:21 PM   #4
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Got ya . Are you wanting to tie each 200 watt inverter into 1 main line and run them to the main breaker . Or are you going to put a inverter in each room each on a seperate breaker ?
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Old 01-13-14, 07:27 PM   #5
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All the microinverters are tied in parallel on the roof and send 2 phase power through a 3 wire cable to the main electrical power and share 1 neutral (white) wire within that cable. I wasn't trying to start a tutorial.

Anyone out there with "experience" that can answer my question?

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Old 01-13-14, 07:33 PM   #6
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I know how to run grid tie off grid and hybrid grid tie there are several ways to run grid tie beings that i have already wired several houses for all all 3 methods ive had experience . But ill now let someone with (experience) answer your question
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Old 01-13-14, 07:43 PM   #7
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I believe you. I just meant that I'd like someone who has had experience with microinverter type systems. Sorry if I offended you.
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Old 01-13-14, 08:27 PM   #8
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Well, I just found an answer to one of my two questions. It's here:
http://enphase.com/global/files/Enph...Vdrop_M250.pdf

I think I have to rate the neutral wire as if its taking the full current of each of the 2 phases and not derate because of the phase angle difference between the two. It is not impossible that all the power would come from one phase only under unusual circumstances and if the power exceeded the derated current capacity of the neutral wire it wouldn't be good. It also gives a table for guage size and number of microinverters with the run in feet of the home run cable so that completely solves that question.

Now I just need to find the type of homerun cable sheathing that's legal to be exposed for a few feet on the roof. Anyone?
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Old 01-14-14, 07:54 AM   #9
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I haven't tried to do this my self, but I do know that most inspectors want it to be in conduit up to the point that it connects to the wiring from the micro inverters and if possible I think that just running conduit is going to be your best choice, that way you can run proper wire for the system that you install instead of trying to predict what will be out there in a year or two.
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Old 01-14-14, 09:54 AM   #10
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Quote:
Originally Posted by Exeric View Post
Well, I just found an answer to one of my two questions. It's here:
http://enphase.com/global/files/Enph...Vdrop_M250.pdf

I think I have to rate the neutral wire as if its taking the full current of each of the 2 phases and not derate because of the phase angle difference between the two. It is not impossible that all the power would come from one phase only under unusual circumstances and if the power exceeded the derated current capacity of the neutral wire it wouldn't be good. It also gives a table for guage size and number of microinverters with the run in feet of the home run cable so that completely solves that question.

Now I just need to find the type of homerun cable sheathing that's legal to be exposed for a few feet on the roof. Anyone?
With a grid-tie system, the solar array is using your power utility as a battery. There is no need to run a heavier neutral wire than the other primary wires. The bulk of your power travels through the 2 line wires at equal current, 120 volts per leg, back to the transformer on the pole. There may be a small amount of current flowing between the neutral of your solar array and main load center panel, but normally this current is 5% or less of your main current.

The neutral wire is there mainly in case you lose a leg of your incoming electric service, so the solar array can run at 120 volts or shut down completely. If the solar array does run, its generation capacity is limited to the panels tied to the active leg. In this case, the neutral wire and the active leg carry equal current.

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