combine dc & ac

Re: combine dc & ac

gino45 wrote: »

Thanks for the response. Already installed is a main panel with 2 100 amp breakers on the meter side, and 2 20 amps on the incoming (from the inverter) side. There are also 2 double empty 'sockets' on each side. Could another 20 amp be added from say, 6 250 watt panels and then connected to one of the 100 amp, or would it need an additional 100 amp also?

I am not a code guy or professional installer--But here is the issue, as I understand.

You have a 100 amp main panel (120/240 VAC split phase power--typical North American Home). And you have a pair of 20 Amp breakers for a 120/240 VAC circuit to your 3kW GT inverter (really 240 VAC only, the neutral carries no useful power).

The NEC allows you to put up to 20% of the main panel's rating (the bus bars specifically) to support GT solar power.

100 Amp panel * 20% = 20 Amp maximum for GT solar power.

You have to check your installation manual for the Inverter--But in round numbers, a 3kW inverter will output:

3,000 Watts * 1/240 VAC = 12.5 amps nominal maximum current

The maximum allowed continuous current on a 20 amp branch circuit is:

20 amps * 0.80 NEC derating = 16 amps

So, at this point, in theory (if allowed by your inspector), you could add ~3.5 more amps of GT solar power or:

240 VAC * 3.5 amps = 840 watts

So--roughly 840 watts (or 3.5 amps) of enphase inverters to your home without installing a new main service (there are some other options--but lets stop here for a moment as they may not be allowed by most inspectors for homes).

It is not a lot of solar panels that you can add right now. And you cannot add another branch circuit to your existing main service (legally/to code).

Now, I'm not planning on doing this myself; however, I am thinking, that I could up my system by a third or 50% fairly cheaply.

You have to check the micro inverter(s) you wish to use--They cannot exceed 3.5 amps in total (so you may have be less, depending on the specific rated output of the micro inverter you wish to use).

FYI, our electric use is low because we have no heating, no air conditioning, and a solar water heater; however, grid electricity presently costs over 40 cents per additional kilowatt.

Conservation is still, usually, your best "first" investment of time and money. A Kill-a-Watt type meter can show an old stereo, TV, fridge, desktop computer, etc. using a lot more power than you first expected.

For example, if you want to get down to ~200 kWH per month (possible if you don't use electricity for cooking/heating/etc.), the average load would be:

200,000 WH per month * 1/30 days per month * 1/24 hours per day = 278 Watt average load 24 hours per day, 30 days per month

It is not unusual for a desktop computer to use 100-200 watts just by itself... use it 12 hours per day and that could be almost 1/3 to 1/2 of your electric bill (in a small home/apartment).

-Bill

Now I am confused. Looking at the pv disconnect box, how is it if I have 14 panels at 7 or 8 amps each, that the dc to ac transfer only has 2 20 amp breakers. I admit to not knowing much about this but say 7 x 14 = 98, and I have 40 amps of circuits breaker? I don't understand.

Remember Power=Voltage*Amperes

Those 14 panels may be organized into 2 parallel strings by 7 panels each... That could be around Vmp-array of 210 volts * 16 amps = 3,360 Watt solar array.

Taking efficiency into account and the 3,000 watt limit by your inverter--The AC output would be:

3,000 watts * 1/240 VAC = 12.5 amps (at 240 VAC)

Amps, without knowing the voltage is only part of the answer... Amps*Volts which equals Watts is the "total" answer...

More or less the power at some Varray*Iarray=Power-array=Vac*Iac (less losses). The GT inverter just takes the DC solar power and "inverts it" to AC power that can be fed back to the utility grid. It is a "power conversion" process (the GT inverter). No energy was created or destroyed.

-Bill

Re: combine dc & ac

No--100 amps main breakers is still 100 amp @ 120/240 VAC service. The two breakers are in series at 240 VAC (100 amps in one breaker and 100 amps out the other--They do not add).

I am a bit surprised at 100 amp service with the electric stove and such... In our area, they seem to do 125 amp minimum service for an all natural gas home (don't know if it is code or not--My home from ~1955 was a 60 amp service).

It is possible that you have larger bus bars in the main panel--You would need an electrician to be sure (or look for the box id plate and see what it says/look up part number on web).

Even if you have a larger box and could put in 200 am service--Then you would need a new permit (I am guessing) and a call to your utility to make sure your service can manage a (much) larger main service.

You have to follow code in how they calculate adding more sources to the panel--But, I don't think you can add another 20 amp PV breaker at all (20 amp for your first pair is already maximum). You might be able to add another ~3.5 amps of Micro Inverters to the existing 20 amp branch circuit--But you would have to work closely with the building inspector to see what he would allow (he could just as easily say no too about adding more micro inverters).

There are other possible options--But probably will not be allowed for residential install anyway (line side tap).

The only option left would be to install a larger main panel. You can install a 200 amp panel and use 175 amp service disconnect--and you will get:
....200 amps * 20% solar allowance + (200 amp - 175 amp service breakers) = 75 amps maximum of 240 VAC solar power.

-Bill

Re: combine dc & ac

You may be lucky there--You need to verify that the bus bars (which all the breakers plug into) is also rated at 125 amps (and not just the meter socket).

-Bill