Electrical Panel & Solar - Center Fed and 100A?

[Dan Z]

Inverter size matters more than panel rating. In your case, a 3800 W inverter will have a 20 A PV breaker. For a center-fed panel with a 100 A main and 125 A bus, adding 20 A of PV is fine.

In my case, I am planning a 5.7 kW system with microinverters (20 panels at 285 each). Two branches. Am I ok with my current panel (adding two 15 A of PV)?

[Glen]

What city? Mircoinverters or string inverter? I think 4.125 is about 17 Amps (so probably two branches for a microinverter system). They fed the wires directly into your main panel?

Dan, I'm the one who pointed you to this forum from NextDoor. Same neighborhood as you. I'm using a SolarEdge 3800 inverter, 15 panels with optimizers on a single string. You're welcome to come by and look over my system.

[Dan Z]

Thanks for the offer Glen! Considering I am planning a microinverter system (for now), I think the differences are significant. So, seeing the electrical will not be very helpful. I will let you know if I change my mind. BTW: I am on Calle Mejor.

[sensij]

In my case, I am planning a 5.7 kW system with microinverters (20 panels at 285 each). Two branches. Am I ok with my current panel (adding two 15 A of PV)?

If you have a centerfed panel with a 125 A bus and a 100 A main breaker, that leaves 25 A available for PV. Two 15 A breakers would not be ok*.

Again, inverter size matters, not solar panel rating. You are planning 20 M250's. At 1.0 A rating each, that is 20 A. Minimum PV breaker is 1.25 the inverter rating, or 25 A. If you combine the strings outside the main panel and then use a 25 A PV breaker, you will be OK. The combiner would need to provide 20 A overcurrent protection per string to protect the 12 AWG trunk cable.

If you don't combine externally, 16 M250's * 1.25 = 20 A. If you have one string of 16 and one of 4, you would need a 20 A breaker and a 5 A breaker to stay under the 25 A allowed. I'm not sure that 5 A two pole breakers exist, but if you change it to a string of 12 and a string of 8, you could run that into a 15 A and a 10 A breaker and be compliant.

In any case, with only 25 A to work with, you can't put more than 20 M250's onto that center fed service panel (*without doing an engineered load calculation to explain why your bus won't get overloaded)

[Glen]

Thanks for the offer Glen! Considering I am planning a microinverter system (for now), I think the differences are significant. So, seeing the electrical will not be very helpful. I will let you know if I change my mind. BTW: I am on Calle Mejor.

I think wiring from the inverter to panel are pretty much the same, no? In any case, sometimes seeing something in person is helpful. I'm on Sitio Borde.

[Dan Z]

To summarize in detail (using strings of 8 and 12)
1) run the two Enphase trunk cables into a weatherproof junction box on the roof and connect them them to two 12/2 with ground AWG (also connect the earth ground running to the panel bonding circuit).
2) run solid conduit out of the junction box and through a sealed roof penetration
3) run two 12 AWG through my attic with flex conduit
4) run the wires through the side wall of the attic and into solid conduit and down the exterior of the house
5) run the wires into the main box using a 15 A and a 10 A

I still want to add a 4 ton AC unit. Is this a problem. Currently, I have 5 open slots (3 above the main and 2 below). I guess two at the top will be used by the PV. I think I may be ok if I do the engineered load calculation because I have no big loads (gas dryer, gas stove, no pool). Do I need to hire someone to do this calculation? I have tried a spreadsheet on the internet, but the terminology is set up for pros and I couldn't figure out my load.

[Glen]

Ah, I wasn't thinking microinverters, yeah, wiring would be different. Note, my installers added two earth grounds, spaced 6 feet apart and ran both back to the inverter

[sensij]

To summarize in detail (using strings of 8 and 12)
1) run the two Enphase trunk cables into a weatherproof junction box on the roof and connect them them to two 12/2 with ground AWG (also connect the earth ground running to the panel bonding circuit).
2) run solid conduit out of the junction box and through a sealed roof penetration
3) run two 12 AWG through my attic with flex conduit
4) run the wires through the side wall of the attic and into solid conduit and down the exterior of the house
5) run the wires into the main box using a 15 A and a 10 A

I still want to add a 4 ton AC unit. Is this a problem. Currently, I have 5 open slots (3 above the main and 2 below). I guess two at the top will be used by the PV. I think I may be ok if I do the engineered load calculation because I have no big loads (gas dryer, gas stove, no pool). Do I need to hire someone to do this calculation? I have tried a spreadsheet on the internet, but the terminology is set up for pros and I couldn't figure out my load.

Your summary looks correct. Keep in mind when running the flex in the attic that clearances need to be maintained with the roof, and the flex needs to be properly supported, maybe labeled if it is considered accessible.

I'm not sure what to make of your slot question. Your PV breakers are two pole (240 V) that need to connect to both phases. That usually means each breaker will consume two slots, so adding a 10 A and a 15 A would take four slots total, leaving only one for the AC. A 4 ton AC is probably a 240 V load as well (30 A breaker?), which would also need two slots and leave you one short. If you have single phase loads that are consuming an entire slot, you should be able to replace the breakers with half-sized breakers and get two circuits per slot, but you'll need to share more about the existing wiring to know. If you are slot-limited, the external string combiner into a single 25 A PV breaker is another way around the problem.

If you want to go the load analysis route, before hiring someone, it would be a good idea to talk to your AHJ about what they want to see to support supplying a centerfed panel with PV over 100% of the bus rating. Maybe you can do it yourself, maybe they'll require a PE stamp, and maybe they won't entertain it at all. With end fed panels, code allows 120% no problem if the PV breaker is at the opposite end from the supply, but anything else is murky.

Glen's suggestion to run 2 ground electrodes is a good one, if you don't already have them or a Ufer. The bonded pair is almost certain to be approved... anything else can become a subject of debate.

[Dan Z]

Below are some photos of my box. There are 6 tied 15A and one 20A. My gut is telling me that I will need the RMA or something like you suggest to add the 4 ton AC. Hopefully not a new box.
Any suggestion on how to reach the AHJ?
Not sure what I have in the way of ground rod. I will check when I get home.

Panel closed.jpg Panel.jpg

[FFE]

That picture and info will get you the answer you need.

[sensij]

Yeah, you aren't going to fit two PV breakers and an A/C circuit in that panel. I've mentioned that you could combine the two PV strings externally and run them into a single 25 A breaker (two pole). That would leave room for the A/C circuit. A relatively painless way to do it is to use a Soladeck junction box to make the pass-through from the roof to the attic. Model 0760-41 is 6" deep, apparently designed for tile. You can put the breakers inside it (there is a kit, if you look) and combine the trunk cable to a single 10/2 + ground cable for the run to the service panel. Since you need to have a pass though of some kind anyway, it isn't really adding much in the way of components, just some cost. (probably $200 or so). If you DIY a panel upgrade, the cost might be comparable, but that is a lot more work. Paying for an electrician to do a panel upgrade could approach $2k.

And, just to revisit what was written earlier... although your plan with FMC is OK, conduit is not usually required in an attic. You can transition to Romex in the Soladeck and run it like any other AC circuit through the attic and house

[Dan Z]

OK. I follow that. The Soladeck looks like a very convenient box! One question on it: If I am putting a 25 A two pole fuse in the main breaker box, why to I need additional branch fuses in series in the solardeck? This seems redundant, and if they ever trip, someone has to go on the roof to re-set them. I would have assumed that just a terminal block to join the conductors would go in the Solardeck. Good to know I can skip the FMC. I'll do a some research on the rules for the Romex routing.

Thanks for all the great advice! I'm sure I will have some more questions, but I think I am about 90% done with respect to knowing what to do.

[sensij]

OK. I follow that. The Soladeck looks like a very convenient box! One question on it: If I am putting a 25 A two pole fuse in the main breaker box, why to I need additional branch fuses in series in the solardeck? This seems redundant, and if they ever trip, someone has to go on the roof to re-set them. I would have assumed that just a terminal block to join the conductors would go in the Solardeck. Good to know I can skip the FMC. I'll do a some research on the rules for the Romex routing.

Thanks for all the great advice! I'm sure I will have some more question, but I think I am about 90% done with respect to knowing what to do.

The Enphase truck cable is 12 AWG, which must be protected by OCPD with a rating no more than 20 A. The problem is the *minimum* required OCPD rating for the combined circuit rated to produce 20 A continuous is 1.25x, or 25 A, too high to protect the trunk cable. So, to protect the trunk cable, additional OCPD is required for each string before they are combined. If Enphase sold 10 AWG trunk cable, it would be good up to 30 A (typically), and individual branch OCPD would not be necessary.

[Dan Z]

Got it. Thanks!

[philips]

..10/2 + ground cable..

I think enphase inverters require the neutral, so 10/3 would be needed. At least the engage cable has a neutral in it. Not sure what it is used for - voltage monitoring maybe?