Posts: 16Registered Users
I'm installing my first system with 2 arrays. I need help understanding the calculations for the back feed breakers.

I have a 200 amp service

I'm installing 2 arrays. I planned to run out from each inverter to their own AC disconnect then to individual meters for the PA Sunshine rebate program and then into the service panel with a backfeed breaker for each array. Forgive me, but I seem to be having brain freeze at the point of interconnection to the service panel. Below are the specs. All assistance is much appreciated.

Solar System Specs:
Array #1
3 strings of 5 - Sanyo HIP-215NKHA5

Fronius IG4000
The nominal output is 16.7A (According to Section 690.8, the output circuit must be sized at 125% of the rated output) Then the safety factor would make it 20.875A.

There is an Outback combiner box mounted on the roof. EMT runs from the combiner through attic to DC disconnect in garage.

8 AWG wiring from the combiner to the DC disconnect (account for heat running through attic to diminish voltage drop)

EMT runs from the DC Disconnect to the inverter located in the basement. (Utility supplied lockbox for key to basement)
10 AWG wire runs from the DC Disconnect to the Fronius inverter.

Array #2
3 strings of 6 with the same setup as described above.

From what I understand my backfeed breakers would need to be 25A for each array. When the code refers to OCPDs supplying power would that be the backfeed breaker? So then I would have 50A supplying power.

"The sum of the ratings of the breakers may not exceed 120% of the rating of the bus bar or the ampacity of the conductor." John Wiles

In a 200 amp service is the bus bar rating 200amps? (I believe it is the terminology that throws me not the concepts/theory) If yes then 120% of 200A is 240A. Then my two 25A backfeed breakers are safe to install, right?

My partner spoke to an electrical inspector. The inspector suggested that he has seen installers with two arrays go into a sub panel with one AC disconnect for panel and then only 1 meter. My partner is not trained in solar PV, but he does have a very good knowledge base, but I don't trust that he accurately described the system we are installing. (Since this is my first post I have to figure out how to upload a photo.)

I'm coming to all of you because my electrician seems to be in never never land when I try to discuss this with him about code.

I took my SEI class last year and there is much to remember and understand, let alone try and decipher the NEC 2008 code book.

If I have missed supplying you information please yell and I will get it to the discussion.

Thanks,
Kim

• Posts: 66Solar Expert ✭✭✭✭

120% of the busbar rating for a 200A service is 240A. 240A would then be the maximum sum total of allowable amps of all breakers that feed power into the main busbar. You cant use two 25A breakers with 200A service and a 200A main breaker. That would total 250A, which is 10A more than allowable. per the NEC.

You will either need to do a line side tap outside
OR
Install a 150A or 175A main breaker in your main panel
(240A - 150A = 90A of allowable backfeed)
(240A - 175A = 65A of allowable backfeed)
Then, install your two 25A backfeed breakers at the BOTTOM of the busbar.

Note: You'll likely find that when you go looking for main breakers on all the usual places, that 175A are more expensive and generally harder to come by than 150A main breakers. A new old stock 150A cuttler-hammer cost me \$60.
• Posts: 16Registered Users

Great! Now I understand what they mean. Since I'm feeding the service panel with 50A from the PV and the utility is feeding it with the 200A from the street then we overload the panel.

Could someone like you write the NEC code?;)
• Posts: 2,297Solar Expert ✭✭✭

You should be able to combine the 2 PV feeds into one, and run a single meter. connection to the panel. The breaker rating would then be the next size up for the 2 amp ratings of the inverters max AC output.

Now as far as de-rating you panel main, you need to consider the max draw that would possibly take place. If your load side can exceed the main de-rated breaker, you will be looking at a service entrance upgrade. At least that is what happened here and was required by the local code guys. We tried to do a line side tap, however the code guys shunned that kind of solution and said the only time they allowed that was a large commercial install.
• Posts: 10,311Solar Expert ✭✭✭✭

sd,
technically when feeding an electric panel with both solar and with the utility ac there will always be more power available within the buss of the panel than the main breaker could handle. the problem is that the total power should not exceed that of the buss rating. one way to insure the buss rating is to lower the main breaker rating, that i assume you would not want to derate your solar input to the panel, which is not always conducive with the home requirements and hence a panel upgrade is then warranted.
• Posts: 480Solar Expert ✭✭✭
solar_dave wrote: »
You should be able to combine the 2 PV feeds into one, and run a single meter. connection to the panel. The breaker rating would then be the next size up for the 2 amp ratings of the inverters max AC output.
Not going to help in this case as he has two inverters rated at 16.7A.

16.7A * 2 = 33.4A * 1.25 = 41.75A

Rounding up he could use a 45A breaker - not that common, though, so he'd likely still end up feeding the main panel with a 50A breaker.

Only way around it is to downsize the main breaker as previously suggested or tap into the feed in between the main meter and main panel.
• Posts: 2,297Solar Expert ✭✭✭
drees wrote: »
Not going to help in this case as he has two inverters rated at 16.7A.

16.7A * 2 = 33.4A * 1.25 = 41.75A

Rounding up he could use a 45A breaker - not that common, though, so he'd likely still end up feeding the main panel with a 50A breaker.

Only way around it is to downsize the main breaker as previously suggested or tap into the feed in between the main meter and main panel.

i think you missed my point. In my case we planned to downsize the main, however with the loads already in the service were over the required downsize value. Code dudes said unacceptable. So if you add up all the loads and then add in the solar amperage it can't be more than 120% of the buss bar rating. Their fear was a max load in off hours constantly tripping the de-rated main. Yeah I know, I would have had to run both AC, units, the stove and oven, clothes dryer and washing machine, pool pump, microwave, every light, hair dryer, TV in the place all at the same time.
• Posts: 971Solar Expert ✭✭✭

For What is worth Pa was still using 2005 code last december when I installed my fronious inverters. Think 2005 nec is easier to meet. I have 2 inverters into a breaker box and 1 240 v output to shutoff switch at my power pole that is for a mobilhome. S:Dlarvic
• Posts: 480Solar Expert ✭✭✭
solar_dave wrote: »
i think you missed my point. In my case we planned to downsize the main, however with the loads already in the service were over the required downsize value. Code dudes said unacceptable. So if you add up all the loads and then add in the solar amperage it can't be more than 120% of the buss bar rating. Their fear was a max load in off hours constantly tripping the de-rated main. Yeah I know, I would have had to run both AC, units, the stove and oven, clothes dryer and washing machine, pool pump, microwave, every light, hair dryer, TV in the place all at the same time.
Eh? It's not as simple as adding up all the loads and seeing if they add up to more than the size of the main breaker or not. Never-mind that the whole point of the NEC allowing 20% busbar rating (as long as you put your backfeed on the opposite end of the main breaker) already accounts for possible bus-bar overloading.

There's a proper way to calculate your minimum main breaker size based on the size of your house, the number of small appliance and laundry circuits and all your hard-wired appliances.

This is typically called a "Load service calculation" or similar. It's spelled out exactly in the NEC.

Up to the first 10,000 VA, the loads get counted at 100%, but after that, most loads (except HVAC) only get counted at 40% of their rated value.

And all your hard-wired appliances only get rated at their nameplate Volt-Amps value, not the size of the circuit they're plugged in to.

The size of your PV system doesn't come in to play in this calculation at all.

For example for my house, I have a 100A main breaker, but probably have close to 200A of circuit breakers plugged in to the main panel (counting only half of the single pole breakers since the main breaker is a 2-pole). But doing a proper load service calc reveals that a 100A main breaker is sufficient (though just barely in my case).
• Posts: 2,297Solar Expert ✭✭✭

What can I say, I can only go by what the local code guys would accept. My 200 amp panel with a 50 amp breaker add for the solar would have overload the bus, reduction to 175amp was insufficient, and a 150amp de-rating would have been required. With 2 AC units, electric stove/oven, electric dryer & washer, ........ 150 amp would not have made it according to the code department.

All of this required a \$4300 service entrance upgrade to satisfy them. It is fine that NEC makes those statements but the ultimate stamp of approval comes from the local code department.
• Posts: 480Solar Expert ✭✭✭

Yes, while the local inspectors have the final say, if you can present an argument using the NEC, most will have a hard time saying no.

200A main breaker w/200A busbar only gives you room for 40A backfeed - NEC is very clear on that so no arguing that you're OK to backfeed 50A there.

A 175A main breaker with a 200A busbar gives you room for 65A of backfed solar power. Why did they say that was insufficient?
• Posts: 16Registered Users

Update:
I just learned that the busbar could possibly be rated larger than the 200A service. I should be able to go check the rating in the morning.

If it is only rated at 200A the inspector wants me to tie in on the line side. Install a small ac panel next to the inverters, feed the panel with (2) 25 amp breakers and then feed out with a 60 amp breaker (#6 THHN) to 1 solar production meter , from there go outside to a 100 amp rated D/S. From there perform a line side tap ahead of the main breaker with a splice kit / bug. The new AC panel inside would have to marked as the solar ac power source along with some labeling on the main panel just for safety.

Let you know what I find and how we proceed.
• Posts: 480Solar Expert ✭✭✭

Nice - line-side-taps aren't that common as many homes have integrated meter/panel boxes which don't give you the room to perform a line-side-tap.

Either way, take some pics and show us how it's done.

PS - It's not that common to be able to determine if the busbar is rated higher than the main breaker. Often, even if the busbar could handle the load, the manufacturer doesn't rate the busbar separately from the enclosure. Hopefully yours is an exception as that makes things much easier to hook up!
• Posts: 2,297Solar Expert ✭✭✭
drees wrote: »
Yes, while the local inspectors have the final say, if you can present an argument using the NEC, most will have a hard time saying no.

200A main breaker w/200A busbar only gives you room for 40A backfeed - NEC is very clear on that so no arguing that you're OK to backfeed 50A there.

A 175A main breaker with a 200A busbar gives you room for 65A of backfed solar power. Why did they say that was insufficient?

I am not sure about that one, I would have to ask the engineer on the project. Perhaps it is the 40C rating on the breaker. mid summer temps here in AZ regularly hit the 40C number.
• Posts: 16Registered Users

Okay, the system is tied in, inspected, producing and metered with the Electric company.

Here are the pictures of the line-side tied in.

Please note that all the wiring mess around the box was not our doing. Our work is very neat and orderly.

• Posts: 16Registered Users

Also, forgot to mention...
Dealing with PECO in getting the keybox issue resolved was ridiculous.

In PA, as approved by the PUC, a homeowner may choose not to have the AC disconnect outside. They can opt to have a lockbox outside with a key to get into the home and access the disconnect.

we chose to have the lockbox and submitted that on the interconnection application. Our application was approved. I guess no one looked to see that lockbox was checked. Come time to interconnect with PECO and I asked what/if any were the specs for the lockbox. I was told "what lockbox?"

There was one guy, electrical engineer, who was a thorn in my side. I finally circumvented him. They had nothing to provide the customer for specs. I ended up purchasing a small AirConditioning disconnect box (5"x7"x2"deep) and pulling the guts out & placing the house key inside.

If anyone ever needs help in dealing with this in the future just contact me.
• Posts: 1,280Solar Expert ✭✭
HTG PVinPA wrote: »
Okay, the system is tied in, inspected, producing and metered with the Electric company.