First Time Commercial Calculations

[roblsmith3]

I'm trying my hand at a large (to me!) commercial project, 60.58kw to be exact. We are on the 2011 NEC code here in NC. We are using 5x SMA SunnyBoy 6000TL inverters, 1x 7000TL, and 2x 7700TL. The max output for the 6000s are 25A each; 7000 is 29.2A; and the 7700s are 32A each. My question is about how to combine everything into a solar subpanel. When I do my calculations, each inverter output will be on a 40A breaker. 8 inverters x 40A = 320 A total.

Am I correct in figuring that I need a 600A Solar SubPanel with a 350A MCB in the subpanel?
This would allow up to 370A of solar to be backfed on the busbar.

Now for the Main Utility AC disconnect.....would I use a 400A disconnect fused at 350A? If so that would mean a conductor size of 500 kcm correct?:blink::?

Any suggestions or verifying my calculations would be greatly appreciated. Thanks everyone!

 

[GoldDigger]

Even on [2011] many inspectors will allow you to use a 400A panel bus and main breaker as long as the subpanel is identified as a combiner only and you install a placard stating not to add any load supplying breakers.
And in [2014] you would be able to use the sum of the rated inverter outputs instead of the sum of the breaker ratings.

 

[Zee]

Even on [2011] many inspectors will allow you to use a 400A panel bus and main breaker as long as the subpanel is identified as a combiner only and you install a placard stating not to add any load supplying breakers.
And in [2014] you would be able to use the sum of the rated inverter outputs instead of the sum of the breaker ratings.

yes, the label should read:

WARNING:​

THIS EQUIPMENT FED BY MULTIPLE SOURCES.​

TOTAL RATING OF ALL OVERCURRENT DEVICES, ​

EXCLUDING MAIN SUPPLY OVERCURRENT DEVICE, ​

SHALL NOT EXCEED AMPACITY OF BUSBAR.​

 

[Zee]

yes, the label should read:

WARNING:​

THIS EQUIPMENT FED BY MULTIPLE SOURCES.​

TOTAL RATING OF ALL OVERCURRENT DEVICES, ​

EXCLUDING MAIN SUPPLY OVERCURRENT DEVICE, ​

SHALL NOT EXCEED AMPACITY OF BUSBAR.​

Above label is per 2014 NEC, if you want to take advantage of a provision that allows you to exceed the 120% limitation.
(can add brkr.s up to busbar rating)

Today you might word label:

PV INVERTER OUTPUT COMBINING (or AGGREGATION or ACCUMULATION) PANEL ONLY!
ADD NO LOADS!
​

Finer point: It helps convince AHJ's, if your panel is also physically just big enough to fit your PV breakers...say a 16-space panel for your 8 @ 2p40A brkr.s.....

 

[Zee]

Even on [2011] many inspectors will allow you to use a 400A panel bus and main breaker as long as the subpanel is identified as a combiner only and you install a placard stating not to add any load supplying breakers.
And in [2014] you would be able to use the sum of the rated inverter outputs instead of the sum of the breaker ratings.

Thanks. I had no idea the inverter output rating counts per 2014 NEC. Could you provide the actual wording? ( I only have the 2011 NEC) I thought it was still the PV ocpd rating, ie pv brkr rating....

 

[Smart $]

...

Any suggestions or verifying my calculations would be greatly appreciated. Thanks everyone!

1? or 3??

 

[Zee]

40 amps is correct as breaker size for the 5000, 6000 and 7700 inverters each.

However, to save you wire size..... and labor and cost:
Consider using as many
3800
7600
and 11400 inverters as possible.
These are optimized to max out the relevant breaker sizes and to keep you at minimum breaker sizes. (20, 40, 60)
Whereas, currently, on each of your 5 @ 6000 inverters you only have 31.25 A continuous Amps.....yet you must go up to 40A breakers on each.....

IS there a technical reason for needing 6000 inverters? Could you use, say 7 @ 7700?
That would be higher Wattage and even fewer amps, 40 fewer amps actually.

 

[Smart $]

...
Whereas, currently, on each of your 5 @ 6000 inverters you only have 31.25 A continuous Amps.....yet you must go up to 40A breakers on each...

Not really. 35A is a standard rating. Not as commonly stocked as 40A, but a standard rating nonetheless.

 

[ggunn]

Not really. 35A is a standard rating. Not as commonly stocked as 40A, but a standard rating nonetheless.

One thing to consider is that if your inverter requires a 35A breaker but you use a 40A breaker instead, you will probably have to size up your conductors. Your conductors must be protected by the breaker, and to do that you can only go up to the next breaker size up from the derated ampacity of your conductors.

 

[GoldDigger]

Thanks. I had no idea the inverter output rating counts per 2014 NEC. Could you provide the actual wording? ( I only have the 2011 NEC) I thought it was still the PV ocpd rating, ie pv brkr rating....

It is in 705.12(D)(2) [2014]:

(2) Bus or Conductor Ampere Rating. One hundred twenty-five percent of the inverter output circuit current shall be used in ampacity calculations for the folIowing:

Where the following are feeders, taps, bus bars, etc. all in separate subsections. And the busbar section is the home of the 120% rule. I do not see an explicit recognition of the 100% rule (not counting feeder breaker) for AC combiner panels, but I believe that it is in there somewhere.

Back in [2011] the same section reads:

(2) Bus or Conductor Rating. The sum of the ampere ratings of overcurrent devices in circuits supplying power to a busbar or conductor shall not exceed 120 percent of the rating of the busbar or conductor.

With the added provision elsewhere that the rating used in upstream panel calculations is that of the first overcurrent device, that is the one closest to the GTI.

 

[PWDickerson]

Can you provide more information about the installation and electrical service at the site? Single phase or three phase? How have you selected the inverter sizes? What modules are you using?

Based on the straight addition of breakers you did in your original post it sounds like it might be a single phase 120/240V service. If not, some of the following may be incorrect.

You have 52.36 kW total inverter capacity (5x6000W + 1x7000W + 2*7680W), and a total inverter breaker capacity of 295A (5x35 + 3x40). It occurs to me that you would do better by using qty. 7 SB7700TL's. Total inverter capacity goes up to 53.76kW, total breaker capacity goes down to 280A, and materials and installation costs go down a little as well. The reduction in breaker capacity is not enough to get you into a 400A combiner panel per the letter of the code, but like others suggest, check with your AHJ and see if you can apply some of the 2014 code sections that would allow a 400A combiner.

Your fused disconnect should be rated at 400 amps, and fused at 300 amps (52.36kW/240V=218.2A*1.25=272.7A --> 300A fuse). 300kcmil CU has a 75 degree ampacity of 285 amps and should be adequate.

 

[ggunn]

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[Carultch]

I'm trying my hand at a large (to me!) commercial project, 60.58kw to be exact. We are on the 2011 NEC code here in NC. We are using 5x SMA SunnyBoy 6000TL inverters, 1x 7000TL, and 2x 7700TL. The max output for the 6000s are 25A each; 7000 is 29.2A; and the 7700s are 32A each. My question is about how to combine everything into a solar subpanel. When I do my calculations, each inverter output will be on a 40A breaker. 8 inverters x 40A = 320 A total.

Am I correct in figuring that I need a 600A Solar SubPanel with a 350A MCB in the subpanel?
This would allow up to 370A of solar to be backfed on the busbar.

Now for the Main Utility AC disconnect.....would I use a 400A disconnect fused at 350A? If so that would mean a conductor size of 500 kcm correct?:blink::?

Any suggestions or verifying my calculations would be greatly appreciated. Thanks everyone!

Make sure that you select the inverter current, to correspond with the grid voltage and grid configuration from the manufacturer's datasheet. Those currents sound like single phase values. 25A = 6kW at 120/240 single phase.

Most commercial applications have three phase. And combining single phase inverters onto a 3-phase bus isn't as straight forward. For single pole interconnections, common at 277V, you add them up as they will connect to each phase, and then select the largest one. For two pole interconnections, common at 208V, it is a bit more complicated. For three phase inverters on a three phase grid, you can simply add them up.

Assuming it is single phase, you can add up inverter currents directly, and then multiply by 125% to get the main breaker and AC combiner busbar.
You do not need to add up the rounding errors as you go, when you round each 125% of current to a breaker size. Once you have a main breaker, or other overcurrent device somewhere in the output circuit, you've sufficiently protected the AC combiner busbar, and the main AC feeder.

5* 25A = 125A
1* 29.2A = 29.2A
2*32A = 64A

(BTW, are you sure that 7.7 kW is 32A? Because 7.6 kW at 240V calculates to exactly 32A)

Total operating current = 218.2A

Multiply by 125% = 272.5A
Round up to the next breaker, and that is a 300A main breaker, which would thus have a 400A bus (since that is what they make).

This AC combining subpanel is dedicated to the PV system, and cannot contain any unrelated loads. . It would need to be labeled to indicate not to add loads. The 120% rule doesn't need to apply, since it is for a "mixed use" panel, usually at your final landing point.

Unless you use a tap or transformer secondary to interconnect, you simply need at least 272.5 amps of wire, which would be 300 kcmil copper. See 240.4(B).