# Solar 120% Rule



## dronai (Apr 11, 2011)

Having to install a 400A service change for the Solar Electrician because of the rating of the existing 200A service bus, and the "Main Breaker's Top fed rule" Customer has a center fed main. 

Looking for the code reference if anyone knows it.


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## ren79eg (May 30, 2010)

705.12 (D) (2) and (7)


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## bbee (Dec 15, 2011)

Do a line side tap, much cheaper and easier.


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## dronai (Apr 11, 2011)

bbee said:


> Do a line side tap, much cheaper and easier.


 
I'm just doing the service change for the Solar co. They have another Electrician doing the invertors, and the rest.

Explain please what you mean by line side tap. Existing is a 200A, and the Solar 2 pole is going to be a 70A 12kw system


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## backstay (Feb 3, 2011)

dronai said:


> I'm just doing the service change for the Solar co. They have another Electrician doing the invertors, and the rest.
> 
> Explain please what you mean by line side tap. Existing is a 200A, and the Solar 2 pole is going to be a 70A 12kw system


Should not be more than 50 amps. The calc should 200 times 120% minus 50 amps equals 170 left for the service.


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## dronai (Apr 11, 2011)

backstay said:


> Should not be more than 50 amps. The calc should 200 times 120% minus 50 amps equals 170 left for the service.


That's why I have to go with a 400A service

Also how did you come up with 50A ? 120% x 200 = 240 I thought that means the remaining 40 is the maximum amount that could be used ?


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## ren79eg (May 30, 2010)

12kw = 50 [email protected] However I think you're right about the 40 amps in addition to the 200 amp main breaker


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## dronai (Apr 11, 2011)

ren79eg said:


> 12kw = 50 [email protected] However I think you're right about the 40 amps in addition to the 200 amp main breaker


The guy ran 2 invertors into a panel backfeeding with 2 35A 2 pole breakers, and then #4cu, to a main disconnect that will tie into the main panel. 

I didn't check his math but it looks like more than 12K


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## ren79eg (May 30, 2010)

FWIW - You don't necessarily "need" a 400 amp service. In addition to doing a line side setup, you could swap in a panel with a 250 amp BUSS rating and use a 200 amp main breaker. IF you could source the parts. That would allow for 100amps of inverter backfeed.


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## ElectricJoeNJ (Feb 24, 2011)

ren79eg said:


> 12kw = 50 [email protected] However I think you're right about the 40 amps in addition to the 200 amp main breaker


It's actually 62.5 amps. You have to rate it for continuous load. Each 6kw inverter is rated at 31.25 so the next breaker size is 35. I agree that a 400 amp service is overkill I would do a 250a with a 200a main or main lug panel with a seperate disco


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## ren79eg (May 30, 2010)

True...just tryuing to figure out where the previously mentioned 50 came from.


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## backstay (Feb 3, 2011)

ren79eg said:


> True...just tryuing to figure out where the previously mentioned 50 came from.


12kW divided by 240 volts is 50 amps. Is that the solar array size or are you calling it 12kW because of the inverter? With losses across the system you will not see that many amps. If the calculated load of the service is less than 170 amps, I say a 200 amp service is enough.


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## dronai (Apr 11, 2011)

ren79eg said:


> FWIW - You don't necessarily "need" a 400 amp service. In addition to doing a line side setup, you could swap in a panel with a 250 amp BUSS rating and use a 200 amp main breaker. IF you could source the parts. That would allow for 100amps of inverter backfeed.


Yep, but two supply houses and a lot of calls, and I couldn't even get a panel with 225A busing ! It has to be a top fed main breaker, and not center fed. That is what will give me the 120% rule to work with. They only had 200A, and then 400A top fed breaker panels.



backstay said:


> 12kW divided by 240 volts is 50 amps. Is that the solar array size or are you calling it 12kW because of the inverter? With losses across the system you will not see that many amps. If the calculated load of the service is less than 170 amps, I say a 200 amp service is enough.


I don't know their load calcs. But I went through it with the inspector already, as far as my requirements, and what he was looking at with the invertors, and the existing panel. The Solar company is now in the process of submitting plans, and getting permits.


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## ElectricJoeNJ (Feb 24, 2011)

backstay said:


> 12kW divided by 240 volts is 50 amps. Is that the solar array size or are you calling it 12kW because of the inverter? With losses across the system you will not see that many amps. If the calculated load of the service is less than 170 amps, I say a 200 amp service is enough.


Unfortunately most inspectors I've dealt with and engineers don't care about the losses on the system or the calculated load. If the inverters rated output is 12kw they want it calculated and installed as such. That would mean a minimum of 70a ocpd and a 250 rated buss with 200a main breaker.


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## backstay (Feb 3, 2011)

ElectricJoeNJ said:


> Unfortunately most inspectors I've dealt with and engineers don't care about the losses on the system or the calculated load. If the inverters rated output is 12kw they want it calculated and installed as such. That would mean a minimum of 70a ocpd and a 250 rated buss with 200a main breaker.


Inverter size has little to do with system generating capability. It a means to transfer energy from generator(panels) to the grid. Inverters are almost always larger than the array that they connect to.


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## ElectricJoeNJ (Feb 24, 2011)

backstay said:


> Inverter size has little to do with system generating capability. It a means to transfer energy from generator(panels) to the grid. Inverters are almost always larger than the array that they connect to.


I understand what your saying and all I'm saying is they don't care about that. The system must be wiring according to the output of the inverter.


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## ElectricJoeNJ (Feb 24, 2011)

backstay said:


> Inverter size has little to do with system generating capability. It a means to transfer energy from generator(panels) to the grid. Inverters are almost always larger than the array that they connect to.


II. Utility-Interactive Inverters
705.60 Circuit Sizing and Current.
(A) Calculation of Maximum Circuit Current. The maximum current for the specific circuit shall be calculated in accordance with 705.60 (A)(1) and (A)(2).
(1) Inverter Source Circuit Currents. The maximum current shall be the maximum rated input current of the inverter.
(2) Inverter Output Circuit Current. The maximum current shall be the inverter continuous output current rating.
(B) Ampacity and Overcurrent Device Ratings. Inverter system currents shall be considered to be continuous. The circuit conductors and overcurrent devices shall be sized to carry not less than 125 percent of the maximum currents as calculated in 705.60(A). The rating or setting of overcurrent devices shall be permitted in accordance with 240.4(B) and (C).
Exception:  Circuits containing an assembly together with its overcurrent device(s) that is listed for continuous operation at 100 percent of its rating shall be permitted to be utilized at 100 percent of its rating.


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## backstay (Feb 3, 2011)

ElectricJoeNJ said:


> II. Utility-Interactive Inverters
> 705.60 Circuit Sizing and Current.
> (A) Calculation of Maximum Circuit Current. The maximum current for the specific circuit shall be calculated in accordance with 705.60 (A)(1) and (A)(2).
> (1) Inverter Source Circuit Currents. The maximum current shall be the maximum rated input current of the inverter.
> ...


Too bad, systems rarely increase in size because module manufacturing changes too rapidly to match new panels to old ones.


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## mertabird (Nov 14, 2013)

I definitely need to read up more about grid tied solar arrays. Seems like solar may truly be just on the horizon.


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## jsmart84 (Nov 20, 2011)

I was a sub for a solar installer for almost 2 years and performed almost 200 hundred Iinstallations. From what I have read yyour better off performing a line side tap between the meter and main breaker in home. We normally use a 18 by 18 by 6 pvc box and 3 barrel insulated lugs . We did start to use a different product that has teeth that bite onto the service.conductors Iin the panel and run out but I was never a fan of them. If you have any questions PM me.


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## macmikeman (Jan 23, 2007)

http://w3.usa.siemens.com/us/internet-dms/btlv/Residential/Residential/docs_Home/SIE_FL_4SOLAR.pdf

60 amps max pv however.. Maybe you could figure out a way to have em lower the amount of panels connected.


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## ElectricJoeNJ (Feb 24, 2011)

macmikeman said:


> http://w3.usa.siemens.com/us/internet-dms/btlv/Residential/Residential/docs_Home/SIE_FL_4SOLAR.pdf
> 
> 60 amps max pv however.. Maybe you could figure out a way to have em lower the amount of panels connected.


60 amps of solar for a house is a pretty sizable system. It will allow a 10k system without having to do a line side tap. Unfortunately around here the inspectors won't allow a smaller system output size based on less panels. The system size is based on the max output of the inverters connected to the service


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## macmikeman (Jan 23, 2007)

ElectricJoeNJ said:


> 60 amps of solar for a house is a pretty sizable system. It will allow a 10k system without having to do a line side tap. Unfortunately around here the inspectors won't allow a smaller system output size based on less panels. The system size is based on the max output of the inverters connected to the service


That might even be code (I have read the pv section at least 50 times and still cannot understand what the hell it says...) however it is the most re-tarded concept there could possibly be injected into the code book. 


On the other hand , since microinverters are tied to each panel, it is another good reason to sticking to using a microinverter system. Add panel- add another microinverter. Remove panel- remove microinverter rating.


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## Semi-Ret Electrician (Nov 10, 2011)

How does one calculate the short circuit rating of equipment tied to a battery bank?


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## backstay (Feb 3, 2011)

Semi-Ret Electrician said:


> How does one calculate the short circuit rating of equipment tied to a battery bank?


 The only thing tied to the batteries are breakers designed for that. One between the battery and the positive system and one between the battery and the charge controller. Or class T fuses.


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## Solargarbage (Jan 28, 2014)

Line tap takes the rule out of play. As far as I know. Done about 10 taps last year all passed no prob.


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## benjdwilson (Jun 12, 2013)

Also could downsize the main breaker, but of course do load calcs first to ensure no nuisance tripping.

Going from 200A to 175A would allow for 65A solar.
Going all the way down to 150A would allow for 90A solar.


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## chicken steve (Mar 22, 2011)

Semi-Ret Electrician said:


> How does one calculate the short circuit rating of equipment tied to a battery bank?



output @ high noon? :blink:

~CS~


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## chicken steve (Mar 22, 2011)

Solargarbage said:


> Line tap takes the rule out of play. As far as I know. Done about 10 taps last year all passed no prob.


I'm drawing a blank on 690, if the jist of sizing is the house demand load PLUS the inverter output for a panel, wouldn't it exist for a service drop as well?

~CS~


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## ElectricJoeNJ (Feb 24, 2011)

chicken steve said:


> I'm drawing a blank on 690, if the jist of sizing is the house demand load PLUS the inverter output for a panel, wouldn't it exist for a service drop as well?
> 
> ~CS~


Doesn't really have much to do with the house demand. The rule is there for actually supplying power to a buss. Breakers supplying power to a buss can't exceed 120% of the buss rating


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## chicken steve (Mar 22, 2011)

Well i think i _get it _Joe

We are just addressing _one_ component, the panel buss rating...

am i correct in _assuming _certain installs would require a 200A panel on a 100A service then?

~CS~


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## ElectricJoeNJ (Feb 24, 2011)

chicken steve said:


> Well i think i get it Joe
> 
> We are just addressing one component, the panel buss rating...
> 
> ...


Yes. On a 100 amp panel you can only backfeed via breaker 20 amps of solar. If you had a 200 amp panel with a 100 amp main you technically could put a 140 amp solar backfeed, but that wouldn't be to code because the solar backfeed can't exceed the rating of the service. So the most you could backfeed on a 100 amp service would be 100 amps.


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