Design4Solar Tutorial: The 120% Rule
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- Опубликовано: 8 сен 2024
- The 120% Rule, also known as 690.64(B)(2), is explained. Examples are given to illustrate how it works in real situations. Practice problems are presented to the student to check comprehension.
I’ve been told to add the backfeed solar breaker to the total load before you subtract the overcurrent device
Thank you so much for your easy-to-understand explanation. Thank you.
Thanks! Been looking for clarity off and on over the past couple weeks. :)
So clear, the best explanation
I read the rule as 120% of the busbar rating. So as an example, I have a 200 amp service feeding a 200A main breaker panel with feedthrough lugs to a downstream 200A breaker panel. The busbar in the main panel is rated at 225 amps. If only one solar breaker was backfeeding in that main panel it should be rated at 225X1.2-200 which would be 70amps. Does that seem right?
Question, is it 120% based on the bus bar rating or the service rating, cause you can have 4/0 alum which is rated for 200A, but panel bus bar and main could be 100A, in that case u go by the bus bar rating and not the feeders rating, is this correct or not?
What happens if the busbar rating is different from the service rating and/or the main disconnect? Example 150 amp service, 200 amp busbar, 125 amp main breaker? How do you DERATE a main service panel?
My understanding is that it's the rating of the BUSBARS THEMSELVES irrespective of the size of the service. So in the 125A service with 100A main, you need to specify the rating of the BUSBARS to find out the actual breaker sizing. Say it's in a 200A panel, you'd have up to 140A of solar you could add.
I hope my question is dumb but.....if the two solar systems produce 10A with safety factor each and the availablity of breaker is 20A so inside the panel i provide two 20A breaker and my main service panel is 100 amps rated does it violate 120 percent rule......??can anybody answer this
In some respect this NEC code makes little sense in the case of a home owner adding solar to help cut down on his use of utility power. The solar power added to the main panel just reduces the power needed from the grid. Any solar power not needed by the home would just flow back out to the grid. The only time this rule makes sense is when the homeowner adds a lot of air conditioner load to a small electric panel like a 60 or 100 amp panel and counts on a lot of solar input to supply power to the buss bar in the main panel. It is too bad there is not technical language to avoid a lot of unneeded panel upgrades or derating of the main breaker just because solar power is being added to the homeowners electrical system.
The purpose of this NEC article (705.12(B)(3)) is to prevent overloading the busbars. Normally, the Main Circuit Breaker protects the busbars. But when you add an additional power supply to it (solar), you might be exceeding the Amperage capacity of the busbars. Theoretically, an overload could possibly happen when the connected loads are drawing more amperage than the busbars are rated to handle. The Main CB might not trip because there's additional power supplied by the solar. The 20% tolerance gives us some leeway, because that situation is unlikely to occur in a properly designed system, with load calculations done per NEC rules (how a service is sized). ...and I presume they're probably also banking on the fact that all electrical equipment has some extra safety margin capacity designed into it.
What does this means? Smaller solar breaker (40A) the back feed slower to the grid compare to the bigger breaker (80A) the back feed is faster back to the grid. Is this to our advantages or same?
THANK YOU MUCH
What if the main service panel is a transformer that wants to suck your grounding rod for more Amperage. Should I wear a "conduit"?
That was a great primer for me. Thank you!
My house has 5 subpanels, and I think I read somewhere that there is different math when considering adding solar to a subpanel. Can you explain how that scenario works?
Looks like my main "box" is rating for 400A, and it has a 200A main breaker on it. A subpanel closer to a ground mount location has a sticker which says 200A max, and it has a 100A main panel shut off breaker.
The panel "box" rating is the busbar rating. Busbars rated for 400Amps that are protected by a 200Amp Main Circuit Breaker, provide the capacity to add 200Amps+ from solar Circuit Breakers. Without looking at it, I'm guessing the solar subpanel has a 200Amp bus, which is protected by that 100Amp Main Breaker. That would mean the bus has 100Amps+ of solar Circuit Breaker capacity. There's a difference between the circuit breaker size and the maximum solar output. Max solar current X 1.25 = circuit breaker. Your solar array might be roughly close to 15kW? I'm curious, what brand name is on the 400A Main Panel? You might find my other comments on this video interesting as well.
Do you have to derate for continuous load for solar? Solar amps x 1.25, then size wire and breaker. Solar is over 3 hrs.
I think the 3 hours you mention is for MOTORS continous duty derating.
Can someone maybe shed some light on where this percentage came from / the deciding factors or test conditions which leads to NEC making this the standard. How long has this been included in the code?
no replies? best Ive seen is 'wiggle room' but this doesnt satisfy me...at all...
@@atomicdmt8763 You might find my comments/replies on this video of interest.
Their a$$. I am waiting for years for someone to explain to me how the busbar can be overrun if total load is less than or equal to 200A and the solar is fed at the other end of the busbar than grid with loads in between the two sources. Cause it can't according to the power flow.
I even dare to raise: not even with 400A loads in between a 200A grid on one side and 200A solar on the other of the busbar - not even in this case will the busbar be overrun provided that load breakers are no higher than 200A
Interesting, I agree with your calculations. NABCEP is using a different formula though, not sure why. I was just reading the NABCEP study guide it had this to say. "For example, consider a 200 A
busbar fed from the top with
a 200 A overcurrent device
from the utility supply. Using
method (c), the maximum
allowable inverter rated
output current feeding the
busbar would be: [(200 A x
1.2) - 200 A]/1.25 = 32 A." The method c they're referring to deals with 2014 code. What do you think of that extra 1.25 division?? www.nabcep.org/wp-content/uploads/2019/05/NABCEP-2019-PV-Certification-Study-Guide.pdf
This video is referring to the maximum overcurrent device that connect to the bubar, but the NABCEP questions asks for the largest inverter output current. The difference in the rule between the 2011 and 2014 NEC is that the 2011 NEC specifies the largest solar overcurrent device that can connect to a busbar, but the 2014 NEC specifies the largest inverter output that can connect to a busbar (the actual current from the inverter). In most cases, you get the same answer with the 2011 and 2014 wording because the overcurrent device is usually 1.25 x the inverter output rating. The 1.25 factor is used for calculating the continuous rating of a wiring and overcurrent devices, solar or otherwise.
WARNING TOO MUCH INFORMATION BELOW!
One instance when the answer is different is when you are using a bi-modal inverter (battery inverter). These inverters usually have a large overcurrent device (60-70A) connected to the busbar on their supply side. The reason for this large overcurrent device is so that the inverter can pass current from the utility directly to a backup loads panel or charge batteries. When the inverter is operating in this manner, it is essentially acting as a load and not power production source. For an inverter like the Schneider XW6848 which has a 28A output, the 2011 (*actually, pre-2011) NEC would only allow a 40A breaker to feed this inverter if the bus was 200A and fed by a 200A main breaker. The inverter manufacturer, however, requires a 60A breaker so the system can pass through utility power to the loads. The 2014 NEC, allows this inverter fed by a 60A breaker to connect to a 200A bus protected by a 200A main breaker since 28A (solar feed) x 1.25 = 35A which is less than 200A (bus rating) x 1.2 - 200A (main OCPD) = 40A. Since the 35A is less than the maximum of 40A specified by the 2014 NEC, the inverter can be connected regardless of the size of the circuit breaker feeding it.
*The 2011 NEC actually had another section (I believe it was somewhere in 690) that allowed this calculation for battery-based systems and this became the rule in the 2014 NEC and the wording with the size of the overcurrent device was removed.
@@Kimandy6862 thank you for your time In sharing this information!
Thank you for sharing this info. Very helpful...
what about a subpanel that fed from a main panel that has a 100 amp for the subpanel?
I am not sure I agree with your explanation here. The intent is to prevent overloading the panel board's bus bars. Using the "service rating" is not correct as the service coming from the utility (and passing through the meter base in most states) is not limited by an over current device, thus it is usually taken as infinite. The main service entrance breaker (the main panel breaker in your example) is what limits the current feeding the panel's bus bars coming from the utility. Your 120% calculation should be taken as this breakers rating or the listed rating of the panel's bus bars. Your example of a "125 amp service" and a "100 amp main breaker" is misleading. In that example, I would assume the 100 amp main is the maximum rating of the bus bars (as most panels have service entrance breakers rated to the maximum bus rating). Thus you should use the 100 amp for the calculation, not the assumed "125 amp service rating". Bottom line, you do not want to be able to draw more than 120% of any panel board's bus bar rating. To do so runs the risk of melting the panel and starting a fire.
I guess using the words "service rating" is probably incorrect is your assertion. I assumed he meant the "service rating" of the panel's busbar. He wrote the number right on top of the rectangular box representing the main panel. So if you adjust the wording to be more clear, everything he did was fine. The example you cite is simply a derated main breaker--not at all uncommon.
Is the bus bar rating same as main service panel rating?
Usually, but you have to be careful. It's possible to have a Main Circuit Breaker of one rating, and the Main Panel (busbars) at a greater rating. Like one example in this video. But he misspoke about that example. What I think he meant to say is there's a 200 Amp service, because there's a 200 Amp Main CB, which has a 225 Amp panel (busbars). The higher rated busbars provide more capacity for adding solar.
It's good but we can't see what your pointing to a lot.
Is the software still available?
my main is rated 200 amp. the 100 amp that feed the subpanel is still the same scenario?
Why isn't it a 100% rating this is a little confusing since I personally think of 100% as the max of any rating/percent. Is the 20% a tolerance that is designed around?
Short answer, yes. Long answer: The purpose of this NEC article is to prevent overloading the busbars. Normally, the Main Circuit Breaker protects the busbars. But when you add an additional power supply to it (solar), you might be exceeding the Amperage capacity of the busbars. Theoretically, that could possibly happen when the connected loads are drawing more amperage than the busbars are rated to handle. The Main CB might not trip because there's additional power supplied by the solar. That's how the overload occurs. The 20% tolerance gives us some leeway, because that situation is very unlikely to occur in a properly designed system, with load calculations done per NEC rules (how a service is sized). ...and I presume they're probably also banking on the fact that all electrical equipment has some extra safety margin capacity designed into. Nobody's going to sell a 100Amp panel that melts down at 100Amps. They'd get sued out of business. If it was me, I'd design it for at least 50% more, or double!
My main service panel is rated at 62 milliamps. What now? I really need one more Christmas light to finish my box off. I meant house.
You can do 0.74 milliamps you bag of shit
I wish I could drive 120% of the posted speed limit. Makes sense I guess.