Photovoltaic Panel Problem...

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lunalilo

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A customers adding a 2-pole 70Amp additional sub-panel to a 200Amp Main panel. I heard there is a 120 percent rule, and with my add-on this will indeed go over this 120 percent, which would be at a safe value of 240 AMPS. I am goin over this Amperage,

1)is this safe?

2)is this 120 percent rule real? If so, what's it all about?
 

don_resqcapt19

Moderator
Staff member
See 690.64(B)(2). In general you have to protect the panel at or below its rating. When you connect a second power source to the panel you are providing a source of additional current. The code rule in 690 permits the total of the main breaker and the solar system breaker to equal 120% of the panel rating.
Also note the rule in 690.64(B)(7) which requires the solar breaker to be on the opposite end of the panel bus from the panel feed.
 

drive1968

Senior Member
Depending on the load, one option would be to switch out the 200 amp breaker for a 150, then you would be under the 20% rule.
 

mlnk

Senior Member
If the main is 200 amps, 120% is 240 amps. If your solar maximum input is over 40 amps then you need to change the 200 to a 175. Is it really 40 amps? This would be about $50,000 in solar work!!! The customer can add whatever load they want: a 70 amp panel and a couple of 100 amp panels or more....all not relevant to the solar 120% rule.
 

acrwc10

Senior Member
The 120% rule is because the colder it is out the more a solar panel can produce. They will go over their listed wattage.
 

hurk27

Senior Member
A customers adding a 2-pole 70Amp additional sub-panel to a 200Amp Main panel. I heard there is a 120 percent rule, and with my add-on this will indeed go over this 120 percent, which would be at a safe value of 240 AMPS. I am goin over this Amperage,

1)is this safe?

2)is this 120 percent rule real? If so, what's it all about?
I'm curious to know if this 70 amp sub-panel is add load or added supply from a solar system?

I understand the supply rule simply because you could possibly over current the main busses, but as a added load the only thing that would happen is the OCPD's would open.
 

drive1968

Senior Member
<<<The 120% rule is because the colder it is out the more a solar panel can produce. They will go over their listed wattage. >>>

It is true that solar panels produce more efficiently when they are cool, but that is totally unrelated to the 120% rule. Solar panels will occasionally produce slightly above their listed wattage, but even if they did so regularly, that would be an argument against allowing the bus to be loaded with breakers at 120% of the bus rating.

The primary reasons for the 120% rule is that many homes have their main breaker at the full rating of their bus, yet the home rarely approaches full load. If the 120% rule weren't in place, homeowners would often be forced into either upgrading their service or downgrading their main breaker whenever they wanted to add even a small solar system.
 
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acrwc10

Senior Member
<<<The 120% rule is because the colder it is out the more a solar panel can produce. They will go over their listed wattage. >>>

It is true that solar panels produce more efficiently when they are cool, but that is totally unrelated to the 120% rule. Solar panels will occasionally produce slightly above their listed wattage, but even if they did so regularly, that would be an argument against allowing the bus to be loaded with breakers at 120% of the bus rating.

The primary reasons for the 120% rule is that many homes have their main breaker at the full rating of their bus, yet the home rarely approaches full load. If the 120% rule weren't in place, homeowners would often be forced into either upgrading their service or downgrading their main breaker whenever they wanted to add even a small solar system.
Do you really believe what you wrote ? The NEC is concerned about the possibility of an "over current" on the buss causing a fire, not that a home owner may have to spend a few dollars to change a panel.
 

drive1968

Senior Member
<<<Do you really believe what you wrote ? The NEC is concerned about the possibility of an "over current" on the buss causing a fire, not that a home owner may have to spend a few dollars to change a panel.>>>

Yes, I believe what I wrote and it is exactly correct. The reason I gave is exactly why the exception is allowed for residential, but not for commercial. The NEC allows the bus to be over by 20% on residential because the load is less likely to be maxed. It has absolutely nothing to do with the panels producing higher watts during cold weather as you suggested. By the way, if you think financial considerations have nothing to do with the code, you would be incorrect.
 

acrwc10

Senior Member
<<<Do you really believe what you wrote ? The NEC is concerned about the possibility of an "over current" on the buss causing a fire, not that a home owner may have to spend a few dollars to change a panel.>>>

Yes, I believe what I wrote and it is exactly correct. The reason I gave is exactly why the exception is allowed for residential, but not for commercial. The NEC allows the bus to be over by 20% on residential because the load is less likely to be maxed. It has absolutely nothing to do with the panels producing higher watts during cold weather as you suggested. By the way, if you think financial considerations have nothing to do with the code, you would be incorrect.
Ya the requirement to put AFCI in proves that.
 

drive1968

Senior Member
The AFCI requirement does show that the large players in the industry have a big say in the NEC code. The solar industry also has a seat at the table and in 2008 pushed to change the 120% rule to also apply to commercial installations if the backfed breaker was placed on the opposite side of the bus from the utility source. 690.64 is an important rule for the solar industry because it removes a significant additional cost during many solar installations. The less financial impediments, the better, as far as the solar industry is concerned as long as safety isn't clearly sacrificed.
 

ty

Senior Member
you are limited to 40amps of PV load in the 200amp loadcenter, as stated above.

You have a couple work-arounds: you can possibly change the main breaker to 150amp as stated above, you can do a line-side tap, you can put a larger service meter taht accepts double lugs and put in a seperate panel for the PV, you can cahnge the existing 200amp loadcenter to a Cutler Hammer 200amp mbl that is the plug-in neutral line, they have a 250amp rated buss that would allow you to put up to 70amps of PV.
 
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