The 120 rule--- IMHO---Misunderstood?

[foggysail]

The NEC rule 705.12-3 states "The sum of 125% of the power source(s) output circuit current and the rating of the overcurrent device shall not exceed the ampacity of the busbar!''

Fair enough with buts! If the added current is located opposite the end of where the bus bars connect to the main breaker, the direction of the current flowing into the bus bar will be opposite in direction of that originating from the main breaker. Those currents are opposite (negatively adds) to that originating from the main breaker.

I believe the rule is fine, it is the interpretation that is faulty. As to a 40 ampere breaker, supposedly the maximum size for solar currents connecting to your 200 ampere panel. It only depends on where in the panel it is connected,

[SunEagle]

The NEC rule 705.12-3 states "The sum of 125% of the power source(s) output circuit current and the rating of the overcurrent device shall not exceed the ampacity of the busbar!''

Fair enough with buts! If the added current is located opposite the end of where the bus bars connect to the main breaker, the direction of the current flowing into the bus bar will be opposite in direction of that originating from the main breaker. Those currents are opposite (negatively adds) to that originating from the main breaker.

I believe the rule is fine, it is the interpretation that is faulty. As to a 40 ampere breaker, supposedly the maximum size for solar currents connecting to your 200 ampere panel. It only depends on where in the panel it is connected,

Your logic may be correct but unfortunately we have to abide by the rules stated in the NEC or run the risk of getting your POCO or AHJ to shut you down.

I have been an EE for more than 40 years and while I don't always agree with what is written or approved by the NEC board I have to "follow the rules" or suffer the consequences.

[foggysail]

Your logic may be correct but unfortunately we have to abide by the rules stated in the NEC or run the risk of getting your POCO or AHJ to shut you down.

I have been an EE for more than 40 years and while I don't always agree with what is written or approved by the NEC board I have to "follow the rules" or suffer the consequences.

I based my above post on how I read the rules in the NEC, yeah another EE here also. But been there done that when it comes to debating with authorities.

[SunEagle]

I based my above post on how I read the rules in the NEC, yeah another EE here also. But been there done that when it comes to debating with authorities.

Something I have also experienced is that even though the NEC states something the AHJ makes the final call even if it is different then what the NEC states.

I have always stated that Rule 1. Whatever the AHJ wants they get. Rule 2 The AHJ is always right.

[bcroe]

The NEC rule 705.12-3 states "The sum of 125% of the power source(s) output circuit current and the rating of the overcurrent device shall not exceed the ampacity of the busbar!''

Fair enough with buts! If the added current is located opposite the end of where the bus bars connect to the main breaker, the direction of the current flowing into the bus bar will be opposite in direction of that originating from the main breaker. Those currents are opposite (negatively adds) to that originating from the main breaker.

I believe the rule is fine, it is the interpretation that is faulty. As to a 40 ampere breaker, supposedly the maximum size for solar currents connecting to your 200 ampere panel. It only depends on where in the panel it is connected,

That is so true. I just refer to that as the belt and suspenders
rule, to avoid suggesting otherwise and getting busted. Out
here in the Wild West, zoned Ag, I have seen stuff running safely
but not necessarily by that interpretation. I slipped by with 15KW
(62A) with a pair of 40A breakers at one end of a 200A box, and
a 125A breaker at the other end. Bruce Roe

[Ampster]

I based my above post on how I read the rules in the NEC, yeah another EE here also. But been there done that when it comes to debating with authorities.

One of your options instead of debate is to derate. If it is a sub panel it would be easier to add a lower Amperage breaker on that panel effectively lowering the maximum current that can come from the grid or the main panel. I have a 200 Amp subpanel and it is fed by a 60 Amp breaker from my hybrid inverter. I only have two 20 Amp breakers from my micros but anticipate adding at least another at 15 Amps back feeding that panel.

[foggysail]

One of your options instead of debate is to derate. If it is a sub panel it would be easier to add a lower Amperage breaker on that panel effectively lowering the maximum current that can come from the grid or the main panel. I have a 200 Amp subpanel and it is fed by a 60 Amp breaker from my hybrid inverter. I only have two 20 Amp breakers from my micros but anticipate adding at least another at 15 Amps back feeding that panel.

I did an analysis based on the maximum output of the 26 Enphase inverters that are defined as a part of the installation. I further derated my 40 ampere breaker that solar connection to the grid uses. And then because there could be continuous current flowing for 3 hours or more, I derated it 20% or 32 amperes. The predicted maximum current will be just under 32 ampere making my system compliant with the NEC. And even doing the analysis, I knew I was dealing with bull poop regarding the 120% load center's bus.

I have no clue why technical people allow &^%$#! non-technical people to establish technical limits and requirements.

[Ampster]

How are you going to find a 32 Amp breaker? Even if you did the micros are a continuous load and that any breaker cannot carry more than 80 percent continuous load of the breaker rating. My earlier comment was meant to imply that you derate the main breaker to pass the 120 percent busbar back feed rule. Two different rules constraining your choices.

[foggysail]

You misread or I poorly explained what I did. The breaker remains 40 ampere. I just derated it 80% to 32 ampere. I don’t have the code book here on my boat so just maybe I did not need to derate it.

The IQ8 Plus has a continuous power rating of 290 watts, I = 290/240 = 1.2 A/inverter. 26 inverters @ 1.2 ampere = 31.4 which is at the limit of a 40 ampere derated breaker. And I am not sure at which current level the breaker needs to be derated in any case. As far as derating the main breaker that gets to be ridiculous! This reason for the 40 ampere maximum sized breaker to back feed into the load center is allowed by the 120 rule. Heck, I AM NOT THE GUY THAT HAS TO APPROVE THE SYSTEM ANYWAY! That is up to the installer. This entire discussion is predicated on a bull poop rule to begin with.

[foggysail]

One more thing while I am in a keyboard pounding mood. It has been awhile (years) since I did my last service upgrade when doing electrical work. But the total amperes allowed by the breaker count has zero to do with the actual main breaker load currents. You have to take into consideration the duty cycles of the panel loads. A stove is never operated with all burners and oven 24 hours a day. A clothes dryer does not operate 24 hours every day. And the list goes on

[bcroe]

Yes so much of energy used by consumers is on an intermittent
basis. No breakers blown, no hot wires. But solar can hit its peak
power every day, for 8 straight hours here. That warm up a feed,
which then cools at night. This continuous cycling can loosen
things up, I found for a couple years I needed to just tighten
everything once or twice a year. not so much any more, and I
have upgraded feeds so there is very little warm up. Bruce Roe

[Jerry Kirwan]

I based my above post on how I read the rules in the NEC, yeah another EE here also. But been there done that when it comes to debating with authorities.

In this situation they 'Own the Football'! It is just like a pedestrian crossing the street, they could have been Right, but Dead Right. Rules & Regulations; Do not mix with Fighting City Hall.

[foggysail]

Yes so much of energy used by consumers is on an intermittent
basis. No breakers blown, no hot wires. But solar can hit its peak
power every day, for 8 straight hours here. That warm up a feed,
which then cools at night. This continuous cycling can loosen
things up, I found for a couple years I needed to just tighten
everything once or twice a year. not so much any more, and I
have upgraded feeds so there is very little warm up. Bruce Roe

Yes, but the solar current is limited to that being back fed through the breaker it is connect to. That current flows in the opposite direction of that entering the panel thereby actually reducing the current entering via the main breaker.

[foggysail]

In this situation they 'Own the Football'! It is just like a pedestrian crossing the street, they could have been Right, but Dead Right. Rules & Regulations; Do not mix with Fighting City Hall.

Quite right! I was recently told there is a movement in Congress to repeal Ohm’s Law!!! People demand protection!

[SunEagle]

.......movement in Congress to repeal Ohm’s Law!!! People demand protection!

LOL. Nice comment