Centerfeed busbar and 120% rule

[Smart $]

But as long as all the breakers are load breakers, there's not a problem with overloading a panel. If the load exceeds the rating, the main breaker will shut it down.

I understand that. Using the sum of load breakers is just an alternative to allow POCO + PV greater than 120% of bus rating.

 

[ggunn]

I understand that. Using the sum of load breakers is just an alternative to allow POCO + PV greater than 120% of bus rating.

But load breakers can come and go after the PV is installed. Personally, I am fine with the 120% rule as it is. It's easy to understand and implement, and there's no gray areas. Except for center fed panels, of course...

 

[Smart $]

But load breakers can come and go after the PV is installed. Personally, I am fine with the 120% rule as it is. It's easy to understand and implement, and there's no gray areas. Except for center fed panels, of course...

And that's the reason for the plaque. Easy for [most] anyone to sum up load breaker ratings. Look at 705.12(D)(2)(3)(c) in 2014. Essentially says the same as what I said except it includes the PV breaker rating in the sum.

 

[Carultch]

I understand that. Using the sum of load breakers is just an alternative to allow POCO + PV greater than 120% of bus rating.

I think it's simple enough. It's tough when you are in a situation where you exceed 20%.

I'm glad that NEC2014 rewrote this rule to prevent rounding errors from being a show stopper. So that 125% of the current is used, instead of 125% of the current rounded up to a corresponding breaker size.


I do wonder about taking advantage of future "relaxed" NEC rules, when the local region hasn't adopted your version of the NEC yet. Such as:
"My system is in New York, still on the 2008 code, and I've got a rounding error situation where 705.12(D) from the 2014 edition would really help. But at the same time, I don't want to make the entire NEC2014 apply, introducing rapid shutdown and AFCI."

Or, another situation would be:
"I'm familiar with the 2014 code, and I'm new to the trade. Therefore I haven't learned any previous editions. If I build to the 2014 NEC where the 2008 NEC applies, can I still pass inspection?"

 

[SolarPro]

If know the history of the 120% rule & that of the "opposite end of the busbar" rule, you can see why there should be no problem applying the 120% rule to a center-fed bus in residential applications.

If you go back to NEC 2005, you'll see that the 120% rule shows up as an exception to 690.64(B)(2):

Exception: For a dwelling unit the sum of the ampere ratings of the overcurrent devices shall not exceed 120 percent of the rating of the busbar of conductor.

So in any residential application, you are allowed to apply the 120% rule. But you are limited to the rating of the busbar in commercial applications under NEC 2008.

This changes in NEC 2008, which expands the 120% rule to apply to all PV systems and applications. However, members of the CMP note that there is a difference between a residential load profile and a commercial load profile.

Therefore, the "opposite ends of the busbar" rule is added in NEC 2011 to address this difference.

In other words, the CMP added the "opposite ends of the busbar" rule to address concerns about the 120% allowance specifically in commercial applications. The CMP has never had any concern about applying the 120% rule in residential applications.

It sounds like this will be clarified in the 2017 cycle of revisions. Not only will the 120% rule apply to center-fed panels in residential applications, but there will be no need for engineering supervision in these residential applications.

 

[jaggedben]

It sounds like this will be clarified in the 2017 cycle of revisions. Not only will the 120% rule apply to center-fed panels in residential applications, but there will be no need for engineering supervision in these residential applications.

Glad to hear that. Fingers crossed.

 

[Zee]

If know the history of the 120% rule & that of the "opposite end of the busbar" rule, you can see why there should be no problem applying the 120% rule to a center-fed bus in residential applications.

If you go back to NEC 2005, you'll see that the 120% rule shows up as an exception to 690.64(B)(2):



So in any residential application, you are allowed to apply the 120% rule. But you are limited to the rating of the busbar in commercial applications under NEC 2008.

This changes in NEC 2008, which expands the 120% rule to apply to all PV systems and applications. However, members of the CMP note that there is a difference between a residential load profile and a commercial load profile.

Therefore, the "opposite ends of the busbar" rule is added in NEC 2011 to address this difference.

In other words, the CMP added the "opposite ends of the busbar" rule to address concerns about the 120% allowance specifically in commercial applications. The CMP has never had any concern about applying the 120% rule in residential applications.

It sounds like this will be clarified in the 2017 cycle of revisions. Not only will the 120% rule apply to center-fed panels in residential applications, but there will be no need for engineering supervision in these residential applications.

good to know! i tie into many center fed panels, and this is good background.

 

[jmc392057]

More clarification needed: Visual Added

More clarification needed: Visual Added

Here's a visual example of a residential center-fed panel to try and provide some clarity (see attached photo). The OCPD at the top (40A 2P) is the PV OCPD. Note the 200A service OCPD in the middle, with most (but not all) of the other load OCPDs below. If we add up the OCPDs below the service OCPD, the total is 140A. Above the service, there's an additional 90A. Given this scenario, it would not be possible for the bus to be overloaded given the location of the PV breaker at the top, and the total sum of other OCPDs below the main service OCPD. Agreed?

 

[ggunn]

Here's a visual example of a residential center-fed panel to try and provide some clarity (see attached photo). The OCPD at the top (40A 2P) is the PV OCPD. Note the 200A service OCPD in the middle, with most (but not all) of the other load OCPDs below. If we add up the OCPDs below the service OCPD, the total is 140A. Above the service, there's an additional 90A. Given this scenario, it would not be possible for the bus to be overloaded given the location of the PV breaker at the top, and the total sum of other OCPDs below the main service OCPD. Agreed?

Are you an engineer? See 705.12(D)(2)(3)(d).

 

[zman990]

This is a classic case of I hate center fed panels bus rated at 200 and main at 200 no solar allowed if u derate main to 175 then 25 amps solar 150 main 50 amps solar.
So do load calculation and put a 150 main in instead of 200.
I personally don't like 150 and never do it with out customer consent because of possible adding of load like hot tubs and electric cars.
Don't want to limit panel when customer can receive 30% off a panel upgrade.
Change it out

Sent from my HTC One_M8 using Tapatalk

 

[pv_n00b]

Here's a visual example of a residential center-fed panel to try and provide some clarity (see attached photo). The OCPD at the top (40A 2P) is the PV OCPD. Note the 200A service OCPD in the middle, with most (but not all) of the other load OCPDs below. If we add up the OCPDs below the service OCPD, the total is 140A. Above the service, there's an additional 90A. Given this scenario, it would not be possible for the bus to be overloaded given the location of the PV breaker at the top, and the total sum of other OCPDs below the main service OCPD. Agreed?

If you assume that none of the breakers will be changed out in the lower section. Unfortunately what you have to look at is what would that lower section look like if every position was switched out with the highest breaker rating available? Would it still be below 200A? Not that anyone would ever do that, but since it's physically possible it has to be planned for in the code.