Center Fed Main Breaker

[dronai]

I know about the 120% rule as it regards to calculating a system size, but I am aware that the inspectors wants to see the main breaker on the opposite end of the bus than the invertor breaker.

So if someone has a center fed breaker, and it's 200A, what's the size of the system that can be complaint ? or is it not even acceptable ?

 

[dronai]

Will inspectors even allow a center fed main breaker ?

 

[GoldDigger]

I know about the 120% rule as it regards to calculating a system size, but I am aware that the inspectors wants to see the main breaker on the opposite end of the bus than the invertor breaker.

So if someone has a center fed breaker, and it's 200A, what's the size of the system that can be complaint ? or is it not even acceptable ?

There are definitely existing center fed residential panels, whether new ones are currently marketed or not.
Unless you have a very agreeable inspector the only way to back feed a center-fed panel is to revert to the 100% rule instead.
(Sum of main and backfeed may not exceed bus ampacity or sum of load breakers may not exceed bus ampacity.)

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[Smart $]

There are definitely existing center fed residential panels, whether new ones are currently marketed or not.
Unless you have a very agreeable inspector the only way to back feed a center-fed panel is to revert to the 100% rule instead.
(Sum of main and backfeed may not exceed bus ampacity or sum of load breakers may not exceed bus ampacity.)

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Isn't that the sum of both the load and PV breakers ratings cannot exceed the bus rating?

 

[jaggedben]

...
Unless you have a very agreeable inspector the only way to back feed a center-fed panel is to revert to the 100% rule instead.
(Sum of main and backfeed may not exceed bus ampacity or sum of load breakers may not exceed bus ampacity.)
....

That's somewhat out of date, fortunately.

See: http://www.nfpa.org/assets/files/AboutTheCodes/70/TIA_70_14_12.pdf

705.12(D)(2)(3)(e) to read as follows
:
(e) A connection at either end, but not both ends, of a center-fed panel board in dwellings shall be permitted where the sum of 125 percent of the power source(s) output circuit current and the rating of the overcurrent device protecting the busbar does not exceed 120 percent of the current rating of the busbar.

Note this TIA was incorporated into the 2017 code as well. So, if you're in California on the 2014 NEC you have a good case to make to your AHJ, although it's best to run it by them before install since their agreement to abide by the TIA is quite optional for them, legally. If you're on the 2017 code you're in a better position to assume that you can use the 120% rule.

Most AHJs I've worked with have been granting installs by the TIA, with the exception, of course, of Palo Alto. :lol:

 

[GoldDigger]

Note that it's hard to seethe theoretical grounds for the TIA since an overload in the opposite section of the center fed bus from the backfeed could draw more than the bus ampacity without either main or backfeed opening.
As long as the sum of the load breakers in the opposite half is less than 100% of bus ampacity then you would be OK, but the TIA does not require that.

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[dronai]

The sum of the load side breakers is based on 100% of the breakers without demand factors applied ?

 

[Smart $]

The sum of the load side breakers is based on 100% of the breakers without demand factors applied ?

It's the sum of breaker ratings. The calculated load is not considered [directly], so neither are demand factors.

 

[jaggedben]

Note that it's hard to seethe theoretical grounds for the TIA since an overload in the opposite section of the center fed bus from the backfeed could draw more than the bus ampacity without either main or backfeed opening.
As long as the sum of the load breakers in the opposite half is less than 100% of bus ampacity then you would be OK, but the TIA does not require that.

What's the theoretical grounds behind the extra 20% in 120%, anyway? I think the 'theory' in the TIA is that if the main breaker has never tripped, and never trips when the intermittent inverter source isn't outputting, then it's highly unlikely that more load will ever be drawn than the bus is rated for. (Also I would guess that more than 50% of the time the situation you described in your last sentence will exist.) Note that the TIA only applies to dwellings. The history of this is that the 120% originally applied only to dwellings and didn't have any opposite end rule. When the 120% was expanded to C&I they made the more stringent opposite end rule, but it was never really so reasonable to be concerned about it in residential service panels.