Main lug only sub-panel backfeeding

[BVNitta]

My first post! OK here's the scoop. Two separate PV inverters (SMA5000 and SMA6000) going into a PV-only sub-panel (main lug only and 100A rated). One with a 25A and the other with a 35A breaker. This feeds into a 125A rated main lug only sub-panel (which has multiple other domestic loads) through a 60A backfed breaker in this sub-panel. This sub-panel is protected by an 80A breaker in a 200A main breaker panel located in a separate building.

The system configuration met with resistance from NY incentive folks who review all system documentation prior to installation. Their concern pertains to the 125A main lug only sub-panel with the 60A PV breaker. My contention was that 690.64 B(2) permits me to size as follows: (80+60) </= 125*1.2. Their statement reads as follows:

"In review of your application, required document(s) are missing or need additional information:

[FONT=&quot]Take another look at the 3-line diagram, the 60 amp PV breaker is a violation of NEC 690.64(B)(2) since it in a main lug only sub panel the sum of the main and the PV breakers cannot exceed 120% of the 80amp sub-feed. If the sub panel had an 80 amp main circuit breaker, then it would be 120% of the 125 amp bus rating."

I take this to mean that if you're backfeeding a main lug only sub-panel you cannot use the 120% of bus rating rule but are restricted to 120% of sub-panel breaker rating? Does this make sense to you folks? Your help is greatly appreciated.

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[jaggedben]

Their wording is a little confusing, but they are right. Note that they said " the sum of the main and the PV breakers cannot exceed 120% of the 80amp sub-feed." They are talking about the feeder conductors between the 80A breaker and the MLO subpanel, which are also subject to the 120% rule. So unless that feeder was sized for at least 116A when it was installed, you are violating that rule.

They suggested the solution for you: " If the sub panel had an 80 amp main circuit breaker, then it would be 120% of the 125 amp bus rating." Hopefully you can just install such a breaker into the MLO panel and re-land the feeders on it. Otherwise you'll have to consider a more difficult option, such as replacing the feeders or finding another place to land.

I take this to mean that if you're backfeeding a main lug only sub-panel you cannot use the 120% of bus rating rule but are restricted to 120% of sub-panel breaker rating?

The rule isn't about an MLO panel per se. But if the feeders to an MLO panel are rated less than the panel itself, then you are restricted to 120% of the feeder rating. In many situations the feeder and breaker rating will be the same so it amounts making the answer to your question 'yes'.

 

[BVNitta]

Thanks for the response! I'll have a look at the feeder conductors and get back to you. Can't recall what they were off the top of my head..

 

[BVNitta]

Hi jaggedben,

I'm trying to understand this '120% of the subpanel feeder conductor rating'. If the feeders are 2AWG and have an ampacity of 130A, this means the code would have me size breakers (on sources supplying power) upto 156A? (Presumably the concern is for a potentially overloaded neutral?)

 

[jaggedben]

Yes, you've got it right.

The wording is in 690.64(B)(2) in 2008 NEC or 705.12(D)(2) in 2011.: "The sum of the amp ratings of overcurrent devices in circuits supplying power to busbar or conductor shall not exceed 120 percent of the rating of the busbar or conductor." 156A does not exceed 120% of 130A.

If the feeder between the 80A breaker and the 125A panel is rated 130A, the utility's objection is not valid.
Note that the restricting factor here would be the 125A panel, but you are still fine with 140A utility plus solar.

The concern is overloading the conductors and busbars themselves. The rule doesn't entirely make sense, but you could look at this other thread.

 

[BVNitta]

To forestall any future objections and meet the letter of the code, I'm thinking to install a main breaker in the main lug only subpanel. As you recall, this is a Murray MLO 125A subpanel and the main breaker needs to be no bigger than 80A. I can't seem to find an 80A main breaker kit which bolts to the busbars. So I will have to use a snap-in 80A breaker and put a hold down kit on it (not sure if I will find an acceptable one..), to backfeed the busbars. Is this acceptable, do you think?

 

[jaggedben]

To forestall any future objections and meet the letter of the code, I'm thinking to install a main breaker in the main lug only subpanel. As you recall, this is a Murray MLO 125A subpanel and the main breaker needs to be no bigger than 80A. I can't seem to find an 80A main breaker kit which bolts to the busbars. So I will have to use a snap-in 80A breaker and put a hold down kit on it (not sure if I will find an acceptable one..), to backfeed the busbars. Is this acceptable, do you think?

That should be acceptable, and is more or less exactly what I would have looked at doing. There are definitely hold-down kits for Murray panels of recent vintage, though there may be couple different types if I recall correctly.