center fed residential panel with Photo Voltaic

[jengrahm]

I recently inspected a 125 amp residential panel with a 6.6 KVA photovoltaic system. It has a 100 amp MCB, and is feeding 2 separate photo voltaic inverter's with 20amp/2 poles. The photo voltaic load side breakers are located at each end of the buss bar, with MCB in the middle. is this acceptable? I know that usually with a center fed panel that the 120% rule now becomes a 100% rule, but does this still apply with a P.V. CB on either end?

 

[ggunn]

I recently inspected a 125 amp residential panel with a 6.6 KVA photovoltaic system. It has a 100 amp MCB, and is feeding 2 separate photo voltaic inverter's with 20amp/2 poles. The photo voltaic load side breakers are located at each end of the buss bar, with MCB in the middle. is this acceptable? I know that usually with a center fed panel that the 120% rule now becomes a 100% rule, but does this still apply with a P.V. CB on either end?

I guess it could be looked at as two sets of busbars each fed by a 100A breaker, so a 20A breaker on each end would conform to the 120% rule, but I dunno. There was a spirited discussion in here about center fed panels and PV a while back, but I don't remember what if anything was the consensus.

Aside: What is the rationale behind the design of a center fed panel in the first place?

 

[GoldDigger]

You cannot treat it as two separate busses for either 100% or 120% rule. Regardless of the physics, you must keep the sum of the center main and both end sets of breakers (or the sum of first inverter breakers) no higher than 100% of the bus rating.
It is true that when looking at the maximum bus current at any one cross section it can never be greater than the sum of the main and the larger of the two end breaker groupings.
There is no way that the 120% rule could be applicable.

 

[jaggedben]

I'd say you'd be within your rights to reject this, the code being somewhat vague. But note that if the loads on either side of the main breaker are less than the rating of the busbar then there is no way that the busbar can be overloaded in that section.

 

[SolarPro]

Here's what will be allowed under NEC 2014 in subsection705.12(D)(3)(d):

Connections shall be permitted on multiple-ampacity busbars or center-fed panelboards where designed under engineering
supervision that includes fault studies and busbar load calculations.

That being the case, the alternative to rejecting this interconnection outright?which is well within your right?might be to request proof of engineering supervision. (Unfortunately, the definition of "engineering supervision" will not make it into the Code until the 2017 cycle. ;-)

 

[shortcircuit2]

Engineering Supervision for a 100amp residential panel? That's alittle over the top IMO. This is in California...so that would be the 2011 NEC? The 2011 NEC doesn't address Center Fed Equipment that I could find.

Probably a 6.6 system may put out 5KW thru the inverter(s). Why 2 inverters anyway for a small system like this.

I don't think adding a 5KW input to a 125amp bussed panel with 100amp Main would ever be a problem.

 

[ggunn]

Engineering Supervision for a 100amp residential panel? That's alittle over the top IMO. This is in California...so that would be the 2011 NEC? The 2011 NEC doesn't address Center Fed Equipment that I could find.

True, the 2011 NEC does not address center fed panels, but how are you going to put the backfed breaker at the opposite end of the busbar from the main when the sum of the main and backfed breaker ratings is between 100% and 120% of the busbar rating (as the NEC stipulates) if the main is in the center of the busbar? Engineering supervision might mean that someone would look at the load distribution and sign off that there would be no way for the loads on the opposite side of the bus from the backfed breaker to draw more than the rating of the main.

 

[GoldDigger]

If you do not qualify for the 120% rule because of the center feed, you are left with the 100% rule either using the bus rating of 125A if you can document it or putting in a smaller main breaker if the load calc allows.
Or, of course, a supply side tap.

 

[jaggedben]

Engineering Supervision for a 100amp residential panel? That's alittle over the top IMO.

I've somewhat idly wondered if I would be able to get an engineer to give me a boilerplate letter stating circumstances under which center fed panels would be okay, that could be used for most simple panels. Haven't actually tried though.

 

[ggunn]

I've somewhat idly wondered if I would be able to get an engineer to give me a boilerplate letter stating circumstances under which center fed panels would be okay, that could be used for most simple panels. Haven't actually tried though.

A center panel fed would not be OK if there were enough load breakers on one side that the main would trip if those breakers were all maxxed at once without PV on the panel, and there was a backfed breaker on the other side. In that case, the side of the panel opposite from that containing the PV breaker could receive the max current from the main plus the max current from the backfed breaker. The 120% rule does not mean that a busbar can carry 120% of its rating.

I would never give you such a letter.

 

[Sierrasparky]

OK
Are you saying that if you have a 100 amp center fed panel you cannot feed with a PV breaker of 20amp input load.

 

[GoldDigger]

If the bus is rated at 125A, you can. Or if you put in an 80A main.
If you blocked off one half of the bus and put the backfeed at the far end of the other and called it engineering supervision....

 

[Sierrasparky]

If the bus is rated at 125A, you can. Or if you put in an 80A main.
If you blocked off one half of the bus and put the backfeed at the far end of the other and called it engineering supervision....

Just thought.
All those 200 amp quad breaker center feed panels won't work as I do not think they are rated higher than 200 amps.
My local will not allow a line side tap. You will need to install a new panel or a seperate one.

 

[Zee]

Just thought.
All those 200 amp quad breaker center feed panels won't work as I do not think they are rated higher than 200 amps.
My local will not allow a line side tap. You will need to install a new panel or a seperate one.

...or just get a 175 Amp main breaker = up to 25 A of PV.
...or just get a 150 Amp main breaker = up to 50 A of PV.

I tie in to centerfed panels and usually the inspector has no issue.
I am ready to downsize the MAIN breaker if the insp. brings it up. Sometimes I just beat the insp. to it and downsize the main brkr. from the get go.

I do tie in on the end of the busbar that has the highest loads.
The other end with, say a 20 amp breaker or similar just cannot DRAW THE BUSBAR RATING PLUS THE PV AMPS.

Finally the center fed panels are often not equally split into two equal length busbar halves. I tie in on the longer busbar half. IF it has the big loads.


( and if you leave the old 200 amp main brkr you removed with customer, he can do what he will.....)

 

[ggunn]

.
The other end with, say a 20 amp breaker or similar just cannot DRAW THE BUSBAR RATING PLUS THE PV AMPS.

But a fault in the panel itself can. The fault can draw the rating of the main plus that of the PV, and if the fault occurs on the busbars on the other side of the main from the PV, all that current flows in that section of the bus. That's why the NEC stipulates that the main and PV breakers be on opposite ends of the bus; in that configuration in the event of a fault no part of the bus sees all the current from both breakers.

 

[Sierrasparky]

...or just get a 175 Amp main breaker = up to 25 A of PV.
...or just get a 150 Amp main breaker = up to 50 A of PV.

I tie in to centerfed panels and usually the inspector has no issue.
I am ready to downsize the MAIN breaker if the insp. brings it up. Sometimes I just beat the insp. to it and downsize the main brkr. from the get go.

I do tie in on the end of the busbar that has the highest loads.
The other end with, say a 20 amp breaker or similar just cannot DRAW THE BUSBAR RATING PLUS THE PV AMPS.

Finally the center fed panels are often not equally split into two equal length busbar halves. I tie in on the longer busbar half. IF it has the big loads.


( and if you leave the old 200 amp main brkr you removed with customer, he can do what he will.....)

Can you even get a Quad 150 or 175 that will replace the 200.

 

[jaggedben]

But a fault in the panel itself can. The fault can draw the rating of the main plus that of the PV, and if the fault occurs on the busbars on the other side of the main from the PV, all that current flows in that section of the bus. That's why the NEC stipulates that the main and PV breakers be on opposite ends of the bus; in that configuration in the event of a fault no part of the bus sees all the current from both breakers.

I don't know why you're talking about a fault instead of about loads. We've talked before in this forum about the possibility of a 'resistive fault', i.e. a steady fault that would somehow act like a resistive load but not draw enough current to trip the main breaker. I don't believe either the 120% rule or the opposite end rule have the purpose of protecting against such an unlikely possibility, which, by itself, would be a far greater fire danger than overloading a busbar by 20%.

I believe the real thinking behind these rules is as follows:

• legitimate loads (not faults) that are to one side of both sources could conceivably add up to more than the busbar rating. This is why the 2014 code allows any amount and placement of interconnected breakers as long as the total sum of non-utility-source breakers does not exceed the busbar rating.
• UL testing of panelboard busbars is not designed to account for most of these esoteric scenarios, so the rules are conservative.

 

[jaggedben]

Can you even get a Quad 150 or 175 that will replace the 200.

They do exist.
E.g.http://www.amazon.com/Murray-MPP2175-240-Volt-175-Amp-Breaker/dp/B0088BEFEK
(Not endorsing or advertising for this vendor.)

A 150 is probably easier to find than a 175.

BTW, 'quad' is off the mark. We are talking about a 4pole breaker meant to be installed with parallel conductors.

You may also run the risk of having an inspector ask if you violate the panelboard listing by replacing main breakers on such panels.

 

[ggunn]

I don't know why you're talking about a fault instead of about loads.

I mentioned a fault because Zee said with load breakers distributed as he stipulated overloading the busbars was impossible. Unlikely, maybe, but impossible, no.