Have a customer who is insisting on a 33 panel system utilizing Enphase M250's. He currently has a 200A main breaker panel with a bus rated also at 200A, allowing me 40A of PV. My 33 panel system calculates out to 41.25A continuous load. I've considered downsizing the main, but am told that my particular breaker/frame combo is no longer made. Here's my question; is there anything in the code that prevents me from sizing my OCPD at 40A instead of 45A (next higher breaker, based on continuous load calculation). The only risk I see here is that the breaker trips because I am slightly undersized based on continuous load. I also doubt that this will ever happen, given the loads in the house and the fact that I am oversizing my wires to compensate for voltage drop. Thoughts?
OK to downsize OCPD
- Thread starter csoc64
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shortcircuit2
Senior Member
- Location
- South of Bawstin
If you are under the 2014 code you should be OK.
33x250 = 8250watts
8250/240volts = 34.375amps
33x250 = 8250watts
8250/240volts = 34.375amps
- Location
- Placerville, CA, USA
- Occupation
- Retired PV System Designer
1. The amount of house load and the wire size have very little to do with preventing nuisance trips from an undersized backfed breaker. In fact high VD becomes high voltage *rise* at the GTI, leading to lower output current for the same wattage.
2. The 2014 code gives you some leeway to perform the 120% rule calculation based on the rated output rather than the actual breaker size, but that may not help you in this case.
3. I do not think the OCPD selection requirement for an inverter output circuit gives you the option of going smaller.
4. Are you calculating output based on the sum of the panel power ratings or on the rated output of the M250s? Clipping can help you.
2. The 2014 code gives you some leeway to perform the 120% rule calculation based on the rated output rather than the actual breaker size, but that may not help you in this case.
3. I do not think the OCPD selection requirement for an inverter output circuit gives you the option of going smaller.
4. Are you calculating output based on the sum of the panel power ratings or on the rated output of the M250s? Clipping can help you.
shortcircuit2
Senior Member
- Location
- South of Bawstin
That's how I figured it in post #2...
He could run the system with the 45amp breaker under the 2014 code.
SolarPro
Senior Member
- Location
- Austin, TX
There may be a way to do this under the 2014 Code. Just keep in mind that Enphase's installation manual only allows 16 M250s per 20A branch circuit (at 240Vac).
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- Location
- Placerville, CA, USA
- Occupation
- Retired PV System Designer
Oops. 33/16 >2
Does it allow only a 20 circuit to be used based on the ampacity of their cable harness?
ggunn
PE (Electrical), NABCEP certified
- Location
- Austin, TX, USA
- Occupation
- Consulting Electrical Engineer - Photovoltaic Systems
I guess then the question becomes whether the AHJ will allow him to do that if they haven't yet adopted the 2014 NEC.
Sierrasparky
Senior Member
- Location
- USA
- Occupation
- Electrician ,contractor
I'd put in the 40 amp and call it a day.
Is there worry that the demand will exceed the 40 and cause tripping.
will the array even produce the 40 amps.
Is there worry that the demand will exceed the 40 and cause tripping.
will the array even produce the 40 amps.
- Location
- Placerville, CA, USA
- Occupation
- Retired PV System Designer
I am a little more concerned about how the OP intends to connect the three micros to the 40A breaker. Via three harnesses and three 20 or 15A breakers in an AC combiner panel?
Prior to [2014] the calculation would have to be based on the sum of the first breakers in line rather than the 40A breaker in the main panel.
And on a cold clear day with just the right addition of clouds outside the direct line you could get 40A. But that would not trip the breaker anyway.
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Correct. Three trunk cable runs into three breakers in an AC combiner box. Total of 33A based on rated output of 1.0A for Enphase M250's. Working off the 2014 code here in Mass.I am a little more concerned about how the OP intends to connect the three micros to the 40A breaker. Via three harnesses and three 20 or 15A breakers in an AC combiner panel?
Prior to [2014] the calculation would have to be based on the sum of the first breakers in line rather than the 40A breaker in the main panel.
And on a cold clear day with just the right addition of clouds outside the direct line you could get 40A. But that would not trip the breaker anyway.
So are you suggesting I could use a 35A breaker here? Doesn't continuous load need to be factored in?
shortcircuit2
Senior Member
- Location
- South of Bawstin
No on the 35 amp breaker. You have to size your OC protection at 125%...in your case...34.375x125%= 42.96amps...so 45amp BKR
We are telling you that you can load side connect your system to the 200 amp panel because under the 2014 code your allowed to use the actual system output for the 120% load side calculation...not the size of the circuit breaker as required in previous code cycles.
shortcircuit2
Senior Member
- Location
- South of Bawstin
Are you sure about that???
I always based a 120% load side connection on the 40amp main panel interconnection...Not on the sum of the combiner panel breakers.
ggunn
PE (Electrical), NABCEP certified
- Location
- Austin, TX, USA
- Occupation
- Consulting Electrical Engineer - Photovoltaic Systems
Golddigger is correct. When the output of inverters is combined in a subpanel and fed into the main panel through another breaker, the 120% rule calculation in the main panel is based on the sum of the ratings of the first breakers encountered by the current from the inverters, and the rating of the backfed breaker in the main panel is irrelevant.
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SolarPro
Senior Member
- Location
- Austin, TX
Shortcircuit is doing the math properly. In the 2014 Code, calculations for load-side interconnects are no longer based on inverter output circuit breaker rating, but rather on 125% of the inverter output circuit current.
shortcircuit2
Senior Member
- Location
- South of Bawstin
How do you guys get that interpretation from 705.12(D)(2) in the 2011 code?
- Location
- Placerville, CA, USA
- Occupation
- Retired PV System Designer
I get it from looking further down at 705.12(D)(7):
Bold emphasis mine.
A combiner panel and then a main panel would constitute panelboards connected in series.
I read that as saying that I carry through the sum of the first OCPD ratings all the way up to the main panel when applying the 120% rule.
A royal pain, IMHO, and fortunately changed in [2014]
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ggunn
PE (Electrical), NABCEP certified
- Location
- Austin, TX, USA
- Occupation
- Consulting Electrical Engineer - Photovoltaic Systems
I believe the issue was what the calculations were before the 2014 Code. In the 2014 Code, as you say, the breaker ratings have been completely removed from the calculation. And there was great rejoicing...
shortcircuit2
Senior Member
- Location
- South of Bawstin
Ok...I understand. Thanks!
jaggedben
Senior Member
- Location
- Northern California
- Occupation
- Solar and Energy Storage Installer
No, he's not doing the math properly, because he's forgetting the 125%. The rated output of an M250 is 1.0A, thus 33AX1.25 = 41.25A which the OP had correct to begin with. We really got sidetracked in this thread with the code cycle discussion because it makes no difference to the OP's situation.
I gather the OP has tried suggesting just dropping the 33rd panel, which would make everything simple.
I also wonder if a supply side connection is an option.
Otherwise we should re-focus on the original question of whether it's allowable to downsize a breaker.
Yes.
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