OK to downsize OCPD

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jaggedben

Senior Member
Location
Northern California
Occupation
Solar and Energy Storage Installer
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.

The OP had the math correct.

And it's 125% of the rated (not actual) system output that goes into the 120% rule, so he still cannot connect load side.
 

SolarPro

Senior Member
Location
Austin, TX
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.

Thanks for getting us back on track. (I saw the 125% come out, but of course the result after that is applied still needs to be 40A or less.)

Two options then: Either lose a module or make a supply-side interconnection.
 

shortcircuit2

Senior Member
Location
South of Bawstin
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?

What panels are being used in your system?
 

jaggedben

Senior Member
Location
Northern California
Occupation
Solar and Energy Storage Installer
Right. And as the 1A output circuit current rating suggests, the M250 is really a 240W inverter.

Not really, since it can output 250W if the voltage is 250 or higher, which is possible and even likely at the end of a string.

(Funny marketing sidenote: Previous Enphase inverters could actually output 10W or so more than their nameplate power, but the M250 actually maxes out at 250W.)

It's always an option. Usually just a matter of convenience and/or cost.

Yes, well, so is a service upgrade, which is presumably the level of cost and inconvenience we're trying to avoid.
 

Smart $

Esteemed Member
Location
Ohio
...

Yes, well, so is a service upgrade, which is presumably the level of cost and inconvenience we're trying to avoid.
There appears no need to 'upgrade' the service for this installation, to make a line-side PV System connection. A minor revision will suffice: a few more parts, a little more labor, done.

Of course, it'd still cost more than eliminating one PV panel... :happyyes:
 

jaggedben

Senior Member
Location
Northern California
Occupation
Solar and Energy Storage Installer
There appears no need to 'upgrade' the service for this installation, to make a line-side PV System connection. A minor revision will suffice: a few more parts, a little more labor, done.

With many meter/main combo service panels there is no code compliant way to do a supply side connection that doesn't cost just as much as upgrading the panel. Also some AHJs don't allow supply side connections.

Of course, it'd still cost more than eliminating one PV panel... :happyyes:

Yup. And yet, I've never been able to convince any customer or salesman to downsize a system once a contract has been signed. :roll: I've headed off a few mistakes of this nature before the sale was made, but never after.

Perhaps the OP can find panels that are 10W higher power and offer the same size (DC) system with one less inverter. That could be a winner all around.
 
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SolarPro

Senior Member
Location
Austin, TX
Not really, since it can output 250W if the voltage is 250 or higher, which is possible and even likely at the end of a string.

True. That does provide some semblance of engineering logic behind their rating convention.

But isn't an inverter's nameplate power rating always the rated power at the nominal grid voltage?

That being the case, I suspect this product is call the M250 simply because the marketing department decided that M250 sounded better than M240. While it's certainly not the first time an inverter vendor rounded its model numbers up, there's a lot less of that going on today as compared to 10 years ago.
 

GoldDigger

Moderator
Staff member
Location
Placerville, CA, USA
Occupation
Retired PV System Designer
But isn't an inverter's nameplate power rating always the rated power at the nominal grid voltage?
Now that is a very good question, which only the manufacturer can answer for sure! But I will try anyway. :)
There are at least three bottlenecks in the power conversion process of a microinverter:

1. The MPPT input circuitry which is driving the DC to DC boost convertor.
2. The DC to DC convertor itself.
3. The DC to AC output stage. This one is likely to be primarily current limited, leading to the output power being grid voltage dependent.

1 and 2 would not necessarily depend on the grid voltage.
Now the spec sheet for the M250 suggests that the same model can be used for either 208V (208Y/120) or 240V (120/240 single phase 3-wire) operation with the same output power limits, which argues that the effective power limit (240W continuous, 250W peak (for how long??)) is really because of 1 or 2.
 

Barbqranch

Senior Member
Location
Arcata, CA
Occupation
Plant maintenance electrician Semi-retired
It may not be possible to go down to 32 panels, because in many solar installations the "look" of the finished product is an important consideration to the homeowner. I had 5 columns of 4 rows installed on my barn. There is no way I would have tolerated the gap tooth look of 19 panels. If I had to reduce the number of panes, I would have gone down to 16.
 

GoldDigger

Moderator
Staff member
Location
Placerville, CA, USA
Occupation
Retired PV System Designer
It may not be possible to go down to 32 panels, because in many solar installations the "look" of the finished product is an important consideration to the homeowner. I had 5 columns of 4 rows installed on my barn. There is no way I would have tolerated the gap tooth look of 19 panels. If I had to reduce the number of panes, I would have gone down to 16.
If the appearance is that important, mounting a 33rd panel but not connecting it electrically could be less expensive than service panel changes or use of a line side tap.
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
If the appearance is that important, mounting a 33rd panel but not connecting it electrically could be less expensive than service panel changes or use of a line side tap.
Even better, some PV sales houses sell modules that have failed final inspection for one reason or other, and they are very cheap. They are electrically dead, of course (they probably don't have junction boxes or leads), but they can be used for aesthetics.
 

SolarPro

Senior Member
Location
Austin, TX
Store an extra working one for later. With module-level monitoring, you'll know when a module fails. But will there be an exact replacement module available when it does?
 

Zee

Senior Member
Location
CA
What panels are being used in your system?

Hold on.
Panel Wattage may in fact be relevant to inverter choice:
if panels are in the 250 W range---> use Enphase M215's---->max allowable is 17 per string.
Now you can do 33 panels. Even 34. Voila.

If panels are above 260 W.....you may be losing some power with M215's only at peak sun moments ...rarely?
However, it is allowable,
safe
and code compliant with M215's.

Even with 270 W panels, I am not even sure 33 @ M215's would make less juice than 32 @ M250's.
 

Zee

Senior Member
Location
CA
OP:
Why the absolute, unbending need for that magical, 33rd panel (an additional 3% power)?

If power is reason why not get a higher W panel? Even a 5 W increment is 33 @ 5 = an extra 165 W, kinda like a 33rd panel.;)
 
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