Maximum breaker allowable in main circuit breaker panel

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dala

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I'm planning on installing an 8 to 10kw pv system. What is the maximum allowable circuit breaker i can install in my circuit breaker panel to tie it into? I have a 200amp main breaker panel and live in Suffolk County, NY.
 
Assuming that the panel bus is rated at 200 amp, the maximum breaker for the solar system would be 40. 690.64, 705.12(D)(2)
 
40 amps only in the whole solar system... wow thats pretty small for such a big place. :roll:
 
Thanks Don for the NEC reference. Suppose i needed a larger breaker because the output from the inverter is greater than 40amps. What's the best/allowable method of tying it into the
200amp main breaker panel?
 
dala said:
Thanks Don for the NEC reference. Suppose i needed a larger breaker because the output from the inverter is greater than 40amps. What's the best/allowable method of tying it into the
200amp main breaker panel?
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A 40A breaker is the biggest.

If it is 10kW @ 240V STC, then 40A is probably the size you need considering the efficiency of a 10kW group of panels converting to AC.
 
dala said:
Thanks Don for the NEC reference. Suppose i needed a larger breaker because the output from the inverter is greater than 40amps. What's the best/allowable method of tying it into the
200amp main breaker panel?
Click to expand...
What is your inverter? The breaker size is the maximum continuous rated output current of the inverter X 1.25, and then up to the next standard breaker size. If that comes out to be more than 40A, then you'll have to come up with a Plan B.

If it's 40A or less, you will want to install a backfeedable breaker (one that does not have "line" or "load" labeling at the terminals) into the service panel at the opposite end of the busbars from the main breaker. Depending on the rules of your AHJ (authority having jurisdiction),you may need to hire a licensed electrician to install it.

If it's more than 40A, you'll have to do some finagling. One thing you can do is use a smaller backfed breaker, which might result in nuisance trips of the breaker when PV output is high. You can upgrade your service (you'll definitely need an electrician for that) to one which gives you enough headroom in the 120% rule to do the install. That will be expensive. You can install a smaller main breaker in your service panel to generate more headroom (ditto on the electrician), but that will risk starving your loads and possible nuisance trips of the main when you have a lot of appliances running. However you do it, you must always be certain the the rating on the breaker is lower than the derated ampacity of the conductors which feed it.

Finally, you can install the PV with a line side tap between your service and the meter, but that is also expensive and will definitely require the services of a licensed electrician.
 
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ggunn said:
One thing you can do is use a smaller backfed breaker,
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That would be a code violation.



You can install a smaller main breaker in your service panel to generate more headroom (ditto on the electrician),
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I have done that in a PV install but it is only an option if you do an Article 220 load calculation to prove that the reduced size is still adequate.

However you do it, you must always be certain the the rating on the breaker is lower than the derated ampacity of the conductors which feed it.
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If you are talking service conductors or main feeders I would point out Table 310.15(B)(6) which allows conductors to be used above their ratings.


Also the poster is an electrical contractor, not a DIY.
 
You can also downsize the main breaker to a 175amp if you do a load check for the residence and do not surpass the 80% rule and if your AHJ is ok with it. If you did that you could use a 65amp breaker.
 
ggunn said:
I thought you could go smaller but not larger, but I'll stand corrected in lieu of running for the code book. ;^)
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I thought I'd gotten into an argument with someone else on this very subject and the conclusion was you could ALWAYS go smaller unless the size was =specifically= required by some section of the code. For example, the 20A SABCs in a kitchen.

On the basis of that, I call "CODE SECTION!"
 
iwire said:
Also the poster is an electrical contractor, not a DIY.
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Yes, but an electrical contractor isn't a licensed electrician :p

Not that it likely matters -- if it's his place, he can likely do it himself and tell his master electrician to explain it all.
 
tallgirl said:
I thought I'd gotten into an argument with someone else on this very subject and the conclusion was you could ALWAYS go smaller unless the size was =specifically= required by some section of the code. For example, the 20A SABCs in a kitchen.

On the basis of that, I call "CODE SECTION!"
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2011 NEC, 690.8 (B)

Ampacity and Overcurrent Device Ratings. Photovoltaic
system currents shall be considered to be continuous.
(1) Overcurrent Devices. Overcurrent devices, where required,
shall be rated as required by 690.8(B)(1)(a) through
(1)(d).
(a) To carry not less than 125 percent of the maximum
currents calculated in 690.8(A).
...
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690.8 (A)(3)

Inverter Output Circuit Current. The maximum current
shall be the inverter continuous output current rating.
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Emphasis mine. On that basis I would say you can't go smaller on the OCPD. However, all of section B quoted is marked as revised in 2011, so if you are on an earlier code cycle the same logic may not apply. I don't have earlier codes here at home.
 
tallgirl said:
I thought I'd gotten into an argument with someone else on this very subject and the conclusion was you could ALWAYS go smaller unless the size was =specifically= required by some section of the code. For example, the 20A SABCs in a kitchen.

On the basis of that, I call "CODE SECTION!"
Click to expand...

On the basis of common sense, I'd ask what the hazard would be, especially if the inverter were underloaded and would likely seldom if ever reach its rated maximum output and the AC conductors from the inverter to the breaker were sized according to code. It seems to me that the worst thing that could happen would be an occasional (if ever) nuisance trip of the backfed breaker, and if the customer were to be OK with that, where's the beef?
 
It was explained to me that the limitation was due to the amount the buss could handle, worst case scenario being full current through both the main and the PV breakers. If the combination of the two sources exceed the rating of the buss they are connected to, there may be problems.

The required positioning of the PV breaker also has to do with the amount of current that could possibly be put on the buss.
 
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