Split bus on a MLO residential main service panel

Joelengrahm

Building Inspector ll
Location
Dixon, Ca
Occupation
City Building Inspector ll
Hello,

I am looking at a split bus 200 amp mlo panel that has existing 50 amp pv backfeed breaker. The PV company is attempting to add an additional 20amp.I believe the 70amps back feed overloads the bus or service wires which company state is 2/0 AL.
any help would be appreciated.
 

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electro7

Senior Member
Location
Northern CA, US
Occupation
Electrician, Solar and Electrical Contractor
Id say it depends on how that busbar is configured. Right now if all of those breakers are on the same bussing then it looks like all the load breakers add up to 220A. Id say in this case that would not be code compliant. Id say the bussing would have the potential to overheat unless there is a load calculation done on the house??

If there is a 200A buss feeding the top where the solar is landed and a separate buss feeding the lower then I would say it is code compliant. There is a 50A load breaker there and 70A solar. Id say all that buss would see is 70A max.

Id be concerned though if the 2/0 AL are service conductors. Thats how I am interpreting your post. They would only be rated at 150A, thats aside from the solar.






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PWDickerson

Senior Member
Location
Clinton, WA
Occupation
Solar Contractor
If the solar backfeed breakers do not have any OCPD's ahead of them, they would be considered line-side connected and not subject to the 705.12 limitations on busbars. This is logical since busbars that aren't protected by a main breaker are already exposed to several thousand amps of short circuit current from the utility. 705.11 (2020 NEC, can't remember the section in earlier code cycles) limits the backfeed of the PV system(s) to the service capacity. Based on the information provided, I would guess the service capacity is 150A. This looks OK to me. I would need to see the panel label to be sure though. Hard to know how that panel is set up with the information provided.
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
If the solar backfeed breakers do not have any OCPD's ahead of them, they would be considered line-side connected and not subject to the 705.12 limitations on busbars. This is logical since busbars that aren't protected by a main breaker are already exposed to several thousand amps of short circuit current from the utility. 705.11 (2020 NEC, can't remember the section in earlier code cycles) limits the backfeed of the PV system(s) to the service capacity. Based on the information provided, I would guess the service capacity is 150A. This looks OK to me. I would need to see the panel label to be sure though. Hard to know how that panel is set up with the information provided.
Earlier versions of the NEC just said that the size of a line side connected PV system could not exceed the size of the service, which was ambiguous. In the 2020 NEC it was changed to say that 125% of the rated inverter current cannot exceed the ampacity of the service conductors.
 

jaggedben

Senior Member
Location
Northern California
Occupation
Solar and Energy Storage Installer
Hello,

I am looking at a split bus 200 amp mlo panel that has existing 50 amp pv backfeed breaker. The PV company is attempting to add an additional 20amp.I believe the 70amps back feed overloads the bus or service wires which company state is 2/0 AL.
any help would be appreciated.
70A does not overload 2/0 AL, so not sure what you're missing. Solar backfeed is not a load.
 

electro7

Senior Member
Location
Northern CA, US
Occupation
Electrician, Solar and Electrical Contractor
Yeah I see that now being a 705.12(A) connection and not 705.12(B). My bad.


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PWDickerson

Senior Member
Location
Clinton, WA
Occupation
Solar Contractor
JBen's code reference is spot on. The load calc is what limits the current in an MLO service panel, not the sum of the load breakers. The sum of the load breakers routinely exceeds the panel/conductor rating. And this is still a line-side connected system since there are no OCPD's upstream of the PV breakers. This is a code compliant installation as far as I can see.
 

Joelengrahm

Building Inspector ll
Location
Dixon, Ca
Occupation
City Building Inspector ll
Thank you all, my original call was for the PV company to provide load calcs, as I require with all supply side connections. In the 2020 nec. This particular panel would be illegal. I informed company that our jurisdiction would from now on require MPU's if they encounter this panel. I have an entire subdivision with them.
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
Thank you all, my original call was for the PV company to provide load calcs, as I require with all supply side connections. In the 2020 nec. This particular panel would be illegal. I informed company that our jurisdiction would from now on require MPU's if they encounter this panel. I have an entire subdivision with them.
Why would the installation of PV require a load calculation? I have been designing PV for more than 12 years - residential and commercial, load side and supply side interconnected - and not once has an inspector called for a load calculation. I don't see why a load calculation would ever be necessary, especially on a supply side interconnection.
 

Joelengrahm

Building Inspector ll
Location
Dixon, Ca
Occupation
City Building Inspector ll
Why would the installation of PV require a load calculation? I have been designing PV for more than 12 years - residential and commercial, load side and supply side interconnected - and not once has an inspector called for a load calculation. I don't see why a load calculation would ever be necessary, especially on a supply side interconnection.
Typically I only ask if they do a derate. However In This instance and many other times. They are trying to install on a subpar or outdated service panel. (Ex. Zinsco with more than 6 handles) So if I can find away to get them to do an upgrade by using the code I will. In the electrical industry, if you touch it, you own it. So if you add a new circuit breaker the rest of the panel should be up to code.
 

jaggedben

Senior Member
Location
Northern California
Occupation
Solar and Energy Storage Installer
That is not ex Zinsco. It's CH which is still being made and sold by Eaton. And I see exactly six handles. If you are still on the 2017 it is totally up to code.
 

Joelengrahm

Building Inspector ll
Location
Dixon, Ca
Occupation
City Building Inspector ll
That is not ex Zinsco. It's CH which is still being made and sold by Eaton. And I see exactly six handles. If you are still on the 2017 it is totally up to code.
Unless it has a single main breaker it " is" outdated, yet still legal.( having one main is safer all around.) In a neighboring jurisdiction, they make you replace all zinsco panels if you have to alter or add a new breaker. I did not agree with them, but it's in the Coty ordinance, which is allowed to be more restrictive. In my last AHJ they disallowed supply side connections. Perfectly within. The AHJ's rights, to do so. Especially since they have proven not to be as safe. Legal but not the safest or the best for the utility.
 

jaggedben

Senior Member
Location
Northern California
Occupation
Solar and Energy Storage Installer
Unless it has a single main breaker it " is" outdated, yet still legal.( having one main is safer all around.) In a neighboring jurisdiction, they make you replace all zinsco panels if you have to alter or add a new breaker. I did not agree with them, but it's in the Coty ordinance, which is allowed to be more restrictive. In my last AHJ they disallowed supply side connections. Perfectly within. The AHJ's rights, to do so. Especially since they have proven not to be as safe. Legal but not the safest or the best for the utility.

A code change that hasn't even taken effect in most of the country makes something outdated? That panel is only about 10 years old (has both the Cutler Hammer and Eaton names on it). That's outdated? Not a widely shared opinion I don't think.

The Zinsco stuff is irrelevant to that panel.
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
Typically I only ask if they do a derate. However In This instance and many other times. They are trying to install on a subpar or outdated service panel. (Ex. Zinsco with more than 6 handles) So if I can find away to get them to do an upgrade by using the code I will. In the electrical industry, if you touch it, you own it. So if you add a new circuit breaker the rest of the panel should be up to code.
Why would a PV installer ever derate the main breaker for a supply side PV interconnection? 705.12(B)(3)(2), aka the 120% rule, doesn't apply to supply side connections.
 
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