PV Subpanel Breakers

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mrizon

Member
Location
Manteca, CA
Hello This is my first post on this forum. I am a Commercial PV Designer working toward my EIT and have been tasked with my first project. I have limited electrician experience (terminate cables and crimping, QC'd wire runs, combiner box wiring contest, etc..) through my experience as a Project Engineer at my previous position. Im not really working toward my Journyman's but I am always interested in improving conditions for my employees so that they may do their work more efficiently and easier (using Lean Six Sigma techniques) so I try to get in the field as much as possible and see what the ground troops do.

I am designing a 171kWDC PV system (located in Los Angeles) and have nailed down the equations and design all the way up to the 400A PV subpanel. My question is regarding backfeeding this subpanel. I have 672 modules (12 modules/string with 56 strings) feeding 2 inverters through Solectria 50kW and 100kW central inverters. (I did calculate excess voltage drop on the farthest string so I added another string of modules to help compensate the power loss due to this). Module specs are Voc:37.7
Isc: 8.9
Vmp: 31.1
Imp:8.4

My challenge is at the inverter output circuit. I output 240.57A and 144.34A (per datasheet) and if I connect these to the subpanel I would need 250A and 150A breaker in this panel. This does not seem right to me because I have only seen up to 80A breakers in these panels. Any help is appreciated, I feel like this is a simple answer but brainfarted.
 

jaggedben

Senior Member
Location
Northern California
Occupation
Solar and Energy Storage Installer
What voltage do the inverters output?
Three phase output (I assume)?

Module specs are irrelevant. You need to look at the inverter output specification on the inverter data sheet. I suspect you're not doing this right. With the exception of microinverters, I've never seen inverter output specified past the decimal point.

Then, you need to multiply the inverter output spec by 125%.

Once you have the correct output numbers you need to do some research with your potential suppliers and look at catalogs to determine what will work, and for what cost.
Equipment and breakers exist for this sort of thing but you're not going to find it at Home Depot.
 
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Perhaps you are not real familiar with panel boards that are more on the commercial side of things? These would have larger frame breakers than you may be used to. The gear guy at a supply house should be able to get you what you need no problem. Hope that helps.
 

mrizon

Member
Location
Manteca, CA
Thank you for the reply. The 100kW inverter outputs 240.57A(max) @240V and the 50kW outputs 144.34A @240V. The tie-in is a 240D/120V 3ph 4Wire Panel rated for 400A and this goes into a switchgear rated for 600A Mains and neutral. I did the calcs and I got a 300A braker for the 100kW and 200A Breakers for the 50kW. Ive never seen these large of a size breaker in subpanels ive worked on so I am a little confused if this is right. Im guessing at this size, these would be the double breakers that use both bus bars




What voltage do the inverters output?
Three phase output (I assume)?

Module specs are irrelevant. You need to look at the inverter output specification on the inverter data sheet. I suspect you're not doing this right. With the exception of microinverters, I've never seen inverter output specified past the decimal point.

Then, you need to multiply the inverter output spec by 125%.

Once you have the correct output numbers you need to do some research with your potential suppliers and look at catalogs to determine what will work, and for what cost.
Equipment and breakers exist for this sort of thing but you're not going to find it at Home Depot.
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
Thank you for the reply. The 100kW inverter outputs 240.57A(max) @240V and the 50kW outputs 144.34A @240V. The tie-in is a 240D/120V 3ph 4Wire Panel rated for 400A and this goes into a switchgear rated for 600A Mains and neutral. I did the calcs and I got a 300A braker for the 100kW and 200A Breakers for the 50kW. Ive never seen these large of a size breaker in subpanels ive worked on so I am a little confused if this is right. Im guessing at this size, these would be the double breakers that use both bus bars
In the first place, you are to use the published maximum current values for these inverters (120A and 240A @ 240V) in sizing conductors and OCPD rather than calculating them. That means a 150A breaker and a 300A breaker. You cannot use a 400A subpanel to combine them because the panel busbar rating must be equal to or greater than the sum of all the breakers per 705.12(D)(2)(3)(c). Unfortunately, that means you need a 600A panel.

There are three (not two) busbars in a three phase panel and you need 3 pole breakers, i.e., breakers that use them all.
 

mrizon

Member
Location
Manteca, CA
In the first place, you are to use the published maximum current values for these inverters (120A and 240A @ 240V) in sizing conductors and OCPD rather than calculating them. That means a 150A breaker and a 300A breaker. You cannot use a 400A subpanel to combine them because the panel busbar rating must be equal to or greater than the sum of all the breakers per 705.12(D)(2)(3)(c). Unfortunately, that means you need a 600A panel.

There are three (not two) busbars in a three phase panel and you need 3 pole breakers, i.e., breakers that use them all.

Thank you ggunn. I should have noted that I am doing redlines for a project completed before I got to the company and the previous designer is unable to be in contact. In any case Thank you for taking the time to respond and it seems to have confirmed my suspicions and has increased my understanding on this subject.
 
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