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Backfeeding thru 50 amp breaker

rockwind1

Natural selection intervention specialist
Joined
Nov 19, 2020
Messages
85
Location
Az
I have a large south facing barn Typical metal roof, it’s 25 feet wide and 50 feet long it’s about a four or five pitch roof, I haven’t measured it. The barn has got number six wire running to it from the house where the main panel is. It’s on a 50 amp breaker. it is a very long run of number six copper wire from my barn up to the main panel on the house probably around 400 to 500 feet but to be clear it’s already in place. I can fit about 15k-16k depending on how I arrange the panels but if I put my inverter in the barn, is there a problem with backfeeding my panel through that number six wire and 50 amp breaker? It’s just the easiest way to mount a bunch of panels 
My house usually uses about 75 kW per day
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You need an electrician.

if you mean 16kw of panels then 16,000 / 240 == 66.6 amps. So you would need an 80amp circuit min to be able to handle 16kw. Even with less than 100% of panel capacity you will be over the limit for a 50amp breaker (40amp sustained) for several hours on sunny days.

The cheapest way would be to setup multiple strings and run say 3 sets of 8ga or 10ga and say put 13 panels on a string (39 total panels around 400w each assuming 37voc + 600v max mppt voltage on the inverter) and run 3 strings and that would put the PV voltage with the MPPT running at about 425V reducing the current by almost a factor of 2 over the 240v circuit amps. With 10ga/12amp per string that would be voltage drop of about 12v(3% of total pv voltage) for 1000ft of 10ga wire. No idea if 3 runs of 10ga + a ground in conduit is less than what would need to be done to your first current 50a circuit to upgrade it to handle the required current.
 
I'm assuming you would have the inverter in the barn back feeding the sub-panel in there with 50amps. They go by the max current the inverter can output. The 120% rule would apply to the sub-panel, and the main-panel can't exceed either when combining utility power and inverter power. The panels would be limited by the inverter limits. If the main panel has a 200 amp busbar limit can't add 50 amps . You have to reduce the main so it comes out to 120% limit of the busbar. 200 x 1.2 - 200amp = 40 amps have to reduce the main breaker to 190 hard to find those. You get the idea same would be applied to the sub-panel since it has a 50 amp breaker would be 125 amps X 1.2 - 50 have 100 limit on the sub-panel.

That is my first thought probably lot of other NEC rules others who know those rules better will come along this is something to go with. If you don't plan on battery storage could go with micro-inverters that solves the rapid shut down requirement and optimizing.
 
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I'm assuming you would have the inverter in the barn back feeding the sub-panel in there with 50amps. They go by the max current the inverter can output. The 120% rule would apply to the sub-panel, and the main-panel can't exceed either when combining utility power and inverter power. The panels would be limited to by the inverter limits.
yes, I should’ve been more specific. I figured I would run the inverter wires over to the sub panel obviously at night I would be counting on my typical grid system. i do have a sub panel in the barn, it’s got some Lightcircuits and some plug-in circuits on it I’ve also got a 50 amp breaker In the sub panel feeding a RV plug. I don’t ever use it unless I plug in my welder. My 400 amp panel up at my house, which is where the 50 amp breaker is that send power down to the barn, is a “solar ready “400 panel My plan was to put some more solar up by the house to feed the main panel obviously with another inverter and I was planning on getting a number of batteries for up there.
 
You need an electrician.

if you mean 16kw of panels then 16,000 / 240 == 66.6 amps. So you would need an 80amp circuit min to be able to handle 16kw. Even with less than 100% of panel capacity you will be over the limit for a 50amp breaker (40amp sustained) for several hours on sunny days.

The cheapest way would be to setup multiple strings and run say 3 sets of 8ga or 10ga and say put 13 panels on a string (39 total panels around 400w each assuming 37voc + 600v max mppt voltage on the inverter) and run 3 strings and that would put the PV voltage with the MPPT running at about 425V reducing the current by almost a factor of 2 over the 240v circuit amps. With 10ga/12amp per string that would be voltage drop of about 12v(3% of total pv voltage) for 1000ft of 10ga wire. No idea if 3 runs of 10ga + a ground in conduit is less than what would need to be done to your first current 50a circuit to upgrade it to handle the required current.
that actually was my plan to have three rows of 13 panels on the roof, I think I could squeeze another three or four panels on there if I configured them differently, but I’m not sure if it’s worth the hassle
 
yes, I should’ve been more specific. I figured I would run the inverter wires over to the sub panel obviously at night I would be counting on my typical grid system. i do have a sub panel in the barn, it’s got some Lightcircuits and some plug-in circuits on it I’ve also got a 50 amp breaker In the sub panel feeding a RV plug. I don’t ever use it unless I plug in my welder. My 400 amp panel up at my house, which is where the 50 amp breaker is that send power down to the barn, is a “solar ready “400 panel My plan was to put some more solar up by the house to feed the main panel obviously with another inverter and I was planning on getting a number of batteries for up there.
You would want to look at the bus bar limit of that 400 amp panel find out how much to reduce the main by if you plan on back feeding both the house solar system and the barn solar. If you want to use a hybrid inverter with battery storage that will limit you to certain models that have the UL 9450A ESS requirement being in Arizona if your with APS, or SRP for sure both require California approved inverters. The EG4 18KPV and EG4 PowerPro are approved in California and Arizona. The 18KPV has a 50 amp inverter output that would work good for both the house and barn. You could get plans created by Greenlancer as well want to get it all approved by the AHJ and the utility first before you order anything. If you never have done any electrical before might want to hire a licensed electrician if you have plans approved with a permit they probably work with you.

I would get it going soon next year no matter what tariffs going to hit Chinese imports best to get it ordered before EOY.
 
If you put an inverter with a battery at the barn, you can export 50a, and store the rest in a battery. You can then discharge 50a using the battery when pv production is low.
 
If you put an inverter with a battery at the barn, you can export 50a, and store the rest in a battery. You can then discharge 50a using the battery when pv production is low.
80% is the NEC allowed max sustained power. So 40a is what can legally be exported.
 
You would want to look at the bus bar limit of that 400 amp panel find out how much to reduce the main by if you plan on back feeding both the house solar system and the barn solar. If you want to use a hybrid inverter with battery storage that will limit you to certain models that have the UL 9450A ESS requirement being in Arizona if your with APS, or SRP for sure both require California approved inverters. The EG4 18KPV and EG4 PowerPro are approved in California and Arizona. The 18KPV has a 50 amp inverter output that would work good for both the house and barn. You could get plans created by Greenlancer as well want to get it all approved by the AHJ and the utility first before you order anything. If you never have done any electrical before might want to hire a licensed electrician if you have plans approved with a permit they probably work with you.

I would get it going soon next year no matter what tariffs going to hit Chinese imports best to get it ordered before EOY.
Im with APS
 
Is the "Barn" breaker in the Main panel interlocked with the Main/Grid breaker such that only one of the breakers can be closed.
In you scenario the "Barn" solar system would be like a generator hookup, the solar system would act as a generator to feed the house main panel which cannot/should not be done with the Grid breaker closed.
 
Split phase or 3-phase?

6 awg Cu and 75 degree C terminals is allowed 65A


A 60A breaker, 80% continuous would be 48A

A CBI magnetic-hydraulic breaker is guaranteed not to trip under 105%, allowing 60A continuous:


OK to have more PV panels, "overpaneled", just select inverter (or configure limit) of 14,400W for 240V split-phase.
With 16kW (STC) of panels it could rarely reach that anyway.
 
You cannot feed the barn panel with 100 amps unless it is 125+ amp rated. 50 amps coming in from the house and 50 amps coming in from the solar is 100 amps feed to the barn sub panel.
 
At 500' and 50 amp you have more than a 9% voltage drop. You may not even be able to keep the inverter running. I would take some measurements on the actual voltage in the barn when you have a large load running.

If you have to run new cable, run the DC up to the house and put the inverter at the house instead of the barn. You can use the existing 6awg for part of the DC if you choose string size correctly.
 

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