pv system connected to sub-panel

[Jest Waitn]

i am installing a pv system that connects to a sub-panel about 120' from the primary load center. the pv system micro-inverters initially connect to a cutoff/junction box at the array and then go to the sub-panel. 12-3 wire is used, which is 4 wires. the panel frames will be connected to an 8' ground rod.

the sub-panel wiring from the primary load center only has 3 wires; neutral is bonded to ground at the primary load center.

what do i do with the ground from the pv system? this is the 50

[SunEagle]

i am installing a pv system that connects to a sub-panel about 120' from the primary load center. the pv system micro-inverters initially connect to a cutoff/junction box at the array and then go to the sub-panel. 12-3 wire is used, which is 4 wires. the panel frames will be connected to an 8' ground rod.

the sub-panel wiring from the primary load center only has 3 wires; neutral is bonded to ground at the primary load center.

what do i do with the ground from the pv system? this is the 50

There still should be an equipment ground wire at the sub-panel which may just be a bare copper wire connected to the back of the panel.

[solarix]

Your situation is not uncommon to have a subpanel erroneously wired with just one conductor for the neutral and ground. It seems that once upon a time, grounding wasn't the issue that it is now. By rights you need to add a ground from your main to your subpanel and separate the neutral and ground there in order to pass inspection - which you need to since you are doing solar. If you don't, you are risking a subtle but potentially dangerous fault condition where a short could occur and not be detected because you have multiple paths to ground.

[Jest Waitn]

There still should be an equipment ground wire at the sub-panel which may just be a bare copper wire connected to the back of the panel.

there is nothing of the sort. this panel was installed circa 1990.

[tyab]

I'm assuming from the 120' distance that this is a detached structure like a garage or barn but it is possible in a large home to have this condition. Prior to 2008 It matters if detached or attached in relationship to the grounded conductor (neutral) and EGC. It also matters if you need a GEC at that subpanel. Lets say we are dealing with a detached setup.

Current code covers this in NEC 250.142(B). That applies for any new construction or modifications to existing setups. Prior to 2008 it was allowed to have detached structures to have a 3 wire setup. They key is what is considered a modification and the absolute authority on this will be your AHJ. This is a common situation that your AHJ deals with and it would be best to have a chat with them about how they want this to be dealt with. Some will allow the existing setup, some may require you to bring that subpanel up to current code which would mean a 4 wire setup.

If there is not one already, you may be required to run a GEC at that detached subpanel that follows 252.52 (and naturally 252.53). Ask your AHJ if they want that (some do, some don't).

Regardless of what you have to do at the sub panel, you need to have a EGC from that sub panel to your array bonded to the other EGC's in the panel. That includes bonding to the EGC's from the micros. And the grounded conductors (neutrals) from the micros need to be run to that subpanel and bonded with the other neutrals.

Your panel frames/rails need to have a GEC per 2014 690.47(D). This code section is dangerous on a structure and many AHJ's are not requiring it - and instead and just asking that be part of the EGC. Ask your AHJ if they want a code compliant 690.47(D) or if they want a safe setup. Lots of links on this issue with the best coming from Mike Holt.

[Jest Waitn]

I'm assuming from the 120' distance that this is a detached structure like a garage or barn but it is possible in a large home to have this condition. Prior to 2008 It matters if detached or attached in relationship to the grounded conductor (neutral) and EGC. It also matters if you need a GEC at that subpanel. Lets say we are dealing with a detached setup.

Current code covers this in NEC 250.142(B). That applies for any new construction or modifications to existing setups. Prior to 2008 it was allowed to have detached structures to have a 3 wire setup. They key is what is considered a modification and the absolute authority on this will be your AHJ. This is a common situation that your AHJ deals with and it would be best to have a chat with them about how they want this to be dealt with. Some will allow the existing setup, some may require you to bring that subpanel up to current code which would mean a 4 wire setup.

If there is not one already, you may be required to run a GEC at that detached subpanel that follows 252.52 (and naturally 252.53). Ask your AHJ if they want that (some do, some don't).

Regardless of what you have to do at the sub panel, you need to have a EGC from that sub panel to your array bonded to the other EGC's in the panel. That includes bonding to the EGC's from the micros. And the grounded conductors (neutrals) from the micros need to be run to that subpanel and bonded with the other neutrals.

Your panel frames/rails need to have a GEC per 2014 690.47(D). This code section is dangerous on a structure and many AHJ's are not requiring it - and instead and just asking that be part of the EGC. Ask your AHJ if they want a code compliant 690.47(D) or if they want a safe setup. Lots of links on this issue with the best coming from Mike Holt.

firstly, thank everyone for the responses. it is good to be supported!

yes, this is a sub panel to a shed that was demolished last year. presently only the slab remains. the shed had an indoor sub panel; i replaced it with an outdoor one. the 3-conductor wire from the primary load center is rather thick, about 1/4" each strand, probably 2 gauge stranded wire. this new sub panel (cutler-hammer ch8L125rp) only has connections for supply and return. no ground connection is present.

the inspector (AHJ) said it is ok to wire the pv system to a sub panel. don't recall if the 3-wire condition was mentioned, but it is shown in the power diagram provided for the permit. this is what i plan to implement: powerdiagram.pdf

this pv system will be frame mounted, not on a roof. this is the first of 3 arrays, for an estimated total of 9kw ac, 3kw ac each array, 12 panels each array, 12 enphase m250 micro-inverters each array. so the maximum continuous output on this will be 36 amps, eventually.

[Mike90250]

Be aware of "Line Loss" with these long runs from the solar, to main panel to utility pole or transformer. Any line loss will reflect a an increase in grid voltage, and at some point, the inverters start to shut down from overvoltage in your long line runs. Solution, shorter runs, heaver cable (or aluminum feed conductors)

[Jest Waitn]

Be aware of "Line Loss" with these long runs from the solar, to main panel to utility pole or transformer. Any line loss will reflect a an increase in grid voltage, and at some point, the inverters start to shut down from overvoltage in your long line runs. Solution, shorter runs, heaver cable (or aluminum feed conductors)

thanks. present total run is < 300' and this is mostly over thick service wire. pv array to sub panel < 100' at the longest point; this is over 12/3. i haven't measured voltage at the sub panel. will do this as soon as the rain is over (don't work with electricity outside when ground is wet).

there is an option for an additional power drop onto the property for the pv system since the pv array is near the transformer. this would be < 75'. don't know how this would work by having pv on an account separate from the supply account. power company might consider this a mini power station.

[McMac]

thanks. present total run is < 300' and this is mostly over thick service wire. pv array to sub panel < 100' at the longest point; this is over 12/3. i haven't measured voltage at the sub panel. will do this as soon as the rain is over (don't work with electricity outside when ground is wet).

there is an option for an additional power drop onto the property for the pv system since the pv array is near the transformer. this would be < 75'. don't know how this would work by having pv on an account separate from the supply account. power company might consider this a mini power station.

Having a meter to send power through and one the use power is often refered to as virtual net metering. but I havn't found a utility or coop in our area that will allow it. Tried to do it to save trenching a 70kw over 600ft but they wouldn't have it.

[McMac]

firstly, thank everyone for the responses. it is good to be supported!

yes, this is a sub panel to a shed that was demolished last year. presently only the slab remains. the shed had an indoor sub panel; i replaced it with an outdoor one. the 3-conductor wire from the primary load center is rather thick, about 1/4" each strand, probably 2 gauge stranded wire. this new sub panel (cutler-hammer ch8L125rp) only has connections for supply and return. no ground connection is present.

the inspector (AHJ) said it is ok to wire the pv system to a sub panel. don't recall if the 3-wire condition was mentioned, but it is shown in the power diagram provided for the permit. this is what i plan to implement: powerdiagram.pdf

this pv system will be frame mounted, not on a roof. this is the first of 3 arrays, for an estimated total of 9kw ac, 3kw ac each array, 12 panels each array, 12 enphase m250 micro-inverters each array. so the maximum continuous output on this will be 36 amps, eventually.

I couldn't view your drawing so let me just make you aware of a couple things based on what I've gathered here.

off the cuff, 36A should be fine in terms of voltage drop. even if its aluminum you are probably around a 2Vac drop and just over 100ft. thats still under 1%. Industry standard is 1% DC Loss and 2% AC loss. but definitely dont want to be over 5% cause you start to make the inverter want to crank up the voltage to overcome the added impedance in order to back feed the grid. The inverter has to stay at 240V + or - 10% and therefore it shuts down above 264V.

However, you are going to run into a problem with the 120 rule, 705.12(D). 2011 said sum of breaker ratings of the back feed and main OCD can be over 120% of the busbar rating and only IF (now this is a big 'IF') the back feed OCD is at the furthest position from the main OCD. but then 2014 said that you need to use 125% of the inverter output plus the main OCD rating to get your sum that can go 120% over the the bus ampacity still with the same big 'IF'. So what does that mean for you? Thought youd never ask .

...2011...
back feed breaker + main breaker cant equal more than 120% of bus.

...at the sub
125A busbar x 1.20 = 150A Allowable. So our sum cant be more than 150A here.

36A would need 40A breaker. assuming 125A main breaker thats a sum of 165A
Option 1. Downsize main breaker to 100A then you're golden here but maybe not at the main, we'll see when we get there get there. Don't forget to apply the appropriate label.
Option 2. Line Tap. I just use polaris blocks to tap the feeders right above the main breaker. Done it a lot and never had a single AHJ question it. But you need to make there is still an OCPD for the wire. now that can come from your AC combiner being a main breaker if it is less than 4ft from lug to lug (don't quote me on that distance) but best practice is to just use a fused disconnect and size the fuses for your wire. This puts your OCP even closer to the interconnection.

...at the main
200A x 1.20 = 240A Allowable
Well for all intensive purposes we have turned the 100 amp supply breaker for the sub-panel into a back feed breaker. If AHJ is on 2011 but has a head on his shoulders he will use the 40A that you probably use for OCP to get a sum of 240A and then your fine. And the same should apply for the line tap too.


...2014...
inverter current x 1.25 + main breaker cant equal more than 120% of bus.

...at the sub
36 x 1.25 = 45A So this is still the same option set. You can downsize the main breaker in the sub to 100A or line tap to make this work. However, this is gonna bite us at the main...

...at the main
200A bus x 1.2 = 240A Allowable
at 125% of inverter current we are looking at 45A. now add the main breaker and we are 245A. No Go.

Option 1. Downsize Main Breaker to 150A. you'll probably be fine with that but I don't know your loads. You could try to find a breaker in between but good luck. It wont be easy if you do.
Option 2. Remove the subpanel feeders from the main panel into a junction box where you can splice then reroute them outside to a 100A fused disco w/ 100A fuse and then carry them back inside and into the main panel and line side tap for the interconnection. don't splice in the panel and go out to the disco cause then you've used your load center as a raceway and that is not allowed. once a wire is in the load center it must be terminated. so you can splice to make wires longer to reach a breaker but not to carry the conductor out of the load center cause then it would just be passing through.
Option 3. Trench 120 feet to the house, hit the disco, then pop inside and make the interconnection.

[sensij]

McMac, how would you justify using a 40 breaker under 2011 NEC? Don't you still need 1.25 x inverter output current, 36 A x 1.25 = 45 A in this case?

The real difference between 2011 and 2014 here is how the backfeed currents are summed. In 2011, if each of the 3 strings is landed in the sub with its own 20 A breaker, 60 A is the number to use for the 120% rule calculation. In 2014, you just use 1.25 x the inverter output circuit for the 120% rule, or 45 A if there are 3 strings with 12 M250s each.

[Jest Waitn]

Having a meter to send power through and one the use power is often refered to as virtual net metering. but I havn't found a utility or coop in our area that will allow it. Tried to do it to save trenching a 70kw over 600ft but they wouldn't have it.

ok. virtual net metering sounds good. what i plan to do is to first become a net metering customer and then try this avenue since it will mean a much shorter distance to the transformer. eventually i plan this to be a 9kw system. this year it will be 3kw with 3kw added each year unless there are problems due to the distance.

70kw over 600' - wow, that's a real system. might run DC the distance, but put fencing along the trench. i'm not a fan of running DC over a distance due to the potential lethality.

[Jest Waitn]

I couldn't view your drawing so let me just make you aware of a couple things based on what I've gathered here.

off the cuff, 36A should be fine in terms of voltage drop. even if its aluminum you are probably around a 2Vac drop and just over 100ft. thats still under 1%. Industry standard is 1% DC Loss and 2% AC loss. but definitely dont want to be over 5% cause you start to make the inverter want to crank up the voltage to overcome the added impedance in order to back feed the grid. The inverter has to stay at 240V + or - 10% and therefore it shuts down above 264V.

this will be three different circuits, each on a 20A breaker at the sub-panel. 12A max current, ~13.2A peak. no real load on the 125A sub-panel, except as PV supply.

this 9kw system might not work connected to the sub-panel for the reasons you state. 9kw production will exceed consumption by a good margin. the 3kw system might work fine, since the distance to the main panel is ~120', the electricity consumed by the home might not result in the over-voltage condition.

so the 9kw plan might NOT be implementable unless virtual net metering is allowed. i will deal with this issue after becoming a net metering customer.

[Jest Waitn]

However, you are going to run into a problem with the 120 rule, 705.12(D). 2011 said sum of breaker ratings of the back feed and main OCD can be over 120% of the busbar rating and only IF (now this is a big 'IF') the back feed OCD is at the furthest position from the main OCD. but then 2014 said that you need to use 125% of the inverter output plus the main OCD rating to get your sum that can go 120% over the the bus ampacity still with the same big 'IF'. So what does that mean for you? Thought youd never ask .

...2011...
back feed breaker + main breaker cant equal more than 120% of bus.

...at the sub
125A busbar x 1.20 = 150A Allowable. So our sum cant be more than 150A here.

36A would need 40A breaker. assuming 125A main breaker thats a sum of 165A
Option 1. Downsize main breaker to 100A then you're golden here but maybe not at the main, we'll see when we get there get there. Don't forget to apply the appropriate label.
Option 2. Line Tap. I just use polaris blocks to tap the feeders right above the main breaker. Done it a lot and never had a single AHJ question it. But you need to make there is still an OCPD for the wire. now that can come from your AC combiner being a main breaker if it is less than 4ft from lug to lug (don't quote me on that distance) but best practice is to just use a fused disconnect and size the fuses for your wire. This puts your OCP even closer to the interconnection.

...at the main
200A x 1.20 = 240A Allowable
Well for all intensive purposes we have turned the 100 amp supply breaker for the sub-panel into a back feed breaker. If AHJ is on 2011 but has a head on his shoulders he will use the 40A that you probably use for OCP to get a sum of 240A and then your fine. And the same should apply for the line tap too.


...2014...
inverter current x 1.25 + main breaker cant equal more than 120% of bus.

...at the sub
36 x 1.25 = 45A So this is still the same option set. You can downsize the main breaker in the sub to 100A or line tap to make this work. However, this is gonna bite us at the main...

...at the main
200A bus x 1.2 = 240A Allowable
at 125% of inverter current we are looking at 45A. now add the main breaker and we are 245A. No Go.

Option 1. Downsize Main Breaker to 150A. you'll probably be fine with that but I don't know your loads. You could try to find a breaker in between but good luck. It wont be easy if you do.
Option 2. Remove the subpanel feeders from the main panel into a junction box where you can splice then reroute them outside to a 100A fused disco w/ 100A fuse and then carry them back inside and into the main panel and line side tap for the interconnection. don't splice in the panel and go out to the disco cause then you've used your load center as a raceway and that is not allowed. once a wire is in the load center it must be terminated. so you can splice to make wires longer to reach a breaker but not to carry the conductor out of the load center cause then it would just be passing through.
Option 3. Trench 120 feet to the house, hit the disco, then pop inside and make the interconnection.

the average KWH usage/day for 15+ years is 23.2KWH. the average/month is 696KWH. the high/day is 45.5KWH; the low/day is 9.6KWH. so this is modest consumption of electricity.

the main panel is a pole mounted 200A 240VAC square d outdoor unit. on the main panel, the well has a 20A breaker. the A/C has a 30A breaker, but it is not in use presently. window units are used. the house has 100A breaker and the sub-panel is fed by a 50A breaker. the total breaker values = 200A, but the effective load is nowhere near this value. the PV array is connected to a 20A breaker at the sub-panel. there is 60A switched cut off (not breaker) at each PV array for convenience. the cutoff is not required by the power company.

i haven't yet done the breakdown of the panels as above, but the ampacity should be fine due to the modest usage.

i believe option 1 is doable, if it needs to be done.

the z brackets arrive tomorrow and the PV panels will be mounted on the PV framework. then all the 'good stuff' happens.

thanks for the support.

[J.P.M.]

OP: NOMB or concern, but a 9kW system for an 8,400 - 8,500 kWh/yr. load seems maybe a bit oversized. How did you determine the array size ? Where are you located ?