Grid Tie in Tucson Az

**solarix** · 07-07-2015, 12:44 AM

Question #1 Is it a relatively simple matter to reorganize the breakers to get a pair of 240V breakers grouped at the bottom of the fuse box?
There is an existing open double at the bottom left with a 15 and 20 amp circuit breakers just above. The 15 and 20 could be moved to two other open slots without regard to phase provide the wire is long enough to support it which it seems it is as everything still stay on the lower portion of the panel.

Yes, move the single breakers up to make room for a double breaker at the bottom left. be careful to relabel as needed.

Question #2 Where is the main breaker? Neither the Main or the subpanel have what I recognize as a Main breaker(my last house had one in Ca and I think they seemed to be common place there). There is a locked box coming out of the ground at the foot of the driveway. I assume this is power as all the other utilities are represented in one way or another outside of this box.

Somewhere in between the meter and the breaker panel, there has to be a main breaker that is readily accessible.
Perhaps in a door below the meter? Need a picture of the meter box.

Follow up to Question #2a

In the absence of a main breaker and the secret lock on the street I'm assuming that the power company has to be called in to switch off the mains inside of that little box.

The main breaker is not going to be locked up.

Follow up to Question #2b

Is it possible to put in a AC disconnect inside of the main panel? The meter and the main panel are separated by little more than 3" of 2" conduit. Basically I'm asking as rewiring the service connection between the meter and the main panel is probably non trivial.

No, it is not OK to put the AC disconnect in the main panel (not listed for that purpose).

TIA.[/QUOTE]

Here is the deal: You want a 10kW inverter - that's going to take some doing on the interconnect.
10kW / 240V = 41.67A (check inv. data sheet, max amps may vary a bit). Since it is continuous duty, the ampacity is 125% of that or 52A. That means a 60A breaker for the solar circuit. Code requires the backfed current to be less than 20% of the busbar rating. 20% of 200A is only 40A. So you need to either get a bigger busbar (upgrade to 400A panel) or downgrade the main breaker to 175A (good luck finding that in the style for this panel) to give you another 25A, or do a line-side tap (before the main breaker which very few panels are listed for). Any of these options can get expensive - even the derated main breaker requires a separate permit to have the utility out to shut down the service to do the changeout.
The panel may be from the 70's but looks in good condition. First try and find the main breaker and see if a 175A version of it can be special ordered (you will also have to justify to the authorities that your load can be serviced by a 175A main). Next option is to replace the whole panel with a "solar ready" 200A panel that has a 225A busbar. Or if you want future expansion, go to a 400A panel. Either of those options generally cost upwards of $1500 installed.
Or you could back off the solar system size and just use a 7.7kW inverter on a 9kW array which just needs a 40A breaker and no modifications to your panel.

**posplayr** · 07-07-2015, 03:54 AM

Originally posted by solarix

Question #1 Is it a relatively simple matter to reorganize the breakers to get a pair of 240V breakers grouped at the bottom of the fuse box?
There is an existing open double at the bottom left with a 15 and 20 amp circuit breakers just above. The 15 and 20 could be moved to two other open slots without regard to phase provide the wire is long enough to support it which it seems it is as everything still stay on the lower portion of the panel.

Yes, move the single breakers up to make room for a double breaker at the bottom left. be careful to relabel as needed.

Question #2 Where is the main breaker? Neither the Main or the subpanel have what I recognize as a Main breaker(my last house had one in Ca and I think they seemed to be common place there). There is a locked box coming out of the ground at the foot of the driveway. I assume this is power as all the other utilities are represented in one way or another outside of this box.

Somewhere in between the meter and the breaker panel, there has to be a main breaker that is readily accessible.
Perhaps in a door below the meter? Need a picture of the meter box.

Follow up to Question #2a

In the absence of a main breaker and the secret lock on the street I'm assuming that the power company has to be called in to switch off the mains inside of that little box.

The main breaker is not going to be locked up.

Follow up to Question #2b

Is it possible to put in a AC disconnect inside of the main panel? The meter and the main panel are separated by little more than 3" of 2" conduit. Basically I'm asking as rewiring the service connection between the meter and the main panel is probably non trivial.

No, it is not OK to put the AC disconnect in the main panel (not listed for that purpose).

TIA.

Originally posted by solarix

Here is the deal: You want a 10kW inverter - that's going to take some doing on the interconnect.
10kW / 240V = 41.67A (check inv. data sheet, max amps may vary a bit). Since it is continuous duty, the ampacity is 125% of that or 52A. That means a 60A breaker for the solar circuit. Code requires the backfed current to be less than 20% of the busbar rating. 20% of 200A is only 40A. So you need to either get a bigger busbar (upgrade to 400A panel) or downgrade the main breaker to 175A (good luck finding that in the style for this panel) to give you another 25A, or do a line-side tap (before the main breaker which very few panels are listed for). Any of these options can get expensive - even the derated main breaker requires a separate permit to have the utility out to shut down the service to do the changeout.
The panel may be from the 70's but looks in good condition. First try and find the main breaker and see if a 175A version of it can be special ordered (you will also have to justify to the authorities that your load can be serviced by a 175A main). Next option is to replace the whole panel with a "solar ready" 200A panel that has a 225A busbar. Or if you want future expansion, go to a 400A panel. Either of those options generally cost upwards of $1500 installed.
Or you could back off the solar system size and just use a 7.7kW inverter on a 9kW array which just needs a 40A breaker and no modifications to your panel.

Solarix: Thanks for your reply

This is a picture of the meter and the buried conduit coming up to the meter box. You can see the 4/0 wires coming through another large conduit on the bottom left of the main panel. The meter box doesn't look like something to open. Perhaps it is?

As to the rest, I need to find out what size the breaker is after I find the main breaker. Any clue?

EDIT: I looked closely at the meter box and used a large flat head screw driver to see if I persuade the lid up. It does not give much at all; as if it never was opened. It could have been painted closed, but jamming the screwdriver end under the front lip of the panel did not get a budge out of it at all.

The bottom has two additional knockouts same size as the pipe coming it. It is not moving.

**solarix** · 07-07-2015, 09:15 AM

Pretty amazing. I'd say you have a service with no main disconnect.
One thing to realize when interconnecting solar is the utility is going to use this opportunity to bring your site up to their current standard. They apparently allowed your service to be this way in back in the seventies, and will allow it forever as long as you don't change anything, but now that you want their permission to add solar, they are gonna want you to bring it all up to date.
You might as well figure on upgrading this service to a solar-ready type breaker panel (if you get a GE one you can reuse the breakers, although all new everything would be prudent). This is going to take a separate permit to have the utility come out and pull the meter, your electrician will have to do the work starting early in the morning and coordinate with the local inspector to check it over and green tag it and then have the utility come back and re-energize it - all the same day.

**solarix** · 07-07-2015, 09:24 AM

After looking closer at your pic of the panel, it looks like there is a 60A breaker feeding the lower section of the panel? It is code to have up to 6 "main" breakers on a panel. I've never seen a panel like this but apparently the top 6 breakers in this panel are all mains, and the 60A one then feeds a separate bus in the lower section. This kind of panel is especially bad at being able to accept a large solar backfeed. Time for a new panel imho.

**posplayr** · 07-07-2015, 02:14 PM

Originally posted by solarix

After looking closer at your pic of the panel, it looks like there is a 60A breaker feeding the lower section of the panel? It is code to have up to 6 "main" breakers on a panel. I've never seen a panel like this but apparently the top 6 breakers in this panel are all mains, and the 60A one then feeds a separate bus in the lower section. This kind of panel is especially bad at being able to accept a large solar backfeed. Time for a new panel imho.

I'm starting to realize that is is probably a practical issue of using smaller main breakers for dedicated large devices like the roof mounted AC unit.

OK so you are saying it is a distributed main circuit breaker. 6 Mains, the shop is shut off here as well with that pair of 100 amp breakers.

What is amazing to me is that the previous owner spent probably close to $100K to put in the 30"x45" brick shop that is 15' high with double 10' rollup doors. It is fully wired for 240 compressors and welders and is served from this main panel (left center of the panel) from a pair of 100 amp breakers and that shop sub panel does not have a main breaker either.

Everything was constructed and permitted in 2000 without requiring a change to the main panel.

Is this unsafe, other than the obvious if there is a fire you have to call the power company to shut off main power?

Just to round this out, here is the 125 amp amp sub panel on a 100 amp main panel main breaker. I'm assuming a solar connection here is out of the questions considering the 4 awg wire and the limited 125 amp rating.

Finally after all the crying, this is still a $1500 job for the solar ready upgrade?

**posplayr** · 07-07-2015, 03:07 PM

I was trampling all over the last post with edits and it would not make any sense so I added another post.
EDIT: I added up 330 amps in breaker amperage in the top part of the box (excluding that little 15 amp one on the left side just above the 100 amp for the shop sub panel. So what is the whole panel rated at? 400 Amp service?

Not knowing any better perhaps, the more I look at this the less problem I see. Based on just the installed mains I see 330 amps which I'll assume means this is more like a 400 amp service(??) and with 20% rule would allow 80 amps back feed which is well above the 60 amp breaker you sized for the 10Kw solar. Since presumably the solar is there to supply the house loads first, it is house breaker that should be fed with the solar in the first place. In theory (if not code), the solar capacity plus the house current draw would never go through that breaker in the first place. How big does the house main need to be increased from 60 amps (to service the house) to accommodate a backfeed 60 amps solar source?

You would not need to add 60 amp for the house load and another 60 amps to supply the grid would you (e.g. 120 amp breaker)? That is not possible.

It looks like this and it seems actually ideal. What could be better?
The 10/15/20 amp circuits are all house loads. The other main breakers are for Furnace, Shop, AC and swamp coolers etc and the house

**posplayr** · 07-07-2015, 06:44 PM

A little more info: My across the street neighbor has a old time electrician as a friend who came over to tell us what was going on(both our panels are identical). He did admit to knowing next to nothing about solar, and then proceeded to lecture me about it for an hour after that.

The main panel is what is known as a "split panel". The buss bar on the upper part of the panel is sufficient to hold several "main breakers" to branch off to major appliances. One of these main breakers (60 amps) is the main for the lower portion of the panel which feeds all of the household light circuits.

Another main breaker (100 amp) is the branch to the 125 amp/ 240V shop panel; It is apparently treated as a separate appliance running off of a 100 Amp ganged pair of breakers as the Furnace is running of it's own main breaker.

The meter is rated at 200W.

The 4/0 wire coming in from the street is as big as it gets and even a 400 amp service panel would used the same wire.

OK What I still don't know:

What is the rating of the current bar?

I'm having a hard time believing you can put 330 amps worth of breakers into a current bar rated at only 200 amps. In addition as recent as the year 2000, the 30"x45' shop was added by simply putting the previously mentioned 100 amp breakers on the same bus bar that was presumable pretty well filled to begin with.

This what I can make of a GE part number for the Main Panel:
GE E531*
TX2420RH

Assuming the current bar is rated at 300 amps(min) and 20% of 200 amps being 60 amps, is there an issue with adding a 60 amps double breaker to the bottom of the split panel which itself if fed by a 60 amp breaker? It seems a good match as per the diagram above. If anything this configuration will always have less current running through the panels and current bar except in the case that the solar is completely shutoff and it simply reverts back to as it is now.

**solarix** · 07-07-2015, 07:02 PM

Well - no, a 400A panel is really a dual 200A panel with two 200A busbars and two 200A main breakers. To use that with solar, you would dedicate one of the busbars to just solar so that the 20% rule doesn't apply. Easily backfeed 60A into the 200A bus. Most of your regular loads would then go on the other 200A bus, much as it is now.
Even though your loads theoretically add up to 330A of load, in actuality you never have them all at the same time and code has equations for figuring just how far you can go on this depending on type of load etc. I'd guess that 330A on a 200A busbar is pushing it though. If it was me, I'd go with a 400A panel and put the 100A subpanel breaker on the solar bus. You can do that as long as the panel is labeled to prevent adding any additional loads to that bus. As long as the straight sum of all the loads doesn't exceed the rating of the busbar, the 20% backfeed rule doesn't apply.

The TX2420 probably means 24 breaker spaces on a 200A busbar.

**posplayr** · 07-07-2015, 10:36 PM

Originally posted by solarix

Well - no, a 400A panel is really a dual 200A panel with two 200A busbars and two 200A main breakers. To use that with solar, you would dedicate one of the busbars to just solar so that the 20% rule doesn't apply. Easily backfeed 60A into the 200A bus. Most of your regular loads would then go on the other 200A bus, much as it is now.
Even though your loads theoretically add up to 330A of load, in actuality you never have them all at the same time and code has equations for figuring just how far you can go on this depending on type of load etc. I'd guess that 330A on a 200A busbar is pushing it though. If it was me, I'd go with a 400A panel and put the 100A subpanel breaker on the solar bus. You can do that as long as the panel is labeled to prevent adding any additional loads to that bus. As long as the straight sum of all the loads doesn't exceed the rating of the busbar, the 20% backfeed rule doesn't apply.

The TX2420 probably means 24 breaker spaces on a 200A busbar.

I have been researching this area of load side v.s. supply side connections. It seems that a guy from NM John Wiles who is from the Institute for Energy and the Environment (IEE) at New Mexico State University is the accepted authority on this stuff.

In doing some reading, including the specific wording of 690.64(B)(2), it appears that there is some apparent difficulty in applying 690.64(b)(2) for a split bus panel. The main question is where/which is the main breaker rating to be added to the PV breaker. This difficulty even extends to misinterpretation of the code by some AHJs, which I assume would be very frustrating.

However, I feel there is some light at the end of the tunnel as the various writings by Wiles I have read, make the underlying theory clear despite instances where the word of the code and its interpretation can lead to confusion. In fact it would appear, that 690.64(b)(2) was written with split bus panels in mind, but with a careless read it would seem to not be so. The end result is confusion.

I'll give a summary later on after I do some more reading. The bottom line is that a 60 amp breaker can be attached to the bottom of the lower panel being itself fed from a 60 amp breaker be in accordance with at least the intent 690.64(B)(2) if not the code itself. I'm basically describing in words the picture above, with the other 5 mains omitted for clarity.

**sensij** · 07-08-2015, 12:18 AM

What code cycle are you on? 705.12 is where the interconnect requirements are in NEC 2011 and NEC 2014.

For the most part, as long as both the top and bottom buses are each rated for 200 A, I think what you are suggesting looks OK. It isn't really any different than a subpanel in series, except that it is housed in the same enclosure.

**posplayr** · 07-08-2015, 02:01 AM

Originally posted by sensij

What code cycle are you on? 705.12 is where the interconnect requirements are in NEC 2011 and NEC 2014.

For the most part, as long as both the top and bottom buses are each rated for 200 A, I think what you are suggesting looks OK. It isn't really any different than a subpanel in series, except that it is housed in the same enclosure.

This is an AIEI article dated July/Aug 2011 written by John Wiles. I assume it is current up through 2011 and it does mention at least a couple of proposed changes since 2005 that were not approved. The general sense I get is that there is and understanding the code is a bit confusing but they are not quite sure how to change it.

A Critical Look at Load Side Utility-Interactive PV Inverter Connections 690.64(B) / 705.12(D)

http://iaeimagazine.org/magazine/2011/07/16/a-critical-look-at-load-side-utility-interactive-pv-inverter-connections-690-64b-705-12d/

This is a quote from the 2005 code dated: 03-27-2009, 06:39 PM

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http://www.inspectionnews.net/home_inspection/electrical-systems-home-inspection-and-commercial-inspection/12675-690-64-b-2005-edition-nec.html

This is that 2005 NEC section. (bold and underlining are mine)
- 690.64 Point of Connection.
- - The output of a photovoltaic power source shall be connected as specified in 690.64(A) or 690.64(B).
- - (A) Supply Side. A photovoltaic power source shall be permitted to be connected to the supply side of the service disconnecting means as permitted in 230.82(6).
- - (B) Load Side. A photovoltaic power source shall be permitted to be connected to the load side of the service disconnecting means of the other source(s) at any distribution equipment on the premises, provided that all of the following conditions are met:
- - - (1) Each source interconnection shall be made at a dedicated circuit breaker or fusible disconnecting means.
- - - (2) The sum of the ampere ratings of overcurrent devices in circuits supplying power to a busbar or conductor shall not exceed the rating of the busbar or conductor.
- - - - Exception: For a dwelling unit, the sum of the ampere ratings of the overcurrent devices shall not exceed 120 percent of the rating of the busbar or conductor.
- - - (3) The interconnection point shall be on the line side of all ground-fault protection equipment.
- - - - Exception: Connection shall be permitted to be made to the load side of ground-fault protection, provided that there is ground-fault protection for equipment from all ground-fault current sources.
- - - (4) Equipment containing overcurrent devices in circuits supplying power to a busbar or conductor shall be marked to indicate the presence of all sources.
- - - - Exception: Equipment with power supplied from a single point of connection.
- - - (5) Circuit breakers, if backfed, shall be identified for such operation. Dedicated circuit breakers backfed from listed utility-interactive inverters complying with 690.60 shall not be required to be individually clamped to the panelboard bus bars. A front panel shall clamp all circuit breakers to the panelboard bus bars. Main circuit breakers connected directly to energized feeders shall also be individually clamped.

This is the extraction from the Wiles white paper.

This is the manner in which many code requirements are formulated. The requirement is written in general terms and then the general requirement is modified by exceptions (restrictions or allowances) or additions to the requirement.

From an engineering point, the basic requirement is sound. A conductor or busbar will be prevented from being overloaded if the rating of that busbar or the ampacity of that conductor is greater than or equal to the sum of the ratings of all overcurrent devices supplying it [see 690.64(B)(2) in 2005 NEC]. Note that the requirement refers only to the rating of the supply overcurrent devices, not to any calculated currents and it does not refer to any load overcurrent devices.

Without referencing more of it , this is a basic guide to how to interpret the code for split bus panels. Generally for load side, this means that the back driven PV breaker is in the lower (distribution ) panel. If it were connected as a back driven main breaker, it would effectively be a supply side connection; at least that is the debate. Setting that aside for the moment, lets consider the 120% rule applied to the the two separate bus bars in the split bus panel.

In the main upper bus bar, the only two power sources are the service (which is limited to infinity) and the breaker value of the lower bus that the PV is attached to. So in my case that is the sum of infinity plus 60 amps which is still infinity. But then careful check of the code wording actually says:

(2) The sum of the ampere ratings of overcurrent devices in circuits supplying power to a busbar or conductor shall not exceed the rating of the busbar or conductor.*

From an engineering point, the basic requirement is sound. A conductor or busbar will be prevented from being overloaded if the rating of that busbar or the ampacity of that conductor is greater than or equal to the sum of the ratings of all overcurrent devices supplying it [see 690.64(B)(2) in2005 NEC]. Note that the requirement refers only to the rating of the supply overcurrent devices, not to any calculated currents and it does not refer to any load overcurrent devices. As the split bus does not have a protection device for the grid power source, you are only left with the PV power coming from the lower panel which can not exceed 60 amps (lower main breaker). For a 120% rule on a 200 amp panel that would be 240 amps and so we are 4:1 below the limit.

No issue with the panel's upper bus! So what is going on here? I could put up to 240 amp PV system in and still meet the code on a 200 amp panel. That is because the current through the panel will in fact be limited by the load breakers and not the PV system breakers nor the grid connection. The only issue can occur due to the PV is if that lower breaker exceeded 240 and there were more than 240 amps of load breakers.

OK now to the lower panel. Here we (as you described) have yet another panel down stream of a main breaker(60 amp). OK so reapply the same rule to that (lower) distribution bus bar. The main is supplying power from the grid as much as 60 amps, the PV potentially can supply 60 amps so under a sufficient load condition there could be 120 amps running through the lower bus. That is still 2:1 below the 120% rule. And it would only occur if the breakers loads exceeded or got to that limit.

So now lets get back to the "supply side" main breaker in this split panel. Is it really a supply side? Lets just say it is not of more so lets say it doesn't matter! The engineering safety theory must still hold. You can not exceed the bus bar rating. What determines the current in the upper bus bar? It is the sum of all the load protections devices on the upper bus bar. In my case this 330 amps and it is clearly exceeds the 200 amp rating of the service. So how do you make an argument that you are just going to throw an additional 60 amp of current in on this 330 amps to stay below 200 amp? It seems like a loosing argument. There are already established formulas for computing an effective load based on the breaker size that is less that the sum of the rated values. Trying to throw the PV (not as per the code) in as on a main breaker under the guise of a supply side source although the current is going through the same bus bar is a theoretically flawed position no matter how you want to twist the code around.

For a PV install on a split bus panel, it is hard to rationalize that a PV system can be connected onto a main as a supply side connection and avoid the 120% rule. And it is going to be even harder to convince someone based on first principles that your are not going to exceed bus ampacity (mainly because the addition of PV is the least of the total currents in the upper bus). And lastly, with a much reduced main breaker for the lower breaker as in my case (60 amps), it is easy to show that the worst case bus current (for a 60 amps PV service) is 120 amps and well below the 120% maximum of 200 amp panel rating even as applied to only the lower part.

One might question if the lower panel is rated to handle the 200 amps? Well it has to as there is nothing that says the upper panel has to have more than one main breaker. So all the 200 amps could potentially come from one single main breaker and go to the bottom distribution panel. Is this not right?

In a nutshell, the distribution panel characterization of the lower panel with a PV connection, is the most straightforward and easiest to provide an engineering explanation as such a convincing argument of compliance with the code. There is also a symmetry to the solution as indicated in the figure above about having the two 60 amp sources feeding the lighting circuits being exactly 1/2 of the 120% rule rating.

I feel better now, unless you can shoot any holes into the argument.

**sensij** · 07-08-2015, 06:51 AM

You won't get any respect from the ahj if you are citing the wrong code. The substance of the requirements is the same, but the language has changed in some cases. Those papers linked are obsolete, but you will find similar discussions of the code section i mentioned that may help your case.

**foo1bar** · 07-08-2015, 01:32 PM

Originally posted by posplayr

In fact it would appear, that 690.64(b)(2) was written with split bus panels in mind, but with a careless read it would seem to not be so. The end result is confusion.

Easy solution - replace the split bus panel.

Yes it's a pain, BUT I think it's your best option. (and makes it easier for next time you want to do something with that panel.)

To determine what amperage main breaker you want, you'll need to determine what size the line is from the utility to your meter. That is likely your limiter (you said it was 4/0 Alum from the meter to the main panel, and it's a short distance, so I don't think that will be your limiter.)

Probably you can find a panel that will fit reasonably well into the space you have. If I were you, I'd spend a little time researching that.

Good luck.

**posplayr** · 07-08-2015, 02:23 PM

Originally posted by sensij

You won't get any respect from the ahj if you are citing the wrong code. The substance of the requirements is the same, but the language has changed in some cases. Those papers linked are obsolete, but you will find similar discussions of the code section i mentioned that may help your case.

Well thanks for that recommendation on getting the up to date codes, but I think I will swim closer to the middle of the channel as I can before pulling raising my head out of the water to quote code. When and if the time comes I'll make sure I have the latest code requirements that are in effect. Do you have an free on-line reference?

My post was less about the code, but asking the fundamental question about what the best type of connection. Based on your prior comment re series panel and the absence of any further issues on the options, I assume you are still in agreement. That the load side breaker in the lower sub panel is still best?

To push the point a bit harder, going to a supply side connection with a new "solar ready" panel is not necessarily any better and much harder to actually show that the bus bar ampacity is not exceeded.

Grid Tie in Tucson Az

Grid Tie in Tucson Az

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