Connect solar combiner to sub panel

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wwilkins

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-I have a 200 amp 240V single phase service.
-I have and existing 100amp sub panel in a remote location feed by #4 copper
-I'm trying to figure out how to how to back feed 60amps of solar that is near the sub panel without installing a new conduit back to the service for a line side tap.

Any ideas?
Thx
 
The way I read the rules is that you can only back-feed 120% of the buss. In your scenario you could back-feed the main through the sub by 60 amps that is over by 20 amps.
 
How much load is actually on the subpanel? If the load is less than 60 amps, change the feeder breaker from 100 to 60 amps and install a back fed 60 in the subpanel for the solar connection.
 
110616-0904 EDT

I do not understand the comments.

What is the breaker size at the main panel, and the #4 wire rating?

Suppose the subpanel has a 60 A resistive load. Also assume the solar inverter produces a PF = 1, almost certainly the case. The solar system will supply all the current to the subpanel load, and no current flows in the #4 wires.

Next turn off all the subpanel loads. Now all the inverter's output flows on the #4 wires.

These are the two extremes. Pick a mid point, 30 A resistive load, and the current on the #4s is 30A with full output from the inverter.


Next consider that a 100 A breaker is at the main and the #4s were rated for 100 A. Apply a 100 A resistive load on the subpanel. With the inverter producing 60 A the current in the #4s is 40 A.


All of the above comments are ignoring other restrictions that might be placed on allowed current.


The problem is more complex to calculate if the subpanel load has a low power factor. If the subpanel load is 60 A pure reactive, a big capacitor, and the inverter is producing 60 A at PF=1, then the current in the #4s is the vector sum. Now the #4s heating is excessive.

.
 
Gar,
The code says that the sum of the ratings of the OCPDs that suppy the panel cannot exceed 120% of the panel rating. The breaker on the line side of the panel feeder and the solar inverter breaker both feed the panel. If the panel is rated at 100 amps and is fed by a 100 amp ocpd, the maximum rating of the solar inverter breaker would be 20 amps. If the load is 60 amps or less you can feed the panel with a 60 amp breaker and backfeed a 60 amp breaker on the bus.
 
don_resqcapt19 said:
Gar,
The code says that the sum of the ratings of the OCPDs that suppy the panel cannot exceed 120% of the panel rating. The breaker on the line side of the panel feeder and the solar inverter breaker both feed the panel. If the panel is rated at 100 amps and is fed by a 100 amp ocpd, the maximum rating of the solar inverter breaker would be 20 amps. If the load is 60 amps or less you can feed the panel with a 60 amp breaker and backfeed a 60 amp breaker on the bus.
Click to expand...

Ok but here is the problem. You now have a scenario that you can push 60 amps back -feed to the main 200 amp panel. This would be greater than the 120% buss max. I would think this is a violation and a potential meltdown that the code is trying to protect from. In my opinion you would need to reduce the feeder breaker from the main to the sub to 40 amps.
 
Last edited:
110616-1203 EDT

Don:

From what you are describing --- the code is trying to prevent too much feed to a panel because the panel could be overloaded on the load side of the equation. So that makes sense.

What restriction would exist if the inverter output, possibly thru a breaker, is connected to the subpanel feeder lines on the input side of the subpanel and feeding a breaker on the input to the subpanel to prevent an overload in the subpanel? This would protect all of the wires and both sub and main panels.

.
 
Gar, here are the 2008 NEC rules for connections.

690.64 Point of Connection. The output of a utilityinteractive
inverter shall be connected as specified in
690.64(A) or (B).

(A) Supply Side. The output of a utility-interactive inverter
shall be permitted to be connected to the supply side
of the service disconnecting means as permitted in
230.82(6).

(B) Load Side. The output of a utility-interactive inverter
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. Where distribution
equipment, including switchboards and panelboards, is fed
simultaneously by a primary source(s) of electricity and
one or more utility-interactive inverters, and where this distribution
equipment is capable of supplying multiple branch
circuits or feeders, or both, the interconnecting provisions
for the utility-interactive inverter(s) shall comply with
(B)(1) through (B)(7).

(1) Dedicated Overcurrent and Disconnect. Each source
interconnection shall be made at a dedicated circuit breaker
or fusible disconnecting means.

(2) Bus or Conductor Rating. The sum of the ampere
ratings of overcurrent devices in circuits supplying power
to a busbar or conductor shall not exceed 120 percent of the
rating of the busbar or conductor. In systems with panelboards
connected in series, the rating of the first overcurrent
device directly connected to the output of a utilityinteractive
inverter(s) shall be used in the calculations for
all busbars and conductors.

(3) Ground-Fault Protection. 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 groundfault
current sources. Ground-fault protection devices used
with supplies connected to the load-side terminals shall be
identified and listed as suitable for backfeeding.

(4) Marking. Equipment containing overcurrent devices in
circuits supplying power to a busbar or conductor supplied
from multiple sources shall be marked to indicate the presence
of all sources.

(5) Suitable for Backfeed. Circuit breakers, if backfed,
shall be suitable for such operation.

FPN: Circuit breakers that are marked ?Line? and ?Load?
have been evaluated only in the direction marked. Circuit
breakers without ?Line? and ?Load? have been evaluated in
both directions.

(6) Fastening. Listed plug-in-type circuit breakers backfed
from utility-interactive inverters complying with 690.60
shall be permitted to omit the additional fastener normally
required by 408.36(D) for such applications.

(7) Inverter Output Connection. Unless the panelboard
is rated not less than the sum of the ampere ratings of all
overcurrent devices supplying it, a connection in a panelboard
shall be positioned at the opposite (load) end from
the input feeder location or main circuit location. The bus
or conductor rating shall be sized for the loads connected in
accordance with Article 220. A permanent warning label
shall be applied to the distribution equipment with the following
or equivalent marking:

WARNING
INVERTER OUTPUT CONNECTION
DO NOT RELOCATE
THIS OVERCURRENT DEVICE
Click to expand...
 
gar said:
110616-1203 EDT

Don:

From what you are describing --- the code is trying to prevent too much feed to a panel because the panel could be overloaded on the load side of the equation. So that makes sense.

What restriction would exist if the inverter output, possibly thru a breaker, is connected to the subpanel feeder lines on the input side of the subpanel and feeding a breaker on the input to the subpanel to prevent an overload in the subpanel? This would protect all of the wires and both sub and main panels.

.
Click to expand...


Gar ;
That is what I was saying. However if the feeder to the sub was breakered at 40 amps to comply then the max load in the dark( no solar) would be 40 amps. if this is ok then the solution works. I would bet this is not the scenario the OP is thinking of.
 
110616-1307 EDT

Sierrasparky:

Yes.


iwire:

Looking at your references it appears that 690.64 B 7 is the critical condition in this instance.

How does 220 fit in as a limitation here, if any?

.
 
Sierrasparky said:
Ok but here is the problem. You now have a scenario that you can push 60 amps back -feed to the main 200 amp panel. This would be greater than the 120% buss max. I would think this is a violation and a potential meltdown that the code is trying to protect from. In my opinion you would need to reduce the feeder breaker from the main to the sub to 40 amps.
Click to expand...
Yes, I did not think about the main panel. The code does not completely address that issue, but I would agree that the 120% rule should apply and it should be calculated by the total of the main breaker and the solar output breaker. That would mean, as you said, use no more than a 40 amp breaker for the inverter, or reduce the main to 180.
 
gar said:
110616-1307 EDT


iwire:

Looking at your references it appears that 690.64 B 7 is the critical condition in this instance.

How does 220 fit in as a limitation here, if any?

.
Click to expand...

I am not sure what you refer to. 220 tells you to go to 690 for PV.
Are you talking about load on the panel? Of course you must calc the load at the panel and provide the proper feeder and overcurrent device. Thus if the load is higher than 40 amps on the sub and you only feed with 40 amps then the breaker will eventually trip. I am not sure it is a practical installation. I realize some backwoods customer could say " I will only use that max load in the sub panel when the PV can generate it." I think that this might be a viable excuse that the code book does not address.
 
110616-1355 EDT

Sierrasparky:

In the references that iwire quoted and in 690.64 B 7 it refers to Article 220. And you are saying 220 refers to 690.

My interpretation of B 7 is that if you have two power feeds to a panel and the sum of their currents is greater than the panel bus rating, then if those feeds are at opposite ends of the bus, and the breakers on each feed individually do not exceed the bus rating, then this is an allowed connection for a PV inverter with the added restriction that labeling is required to indicate that feed points must not be moved. Would seem to allow the breaker for the feeder from the main panel to be at the main panel.

On the theory of why this end point feeding should be OK is:
(1) Neither feeder source can exceed the bus rating based on its individual breaker size.
(2) If only one source is supplying current to the loads, and since its breaker is sized no more than the bus rating, then the maximum bus current will not be exceeded.
(3) If the conditions exist that both sources are feeding current to the bus, then on the bus there is a minimum voltage point between the two ends. To one side of this point one source is supplying current to the loads on that side, and this current does not exceed the bus rating. On the other side of the point the same type of condition exists for the other source. Thus, no place on the bus does the current exceed the bus rating.

.
 
Gar
I am not sure I understand that section or what you thinking. From what I interpet of the code is that in any case you cannot exceed 120% rating of the buss or conductors. I think section is saying if you exceed the buss rating ( but not over 120%) place the breakers at opposite ends. It is my thinking that they want you to do this to keep the load down at the connection points.
 
110616-1723 EDT

Sierrasparky:

I believe that under 690.64 that for this application
B applies and under it
1, 2, and 7 are of importance for the original post question.
2 sets a limit to 7 even when power is fed in at both ends of the subpanel.

So replace the subpanel with a 200 A subpanel. Feed the inverter into the far end of the bus. Now you could have a total of 240 A feeding the panel. Or a 150 panel allows 180 total. And as was stated early on if the feeder is limited to 60 A, then the problem is solved with the inverter breaker at the far end of the panel.

.
 
Gar,

Ok so you replace the sub panel with a 200 amp unit, what size will be the feeder to the sub. If you plan on using a 60 amp feeder you will still be non-compliant.
I don't see what you will gain from this. If you want 60 amps of PV then you need to install a resi 400 (320) amp main panel. Otherwise you need to install a seperate service for the PV with no loads on it.

This is the way I was taught in the PV class I took.

I hear there maybe some new meter main panels that have oversized bussing just for this purpose. I have not seen any myself though!
 
110616-2120 EDT

Sierrasparky:

We are lacking critical information about what the actual expected peak current load may be on the main panel and the subpanel. If these actual loads are less than the panel ratings, then changing breaker sizes may allow use of one or both of the panels.

If you are a gas and electric house vs an all electric house, and have a 200 A panel, the maximum loading may never exceed 150 A or even maybe 100 A.

I practically never exceed a peak of 6 KW. My daily average is about 40 KWH, or an average load of 1.67 KW, but during the day the average may be in the 2 to 3 KW range. So my 200 A panel is lightly loaded.

If you are in a low cost natural gas region there is no way you want to use electricity for major heating purposes. Per KWH our gas cost is about 1/3 that of electricity. Geothermal may be one exception, but the capital cost is very high.

We need more information from wwilkins.

.
 
gar said:
110616-2120 EDT

Sierrasparky:

We are lacking critical information about what the actual expected peak current load may be on the main panel and the subpanel. If these actual loads are less than the panel ratings, then changing breaker sizes may allow use of one or both of the panels.



We need more information from wwilkins.

.
Click to expand...

Yes that is correct.
The theme I was getting at is that you would need to reduce the size of the main breaker. The problem is that many Res services don't have much selection of breakers. For example the 200 amp panel might only be able to accept a 150 amp breaker and nothing in between. You have 2 issues how does the home calculate at 150 amps. Will the AHJ allow you to reduce the main breaker when he knows you might put back the 200.
I realize the latter is a bit extreme but I have seen inspectors get unreasonable.
 
wwilkins said:
-I have a 200 amp 240V single phase service.
-I have and existing 100amp sub panel in a remote location feed by #4 copper
-I'm trying to figure out how to how to back feed 60amps of solar that is near the sub panel without installing a new conduit back to the service for a line side tap.

Any ideas?
Thx
Click to expand...

You may have options, but there are conditions...

Option 1: Do load calculations for the subpanel to determine if it the breaker feeding it can be replaced with a 60amp breaker. If yes, you may install the 60amp solar breaker in the subpanel, at the opposite end of the busbars from which they are fed, with a label not to move the solar breaker. (If you can downsize the breaker feeding the subpanel to 40amps, the location of the solar breaker in the subpanel does not matter.)

Option 2: Do load calcuations on the whole house to see if the main breaker can be downsized to 175amps. (You may also need to verify that you can do this in the main panel without violating its listing.) If yes, you may install the 60amp solar breaker in the main panel, at the opposite end of the busbars from which they are fed, with a label not to move the solar breaker. (If you can downsize the main breaker to less than 140amps, the location of the solar breaker in the mainpanel does not matter.)

Option 3: Line-side tap. As you can see, options 1 and 2 are tall orders, especially option 2.

BTW, I agree with the conclusions gar and sierra have reached, i just thought I'd lay out the practical options.
 
jaggedben said:
You may have options, but there are conditions...

Option 1: Do load calculations for the subpanel to determine if it the breaker feeding it can be replaced with a 60amp breaker. If yes, you may install the 60amp solar breaker in the subpanel, at the opposite end of the busbars from which they are fed, with a label not to move the solar breaker. (If you can downsize the breaker feeding the subpanel to 40amps, the location of the solar breaker in the subpanel does not matter.)


.
Click to expand...

Just a note option 1 will never be complaint alone. You must do so in combination with portions of option 2. The confusing part that is misinterpeted is that in no case can you feed the main panel with 200 amp from the utility and 60 amps from the PV as the bus is mostlikely rated only at 200amps. 200 x 120% = 240amps max. In other words you must reduce the size of the main or the breaker to the sub. If you reduce the breaker feeding the sub from the main to 40amps then you have only 40amps max load during non production hours. The concern is does the load on the sub exceed 40 amps. If so this option is not available. Of course you can add a load shedding control to the mess if this is what you want. The draw back is that you are maxed out at 40 amps that will backfeed the utility and you may trip the feeder breaker when production exceeds the usage over 40 amps.

I hope I did not confuse the situation further as I am not trying to.
 
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