Main Panel supply side connection & 120 % rule questions.

[davidj792]

The main panel is rated 200amps w/ no main breaker, has bus bar w/qty.6 locations for breakers. Currently contains qty.2- 100amp breakers feeding sub-panels. Qestion 1, If qty.2 seperate 40amp pv breakers are added to this bus bar, this would be considered a supply-side connection and not subject to the 120% rule, correct?? Question 2, per 2011 NEC 705.12 (A), am I correct in interpreting that the qty.2 new pv breakers ( 40amp each ), don't exceed the rating of the service conductors rated 200amps, and the existing 100amp feeders for the subpanels aren't included in that calculation.??

 

[kwired]

I can't answer this from the PV side of things, which I believe is also the source of this "120%" rule, but will say that in general where there is 2-6 disconnecting means the load calc is more important than looking at the sum of the ratings of the breakers. Your 200 amp panel could have 6-100 amp breakers installed as mains and supplied by conductors of less than 200 amps as long as load calculation doesn't exceed conductor ampacity or the bus ampacity.

 

[davidj792]

Thank you for your reply. Yes I understand when calculating load calcs for a service. I'm wanting to confirm the interpretation that when installing a PV system, if there is no single main breaker and the panel is configured w/ a bus bar for a total of 6 breakers and you use 2 of these locations for a pv system connection, this would be considered a line-side (or supply side ) connection and the rules of 705.12(A) would apply, and the 120% rule would not apply since this isn't a load-side connection. That's how I understand the interpretation and am looking for a second opinion.

 

[GoldDigger]

Thank you for your reply. Yes I understand when calculating load calcs for a service. I'm wanting to confirm the interpretation that when installing a PV system, if there is no single main breaker and the panel is configured w/ a bus bar for a total of 6 breakers and you use 2 of these locations for a pv system connection, this would be considered a line-side (or supply side ) connection and the rules of 705.12(A) would apply, and the 120% rule would not apply since this isn't a load-side connection. That's how I understand the interpretation and am looking for a second opinion.

Shooting from the hip, which is not necessarily good on a Saturday morning.....:

The 120% rule is applied to the bus in a panel and to the wires in a feeder or a line side wire (not calling it a tap at this point, since we are not limiting where it goes.) Since there is no single main breaker, you do not have any current limit in power supplied to the bus except whatever OCPD is present on the POCO side. So you now have a potentially unlimited supply of current to the bus from POCO, but at least the size of the service. And you have the already installed 200A of load in the breakers feeding the subpanels. You are not required to provide overcurrent protection to the service, but you are required to protect the bus. The problem is that with more spaces available in the panel, more load could be added and the current from the service plus the current from the PV together would then exceed the bus rating.

Since you are connecting to the bus rather than to the service wires, this is NOT a line side tap for the purpose of the 120% rule, and since there is no main, I am not sure how you would be able to apply the 120% rule correctly. And if you could, the total of 80A of the PV would exceed the 120% limit.
Bottom line: I do not see any way that you can legally do this and the PV would have to come into a second panel connected to the service directly (a true line side tap) or a main breaker would have to be installed in the panel so that the 120% calculation could be done.

 

[jaggedben]

The main panel is rated 200amps w/ no main breaker, has bus bar w/qty.6 locations for breakers. Currently contains qty.2- 100amp breakers feeding sub-panels. Qestion 1, If qty.2 seperate 40amp pv breakers are added to this bus bar, this would be considered a supply-side connection and not subject to the 120% rule, correct??

That is correct.

Question 2, per 2011 NEC 705.12 (A), am I correct in interpreting that the qty.2 new pv breakers ( 40amp each ), don't exceed the rating of the service conductors rated 200amps, and the existing 100amp feeders for the subpanels aren't included in that calculation.??v

Yes that sounds correct. The rating of the service is whatever is the 'weakest link' among the service conductors, the main panel busbar (when there's no main breaker), and whatever else may be between the service point and the service disconnecting means, such as a meter base.
The existing load breakers do not tell you what the rating of the service is, although they are often the same.

The 120% rule is applied to the bus in a panel and to the wires in a feeder or a line side wire (not calling it a tap at this point, since we are not limiting where it goes.)

The 120% rule applies only to load side connections. It has no application in the OP's case.

The problem is that with more spaces available in the panel, more load could be added and the current from the service plus the current from the PV together would then exceed the bus rating.

More load could be added regardless of whether a PV system was present, and the danger would be the same. As long as the calculated load does not exceed the rating of the service, the installation is compliant. I'm not sure why the code allows this (230.90(A)Exception 3), but it does.

Since you are connecting to the bus rather than to the service wires, this is NOT a line side tap for the purpose of the 120% rule,

No, that is wrong. 705.12(A) refers to "the supply side of the service disconnecting means". If the disconnecting means and OCPD for the PV system is a service disconnecting means (i.e. it has no other service disconnecting means on the line side of it), then it is a supply side connection. Whether it is done as a tap or at a breaker on a busbar is irrelevant.

Bottom line: I do not see any way that you can legally do this and the PV would have to come into a second panel connected to the service directly (a true line side tap) or a main breaker would have to be installed in the panel so that the 120% calculation could be done.

You are wrong. The way he is doing it is a valid supply side connection, and is both compliant and safe.

 

[PWDickerson]

I asked the same question of my AHJ (who is pretty sharp with solar) last year, and this was his response.

In this case the sum of all existing circuit breakers and the new PV inverter output circuit shall not exceed 120% of the bus bar rating per 690.64 (B) 2 and the PV inverter output circuit breaker shall be located at the opposite end of the bus from the line terminals per 690.64 (b) 7. This is common problem found in older homes with split bus panels. Even thought the panel does not have one main disconnecting means, the 2-6 disconnects that are drawing power from the same bus (grouped in one enclosure) are the service disconnecting means as per NEC 230.71(A). Overloading the panel busing is prevented by keeping the load between the supply points. If the existing service disconnects were in separate enclosures, I would say that a added service disconnecting means for PV inverter output connection is located ahead of the service disconnecting means per NEC 690.64 (A).

 

[davidj792]

I guess the real question here is, is this a supply-side connection ( then 690.64 (A)) would apply, OR, is this a load-side connection ( then 690.64 (B) 1 thru 7 ) would apply. I don't believe the panel configuration would be considered a "split bus panel", It's a 200amp w/ underground service conductors terminating to a bus bar with the capacity of a total of 6 breakers ( no main breaker, and no other bus bars such as would be on a split bus panel ). The PV system terminates to a qty. 2 of these spaces.....so I see this termination as a supply-side connection. Is it correct that each of the 6 breakers would be considered a means of service disconnect, so they aren't on the load-side? Isn't it also correct that the means of the service disconnect is part of the service and considered to be on the line-side of the service. Thanks for all of the responses, I appreciate all of the input...just want to get this right.

 

[don_resqcapt19]

If there is no main, and the breakers in this panel are the service disconnects, then the PV connection is a supply side connection and only limited by the rating of the service.

 

[ggunn]

I asked the same question of my AHJ (who is pretty sharp with solar) last year, and this was his response.

In this case the sum of all existing circuit breakers and the new PV inverter output circuit shall not exceed 120% of the bus bar rating per 690.64 (B) 2 ...

If your AHJ is counting load breakers then he is mistaken. It is only the sum of breakers feeding the bus which are considered in the 120% rule; the load breakers are irrelevant to the calculation.

 

[jaggedben]

I asked the same question of my AHJ (who is pretty sharp with solar) last year, and this was his response.

In this case the sum of all existing circuit breakers and the new PV inverter output circuit shall not exceed 120% of the bus bar rating per 690.64 (B) 2 and the PV inverter output circuit breaker shall be located at the opposite end of the bus from the line terminals per 690.64 (b) 7. This is common problem found in older homes with split bus panels. Even thought the panel does not have one main disconnecting means, the 2-6 disconnects that are drawing power from the same bus (grouped in one enclosure) are the service disconnecting means as per NEC 230.71(A). Overloading the panel busing is prevented by keeping the load between the supply points. If the existing service disconnects were in separate enclosures, I would say that a added service disconnecting means for PV inverter output connection is located ahead of the service disconnecting means per NEC 690.64 (A).

Besides the fact that this isn't what the code says...
The part in red is just wrong. If there is no main breaker, then the only thing preventing the bus from being overloaded by the utility is the load breakers. Their position, and the presence or lack thereof of a PV breaker, is irrelevant.

 

[jaggedben]

I guess the real question here is, is this a supply-side connection ( then 690.64 (A)) would apply, OR, is this a load-side connection ( then 690.64 (B) 1 thru 7 ) would apply. I don't believe the panel configuration would be considered a "split bus panel", It's a 200amp w/ underground service conductors terminating to a bus bar with the capacity of a total of 6 breakers ( no main breaker, and no other bus bars such as would be on a split bus panel ). The PV system terminates to a qty. 2 of these spaces.....so I see this termination as a supply-side connection. Is it correct that each of the 6 breakers would be considered a means of service disconnect, so they aren't on the load-side?

You have this all correct. Either it's on the "supply side of the service disconnecting means", or it's not. If it's on the supply side, then none of the load side rules apply. That's the way the code is written.

Isn't it also correct that the means of the service disconnect is part of the service and considered to be on the line-side of the service. Thanks for all of the responses, I appreciate all of the input...just want to get this right.

I would simply put it this way: If it is not de-energized when the service disconnecting means is opened, then it is on the supply side. It's correct that any busing on the supply-side is considered part of the service equipment.

 

[ggunn]

You have this all correct. Either it's on the "supply side of the service disconnecting means", or it's not. If it's on the supply side, then none of the load side rules apply. That's the way the code is written.



I would simply put it this way: If it is not de-energized when the service disconnecting means is opened, then it is on the supply side. It's correct that any busing on the supply-side is considered part of the service equipment.

Just so I have this straight...

If I have a pad mounted medium voltage to 208/120Y transformer connected to a MLO MDP with no OCP or disconnecting means between them, I can connect any size PV breaker I want up to the size of the service (the rating of the MDP busbars) anywhere I want in the MDP, but if the MDP has a main breaker the 120% rule comes into play, and (assuming that the main breaker rating and the busbar rating are the same) I can only install a PV breaker that is 20% the rating of the MDP and it has to go at the opposite end of the busbar from the main. Correct?

My brain hurts.

 

[GoldDigger]

Just so I have this straight...

If I have a pad mounted medium voltage to 208/120Y transformer connected to a MLO MDP with no OCP or disconnecting means between them, I can connect any size PV breaker I want up to the size of the service (the rating of the MDP busbars) anywhere I want in the MDP, but if the MDP has a main breaker the 120% rule comes into play, and (assuming that the main breaker rating and the busbar rating are the same) I can only install a PV breaker that is 20% the rating of the MDP and it has to go at the opposite end of the busbar from the main. Correct?

My brain hurts.

My brain hurt too, until I realized that in addition to the difference of whether there is a main breaker or not (which has not logical safety effect) there is a difference in what can be done with load supplying breakers in the panel.
If there is a main breaker, the sum of the feeder and branch circuit breakers supplying loads can exceed the rating of the main breaker. This assumes that the calculated or expected load total will still be less than the main breaker rating. So you have to allow for an arbitrary amount of load. Hence the 120%, opposite end rules.
I think that if there is no main breaker, the sum of the load breakers cannot exceed the service rating. (Looking for code section.) That is the practical difference. The panel bus can still be overloaded, in fact even more severely than in the previous case, but only if a non-compliant combination of load breakers is installed in the panel.

 

[jaggedben]

I think that if there is no main breaker, the sum of the load breakers cannot exceed the service rating. (Looking for code section.)

I referenced it a few posts back: 230.90(A)Exception 3
The breakers can exceed the service rating, as long as the calculated load is less. This is only true of services with 2-6 disconnects.

That is the practical difference. The panel bus can still be overloaded, in fact even more severely than in the previous case, but only if a non-compliant combination of load breakers is installed in the panel.

True. And again, the PV is irrelevant to that.

 

[ggunn]

My brain hurt too, until I realized that in addition to the difference of whether there is a main breaker or not (which has not logical safety effect) there is a difference in what can be done with load supplying breakers in the panel.
If there is a main breaker, the sum of the feeder and branch circuit breakers supplying loads can exceed the rating of the main breaker. This assumes that the calculated or expected load total will still be less than the main breaker rating. So you have to allow for an arbitrary amount of load. Hence the 120%, opposite end rules.
I think that if there is no main breaker, the sum of the load breakers cannot exceed the service rating. (Looking for code section.) That is the practical difference. The panel bus can still be overloaded, in fact even more severely than in the previous case, but only if a non-compliant combination of load breakers is installed in the panel.

Soooo... I'll take that as a yes?

 

[GoldDigger]

I referenced it a few posts back: 230.90(A)Exception 3
The breakers can exceed the service rating, as long as the calculated load is less. This is only true of services with 2-6 disconnects.
True. And again, the PV is irrelevant to that.

And any problems associated with bus overloads involving additional current from the PV will only happen if too much load is added, with either a MLO or MCB panel. But if it is an MCB panel, application of the 120% rule will prevent bus overloads resulting from non-compliant loads, while with the MLO panel it simply does not apply.
One serious concern is the effect of non-bolted fault currents on the branch or feeder side instead of normal loads. In those cases the MCB/120% setup will be unconditionally safe with respect specifically to the panel bus loading, while the MLO setup will not be. But hopefully such conditions will be transient only.
To me that is a concise summary of the practical implications of the whole discussion. YMMV.