Solar Backfeed Issue

[david]

Let me try putting the above in a different way:

In a service panel with multiple breakers, we accept that there is no single breaker that is protecting the panel bus and rely on the NEC service size calculation to protect it. Adding PV breakers doesn't change that.

In all other panels, we require the bus to be protected by a breaker. Adding a PV source breaker to such a panel could circumvent the bus protection provided by that breaker, so that possibility is regulated by the 120% rule.

At least, that is my understanding of the rules.

Cheers, Wayne

i think this goes to my point that a main lug panel arrangement really is not addressed in the rules

So current flowing through a main breaker to a bus that has PV source connections needs certain breaker/buss arrangements and 120% consideration to prevent two sources of current flowing to overload (over supply ) a bus.

Remove the main breaker, supply the bus with a main lug and current flow is now going to be different
( of no concern) since it does not have to pass through a main breaker to the buss.

is there somehow a big difference on how the current reacts just because it passes through a main breaker

I guess I’m not able to see why the buss concerns would not be the same in both.

If the answer is there is no concern for overloading the buss for a main lug supplied panel (protected by sizing for calculated load) then there should be equal or less concern for a main breaker supplied panel, which is also sized for calculated load

In all other panels, we require the bus to be protected by a breaker. Adding a PV source breaker to such a panel could circumvent the bus protection provided by that breaker, so that possibility is regulated by the 120% rule.At least, that is my understanding of the rules.

as an explanation to explaining why the rule is in the code that could make sense, but you guys have been pretty diffident that the buss is already protected from overload.

 

[GoldDigger]

IMHO an MLO panel is perfectly well covered.
If it is a service panel the above discussion applies.
If it is an MLO sub then you use the feeder breaker in the 120% rule calculation.

 

[david]

Let me try putting the above in a different way:

In a service panel with multiple breakers, we accept that there is no single breaker that is protecting the panel bus and rely on the NEC service size calculation to protect it. Adding PV breakers doesn't change that.

In all other panels, we require the bus to be protected by a breaker. Adding a PV source breaker to such a panel could circumvent the bus protection provided by that breaker, so that possibility is regulated by the 120% rule.

At least, that is my understanding of the rules.

Cheers, Wayne

I’m willing to except that it just the bell that Don keeps ringing cautioning that most distribution panels are not service rated unless they are supplied through a main breaker. The important's of his caution just became that much clearer.

 

[wwhitney]

So current flowing through a main breaker to a bus that has PV source connections needs certain breaker/buss arrangements and 120% consideration to prevent two sources of current flowing to overload (over supply ) a bus.

Remove the main breaker, supply the bus with a main lug and current flow is now going to be different
( of no concern) since it does not have to pass through a main breaker to the buss.

is there somehow a big difference on how the current reacts just because it passes through a main breaker

The oddity is that the NEC allows services without a main breaker. Such a service is clearly less protected than one with a main breaker. I'm not clear why this is allowed, but it is.

Given that, a main breaker protected service is protected from overload by the main breaker and the calculation. The 120% rule on PV breakers addresses the possibility the additional source of power would undermine the protection from overload provided by the main breaker.

On the other hand a MLO panel service with multiple breakers is only protected from overload by calculation. Adding PV breakers isn't going to reduce or undermine that protection, so there is no need for a 120% rule.

Lastly, consider a service where the service lateral conductors enter a gutter with multiple "taps" to main breaker service panels. A PV disconnect with breaker is set next to these service panels and the PV conductors run to another tap in the gutter. This is definitely an acceptable arrangement with a line side PV connection, yes? This electrical arrangement of service conductors is identical to using a MLO panel instead of the gutter, the only difference is whether the conductors are individual wires or busbars.

Cheers, Wayne

 

[ggunn]

Lastly, consider a service where the service lateral conductors enter a gutter with multiple "taps" to main breaker service panels. A PV disconnect with breaker is set next to these service panels and the PV conductors run to another tap in the gutter. This is definitely an acceptable arrangement with a line side PV connection, yes?

Maybe. Some AHJ's won't let you use a breaker, even an enclosed lockable one, as a disconnecting means for a supply side interconnection.

 

[david]

The oddity is that the NEC allows services without a main breaker. Such a service is clearly less protected than one with a main breaker. I'm not clear why this is allowed, but it is.Given that, a main breaker protected service is protected from overload by the main breaker and the calculation. The 120% rule on PV breakers addresses the possibility the additional source of power would undermine the protection from overload provided by the main breaker.On the other hand a MLO panel service with multiple breakers is only protected from overload by calculation. Adding PV breakers isn't going to reduce or undermine that protection, so there is no need for a 120% rule.Lastly, consider a service where the service lateral conductors enter a gutter with multiple "taps" to main breaker service panels. A PV disconnect with breaker is set next to these service panels and the PV conductors run to another tap in the gutter. This is definitely an acceptable arrangement with a line side PV connection, yes? This electrical arrangement of service conductors is identical to using a MLO panel instead of the gutter, the only difference is whether the conductors are individual wires or busbars.
Cheers, Wayne

The oddity is that the NEC allows services without a main breaker. Such a service is clearly less protected than one with a main breaker. I'm not clear why this is allowed, but it is. Given that, a main breaker protected service is protected from overload by the main breaker and the calculation. The 120% rule on PV breakers addresses the possibility the additional source of power would undermine the protection from overload provided by the main breaker.

On the other hand a MLO panel service with multiple breakers is only protected from overload by calculation. Adding PV breakers isn't going to reduce or undermine that protection, so there is no need for a 120% rule. Cheers, Wayne

I know I get it, In both of these situations service rating sized for calculated load.

In the main breaker panel the amount of allowable load side connected PV source normally is in the 20% range of the main breaker that is sized close to the buss rating of the panel current flow on the bus is based on the load. The amount of PV current is kept in check by the 20 % rule so if calculated load is ignored and a load is added to the bus that exceeds the service sizing design the most current support the increased load can get from the PV source would be in the 20% range . Current in both directions onto the buss around 80% from the utility and 20% from the PV source. When current on the buss reaches the range of 120% of the main breaker rating the main should open and current flow from both sources is interrupted.

.Lastly, consider a service where the service lateral conductors enter a gutter with multiple "taps" to main breaker service panels. A PV disconnect with breaker is set next to these service panels and the PV conductors run to another tap in the gutter. This is definitely an acceptable arrangement with a line side PV connection, yes? This electrical arrangement of service conductors is identical to using a MLO panel instead of the gutter, the only difference is whether the conductors are individual wires or busbars. Cheers, Wayne

Current flow is in one direction through the mains so each bus current is limited by the over current breaker ahead of the buss. The PV source contribution can equal the service rating. Line side connection rules seem to make the assumption that current flow will be in one direction to the distribution panels. The PV source current to the utility grid does not pass through load distribution equipment

On the other hand a MLO panel service with multiple breakers is only protected from overload by calculation. Adding PV breakers isn't going to reduce or undermine that protection, so there is no need for a 120% rule. Cheers, Wayne

But is also unique when defined as a line side connection. The line side PV source connection point directly to the buss being the only circumstance on a line side connection current flow is in both directions on the buss. No code mandated guidelines to where the PV source breakers should be installed on the buss. Perhaps if the PV source was tied to the middle of the buss it would make little difference or to the top of the buss close to the main lugs.

100 Amp main lug panel 100 amp service.
We supply 100 amp buss in a panel with 100 amps of PV source line side rules limitation, in the middle of the panels buss.

We have a pretty constant current flowing to the utility grid on the 100 amp rated buss , perhaps in the 100 amp range, the buss is supplying calculated load, the difference I see the PV current on the buss is not just flowing toward calculated load it is also flowing toward the utility grid load. by the line side load rules it could be 100 amps only limited by the service rating size.

I do not claim to be an expert as you guys may be, so maybe all this doesn’t matter, it just that the line side connection rules didn’t seem like they were intending for that much PV current to be flowing on a buss that was also supplying load to branch circuits and feeders.

Anyhow I think we all have come to an agreement there are code issues that need to be addressed with what the OP posted and the concerns the OP had with that installation involving a main lug panel for the service disconnect.

 

[ggunn]

Anyhow I think we all have come to an agreement there are code issues that need to be addressed with what the OP posted and the concerns the OP had with that installation involving a main lug panel for the service disconnect.

My main concern is that when the conductors were rerouted they turned the load breaker that now feeds what was the service panel into service equipment. Is it listed for that use? Does its AIC rating meet the available fault current from the transformer?

 

[jaggedben]

is there somehow a big difference on how the current reacts just because it passes through a main breaker

I guess I’m not able to see why the buss concerns would not be the same in both.

It's not the current flow that's different it's the rules.

An MLO service panel is restricted in what breakers can be put in it. You can have no more than six and the calculated load must be less than the rating of the service. This protects the busbars according to the NEC and the PV does nothing to undermine that concept.

An MLO subpanel elsewhere does not have any such restrictions, and is supposed to be protected by the main breaker ahead of it. Otherwise it can be filled chock full with load breakers more than it's rated for. So adding PV undermines the concept of how the busbar is protected, and that's why we have restrictions on how much PV can be added and where.





If the answer is there is no concern for overloading the buss for a main lug supplied panel (protected by sizing for calculated load) then there should be equal or less concern for a main breaker supplied panel, which is also sized for calculated load



as an explanation to explaining why the rule is in the code that could make sense, but you guys have been pretty diffident that the buss is already protected from overload.[/QUOTE]

...
I do not claim to be an expert as you guys may be, so maybe all this doesn’t matter, it just that the line side connection rules didn’t seem like they were intending for that much PV current to be flowing on a buss that was also supplying load to branch circuits and feeders.

The only line side rule is that the PV can't exceed the service.

Aren't you an inspector? You should try to be an expert...

Anyhow I think we all have come to an agreement there are code issues that need to be addressed with what the OP posted and the concerns the OP had with that installation involving a main lug panel for the service disconnect.

I don't exactly agree. To me the main issue is that they reconfigured a meter/main combo in a fashion that probably violates the listing and where everything cannot be properly bonded and separated without destroying factory assembled components.

They seemingly also used an MLO panel that was not service rated, or not service rated if used as an MLO. A lot of MLO panels do require in their instructions that a main breaker is installed with a hold down if its used as service equipment. The OP never actually confirmed this was the case though.

 

[jaggedben]

By the way here is an example of an MLO meter main combo that is intended as service equipment. The instructions for this model state to install no more than six circuit breakers. Any PV breaker installed in this panel is a supply side connection, and it doesn't matter where it goes.

 

[david]

.

I'm certainly not defining the solar connections as load side in the OP's picture. I've said exactly the opposite. Your last sentence is correct in my opinion, although some would debate whether the solar breakers count as 'service disconnects'. They are, at any rate, not on the load side of a service disconnecting means, so none of the load side rules apply, including the 120% rule.

Whether the two solar handles count towards the six handle for service disconnects is a more open question. People have argued for many pages on this forum about whether the solar breakers would count as service disconnecting means.

Let's not get into the discussion of whether PV counts towards the six handles. It's not at issue here and really isn't as clear in the code.

An MLO service panel is restricted in what breakers can be put in it. You can have no more than six and the calculated load must be less than the rating of the service. This protects the busbars according to the NEC and the PV does nothing to undermine that concept.

By the way here is an example of an MLO meter main combo that is intended as service equipment. The instructions for this model state to install no more than six circuit breakers. Any PV breaker installed in this panel is a supply side connection, and it doesn't matter where it goes.

690.14 (4)“Maximum Number of Disconnects. The photovoltaic system disconnecting means shall consist of not more than six switches or six circuit breakers mounted in a single enclosure, in a group of separate enclosures, or in or on a switchboard.”

So if a put all this together I could expect to see for example most likely commercial type service rated type MDP, not the residential type MLO panel service rated requiring a main breaker to be in compliance with the service rating. And not the one in your last picture that states no more than six breakers allowed.

Commercial MDP I could expect to see up to six 230.82 utility service disconnects and up to six 690.14(4) PV source disconnects in a main lug MDP and this is line side connection the PV source current can equal the service rating I can look at that and walk away with out any concern?

 

[ggunn]

. Commercial MDP I could expect to see up to six 230.82 utility service disconnects and up to six 690.14(4) PV source disconnects in a main lug MDP and this is line side connection the PV source current can equal the service rating I can look at that and walk away with out any concern?

Whether PV handles count toward the six maximum utility service discos is a matter of interpretation and the details should be hashed out with your AHJ beforehand. As I have previously said, for example, one AHJ I work with doesn't count a PV handle as contributing to the six handle rule but they will not allow the PV to land on a breaker in an MLO MDP if there are already six service breakers in the panel. I haven't asked them if their position would be the same if there weren't already six handles in the MDP but reading through their responses to me it appears that it wouldn't change their position.

 

[wwhitney]

100 Amp main lug panel 100 amp service.
We supply 100 amp buss in a panel with 100 amps of PV source line side rules limitation, in the middle of the panels buss.

We have a pretty constant current flowing to the utility grid on the 100 amp rated buss , perhaps in the 100 amp range, the buss is supplying calculated load, the difference I see the PV current on the buss is not just flowing toward calculated load it is also flowing toward the utility grid load. by the line side load rules it could be 100 amps only limited by the service rating size.

If you have a 100 amp service with a 100 amp MLO panel bus, loads that draw no more than 100 amps, and 100 amps of PV source connected, there is no way to overload the bus. Any arrangement of the load breakers and PV breakers will still give you less than 100 amps flowing through the bus at every point on the bus.

Cheers, Wayne

 

[wwhitney]

Maybe. Some AHJ's won't let you use a breaker, even an enclosed lockable one, as a disconnecting means for a supply side interconnection.

Do they have an NEC reference, or is this a local amendment?

Cheers, Wayne

 

[ggunn]

Do they have an NEC reference, or is this a local amendment?

Cheers, Wayne

It's a decision by the chief inspector for the AHJ.

 

[JasonC]

I have seen this done before and here is my biggest concern. The Bus in the new Main Distro Panel has no OCPD, it currently has unlimited potential from the utility. Conductors are coming from the meter into the MLO, then to the old main panel. That is a major hazard for life safety.

 

[jaggedben]

I have seen this done before and here is my biggest concern. The Bus in the new Main Distro Panel has no OCPD, it currently has unlimited potential from the utility. Conductors are coming from the meter into the MLO, then to the old main panel. That is a major hazard for life safety.

It's no different from all the services in the world that have no OCPD ahead of the service disconnect. It's well within NEC rules.

 

[ggunn]

It's no different from all the services in the world that have no OCPD ahead of the service disconnect. It's well within NEC rules.

True, but that MLO panel has to be service rated, does it not?

 

[jaggedben]

True, but that MLO panel has to be service rated, does it not?

Yes. Many of them are service rated on condition they are installed with a main breaker. Don't ask me why.

 

[ggunn]

Yes. Many of them are service rated on condition they are installed with a main breaker. Don't ask me why.

If it is behind a breaker why does it have to be service rated?

 

[ggunn]

If it is behind a breaker why does it have to be service rated?

Sorry. I asked you why when you told me not to.