Location Back fed breaker.

[Seekthetruth]

Opposite end of bus in outside meter load center combination or feed through interior panel with household loads?

 

[Dennis Alwon]

IMO, The first method is probably the easiest.

 

[jaggedben]

Not really enough information given. Either could be okay, or not, depending on missing details.

 

[Seekthetruth]

Not really enough information given. Either could be okay, or not, depending on missing details.

What information is missing? 705 D 2 3 b "Opposite end of busbar that contains load" ? What if the panel inside the house contains all of the load

 

[jaggedben]

What information is missing? 705 D 2 3 b "Opposite end of busbar that contains load" ? What if the panel inside the house contains all of the load

How can there be no loads in the meter main? How is the interior panel being fed? I gather you are sayingit is being fed by feed-through lugs in the meter main. That's usually a problem, since the feeder would in that case need to be sized for the combined current of the main and the PV. A solution could be to downsize the main, but that depends on the size and the load. So missing information in that case would be the size of the feeder wire, size of the main breaker, max output current of the PV system, and calculated load of the home.

 

[Seekthetruth]

How can there be no loads in the meter main? How is the interior panel being fed? I gather you are sayingit is being fed by feed-through lugs in the meter main. That's usually a problem, since the feeder would in that case need to be sized for the combined current of the main and the PV. A solution could be to downsize the main, but that depends on the size and the load. So missing information in that case would be the size of the feeder wire, size of the main breaker, max output current of the PV system, and calculated load of the home.

Can't downsize main because of the load. It is feed through lugs to a 200 amp MLO panel. Should the let's say 40 amp back fed breaker be at the end of the bus of the MLO panel? I guess the "Opposite end of busbar that contains load" needs clarification. Thanks for your help

 

[CS2]

Can't downsize main because of the load. It is feed through lugs to a 200 amp MLO panel. Should the let's say 40 amp back fed breaker be at the end of the bus of the MLO panel? I guess the "Opposite end of busbar that contains load" needs clarification. Thanks for your help

Seems like you want a million dollar answer with a 10 cent question. Please elaborate. We're here to help.
Do you have any photos or an SLD to share?

 

[jaggedben]

Can't downsize main because of the load. It is feed through lugs to a 200 amp MLO panel. Should the let's say 40 amp back fed breaker be at the end of the bus of the MLO panel? I guess the "Opposite end of busbar that contains load" needs clarification. Thanks for your help

With what you've described I'd say yes, it should go at the opposite end of the MLO panel. (40A PV breaker is the biggest you can go with this setup).

 

[jaggedben]

(Duplicate)

 

[Seekthetruth]

With what you've described I'd say yes, it should go at the opposite end of the MLO panel. (40A PV breaker is the biggest you can go with this setup).

Thanks again

 

[Carultch]

What information is missing? 705 D 2 3 b "Opposite end of busbar that contains load" ? What if the panel inside the house contains all of the load

It doesn't matter how much of the service's load the busbar in question contains. The same logic applies whether it contains the whole house load, or whether it only contains about half of the house's load, or whether it just serves a remote building's load. The idea is that you feed the busbar from opposite ends, such that the current diminishes to zero at a point part way, before it locally exceeds the busbar rating.

If only first principles of electricity applied, you might think it could be a 200% rule by the same logic. Because fed from 200A at the top and 200A at the bottom, the current is opposite and cancels out. The reason why it isn't a 200% rule, is the heating among the branch breakers. Feed a 200A panelboard with 200A from both sides, and you have 400A flowing among all the branch breakers, even if it never exceeds 200A on any cross section of busbar. Load panelboards are routinely populated with branch breakers that add up to a lot more than the panel's ampacity, so you don't want the excess heat among the breakers to go "unnoticed" by the main supply. 120% is how this rule was first written, to design against this possibility.

One thing that is inferred, but isn't directly stated in these busbar interconnection rules in 705.12, is that if you are interconnecting anywhere on the load side of any panel's main supply breaker, an interconnection rule for protection of the busbar needs to apply. So if it is connected as a feeder-tap, or at a subpanel, a rule for protecting every busbar encountered upstream of this panel needs to be considered, all the way to the service point. Local load on the subpanel where you are interconnecting may temporarily be zero, while the system is at full capacity. This is why it is good practice to aim for the "main-ist" panel that is practical to use. You also have more ampacity available in the margin of the 120% rule,for system capacity, with larger panelboards.

 

[Seekthetruth]

It doesn't matter how much of the service's load the busbar in question contains. The same logic applies whether it contains the whole house load, or whether it only contains about half of the house's load, or whether it just serves a remote building's load. The idea is that you feed the busbar from opposite ends, such that the current diminishes to zero at a point part way, before it locally exceeds the busbar rating.

If only first principles of electricity applied, you might think it could be a 200% rule by the same logic. Because fed from 200A at the top and 200A at the bottom, the current is opposite and cancels out. The reason why it isn't a 200% rule, is the heating among the branch breakers. Feed a 200A panelboard with 200A from both sides, and you have 400A flowing among all the branch breakers, even if it never exceeds 200A on any cross section of busbar. Load panelboards are routinely populated with branch breakers that add up to a lot more than the panel's ampacity, so you don't want the excess heat among the breakers to go "unnoticed" by the main supply. 120% is how this rule was first written, to design against this possibility.

One thing that is inferred, but isn't directly stated in these busbar interconnection rules in 705.12, is that if you are interconnecting anywhere on the load side of any panel's main supply breaker, an interconnection rule for protection of the busbar needs to apply. So if it is connected as a feeder-tap, or at a subpanel, a rule for protecting every busbar encountered upstream of this panel needs to be considered, all the way to the service point. Local load on the subpanel where you are interconnecting may temporarily be zero, while the system is at full capacity. This is why it is good practice to aim for the "main-ist" panel that is practical to use. You also have more ampacity available in the margin of the 120% rule,for system capacity, with larger panelboards.

Thanks for your input. So do you agree with Jaggedben about going on the end of the MLO panel fed by feed through lugs?

 

[Carultch]

Thanks for your input. So do you agree with Jaggedben about going on the end of the MLO panel fed by feed through lugs?

MLO panels are a tricky situation, when there is no main supply OCPD. If the main OCPD is externalized, the external main OCPD rating acts as if it were a main breaker for the 120% rule. But what if there is no main OCPD protecting the panel as a whole?

It used to be allowed that you could have up to 6 service disconnects as branch breakers in an MLO panel. In concept, you could have 5 existing service disconnects in the same MLO panel, and the interconnected source would be the 6th service disconnect. The only busbar protection rule that could apply, would be the rule about adding up the branch breakers, limited to the busbar rating. It is plausible that the branch breakers as main disconnects would already add up to the busbar rating, giving no headroom to interconnect.

You can no longer do that for your service panel as of NEC 2020. The new rules for service disconnects require them to either be in dedicated enclosures, or in dedicated partitioned sections of equipment such as a switchboard. How to work with an existing grandfathered situation, isn't specified.

I'm not entirely sure what rules would govern how to use feed-thru lugs, but given an interconnection strategy through subfeed lugs, it would work just like any other 120% rule application. As an example, consider a 600A panel/600A main, with a desire to backfeed a 100A system. Put a 100A fused disconnect adjacent to it, and connect it to the subfeed lugs. Does one need 600A worth of wire between these units, or could a 10 ft tap rule be allowed to apply?

 

[GoldDigger]

MLO panels are a tricky situation, when there is no main supply OCPD. If the main OCPD is externalized, the external main OCPD rating acts as if it were a main breaker for the 120% rule. But what if there is no main OCPD protecting the panel as a whole?

It used to be allowed that you could have up to 6 service disconnects as branch breakers in an MLO panel. In concept, you could have 5 existing service disconnects in the same MLO panel, and the interconnected source would be the 6th service disconnect. The only busbar protection rule that could apply, would be the rule about adding up the branch breakers, limited to the busbar rating. It is plausible that the branch breakers as main disconnects would already add up to the busbar rating, giving no headroom to interconnect.

You can no longer do that for your service panel as of NEC 2020. The new rules for service disconnects require them to either be in dedicated enclosures, or in dedicated partitioned sections of equipment such as a switchboard. How to work with an existing grandfathered situation, isn't specified.

I'm not entirely sure what rules would govern how to use feed-thru lugs, but given an interconnection strategy through subfeed lugs, it would work just like any other 120% rule application. As an example, consider a 600A panel/600A main, with a desire to backfeed a 100A system. Put a 100A fused disconnect adjacent to it, and connect it to the subfeed lugs. Does one need 600A worth of wire between these units, or could a 10 ft tap rule be allowed to apply?

If there is no main OCPD protecting the panel, either in the panel or upstream, then the panel bus conductors must be service conductors and the back fed breaker would be a line-side tap. For that case, the limit on PV backfeed is the rating of the bus bar or the nominal capacity of the service, whichever is smaller.

 

[GoldDigger]

...
...
It used to be allowed that you could have up to 6 service disconnects as branch breakers in an MLO panel. In concept, you could have 5 existing service disconnects in the same MLO panel, and the interconnected source would be the 6th service disconnect. The only busbar protection rule that could apply, would be the rule about adding up the branch breakers, limited to the busbar rating. It is plausible that the branch breakers as main disconnects would already add up to the busbar rating, giving no headroom to interconnect.
...
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I would argue that in the case you propose, the sum of the branch breakers is a limit on the load connections to the bus bar, and regardless of whether the incoming power is that passed by the POCO service or the sum of the POCO service and the PV backfeed the bus is still effectively protected. If you somehow ended up needing to determine the maximum fault current available for SCCR ratings, etc., the 200A supplied by the PV would be negligible, even without considering that the grid interactive inverter would very quickly stop sourcing current into the fault.

 

[jaggedben]

I'm not entirely sure what rules would govern how to use feed-thru lugs, but given an interconnection strategy through subfeed lugs, it would work just like any other 120% rule application. As an example, consider a 600A panel/600A main, with a desire to backfeed a 100A system. Put a 100A fused disconnect adjacent to it, and connect it to the subfeed lugs. Does one need 600A worth of wire between these units, or could a 10 ft tap rule be allowed to apply?

The 2020 NEC addresses feed through lugs explicitly. (Previous code cycles did not.) I think I can take some credit for submitting a public input that resulted in a sensible rule, although I admit that working through the logic of it might be daunting for those not previously familiar with Article 705.

The section is 705.12(B)(3)(6). It reads as follows:

Connections shall be permitted on busbars of panelboards that supply lugs connected to feed-through conductors. The feed-through conductors shall be sized in accordance with 705.12(B)(1). Where an overcurrent device is installed at the supply end of the fed-through conductors, the busbar in the supplying panelboard shall be permitted to be sized in accordance with 705.12(B)(3)(1) through 705.12(B)(3)(3).

705.12(B)(1) is the rule for connecting through a feeder tap.
705.12(B)(3)(1) - (3) are the rules for connecting via a breaker in a panelboard.
So the upshot is that it is saying that if you have a nearby fused disconnect for the feeder, as you suggest, the lugs can be treated like they are a breaker. If not, they must be either opposite end, or the feeder must be sized for the sum of all possible currents.

@Seekthetruth I should clarify that my answer in post #8 is dependent on the feed-through lugs being at the opposite end of any busbar that the main breaker is feeding. I should have mentioned that explicitly. Hopefully it was a reasonable assumption since that is the configuration I've usually seen on 200A panels.