Solar panel interconnection

[shortcircuit1]

Hello,

I am learning a bit about solar panel interconnection with the grid but i am just having trouble at one location.

Lets say you have two strings of panels connected to two inverters. From the inverters they go to a combiner which will have one output which gets connected to a meter and then a disconnect. Everything looks good until here.After this it feeds a panel which is main lugs only. Now my question is there are already wires terminated onto those lugs from utility side. Where would the cables from the disconnect terminate onto cause they are already used by the normal feed?

Second question is lets say you end up with a main backfeed breaker even then how is the termination done on the breaker with two sources of power supply? I was thinking of having an ATS at this point where one source lands to normal side and the other lands to solar side and the load side is connected to the panel.Am i missing something with this idea or is it just not right?

 

[jaggedben]

I think you are overthinking things.

In terms of physical aspects (lugs, breakers, tap connectors, etc.) an interactive inverter can usually be connected just like any similar size circuit would be. If you have a panelboard to connect to, it can go on a circuit breaker. Doesn't matter if the panel is MLO or main breaker. No kind of ATS is necessary for an inverter that is only grid-tied (a.k.a. 'utility interactive').

In terms of overcurrent protection and the code, there are other limits. Read 705, carefully, particularly 705.12

Some other aspects of your questions are hard to comment on without knowing exactly the equipment you are dealing with. For example, whether the MLO panel is service equipment or a subpanel.

 

[shortcircuit1]

I think you are overthinking things.

In terms of physical aspects (lugs, breakers, tap connectors, etc.) an interactive inverter can usually be connected just like any similar size circuit would be. If you have a panelboard to connect to, it can go on a circuit breaker. Doesn't matter if the panel is MLO or main breaker. No kind of ATS is necessary for an inverter that is only grid-tied (a.k.a. 'utility interactive').

In terms of overcurrent protection and the code, there are other limits. Read 705, carefully, particularly 705.12

Some other aspects of your questions are hard to comment on without knowing exactly the equipment you are dealing with. For example, whether the MLO panel is service equipment or a subpanel.

Thanks and i looked at couple topics on internet and got a better understanding. But still i have one question. Lets say you have a solar back feed breaker located at the bottom of the subpanel then how does the load know when to draw from the solar power first before grid power? If the power is coming into the breaker what makes solar power to be the first choice?

 

[jaggedben]

Thanks and i looked at couple topics on internet and got a better understanding. But still i have one question. Lets say you have a solar back feed breaker located at the bottom of the subpanel then how does the load know when to draw from the solar power first before grid power? If the power is coming into the breaker what makes solar power to be the first choice?

The load doesn't need to know. The inverter supplies current synced to the same voltage and frequency as the utility. Once that current reaches a node where flows toward a load, it is indistinguishable from current from the utility. That's the magic. The only kind of device that might 'notice' a difference is a meter on a feeder or service, which is set up to expect current to correspond to voltage in a 'forward' direction, and now might see it 'backwards'.

A key part of the concept here is that extra solar backfeeds the grid, which (so far) has always had enough load to absorb all the current that the solar can put out.

 

[shortcircuit1]

The load doesn't need to know. The inverter supplies current synced to the same voltage and frequency as the utility. Once that current reaches a node where flows toward a load, it is indistinguishable from current from the utility. That's the magic. The only kind of device that might 'notice' a difference is a meter on a feeder or service, which is set up to expect current to correspond to voltage in a 'forward' direction, and now might see it 'backwards'.

A key part of the concept here is that extra solar backfeeds the grid, which (so far) has always had enough load to absorb all the current that the solar can put out.

When does the current flow from solar to grid?Sorry if its a dumb question.

 

[jaggedben]

When does the current flow from solar to grid?Sorry if its a dumb question.

When the solar produces more than the service is using.

 

[ggunn]

I think you are overthinking things.

In terms of physical aspects (lugs, breakers, tap connectors, etc.) an interactive inverter can usually be connected just like any similar size circuit would be. If you have a panelboard to connect to, it can go on a circuit breaker. Doesn't matter if the panel is MLO or main breaker.

Most AHJs I deal with will not allow me to interconnect through a backfed breaker in an MLO panel. Some are OK with IPCs (Insulation Piercing Connectors) on the line side of the panel even if it is MLO, but others will make us install a main breaker irrespective of whether the interconnection is load or line side interconnected.

 

[jaggedben]

Most AHJs I deal with will not allow me to interconnect through a backfed breaker in an MLO panel. Some are OK with IPCs (Insulation Piercing Connectors) on the line side of the panel even if it is MLO, but others will make us install a main breaker irrespective of whether the interconnection is load or line side interconnected.

Yes, well, you are in Texas. :lol::lol: Everywhere else, as far as I know, people basically follow the NEC. I've never had an AHJ not allow me to interconnect to an MLO panel with a breaker.

I think the OP is trying to understand the basic system function as much as AHJ requirements.

 

[shortcircuit1]

Thanks!I think i have a better understanding on the operation of solar and the whole back feed scenario.

 

[Andy Delle]

Thanks and i looked at couple topics on internet and got a better understanding. But still i have one question. Lets say you have a solar back feed breaker located at the bottom of the subpanel then how does the load know when to draw from the solar power first before grid power? If the power is coming into the breaker what makes solar power to be the first choice?

Solar inverters without a backup battery are strict current sources, not voltage sources. They basically push the output voltage up until the current flows to the load which is the panel and the utility. This is why the inverter has to shut down with the utility missing and also why you can't use them with an emergency generator. The inverters that have battery backup and/or the ability to work with a local generator are a different design and more expensive.