Off grid with hybrid

[Pjw88]

Hello im new to solar and have been doing as much learning as possible but I was wondering if someone could shed some light on a question. Currently I’m looking to develop an off grid system without net metering or export and exploring my options. Essentially while I’m interested in off grid options it seems hybrid systems offer more output/deatures in single units however often require supply/line taps or large backfed breakers. My main panel is 200amp which feeds a 100amp MLO subpanel. I believe the bus bar is 200amp rated. Is it possible to use a smaller 40amp breaker for back feed into the inverter which then would connect to the sub panel via transfer switch/interlock with the 100amp? I currently do not have a net metering contract and would be not interested in exporting to the grid.

 

[BentleyJ]

The way the question is worded is not completely clear regarding terminology. Did you wish to use the Off-Grid Inverter to power both the main panel and the subpanel or only the subpanel? Seems kind of odd that a 100A panel would have 200A bus bars maybe a 125A bus bar.

Newer so called solar ready panels sometimes have 225A bus bars with a 200A main (or 125A bus with 100A main) so a larger grid-tie system can be connected per the 120% rule.

 

[Pjw88]

The way the question is worded is not completely clear regarding terminology. Did you wish to use the Off-Grid Inverter to power both the main panel and the subpanel or only the subpanel? Seems kind of odd that a 100A panel would have 200A bus bars maybe a 125A bus bar.

Newer so called solar ready panels sometimes have 225A bus bars with a 200A main (or 125A bus with 100A main) so a larger grid-tie system can be connected per the 120% rule.

Ideally I’d like to use a hybrid (solark 12/15k or eg418k) rather than a true off grid to accept 40amp from main panel into grid input and have the load go to the MLO subpanel via interlock or transfer switch. The reason I need to use 40amp is to avoid overloading the bus bar

 

[BentleyJ]

So that's a straight forward set up. Since the Subpanel is a main lug only would have to install a transfer switch to prevent both sources of power from energizing the subpanel at the same time. A simple way to do that would be with a 60A double throw safety switch.

EDIT: the 100A breaker would have to be reduced to 60A or a larger switch could be used.

The other choice would be get a main breaker kit and breaker interlock accessory so only one breaker could be on at a time.

 

[Pjw88]

So that's a straight forward set up. Since the Subpanel is a main lug only would have to install a transfer switch to prevent both sources of power from energizing the subpanel at the same time. A simple way to do that would be with a 60A double throw safety switch.

EDIT: the 100A breaker would have to be reduced to 60A or a larger switch could be used.

The other choice would be get a main breaker kit and breaker interlock accessory so only one breaker could be on at a time.

Great but would a 40amp breaker for backfeed into the main panel still work to draw from the grid and assist in subpanel if going through an inverter? Would that setup be acceptable or would I absolutely need to upgrade to 60amp feeder breaker thereby requiring me to derate my main breaker to 150-175amp in order to meet nec code? Does the code apply if you don’t intend on back feeding?

 

[BentleyJ]

Lets start with terminology in bold.
The proposed 40A breaker would be connected to the AC Input on the Inverter. It would only be back-feeding if solar was producing more than was being consumed by the sum of loads in the subpanel and battery charging. To prevent grid feed-back (sell back) it is necessary to install CT's on the incoming main breaker L1 & L2. The 120% rule would apply here since the inverter is a 2nd source of power to the main panel.

The Inverter Pass-Through current to the subpanel is reduced to the 40A breaker size.

 

[timselectric]

Does the code apply if you don’t intend on back feeding?

No, but........
According to your statements, you are intending to backfeed.

 

[Pjw88]

Thank you

Lets start with terminology in bold.
The proposed 40A breaker would be connected to the AC Input on the Inverter. It would only be back-feeding if solar was producing more than was being consumed by the sum of loads in the subpanel and battery charging. To prevent grid feed-back (sell back) it is necessary to install CT's on the incoming main breaker L1 & L2. The 120% rule would apply here since the inverter is a 2nd source of power to the main panel.

The Inverter Pass-Through current to the subpanel is reduced to the 40A breaker size.

Thank you for clarification and I believe I understand the concept of backfeed breaker sizing for mitigating overload of the bus bar. But in a situation say with a solark limited to home or any hybrid can guarantee zero energy is fed back into the main panel does this 120% rule still apply?

 

[Pjw88]

Thank you

Thank you for clarification and I believe I understand the concept of backfeed breaker sizing for mitigating overload of the bus bar. But in a situation say with a solark limited to home or any hybrid can guarantee zero energy is fed back into the main panel does this 120% rule still apply?

No, but........
According to your statements, you are intending to backfeed.

understandable. So a 40amp breaker it is unless I derate. Thank you for clarifying.