Connecting to a Subpanel (Fundamental Confusion)

[Andrew Catamount]

Greetings, ( I have been working in PV for over 2- years and started working as a residential designer)

I have a basic question regarding PV connections to subpanels... and where to find the supporting information...

My example: PV going on garage roof. There is a 125a subpanel with a 60a feed. This is coming from a 200amp main panel with a 200amp main..

The continuous output of the inverter requires a 40 amp breaker at the subpanel... The sales team new immediately that we were o.k at the subpanel... can you explain to me the connection requirements when considering subpanels... I recall one situation where there were 3 subpanels and the main... Id like to know how to analyze these scenarios in the future...


I understand the 120% rule but have some fundamental confusion regarding subpanels and feeders... essentially

Thanks in advance

 

[Carultch]

Greetings, ( I have been working in PV for over 2- years and started working as a residential designer)

I have a basic question regarding PV connections to subpanels... and where to find the supporting information...

My example: PV going on garage roof. There is a 125a subpanel with a 60a feed. This is coming from a 200amp main panel with a 200amp main..

The continuous output of the inverter requires a 40 amp breaker at the subpanel... The sales team new immediately that we were o.k at the subpanel... can you explain to me the connection requirements when considering subpanels... I recall one situation where there were 3 subpanels and the main... Id like to know how to analyze these scenarios in the future...


I understand the 120% rule but have some fundamental confusion regarding subpanels and feeders... essentially

Thanks in advance

The 120% rule applies to all subpanels that have both loads and PV source.

This applies to the first subpanel at the point of interconnection, and all sub & main panels upstream from it. So it is generally an advantage to interconnect at the most-main possible panel, so you don't need to repeat this constraint.

The way that the 120% rule works, is that PV breaker + Main supply breaker is limited to 120% of the busbar rating. In NEC2014, they revised the text so that you don't need to let rounding errors be a showstopper. Instead of PV interconnection breaker rating, you use the calculation result that selects it (125% of PV output current), before rounding it up to the standard size that you actually use.

 

[ggunn]

Greetings, ( I have been working in PV for over 2- years and started working as a residential designer)

I have a basic question regarding PV connections to subpanels... and where to find the supporting information...

My example: PV going on garage roof. There is a 125a subpanel with a 60a feed. This is coming from a 200amp main panel with a 200amp main..

The continuous output of the inverter requires a 40 amp breaker at the subpanel... The sales team new immediately that we were o.k at the subpanel... can you explain to me the connection requirements when considering subpanels... I recall one situation where there were 3 subpanels and the main... Id like to know how to analyze these scenarios in the future...


I understand the 120% rule but have some fundamental confusion regarding subpanels and feeders... essentially

Thanks in advance

Either 125% of the maximum inverter output (NEC 2014) or the value of the first OCPD the inverter is connected to (prior NEC) is used to calculate conformance to the 120% rule in every cascaded panel all the way back to the service. Either 125% of the inverter output or the rating of that first OCPD has to fit into the headroom (difference between the rating of OCPD feeding the bus and 120% of the rating of the bus) of every panel between the inverter and the service.

In your case the subpanel is OK (125A bus fed by 60A breaker has 90A headroom) and the main is OK (200A bus fed by 200A OCPD has 40A headroom).The PV breaker must be at the opposite end of the busbar from the feed in the subpanel, and the feed to the subpanel must be at the opposite end of the busbar from the OCPD in the main panel.