120% rule for load side interconnection using combiner box

[PVMatt]

Hi, I hope you can clarify ones and forever for me/us the following situation:

Let’s assume we have a system with 30 Enphase M250 (10 per branch). We use a combiner box and land each branch with a 20 amp breaker in that combiner box. From there we run one conductor to the main service panel (200 amp bus and 200 amp main breaker) and land this conductor on bottom of el. panel with a labeled “solar” breaker.
Can this “solar” breaker be rated at 40 amps (3x10x1.25) OR do we have to use a 60 amp breaker to do interconnection in main service panel?
You know where I am going with this; 60 amp will require an el. panel upgrade.

In other words; does the 120% rule apply to the max current possible from pv system or is it the sum of breaker rating in combiner box?

To what NEC code do I refer when I have this discussion with the inspector? We have several inspection failed because they summed up the breaker rating and failed us for over amping the el. panel. Here in Florida we still use NEC2011 but are adopting to 2014 now.


Much appreciated,

 

[Carultch]

Hi, I hope you can clarify ones and forever for me/us the following situation:

Let’s assume we have a system with 30 Enphase M250 (10 per branch). We use a combiner box and land each branch with a 20 amp breaker in that combiner box. From there we run one conductor to the main service panel (200 amp bus and 200 amp main breaker) and land this conductor on bottom of el. panel with a labeled “solar” breaker.
Can this “solar” breaker be rated at 40 amps (3x10x1.25) OR do we have to use a 60 amp breaker to do interconnection in main service panel?
You know where I am going with this; 60 amp will require an el. panel upgrade.

In other words; does the 120% rule apply to the max current possible from pv system or is it the sum of breaker rating in combiner box?

To what NEC code do I refer when I have this discussion with the inspector? We have several inspection failed because they summed up the breaker rating and failed us for over amping the el. panel. Here in Florida we still use NEC2011 but are adopting to 2014 now.


Much appreciated,

In 2014, it is 125% the sum of the inverter output currents. So it is the value that you calculate before rounding up to an available breaker.

If you combine in a dedicated panel first, aggregated onto a main circuit, you do not need to accumulate rounding errors, once you pass through the subpanel's main breaker.

 

[PVMatt]

Thanks, But...

Thanks, But...

In 2014, it is 125% the sum of the inverter output currents. So it is the value that you calculate before rounding up to an available breaker.

If you combine in a dedicated panel first, aggregated onto a main circuit, you do not need to accumulate rounding errors, once you pass through the subpanel's main breaker.

Thanks, but I am still not clear on that. If you look at the example I described above, what would you say is the required minimum breaker I need to use at the MEP? Note: I am not useing a main breaker in the sub panel. Just combining the 3 20 amp breakers on the bus bar and run a conductor from there to main electrical panel(MEP).

 

[Carultch]

Thanks, but I am still not clear on that. If you look at the example I described above, what would you say is the required minimum breaker I need to use at the MEP? Note: I am not useing a main breaker in the sub panel. Just combining the 3 20 amp breakers on the bus bar and run a conductor from there to main electrical panel(MEP).

If you are combining three 20A circuits, each of which only "needs to be" a 16A circuit, then you would only need a 50A circuit for the aggregate power of the system.

By "needs to be a 16A circuit", what I mean is that 125% of the continuous current is 16A. You'd have to use a 20A circuit, because that is what is available. But you don't need to accumulate the rounding errors for the aggregate system output, when you combine three of them, if you use a master OCPD. The master OCPD could be a breaker at the interconnection point, a fused disconnect in the middle of the circuit, and/or a main breaker in the dedicated PV panelboard.

 

[PVMatt]

If you are combining three 20A circuits, each of which only "needs to be" a 16A circuit, then you would only need a 50A circuit for the aggregate power of the system.

By "needs to be a 16A circuit", what I mean is that 125% of the continuous current is 16A. You'd have to use a 20A circuit, because that is what is available. But you don't need to accumulate the rounding errors for the aggregate system output, when you combine three of them, if you use a master OCPD. The master OCPD could be a breaker at the interconnection point, a fused disconnect in the middle of the circuit, and/or a main breaker in the dedicated PV panelboard.

Thanks. I other words, if I satisfy the 125% rule on my AC Sub-Panel (in our example I would combine all three branches (20amp breaker each) in a 150amp rated sub-panel with a 60amp main breaker, and than interconnect this combined conductor to the main service panel, will I have to apply the 125% rule at this interconnection point again? Meaning I have to use a 60 amp breaker on a at least 275amp main service panel? Whats the NEC code reference for that?

 

[Carultch]

Thanks. I other words, if I satisfy the 125% rule on my AC Sub-Panel (in our example I would combine all three branches (20amp breaker each) in a 150amp rated sub-panel with a 60amp main breaker, and than interconnect this combined conductor to the main service panel, will I have to apply the 125% rule at this interconnection point again? Meaning I have to use a 60 amp breaker on a at least 275amp main service panel? Whats the NEC code reference for that?

In your AC Combiner, you are better off using the rule that states "the sum of the overcurrent devices excluding the main supply shall not exceed the ampacity of the busbar". It is overkill to use any panel with a busbar more than 100A in your application. 705.12(D)(2)(3)(c)

At your point of interconnection, that is where you use the 120% rule. 705.12(D)(2)(3)(b). If you have a 200A panelboard busbar, and 60A of a fully utilized PV interconnection breaker, your main breaker needs to be brought down to 175A (proven with a load calculation or meter interval data). Given a 200A main panel with a 200A main breaker, you are limited to 40A of interconnnection breaker (32A of operating current). The 120% rule applies at all panelboards from the point of interconnection to the service point, so it is in your interest to connect at as main as possible of a panelboard. If you interconnect at a subpanel with both loads and the PV source, you have to consider the possibility that all of its local load temporarily diminishes to zero when the PV system is at full power. The full power will then flow to the main panel, and that main panel has to have a code compliant interconnection.

 

[PVMatt]

In your AC Combiner, you are better off using the rule that states "the sum of the overcurrent devices excluding the main supply shall not exceed the ampacity of the busbar". It is overkill to use any panel with a busbar more than 100A in your application. 705.12(D)(2)(3)(c)

At your point of interconnection, that is where you use the 120% rule. 705.12(D)(2)(3)(b). If you have a 200A panelboard busbar, and 60A of a fully utilized PV interconnection breaker, your main breaker needs to be brought down to 175A (proven with a load calculation or meter interval data). Given a 200A main panel with a 200A main breaker, you are limited to 40A of interconnnection breaker (32A of operating current). The 120% rule applies at all panelboards from the point of interconnection to the service point, so it is in your interest to connect at as main as possible of a panelboard. If you interconnect at a subpanel with both loads and the PV source, you have to consider the possibility that all of its local load temporarily diminishes to zero when the PV system is at full power. The full power will then flow to the main panel, and that main panel has to have a code compliant interconnection.

THANK YOU-I think I got it now