CodeQuandary
Member
- Location
- Washington DC
I want to be sure I understand this point. Here's what I believe to be true.
With a 200A service and 200A panel/bus, it would not be possible to add any PV EXCEPT with this provision. The maximum would be 40A (200*1.2 - 200) located at the far end of the bus from the service.
With a 200A service and a 225A panel/bus, it would be possible to add a 25A OCPD for PV anywhere in the panel (the 100% "rule": 225 - 200), or 70A at the far end of the bus (225*1.2 - 200).
Please clarify any mistakes I've made.
I'm also in the midst of a knock-down-drag-out about the "bottom of the bus" point. His point is that it doesn't make sense, there's no way that position on the bus makes any difference, so long as the bus is rated sufficient.
Here's the only logic I've come up with: a panel can be (and routinely is) "overloaded", that is, due to load calculations etc. the total of load OCPD's in a panel exceed the service/main breaker. Generally that shouldn't matter, in a worst-case scenario the main breaker protects the bus from overload/overheating (due to the fact that the rating of the bus is at least as large as the main). HOWEVER: adding another supply (PV) means that any loads AFTER the PV breaker (on the other side of it from the main service) can draw from both. In the extreme case, consider putting ALL the loads after the PV breaker (so PV is at the top of the bus): they can draw up to the total of PV + service. Which, unless one is sticking with the 100% rule (no PV breaker greater than the bus rating - service), will overload the bus (in this extreme example).
That's good as far as it goes. Moving the PV to the bottom of the bus helps resolve that, but is there any particular justification to the 20% excess (that is, the 120% rule) specifically? Is it just a rule of thumb style number? I don't quite see why it's not 150%, or 200%? Is it just a safety margin thing, and is there any reason for THAT safety margin?
Thanks so much for any help getting this nailed down. I'm new to this level of analysis, so any help with concepts OR jargon/languaging will be appreciated. I need to be able to explain this stuff coherently. For example, in talking with another inspector, I mistakenly called the PV OCPD a "load", and he bristled, and thought it was wiser to call it a disconnect or supply OCPD so as not to confuse the issue. Helpful clarification...
With a 200A service and 200A panel/bus, it would not be possible to add any PV EXCEPT with this provision. The maximum would be 40A (200*1.2 - 200) located at the far end of the bus from the service.
With a 200A service and a 225A panel/bus, it would be possible to add a 25A OCPD for PV anywhere in the panel (the 100% "rule": 225 - 200), or 70A at the far end of the bus (225*1.2 - 200).
Please clarify any mistakes I've made.
I'm also in the midst of a knock-down-drag-out about the "bottom of the bus" point. His point is that it doesn't make sense, there's no way that position on the bus makes any difference, so long as the bus is rated sufficient.
Here's the only logic I've come up with: a panel can be (and routinely is) "overloaded", that is, due to load calculations etc. the total of load OCPD's in a panel exceed the service/main breaker. Generally that shouldn't matter, in a worst-case scenario the main breaker protects the bus from overload/overheating (due to the fact that the rating of the bus is at least as large as the main). HOWEVER: adding another supply (PV) means that any loads AFTER the PV breaker (on the other side of it from the main service) can draw from both. In the extreme case, consider putting ALL the loads after the PV breaker (so PV is at the top of the bus): they can draw up to the total of PV + service. Which, unless one is sticking with the 100% rule (no PV breaker greater than the bus rating - service), will overload the bus (in this extreme example).
That's good as far as it goes. Moving the PV to the bottom of the bus helps resolve that, but is there any particular justification to the 20% excess (that is, the 120% rule) specifically? Is it just a rule of thumb style number? I don't quite see why it's not 150%, or 200%? Is it just a safety margin thing, and is there any reason for THAT safety margin?
Thanks so much for any help getting this nailed down. I'm new to this level of analysis, so any help with concepts OR jargon/languaging will be appreciated. I need to be able to explain this stuff coherently. For example, in talking with another inspector, I mistakenly called the PV OCPD a "load", and he bristled, and thought it was wiser to call it a disconnect or supply OCPD so as not to confuse the issue. Helpful clarification...