NEC 2020 705.12(B)(3)(3) Sum Rule

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DTran86

Member
Location
Houston
Occupation
PV + ESS Inspection
Hi everyone,

I wanted to get feedback on the sum rule for PV + ESS systems. Let's say you have a PV+ESS gateway system that is interconnected via a 100A backfed connection at a 225A/200A panel with almost all the loads from MSP moved downstream to a back up panel, sometimes referred to as Hawaiian tie-in. Do you count those loads in the back up panel during calculation of sum rule? Or only the loads in the MSP that are left with your PV+ESS source?
 
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Carultch

Senior Member
Location
Massachusetts
DTran86 said:
Hi everyone,

I wanted to get feedback on the sum rule for PV + ESS systems. Let's say you have a PV+ESS gateway system that is interconnected via a 100A backfed connection at a 225A/200A panel with almost all the loads from MSP moved downstream to a back up panel, sometimes referred to as Hawaiian tie-in. Do you count those loads in the back up panel during calculation of sum rule? Or only the loads in the MSP that are left with your PV+ESS source?
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No, you do not need to count breakers in a subpanel of the panelboard in question. Once all those breakers are behind a breaker protecting the feeder, only the breaker protecting the feeder counts toward this sum. An example of where you would need to count subpanel breakers individually for this rule, is if the panel in question had unprotected subfeed lugs, that feed a main-lug-only subpanel.

The underlying theory behind this rule, is that no matter what the mixture of sources and loads, the maximum you can supply to this panelboard, or draw from this panelboard, is the sum of its branch breakers on each of its busbars. It is a busbar-by-busbar sum, so two otherwise-identical single pole breakers on opposite busbars, only count once. You have a sum for the black wire busbar, and a sum for the red wire busbar, and whichever is larger, is what governs.
 
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