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Electrician
- Location
- Southern California
- Occupation
- Licensed Electrician and General Contractor
I’m a little confused on 704.12B(3)(3)’s intent when we factor in a BESS.
Scenario: 400A MSP with 400A main breaker.
Battery backup IQ system controller 2 (Enphase in this case) is fed by a 200A breaker off of the 400A bus.
Existing loads (280A) remain on the 400A bus and backup loads are moved to a new backup loads panel.
The BESS and PV are 83.5A after multiplying the inverter output of both by 125%.
The sum of all breakers rule says not compliant because the existing loads remaining on the 400A bus (280A) plus the 200A backup system breaker are > 400A. But are they really? The inverter max continuous output is 83.5A. There is no scenario where more than 400A could ever pass through the bus bar.
If the 280A loads were at max, and the backup system consumed more than 120A — the main breaker trips.
If the PV system and ESS are outputting max, there isn’t more than 280A of load on the bus bars, so the bus bar rating isn’t exceeded.
If the backup loads panel was at max, and the existing loads were at max, the main breaker would trip.
I don’t understand why the sum of all breakers rule wouldn’t be clarified to say that it’s the sum of all load breakers excluding the battery backup system breaker itself, there’s never a scenario where the backup system breaker can supply and consume simultaneously — it’s either consuming from the bus or supplying up to its max output, which in both scenarios never exceeds the bus capacity.
Scenario: 400A MSP with 400A main breaker.
Battery backup IQ system controller 2 (Enphase in this case) is fed by a 200A breaker off of the 400A bus.
Existing loads (280A) remain on the 400A bus and backup loads are moved to a new backup loads panel.
The BESS and PV are 83.5A after multiplying the inverter output of both by 125%.
The sum of all breakers rule says not compliant because the existing loads remaining on the 400A bus (280A) plus the 200A backup system breaker are > 400A. But are they really? The inverter max continuous output is 83.5A. There is no scenario where more than 400A could ever pass through the bus bar.
If the 280A loads were at max, and the backup system consumed more than 120A — the main breaker trips.
If the PV system and ESS are outputting max, there isn’t more than 280A of load on the bus bars, so the bus bar rating isn’t exceeded.
If the backup loads panel was at max, and the existing loads were at max, the main breaker would trip.
I don’t understand why the sum of all breakers rule wouldn’t be clarified to say that it’s the sum of all load breakers excluding the battery backup system breaker itself, there’s never a scenario where the backup system breaker can supply and consume simultaneously — it’s either consuming from the bus or supplying up to its max output, which in both scenarios never exceeds the bus capacity.