120% rule on backup panel, Power Limit Feature
- Thread starter Zyb
- Start date
wwhitney
Senior Member
- Location
- Berkeley, CA
- Occupation
- Retired
The specs you quoted say that the maximum continuous charge current is 20.8A per PW3. So considering them as loads, your panel has 4*30 + 2*20.8 = 161.6A of continuous load. That's too much for a 200A breaker which isn't 100% rated (and it won't be). If you can configure the PWs to jointly never charge at more than 40A from the grid, that would take care of the issue.
The subject heading mentions the 120% rule. So considering the PW3s as sources, if they can jointly act as a PCS to export at most 32A continuous, then the panel shown would comply with the 120% rule. I'm unclear on what certifications the NEC requires a PCS to have (which may vary depending on the 2020 NEC vs the 2023 NEC), and what certifications the PW3 has.
Seems like limiting the PW3s to export at most 32A at any time would be an undesirable limitation. Given the loads in the panel shown, that limitation would apply even when off-grid. Putting the EVSEs in a separate panel is probably a better design.
Cheers, Wayne
The subject heading mentions the 120% rule. So considering the PW3s as sources, if they can jointly act as a PCS to export at most 32A continuous, then the panel shown would comply with the 120% rule. I'm unclear on what certifications the NEC requires a PCS to have (which may vary depending on the 2020 NEC vs the 2023 NEC), and what certifications the PW3 has.
Seems like limiting the PW3s to export at most 32A at any time would be an undesirable limitation. Given the loads in the panel shown, that limitation would apply even when off-grid. Putting the EVSEs in a separate panel is probably a better design.
Cheers, Wayne
Elect117
Senior Member
- Location
- California
- Occupation
- Engineer E.E. P.E.
Unless the gateway can monitor and control the sources and/or the loads, then you are over. If the gateway can monitor such that nothing exceeds the 200A busbar rating than you will be okay.
Typically the PCS would need to be the panelboard itself to meet this application (see SPAN panelboards). The location of the Tesla gateway leads me to believe it would only monitor the amps being seen at the conductors entering/leaving the board and not within the board itself.
Typically the PCS would need to be the panelboard itself to meet this application (see SPAN panelboards). The location of the Tesla gateway leads me to believe it would only monitor the amps being seen at the conductors entering/leaving the board and not within the board itself.
tallgirl
Senior Member
- Location
- Glendale, WI
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- Controls Systems firmware engineer
Putting one or more of the EVSEs upstream of the gateway would also allow the system to not get sucked dry. I see that a lot of the OPs posts are about PV systems. What are their plans in the event all 4 chargers are in use and the grid fails for a prolonged period of time?
wwhitney
Senior Member
- Location
- Berkeley, CA
- Occupation
- Retired
If only Tesla EVs are connected, then they and the Gateway can be configured to limit/suspend charging during a grid outage. IIRC you can set a "reserve level," so the Tesla EVs only charge off-grid when the battery capacity is above that level.
But without such a mechanism, I agree with you that EVSEs should not generally be backed up automatically.
Cheers, Wayne