busbar ratings with an ESS

[JoeNorm]

When installing an ESS+solar is it always necessary to factor in the output of the ESS when considering the 120% rule of the busbar?

For instance, if you have an ESS that can output 40amps and you have 40amps of rooftop solar would I need to count the whole unit as an 80amp back-feed?

Does it matter whether the unit is set for time-of-use or just backup mode?

 

[wwhitney]

Yes, yes, no.

Unless the ESS is appropriately listed and configured as a PCS (Power Control System) under 2020 NEC 705.13.

Cheers, Wayne

 

[rainwater01]

Yes, yes, no.

Unless the ESS is appropriately listed and configured as a PCS (Power Control System) under 2020 NEC 705.13.

Cheers, Wayne

Would it depend on the system? The Generac pwrcell ess and solar panels connect to the inverter and the inverter connects to the 40 amp breaker in the panel. It can’t output more than 7600 watts no matter what the source.


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[wwhitney]

Would it depend on the system? The Generac pwrcell ess and solar panels connect to the inverter and the inverter connects to the 40 amp breaker in the panel. It can’t output more than 7600 watts no matter what the source.

Yes, what matters is the inverter rating(s). So if the ESS and the PV share a single inverter with a maximum continuous output current of 32A, then it could go on a single 40A breaker. That's the DC coupled solution.

Or in Tesla's PW+, there are 2 AC-coupled inverters in a single unit listed as a PCS (I assume) to never export more than 32A while on grid. That's the AC coupled solution. Not sure which type Generac Pwrcell is.

Thanks for pointing out that recent integrated products for PV + ESS avoid the necessity to count both systems separately.

Cheers, Wayne

 

[rainwater01]

Yes, what matters is the inverter rating(s). So if the ESS and the PV share a single inverter with a maximum continuous output current of 32A, then it could go on a single 40A breaker. That's the DC coupled solution.

Or in Tesla's PW+, there are 2 AC-coupled inverters in a single unit listed as a PCS (I assume) to never export more than 32A while on grid. That's the AC coupled solution. Not sure which type Generac Pwrcell is.

Thanks for pointing out that recent integrated products for PV + ESS avoid the necessity to count both systems separately.

Cheers, Wayne

Generac is dc coupled. 380-420 Volts DC for the battery and PV.


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[pv_n00b]

For DC-coupled PV+BESS you go by the AC inverter rating since that's what sets the back feed. For AC coupled systems, you go by the sum of the PV and BESS inverters under the 2017 and earlier NEC. This has been a problem since the actual BESS output is controlled and you will not get both the PV system and the BESS outputting maximum current to the POCC and back to the utility. But the NEC did not have an allowance for that until the 2020 version added it. Before that, it was a discussion with the AHJ to have them approve the design of the system with a PCS preventing full output of both systems together.

 

[jaggedben]

Succinctly...

Yes, DC coupled systems can have an advantage of not adding more inverter output that needs to be considered by the code. AC coupled systems need to be listed as Power Control Systems to be equivalent.

That said, there can be situations where you may want to output from both PV and ES at full power. AC coupled systems generally do better at that.

 

[JoeNorm]

So how did the 2020 code address this problem? I am also wondering how many places in the country allow "time of use" and let these systems dump full output to the grid?

We are dealing primarily with Enphase and Sunpower systems now so both AC coupled.

 

[jaggedben]

So how did the 2020 code address this problem?

By permitting Power Control System output to be used in the 705.12 calcs.

We've been doing it on the 2017 in CA, AHJs seem to be fine with it.

 

[pv_n00b]

So how did the 2020 code address this problem? I am also wondering how many places in the country allow "time of use" and let these systems dump full output to the grid?

It depends on the utility. The issue is that NEM is limited to renewable energy. Using TOU and charging a BESS from the grid when the energy cost is low and discharging to the grid under NEM when energy is expensive is a misuse of NEM and typically not allowed. So either the system has to be designed so the BESS never exports to the grid or it must be designed so that the BESS can't charge from the grid and can only charge from the PV system. Due to the complexity and cost, this usually means that PV+BESS systems that allow charging the BESS from the grid end up being non-export. With over 3,000 utilities in the USA I'm sure it's allowed somewhere but not often.

 

[jaggedben]

The grid export question, while interesting, is pretty irrelevant to the 705 issues raised with AC coupled systems, because you still have to consider the possibility of full system output to the house load.

 

[JoeNorm]

The grid export question, while interesting, is pretty irrelevant to the 705 issues raised with AC coupled systems, because you still have to consider the possibility of full system output to the house load.

The house load in backup mode?

 

[jaggedben]

The house load in backup mode?

No, on grid.

 

[pv_n00b]

The grid export question, while interesting, is pretty irrelevant to the 705 issues raised with AC coupled systems, because you still have to consider the possibility of full system output to the house load.

Unless the BESS output is constrained by a PCS. Typically the PCS will monitor the input to the panel from the grid and the PV system and can be set to restrict the addition of any BESS output to keep the total under a specific amperage. Then the total input to the panel is not the full output of the PV system and BESS.

 

[JoeNorm]

I think I am more confused than when I first asked and I just read all of 705.

What classifies a piece of equipment as a PCS?

We are installing all of our ESS's in a "backup" configuration. They are all AC coupled. I would think that while they are grid connected they will not push anymore power to the grid then they have solar output connected to them. What am I missing?

Is it simply because they have the ability and they may not be listed as a PCS?

It is not until they enter backup mode that they would be using the their inverter and the available solar combined, but this should not matter because there is now only one source of power.

Thanks for your patience

 

[wwhitney]

Is it simply because they have the ability and they may not be listed as a PCS?

Yes, to my understanding.

Cheers, Wayne

 

[wwhitney]

BTW, this tech brief from Enphase discusses the PCS options for Enpower, including limiting the current export from Enpower to comply with the 120% rule to avoid having to upgrade/reconfigure the main panel:

https://enphase.com/sites/default/files/2021-05/PCS-Tech-Brief.pdf

Cheers, Wayne

 

[jaggedben]

Unless the BESS output is constrained by a PCS. Typically the PCS will monitor the input to the panel from the grid and the PV system and can be set to restrict the addition of any BESS output to keep the total under a specific amperage. Then the total input to the panel is not the full output of the PV system and BESS.

Hmmm....

First, so far none of the PCS systems I've worked with allow you to do that. They all monitor the system output with CTs at a certain point and must limit the output at that point. They cannot monitor the input to a loadside busbar while also monitoring the grid iport at the same time and make the latter part of PCS operations. I don't see why it isn't possible to do what you describe, but so far it does not seem 'typical' to measure grid imports as part of PCS operation.

Second, thinking like an overly conservative member of a CMP, you could still have a scenario where the system is configured to measure the house load and output battery to cover it, and the house load goes above the busbar rating, and the battery, PV, and utility are all feeding more into the busbar (from opposite ends, to be sure) than it's rated for. This has be prevented either with sufficient busbar rating or PCS. It's the same as the situation we've always had with just solar, just taken to another level of extreme with ESS. But this point is all this is still the case without having to do with the ESS exporting to the grid.

 

[jaggedben]

What classifies a piece of equipment as a PCS?

It is listed as such. The instructions will say what you can do with it. Typically what it means is that the total system (whether PV+ES or just one of them) can limit its output at certain measured point to less than full nameplate. Theoretically it could still output full nameplate while limiting the output at the measured point, as long as part of the output goes to load that's in-between.

 

[jaggedben]

We are installing all of our ESS's in a "backup" configuration. They are all AC coupled. I would think that while they are grid connected they will not push anymore power to the grid then they have solar output connected to them. What am I missing?

You are missing, as I've been trying to explain above, that grid export (or lack thereof) isn't the paramount factor.

Example:
200A MCB/ 200A busbar service panel. Connected at the bottom is a breaker that goes to a microgrid with a 30A solar breaker and 30A ES breaker, let's say nameplate 24A each. There are CTs at the service entrance and the ES is programmed to cover as much load as it can but not to output so much power that it exports to the grid. But in this scenario (without PCS) there may be nothing to prevent 260A being drawn on the service panel between the three sources combined. Unrealistic, to be sure, but the code-making panel wrote the 120% rule to stop this. On the systems I work with, you'd have to move your CTs to be on the feeder that connects to the micro-grid, and set the PCS programming to prevent the ES and PV combined from outputting more than 32A together on that feeder.