Inspector Wants A Load Calc For Battery Backup

[sketchy]

I've never been asked this until now. We use Enphase and Tesla batteries almost exclusively and they're always in conjunction with PV. He's saying microgrid systems fall under 702.4(B). I say that 706.3 tells me I don't need to (this is 2017 code). Any input would be appreciated.

 

[wwhitney]

What part of Article 706 provides guidance contrary to 702.4(B)? If none, then 706.3 is immaterial.

Cheers, Wayne

 

[don_resqcapt19]

I agree with the inspector if you are using the ESS as an optional standby system and have an automatic transfer system. 702.4(B)(2) applies.
If you have a manual transfer system, no load calculation is required as 702.4(B)(1) governs.

 

[jaggedben]

Your best bet may be to argue that 702 does not apply and instead article 710 applies. 710.15(A) is a much less stringent requirement. It's not an ironclad argument. You can point to language in 710 that seems to make it apply, and maybe that's enough. But he may argue that doesn't mean 702 doesn't also apply. The two articles seem to overlap applicability.

 

[jaggedben]

705.150 might help make the argument, although the language in the 2020 NEC (renumbered 705.50) matches up better with 710 in that code cycle than in 2017.

 

[wwhitney]

You could also try arguing that the scoping statement in 702.1 restricts it to engine generators in the "permanently installed" case:

"The systems covered by this article consist of those that are permanently installed in their entirety, including prime movers, and those that are arranged for a connection to a premises wiring system from a portable alternate power supply."

A battery ESS has no prime mover.

Cheers, Wayne

 

[don_resqcapt19]

You could also try arguing that the scoping statement in 702.1 restricts it to engine generators in the "permanently installed" case:

"The systems covered by this article consist of those that are permanently installed in their entirety, including prime movers, and those that are arranged for a connection to a premises wiring system from a portable alternate power supply."

A battery ESS has no prime mover.

Cheers, Wayne

I see nothing there that actually says an optional standby system must have a prime mover...it simply requires that where the stand by system has a prime mover, that the prime mover be permanently installed.

 

[wwhitney]

I see nothing there that actually says an optional standby system must have a prime mover...it simply requires that where the stand by system has a prime mover, that the prime mover be permanently installed.

I favor that interpretation as well, but the language is suggestive so I thought it could sway an inspector.

What's the reason for the 702.4(B) requirement on automatic transfer switches anyway? Is it that an overloaded engine generator can damage itself or the equipment connected to it? If so, I'm under the impression that the overload behavior of a grid-forming inverter is a more graceful failure, in which case it would make sense that it be exempt from 702.4(B).

One more argument: 702.4(B)(2) covers "Automatic Transfer Equipment". That's not a defined term, but we do have: "Switch, Transfer. An automatic or nonautomatic device for transferring one or more load conductor connections from one power source to another."

That's not how a hybrid grid-interactive/grid-forming inverter on a battery ESS works, it's more of an on-line standby. The grid is disconnected by the MID, but the inverter is connected all the time. So in that sense there is no transfer of "connections from power source to another."

Cheers, Wayne

 

[zbang]

One could certainly argue that there's no transfer switch at all (no mechanical switching), not is it a "standby" system as it's always active. I tend to think that 710 is more appropriate than 702.

 

[don_resqcapt19]

I favor that interpretation as well, but the language is suggestive so I thought it could sway an inspector.

What's the reason for the 702.4(B) requirement on automatic transfer switches anyway? Is it that an overloaded engine generator can damage itself or the equipment connected to it? If so, I'm under the impression that the overload behavior of a grid-forming inverter is a more graceful failure, in which case it would make sense that it be exempt from 702.4(B).

One more argument: 702.4(B)(2) covers "Automatic Transfer Equipment". That's not a defined term, but we do have: "Switch, Transfer. An automatic or nonautomatic device for transferring one or more load conductor connections from one power source to another."

That's not how a hybrid grid-interactive/grid-forming inverter on a battery ESS works, it's more of an on-line standby. The grid is disconnected by the MID, but the inverter is connected all the time. So in that sense there is no transfer of "connections from power source to another."

Cheers, Wayne

Not sure if there is any valid technical reason. If the optional supply is overloaded the breaker should trip or the prime mover stall. I don't see a hazard when an optional standby system fails.

No matter what you call it, the switch performs the function of the automatic transfer switch in that it cuts off the connection to the utility and continues to supply the load. It does that without human intervention, making it an automatic function.

 

[Chamuit]

I do load calcs for these and just make them fit.

 

[sketchy]

What I don't want is to set a precedence for having to do this every time. I don't think 702 applies.

These ESS/PV systems aren't like a generator with a constant source of fuel. Some days the battery will be at 20% because it was used up over night but the sun is shining and the PV is kicking ass. If the grid goes down there's no issue. But maybe one day the battery is at 100% and there's a snow storm and the grid goes down. Now you could run out of fuel (ESS) really quick without any sun to replenish it. What loads are backed up shouldn't matter to the AHJ, it's really just a convenience for the customer.

 

[pv_n00b]

I have seen large commercial BESS projects that are sold based on little more than here is the customer's budget, here is how much BESS they can buy for that, let's build it and then see what loads can run on it before it is overloaded. There is no, what loads do you want to backup? What are the kW and kWhr ratings those loads represent? What is the BESS rating needed to support that loading?
I have not run across an AHJ yet who wants a load calc for a BESS system.

 

[WaveGuide]

I think it's good to start by looking at the overall framework of the 7xx chapters (this from the 2023 code):

700 Emergency Systems​

701 Legally Required Standby Systems
702 Optional Standby Systems​

705 Interconnected Electric Power Production Sources​

706 Energy Storage Systems​

​

708 Critical Operations Power Systems​

​

710 Stand-Alone Systems​

2023 702.1 "...applies to the installation and operation of optional standby systems."
2023 706.1 "... to store and provide energy during normal operating conditions."

The words that jump to me are: standby vs normal. Standby means occasionally. Normal means every day.

The Enphase power is normal, thus 705. Enphase systems are not used occasionally, thus it's not standby, i.e. not 701 nor 702.
The Tesla batteries power is normal, thus 705 and 706. Tesla batteries are not used occasionally, thus it's also not 701 nor 702.

Anyone who knows anything about the Tesla batteries and the Enphase products knows that these are most definitley NOT "standby systems". Not at all! Rather these both constantly are able to slosh energy into and out of your system on a moment to moment basis depending on need. So therefore 706 applies and 702 is for older, simpler standby systems. So the only thing that can be said is that your inspector doesn't know very much about ESS nor Interconnected systems. I'd say stick to your guns, and say to this inspector, "would it be crazy to ask for them to read up on these newer systems and then take another look at this?"

 

[wwhitney]

I think it's good to start by looking at the overall framework of the 7xx chapters (this from the 2023 code)

Your analysis is sound for an ESS that does not have an MID or grid forming inverter. But the argument that an MID and grid forming inverter allows an additional standby functionality, which would then be subject to Article 702, is plausible on the face of it. You haven't addressed that in particular.

I.e. the ESS can have both a normal operating mode, and an optional standby operating mode. Why shouldn't it be subject to the respective articles for the respective modes?

Cheers, Wayne

 

[WaveGuide]

Your analysis is sound for an ESS that does not have an MID or grid forming inverter. But the argument that an MID and grid forming inverter allows an additional standby functionality, which would then be subject to Article 702, is plausible on the face of it. You haven't addressed that in particular.

I.e. the ESS can have both a normal operating mode, and an optional standby operating mode. Why shouldn't it be subject to the respective articles for the respective modes?

Good point! Why indeed?

1) I would first ask, what is the generally intended use of a Tesla battery? One's main breaker *could* be used to turn the house lights off at night, but this is not the generally intended purpose of that breaker. Similarly, I would argue that it would be exceptionally rare for the generally intended use of a Tesla battery, given the advanced features of those batteries, to be as a backup. There are much cheaper solutions for that.

2) Furthermore, I would ask what is the purpose of doing this calculation? In other words, is there any public safety involved with the result of such a calculation, or as I suspect, is this just big brother telling us what to do because he can? I doubt that there is any public safety involved in the size of such a battery or solar system, .. although in a stretch, one could argue that by helping cut down on global warming gasses, a much bigger battery makes us all safer. I bristle when code is used for other than it's intended purpose: for safety.

 

[tortuga]

Interesting discussion, just how does one of these Tesla or Enphase systems handle a overload in island mode?
Say someone clueless is staying in the house and the power goes out and they crank the AC, turn on the electric range and use the on demand electric water heater?

 

[jaggedben]

Interesting discussion, just how does one of these Tesla or Enphase systems handle a overload in island mode?
Say someone clueless is staying in the house and the power goes out and they crank the AC, turn on the electric range and use the on demand electric water heater?

It shuts down, waits a couple seconds, then attempts to restart. If the load isn't removed it will shut down again and wait longer. Eventually after some number of attempts it may stop trying.

 

[sketchy]

VICTORY!!! After going above the inspector's head, I got them to see things my way, ie the right way. However, in his emailed response he made sure to tell me: "But that doesn’t change the situation that we can request any documentation we deem necessary for our work." They always have to be right...

Interesting discussion, just how does one of these Tesla or Enphase systems handle a overload in island mode?
Say someone clueless is staying in the house and the power goes out and they crank the AC, turn on the electric range and use the on demand electric water heater?

I recently had this issue and the Powerwall kept shutting down because the AC compressor was going bad and drawing over 70A on start up. No damage done to the ESS or PV because of the built in fail safe.

 

[ggunn]

VICTORY!!! After going above the inspector's head, I got them to see things my way, ie the right way. However, in his emailed response he made sure to tell me: "But that doesn’t change the situation that we can request any documentation we deem necessary for our work." They always have to be right.

I feel your pain. In a meeting with an inspector and his (non technical) boss a while back where I had challenged the inspector on a point of code, I was right and he was wrong. When I showed him the code language that was clearly in opposition to his position, he just fell back on, "I am the AHJ and I reserve the right to interpret or revise the NEC any way I see fit." His boss just looked at me and shrugged. End of meeting.

Many in here will say that he can't do that. In an ideal world that would be true; in the real world, not so much.