400Amp Gateway workaround

[fresnoboy]

I mean if the worst thing possible is you buy something that pays itself back approximately over its lifetime, you made a decent choice. Most luxuries do not do this.

So for the cost of prepaying for the power over 10 years-ish you get the ability to run somewhat independently of the grid, or in total battery backup mode and consume something like 40 kWh every day? That's pretty awesome living if I say so myself

Well, I have a backup generator already, so that's not the biggest motivator. If it would work well with the generator, and provide immediate power until the batteries were exhausted etc.., then that's a different animal, but we discussed why this is hard with the 200A limitation the GW's have.

I suppose it's a good bet that PG&E is only going to get more rapacious on prices given bankruptcy, paying off victims, and the state forcing people to move to electric heat instead of gas, but that may not mean the spread increases and may not offset the loss of capacity over 10 years.

 

[Vines]

Certainly it wont work for everyone, maybe you are in that camp. Having backup power already blows my whole back of the envelope calculation out of the water lol.

If they come out with a 400A gateway with generator support I am sure there are some customers like you would be happy. Honestly its not a huge market so I think it may be a while, Tesla focuses on area under the curve. Even among 400A services, most are 2x200, so a single 400A gateway is not ideal anyway. I could see Tesla working instead on a bridge of some sort that could allow 200A gateways to be used across multiple phases or stacked. I really have no idea.

The worldwide market is 3 phase, and they want to go larger as well. Instead of adding Amps, they want just additional phases.

Forcing any of the other solutions on your situation doesn't serve you it sounds. All of them are a compromise, but with enough powerwalls they can work.

You could feed a 400A ATS from the load side of the Gateway, with a good chunk of Powerwalls. 5 Powerwalls will provide about 100A continuous, or about 150A as a peak. However if your home runs more than 200A as an average its not a great solution. I don't know what your loads look like but it sounds like your home sucks a lot of power.

 

[fresnoboy]

Not sure what "a lot" of power is. Before the solar was switched on, we were burning about 90 KWh on a hot day in the peak of summer with HVAC etc, pool equipment running max out, and an EV that needs charging every day, etc... That's a lot of energy, but only about 15 amps average load through the day. The HVAC units start at 5A, so there is no LRA problem here needing hundreds of amps to start up.

Again, part of this is how the NEC requires load calculation even in an all LED house. I doubt we've ever come close to 200 Amps in terms of peak current - once I get some CT's on the main service feed and the master distribution panel breakers, I'll have hard data on peaks. But by the first way the electricians did the calcs, they said we needed a 600A feed! That whole process was crazy and added a ton of cost, but you can't fight code.

And the generator we have is only 30 KW, and it seems to handle things well. There was no code requirement to use the NEC calcs on generator selection!

I am not suggesting that Tesla needs to build tons of different SKU's to deal with odd customers like me. But ATS's are commodities, and it would be nice if Tesla built gateways that could control a 3rd party ATS instead of having to handle all these configs with just one box.

thx
mike

 

[Vines]

Sure, thing, hopefully in the future a 400A Gateway is a thing Tesla offers the US market, and a 3 phase 200A version of the rest of the world, and all this workaround stuff is a moot point.

 

[fresnoboy]

Just to my point about making it easy to use a 3rd party ATS, Generac builds exactly that for it's Powerwall equivalent system: https://www.generac.com/GeneracCorp...ontent/Clean Energy/PWRCell_ATS_SpecSheet.pdf

It's really just a software driven control interface. I confess I don't know why Tesla doesn't do something that that product.

mike

 

[wwhitney]

And the generator we have is only 30 KW, and it seems to handle things well. There was no code requirement to use the NEC calcs on generator selection!

Maybe you were subject to an earlier version of the NEC, or maybe your planning department missed it, but there is now such a requirement for automatic transfer switches. 702.4(B)(2). Load shedding is allowed, though.

Cheers, Wayne

 

[Patrick66]

Maybe you were subject to an earlier version of the NEC, or maybe your planning department missed it, but there is now such a requirement for automatic transfer switches. 702.4(B)(2). Load shedding is allowed, though.

Cheers, Wayne

The generator itself can be sized for the load, but agree in the ATS. Holdover from wet-stacking concerns on diesel gensets.

 

[fresnoboy]

The ATS is 400A just like the service feed. That's code too, and pretty easy to deal with.

 

[wwhitney]

Sorry, my statement wasn't clear. When there is an automatic transfer switch, the generator is required to be sized for the NEC calculated load attached. 702.4(B)(2). Load management/load shedding can be used to reduce which loads need to be included in the calculation..

Cheers, Wayne

 

[fresnoboy]

Sorry, my statement wasn't clear. When there is an automatic transfer switch, the generator is required to be sized for the NEC calculated load attached. 702.4(B)(2). Load management/load shedding can be used to reduce which loads need to be included in the calculation..

Cheers, Wayne

Wow, I don't think any residential genset install is consistent with that. And why wouldn't it apply to powerwalls as well?

 

[wwhitney]

I have wondered that about the Powerwalls. I guess not many jurisdictions are enforcing 702.4(B)(2) for single family residences. I excerpted the (2014) text below.

Cheers, Wayne

702.4(B)(2) Automatic Transfer Equipment. Where automatic transfer equipment is used, an optional standby system shall comply with (2)(a) or (2)(b).
(a) Full Load. The standby source shall be capable of supplying the full load that is transferred by the automatic transfer equipment.
(b) Load Management. Where a system is employed that will automatically manage the connected load, the standby source shall have a capacity sufficient to supply the maximum load that will be connected by the load management system.

 

[fresnoboy]

My genset has an overcurrent shutdown feature, as I am sure the GW does as well. Maybe that qualifies under "load management"?

This issue never came up with my AHJ, only that the ATS had to have the full rating of the service.

 

[power.saver]

Wayne,

But is the gateway really an ATS?

A traditional ATS would switch from grid power to generator power, like a Form C switch (or combination of switches to achieve that).

In the gateway, the switch is a contactor, like a Form B switch, and simply opens the grid connection. The "generator" is already connected with the load as an active AC inverter.

Does that make a difference?

 

[wwhitney]

But is the gateway really an ATS?

That's an excellent question. Fortunately the NEC has definitions to help resolve this sort of question:

Switch, Transfer. An automatic or nonautomatic device for transferring one or more load conductor connections from one power source to another.

Certainly the Powerwalls may not be producing power at the time of a grid failure, in which case the Backup Gateway will open up and the Powerwalls will start producing power. So that transfers the connected loads from one power source to another. And I think the "one or more" language take care of the issue that the power is entering the wiring at two different points before and after the transfer.

So I would say it is a transfer switch, both in spirit and by the letter.

Cheers, Wayne

 

[fresnoboy]

Mike Holt has an excellent video on this topic that can be found here:

I have found his stuff really helps in understanding the NEC and what the NEC was trying to accoplish in a particular part of the code.

 

[fresnoboy]

PS The key point the video makes is that you can use historical data on ACTUAL load, so that can be used to size the generator adequately. You can get that from the electric utility, so that probably is pretty easy to do.

For a large genset like mine, that works fine, but I don't know how many single or dual Powerwall installations would be able to handle the peak actual load over the last 30 days etc...