PW+ Grid Charging with 100amp Service

[twsmit]

Just pulled the trigger with Tesla on 12.4kWh PV w/ 2x PW+ and GW 2.0. I have underground 100amp service in NorCal with PG&E. The cost to trench is prohibitive so leaving original service equipment alone. As one might guess, 100amps is adequate but doesn't leave a lot of headroom.

My question is if playing the TOU arbitrage game and grid charging my PW is there risk of using all my service amps and tripping the main breaker?

My understanding is each PW+ can pull ~32 amps each to recharge. So ~64 amps from the grid worst case scenario. If I fire up my AC, electric oven on top of the PWs etc... does the GW automatically slow down the rate of grid charging? Presumably the equipment is programmed to know I only have 100amp service?

Thanks in advance! I hadn't originally contemplated the possibility of the PW being a major draw from the grid, but with Storm Watch and time based grid charging seems a possibility they may start drawing heavily from the grid at the most inopportune time.

 

[jjrandorin]

max grid draw for your powerwalls is something that can be set by tesla. I am not sure what criteria they use to calculate what to set it at, but (for example) my two powerwalls will only charge at 3.3kW combined charging rate when charging from the grid. Tesla set this "somewhere" based on "something" and I am not technical enough to know what those criteria are.

I just know they can set it, so you shouldnt worry about "charging your powerwall too much from the grid". I am also not sure what triggers being able to charge from the grid at all, so if this is a must have for you (grid charging) you might want to ensure its going to be there for you. Its not, for everyone.

 

[power.saver]

max grid draw for your powerwalls is something that can be set by tesla. I am not sure what criteria they use to calculate what to set it at, but (for example) my two powerwalls will only charge at 3.3kW combined charging rate when charging from the grid. Tesla set this "somewhere" based on "something" and I am not technical enough to know what those criteria are.

I just know they can set it, so you shouldnt worry about "charging your powerwall too much from the grid". I am also not sure what triggers being able to charge from the grid at all, so if this is a must have for you (grid charging) you might want to ensure its going to be there for you. Its not, for everyone.

Was that during StormWatch or true Grid Charge setting? I know during StormWatch my PW charge at a reduced rate, 3.3kW. But when Grid Charging came out, I tried it once and they were charging at 10kW for 2 PWs. That was much higher that I like, so I haven't used it since.

I wonder if @Vines or anyone else knows if this can be set in the Installer menu. I'd prefer it if mine were lower than 5kW per PW.

 

[jjrandorin]

It was regular grid charging, not stormwatch charging. My 2 powerwalls only grid charge at a rate of 3.3 kW (combined for the 2).

 

[CrazyRabbit]

Gateway has PCS so as not trip your main breaker. Do ask that they install equipment, perhaps 200 amp disconnect and 200 amp wire, so you can upgrade later and not have to rip and replace.

 

[twsmit]

Gateway has PCS so as not trip your main breaker. Do ask that they install equipment, perhaps 200 amp disconnect and 200 amp wire, so you can upgrade later and not have to rip and replace.

Thank you!

 

[twsmit]

Wanted to share an update. Just got PTO and tested out grid charging.

My Powerwall and Model Y started recharging exactly at midnight. Pulled ~21kW from the grid for a sustained period of time.

That’s 90% of my service on 100amps. Is this something to be worried about or escalate?

Everything I read is to keep power under 80% of max theoretical load. I expected the power walls would drop once they detected a significant house load, but that didn’t happen.

 

[Eric33432]

Wow, I would not want that. Sounds like a good way to burn up your service or service drop, especially if it is older or in marginal condition.

90 amps is not the problem. The problem is 90 amps for hours and hours.

 

[mongo]

That’s 90% of my service on 100amps. Is this something to be worried about or escalate?

Everything I read is to keep power under 80% of max theoretical load. I expected the power walls would drop once they detected a significant house load, but that didn’t happen.

Continuous duty loads are defined as on for 3 hours or longer. The screenshot is around 1.5 hours Sunday, how long was it at that load Saturday?

 

[twsmit]

Continuous duty loads are defined as on for 3 hours or longer. The screenshot is around 1.5 hours Sunday, how long was it at that load Saturday?

I turned grid charging off the next day so no more data point. Yeah I guess technically 1.5 hours is not a “sustained load” but what makes me nervous is that I have no control and my family will be asleep.

 

[mongo]

I turned grid charging off the next day so no more data point. Yeah I guess technically 1.5 hours is not a “sustained load” but what makes me nervous is that I have no control and my family will be asleep.

NEC rules are designed to be safe. If everything is installed correctly, there is nothing to worry about.
Though you could verify with Tesla/ installed that the system is configured for 100A service, they may also be able to reduce the max draw to 80A.

 

[tomuo]

The 80% continuous load rule is for individual circuits though, right?
It doesn't apply to the grid connection?

 

[mongo]

The 80% continuous load rule is for individual circuits though, right?
It doesn't apply to the grid connection?

Good point, a 400 Amp electrical service typically has a 320A continuous rating, but a 200Amp service is 200A continuous.
NEC allows the service conductors to be sized to 83% of the circuit ampacity.
So @twsmit, you could check what your meter says CL100 would mean it can do 100 continuously (but then it's a question of the conductirs used, unless the main panel is integrated with the meter).

 

[twsmit]

Good point, a 400 Amp electrical service typically has a 320A continuous rating, but a 200Amp service is 200A continuous.
NEC allows the service conductors to be sized to 83% of the circuit ampacity.
So @twsmit, you could check what your meter says CL100 would mean it can do 100 continuously (but then it's a question of the conductirs used, unless the main panel is integrated with the meter).

Main panel is combo with meter on a 100amp breaker.

 

[mongo]

Main panel is combo with meter on a 100amp breaker.

Excellent! If the meter says CL100, your 90 Amp plot is a non-issue.

 

[twsmit]

Excellent! If the meter says CL100, your 90 Amp plot is a non-issue.

Can you explain in layman’s terms? I’m encouraged by what you wrote but don’t understand.

What I have is as follows: 125amp service in a combo meter with two breakers. 70 amp breaker goes to AC, 100amp breaker goes to the rest of my loads and PW.

So is putting >90amps on my 100amp breaker somehow okay?

 

[mongo]

Can you explain in layman’s terms? I’m encouraged by what you wrote but don’t understand.

What I have is as follows: 125amp service in a combo meter with two breakers. 70 amp breaker goes to AC, 100amp breaker goes to the rest of my loads and PW.

So is putting >90amps on my 100amp breaker somehow okay?

Ahhhh....
@wwhitney or @Vines would have better input than myself.

 

[wwhitney]

What I have is as follows: 125amp service in a combo meter with two breakers. 70 amp breaker goes to AC, 100amp breaker goes to the rest of my loads and PW.

OK, here's a hopefully comprehensive if lengthy response. From the utility transformer, you have:

(A) Utility owned service conductors
(B) Customer owned service conductors
(C) Utility meter
(D) A service busbar
(E) Two separate service disconnect breakers on the service busbar

At least, that's my interpretation of your description, that there's no breaker between (C) and (D). That's a not uncommon arrangement in my area. If I'm mistaken about that, then much of the following changes.

Partial aside: If you had a single breaker that when flipped would kill all the power to the premises, that breaker's rating would be your service rating. But in the above arrangement, there is no such breaker, so it is a bit harder to determine your service rating. I would say it is the minimum of the non-continuous rating of the utility meter (C), 1/83% of the ampacity of (B), and 1/83% of the ampacity of (D). That 1/83% is an oddity of the NEC for services/feeders that supply a dwelling unit.

Now, as to your maximum allowable continuous export, it should be the minimum of the ampacity of (B), the continuous rating of (C), the ampacity of (D), and 1/125% of the rating of the 100A breaker you are exporting through, or 80A. Since your service rating is presumably at least 100A, (B) should be at least 83A. (C) will be 100A for a CL100 meter (those letters should appear on the meter face). (D) is presumably 125A, per your description.

So then the 1/125% of the 100A breaker = 80A is the limiting factor. You should probably have your installer configure the Gateway to limit export to 80A, which is easily done in software, I believe (perhaps even remotely by Tesla).

On the other hand, the only downside to running more than 80A continuously through the 100A breaker is that it might trip. If it's 100F outside, and your service panel is directly in the sun, that's pretty likely to happen if running 80A through the breaker for over 3 hours. Conversely at night it's not that likely to happen. So a practical question is what is the next weakest link? (B) might be rated only 85A, in which case you'd have a 100A service, and in which case the conductors supplied by the 100A breaker may only be rated 85A (e.g. #4 Cu). Or the same for 90A (e.g. #2 Al). But if (B) is #3 Cu or #1 Al, then it is rated for 100A, and that's not an issue.

The upshot is that if (B) is #3 Cu or #1 Al, and your 100A breaker is not tripping, there's not much downside to running 90A export through it. On the third hand, it's not really necessary, either, so again you should probably have your installer configure the Gateway to limit export to 80A.

Cheers, Wayne

 

[mongo]

OK, here's a hopefully comprehensive if lengthy response. From the utility transformer, you have:

(A) Utility owned service conductors
(B) Customer owned service conductors
(C) Utility meter
(D) A service busbar
(E) Two separate service disconnect breakers on the service busbar

At least, that's my interpretation of your description, that there's no breaker between (C) and (D). That's a not uncommon arrangement in my area. If I'm mistaken about that, then much of the following changes.

Partial aside: If you had a single breaker that when flipped would kill all the power to the premises, that breaker's rating would be your service rating. But in the above arrangement, there is no such breaker, so it is a bit harder to determine your service rating. I would say it is the minimum of the non-continuous rating of the utility meter (C), 1/83% of the ampacity of (B), and 1/83% of the ampacity of (D). That 1/83% is an oddity of the NEC for services/feeders that supply a dwelling unit.

Now, as to your maximum allowable continuous export, it should be the minimum of the ampacity of (B), the continuous rating of (C), the ampacity of (D), and 1/125% of the rating of the 100A breaker you are exporting through, or 80A. Since your service rating is presumably at least 100A, (B) should be at least 83A. (C) will be 100A for a CL100 meter (those letters should appear on the meter face). (D) is presumably 125A, per your description.

So then the 1/125% of the 100A breaker = 80A is the limiting factor. You should probably have your installer configure the Gateway to limit export to 80A, which is easily done in software, I believe (perhaps even remotely by Tesla).

On the other hand, the only downside to running more than 80A continuously through the 100A breaker is that it might trip. If it's 100F outside, and your service panel is directly in the sun, that's pretty likely to happen if running 80A through the breaker for over 3 hours. Conversely at night it's not that likely to happen. So a practical question is what is the next weakest link? (B) might be rated only 85A, in which case you'd have a 100A service, and in which case the conductors supplied by the 100A breaker may only be rated 85A (e.g. #4 Cu). Or the same for 90A (e.g. #2 Al). But if (B) is #3 Cu or #1 Al, then it is rated for 100A, and that's not an issue.

The upshot is that if (B) is #3 Cu or #1 Al, and your 100A breaker is not tripping, there's not much downside to running 90A export through it. On the third hand, it's not really necessary, either, so again you should probably have your installer configure the Gateway to limit export to 80A.

Cheers, Wayne

Thanks Wayne!

It doesn't impact most of the analysis, but the concern was exceeding 80 Amps of usage/ import. Two PWs would be fine even at 100% export, I think.

I suppose there is also an interaction between AC usage on the service feed and PW+house consumption as the gateway likely doesn't see the AC breaker. In that case, is the service sizing (both 100 and 125 have been mentioned) more critical?
(And is air conditioning a continous load?)

 

[wwhitney]

It doesn't impact most of the analysis, but the concern was exceeding 80 Amps of usage/ import.

Right, thank you. That's disallowed by the book if the >80A is a continuous load (since 100% rated breakers are not seen in residential work and don't exist at the 100A size). And I guess PW charging and EVSE usage are both considered continuous. So the PW should be reconfigured to either (a) limit the current through the 100A breaker to 80A or (b) limit the current through the service to 80% of its rating.

On the practical front, my comments in the penultimate paragraph of my last post still apply.

I suppose there is also an interaction between AC usage on the service feed and PW+house consumption as the gateway likely doesn't see the AC breaker.

Depends on whether the installation added CTs on the 70A breaker or not.

In that case, is the service sizing (both 100 and 125 have been mentioned) more critical?

So, when you have multiple service disconnects like this, the service conductors are protected only by a load calculation, not by OCPD. That is, the sum of the service OCPD ratings is allowed to exceed the service rating as it does in this case (it's not a 170A service).

Then if the GW is blind to the 70A A/C load (no CTs), I would say the thing to do is to determine the service rating, subtract the A/C's MCA nameplate rating (which will be much less than the 70A breaker size, although when looking at the nameplate, confirm that "MOCP" or "Max OCPD" is at least 70A), and set the GW to limit import to 80% of the result (or 80A, if that is less)

If the GW isn't blind to the A/C load, then just set the G/W to limit the total import to 80% of the service rating.

(And is air conditioning a continous load?)

As a motor-type load, feeder and OCPD sizing includes a 125% factor on the largest motor full load current. That 125% factor is not a continuous use factor per se (because it applies only to the largest motor), but a starting surge current factor (on the theory that simultaneous starting is unlikely). As a result it would also cover the possibility of continuous use, though, for any single motor at a time.

But for a motor in good working order to actually be a continuous electrical load, it would need to be no more efficient than implied by the motor full load current tables in the NEC, and mechanically loaded at its full HP rating for over 3 hours.

Cheers, Wayne