# How to do load calculation for adding Tesla wall charger to a sub panel

I have 200 amp service. The outside main service panel ( 200 amp ) is of the type : meter main MLO Distribution panel with no breaker in between the meter ( supply side ) and the panel itself. I has two breakers on the load side. One 100 amp breaker feeds the two outdoor AC condenser units and another 100 amp breaker feeds a sub-panel in the garage. The panel also has 10 kw solar back feed through a 50 amp OCPD. There is no way a new load can be added to the outside main panel due to its type as mentioned above and due to solar back feed , even though it has physical space. In fact, Tesla solar has put a red sticker in the panel saying "Caution: No new load to be added".

The sub panel has typical electrical loads of a 3375 sq ft home , like a 240V dryer and oven ( the only two 240V loads ) , kitchen circuits , microwave, dishwasher, cloth washer, indoor hvac air handlers ( gas heating ) and a whole house fan. There is no other major electrical load like electric range or water heater etc.

I am looking to add a circuit for Tesla Wall charger to this sub panel and need to do a load calculation to see if this panel can support one more 50-60 amp OCPD for a 40-48amp wall charger circuit. The panel itself is 200 amp capable although feed is 100 amp only from the main panel and has plenty of space for new breakers.

I tried following the guidelines of my city from load cacl to calculate the load, but gotten throughly confused as to how to apply these to my situation. This link explains to add up the load for the entire house as served from the outside main panel and with this approach, I find enough capacity to add 60 amp OCPD to the system.

But the new circuit is being added to the sub panel which has only 100 amp supply. If I apply the above load calculation approach for just the sub panel loads ( i.e. not counting the AC load and treating the supply as 100 amp service ) , then sub panel appears to be already overloaded leaving no scope for adding wall charger circuit.

I have consulted two electricians who will be doing the install after pulling the permit and one says that sub panel may be over loaded without any calculation , while the other did a calculation to confirm that a 50 amp OCPD can be added. This 2nd electrician says that load calculation is done with the main service of 200 amp as total capacity as explained on city's web site in the link above and hence his recommendation.

I can go with the 2nd electrician , but concerned what if the inspection fails . Myself being an electrical engineer, my common sense says that the sub panel with 100 amp feed and its existing load only should be considered for load calculation despite main service being 200 amps. But I also see that the wall charger circuit and other major loads on the panel ( dryer and electric oven ) will never run together.

Requesting experts on the forum to help enlighten me on this issue .

• and you solemnly swear they will newer run together ? Inspectors do not buy that Commented Apr 10, 2023 at 8:30
• I am not the expert you seek, but I would think you need to do and conform to both load calcs. What would be the point of load calcs if you could legitimately just hide some loads behind a curtain or split them up into ones that either count or don't count, according to your own design? Commented Apr 10, 2023 at 10:37
• Can you post the kW rating of your oven and how many kitchen outlet circuits you have? Commented Apr 10, 2023 at 11:42
• Queue Harper's rant against 50a wall charger circuits in 3... 2... 1... Seriously, though, please search this site for `EVSE` and you'll find many, many answers addressing why you likely do not need a 50a charger for your car. If you read through those and find you actually need that much charging or just simply want it for bragging rights, come on back. Otherwise, you can delete your question because you'll discover it's a moot point. Commented Apr 10, 2023 at 13:17

## There's not even the slightest problem here. The question is, how would you like to solve it?

No Load Calculation needed if you don't want to do one. However, it may not work quite as you may have expected - this is not your father's battery charger and EVs have lots of new tricks.

You need to get used to this idea: that the EVSE tells the car how fast to charge. Really. It works like that.
Yes, I know it's astonishing, but think about what SAE was thinking in the 2000s when they were designing these standards. "it's the 21st century for Pete's sake! We have modern miracles like 1980s era microcontroller tech!" is what they were thinking.

My first advice is "electricians are wrong". They specialize in general electrical and don't understand EV uniqueness and want to give you costly service upgrades (not least because they make more money when you do that). Worse, the group-think in the EV community is "get the biggest charge possible" which is not only bonkers, but actually deters adoption of EVs.

...the situation is actually even better than Alec states in that last video, because Alec doesn't really touch on Power Sharing or Load Limiting, which are incredibly powerful. Alec calls those things "chargers" - They aren't so I won't call them that. The EVSE's job is to tell the car the safe amps at any given moment... and go CLACK!

## Forget the Load Calculation. Use Grid Limiting.

Here's a secret: The average American home draws 1.2 kW or 5 amps @ 240V. Don't take my word on it, look at kWH on your monthly electric bill and divide by 720 hours. Yet your house rocks 100A (24kW) and can go as high as 200A (48kw). How can it possibly average 5 amps? Because most of the time, your house takes far under 5 amps. Often under 1 amp. Whole house. Really.

And that means, there's PLENTY of headroom to charge EVs the vast majority of the time, just not ALL - and that "not all" problem has traditionally been a huge barrier.

But this is the 21st century! We have 1980s-era microcontroller tech!

So it's as simple as "put a clamp meter on the panel, and have the EVSE prevent the load from exceeding 80 amps in this panel". And then the EVSE adjusts the amp signal to the car, accordingly, and the car obeys. This happens dynamically on the fly - you can watch it happen exactly here (29:29) in the previously mentioned video.

That was easy!

"Surely there must be a gotcha." Yes - limited choice and higher cost EVSE, and requires a bit more skill to set up. However, the real upside is, usually we try to convince you that you don't NEED a 50/60 amp circuit and a 20A circuit is more practical to provision? You get to ignore that advice! Go ahead and provison 60A! There's no harm in that because your house has 60A to spare most of the time. And now, we know when that is.

Since the EVSE will back off charge rate - to zero if necessary - that means it counts for 0 amps in the Load Calculation for that panel.

That was easy!

So... even if your house panel's Load Calculation will be hostile to adding an EV ... it'll work with this approach.

The best kit on the North American market today to use this feature is the Emporia EVSE with matching Emporia VUE Home Energy Monitor. (in Europe look at the Myenergi Zappi). As J1772 (normal plug) EVSE's go, this is actually a pretty good bargain. It doesn't compare to the subsidized \$400 Tesla Wall Connector unfortunately, and you'll need Tesla's adapter to make it fit your car. And, boo hoo, it won't automatically open the charge port door when the plug gets near it - life is rough!

On the upside, you get a home energy monitor. So there's that :) Also, the Emporia system (Zappi also) supports solar management, so you can set up your system to preferentially dump solar into the EV battery instead of selling it back to the grid. That's a winner for some solar tariffs (e.g. non-net-metering).

AFAIK the Tesla Wall Connector does not support doing grid limiting or solar-preference with a current transformer or home energy monitor. More's the pity - it does a fantastic job with Power Sharing, another power management strategy for multiple EVs on limited current allocation.

## Or... um..... You don't need a 50/60A circuit.

And we're back to our stock advice and the one from the "bonkers" video I mentioned above - hardly anyone needs 60A (people who do, don't buy EVs because it isn't really practicable - either to drive 400 miles a day, or to use an EV if you do.)*

Now we must do the Load Calculation,and your city has given you one of the best I've seen. (in terms of being most clear and understandable). That said, their form has errata and let's fix them.

• The solar is disregarded entirely.
• They give you default numbers for range and oven. Cross those out and use the actual numbers off your oven nameplate (I gather the range is gas).
• For "Appliance Circuit - 1500 watts" they actually mean only certain very specific circuits: the 1 mandatory 20A circuit in the laundry room, and the 2 or more 20A general-use receptacle circuits in the kitchen intended for plug-in typically countertop appliances, although wall receptacles in kitchen, dining and pantry count too. A dedicated circuit e.g. to a built-in microwave does not count. If you have a dedicated fridge circuit, use the nameplate watts of the fridge (which is quite small).
• Feel free to override the default numbers for most appliances with the actual nameplate number off the appliance.

Now. You do a separate Load Calculation for each subpanel to make sure it is properly sized. Frankly I'm not worried about the A/C only panel; your main subpanel will be the pinch point here.

Then, you do another Load Calculation for the whole house to assure the service is properly sized.

I imagine you're able to "spreadsheet" the largest number possible in column 3 that will work in all calculations.

Now, we must proceed carefully. The column 3 number is derated 125% in the Load Calc, yet the EVSE wants to know the breaker size, and will also apply that derate. E.g. if you tell the EVSE that it's on a 20A breaker, it will tell the car to take 16 amps. So we must "un-derate" it when choosing breaker size. let's say you found that 5014 watts is the most you can put in column 3 without putting amps over 100A. In this spreadsheet, that means car actual charging at 5014 watts or 20.9 amps. We need to multiply that by 125%, giving 26 amps, for the largest breaker we can put on that EVSE. So you commission the EVSE as being on a 25A breaker; it limits the car to 4800 watts, which works.

## Hey. What about putting the EVSE in the A/C panel instead?

Wait, what? Oh sure. You hadn't mentioned it so I assume it's not practicable, but yeah, if you can do it, go for it. All the above stuff applies - the Load Calculation or the Emporia with grid limiting.

If you want to have the Emporia preferentially charge with solar, then the Emporia VUE will need to have a current transformer installed on the solar feed. I don't know if Emporia supports multiple VUE's on the same house, otherwise you'd need to find a shielded route for the current transformer cable - it must be inside Class I wiring methods e.g. inside conduit or using AC power rated cable like UF-B or SEU.

* Though, this does beg the question - people who drive around hundreds of miles maintaining DC fast chargers - what do they drive? Probably a Transit van or F150 non-Lightning, since they seem to always find themselves at DC fast chargers which are broken.

• what do they drive - If you move to a new town and there are two barbers, which one should you go to? The one with the worse haircut, because he cuts the other barber's hair. Commented Apr 10, 2023 at 22:18
• Enlightening answer. Emporia does support multiple Vues on one house so long as one of them has access for the main CTs to wires that feed them all. The others will have their main CT ports unused. This could be achieved in OP's case at the main lugs. All Vues need to be in breaker panels (or similar) powered by both phases, and with access to WiFi. Commented Apr 11, 2023 at 2:24

You don't need 50A (or even 40A, and probably not even 30A) to charge your Tesla in a reasonable amount of time each night, unless you do a huge amount of daily driving. If you need extra once in a while, that's what Superchargers are for.

So the challenge is not to figure out how to get 50A. It is to figure out how much you can safely and legally get, and then size the EVSE to match.

The first step is two load calculations. One for the entire house to determine if your utility service (and the main panel) can handle it. For the moment, we'll assume that provides at least 30A, quite likely even 50A. The second is for the panel where the load will be installed. Which is where it gets tricky.

There are some complicated rules regarding solar. Solar power is going the opposite direction of your loads, so in some ways it shouldn't matter at all. But there are rules having to do with the bus capacity of the panel, and the end result is that you can indeed have a situation where after solar is added you can't add anything else. That may be the case here. Or it may be that the solar installers play it safe - e.g., assume 2 x 100A = 200A total (or 160A continuous if everything was done properly, ha ha ha) and size the solar accordingly, not leaving much room for expansion.

So I think that 100A for A/C may be the key. Is that 100A connected directly to the A/C units? I doubt it. If it is then add up the nameplate ratings of the attached units and add 25%, and that is the load to be used, not 100A. Alternatively, if the 100A breaker feeds a subpanel, do a load calculation on that subpanel. If the loads are just the two A/C units plus a convenience circuit then you may well have spare capacity there. It may be that you can downgrade that 100A breaker to 70A or 60A and thereby free up enough space in the main panel to satisfy the solar installers (you'd need them to come back and remove their red sticker or an inspector will likely reject you adding anything to the panel) and use the balance to charge your EV.

If that doesn't work then the EVSE will need to go in your overloaded "runs everything in the house but the A/C" subpanel. A proper load calculation there could show:

• Already oversubscribed. If that's the case then you can't add anything and may need to make some other changes for safety's sake.
• Close to capacity. If that's the case, then see what the balance is. If you can 20A @ 240V then that is enough for most people. According to Tesla, you should get on the order of 12 - 15 miles added per hour of charging at 20A (16A actual). If you charge at least 8 hours a night then that gets you at least 100 miles per day. The typical number I have heard for US drivers for a long time (long before EVs) is 12,000 miles a year, which is around 33 miles per day. Assuming that your days are not all average but you are within that approximate total then you'll be just fine adding 100 miles of range per night.

You're not going to get 50A of capacity from a 100A panel on a large house - just not going to happen. But 20A? That's possible.

• Ok remind people they don't necessarily need 12kW charging stations at home. But I think the point is wildly overstated in these forums. 1) A 25 mile urban commute with heavy traffic, A/C and aggressive driving is something millions of people do. Not me, but I wouldn't rely on a 20A charger for that. 2) If you're paying an electrician to zap up your garage for the future, you're not going to get ready for two cars? Two cars sharing 20A and you come home from work nearly depleted and your teen takes the car out, and you what just pray? OP, with 200A, should be able to do better. Commented Apr 10, 2023 at 14:30
• @jay613 You have to work with what you have. An upgrade past 200A gets expensive for a bunch of reasons. Which is why my first recommendation is not "work within the 100A house subpanel" but "see if there is a reasonable way to cut down the 100A A/C so that you can get this on the main panel". But if all you can get is 20A, it is workable for most people, most of the time. Commented Apr 10, 2023 at 14:32
• Yeah, that's actually a real problem, the network peak happens because people drive home, crank their A/C and start cooking. If they plug in their EV also, that'll be a real mess. But y'know, once people get settled with EVs, they'll realize that when they plug in at the end of the day their battery is at 83% (or 70% in my case with my old 40-mile-each-way hell commute), and they'll go "y'know, that doesn't really need to charge Right Now". Commented Apr 10, 2023 at 20:45
• @jay613 I used to have a 40 mile each way commute in the Bay Area (talk about hell commute) and a 20A circuit would have amply served my needs. Your opinion is just range anxiety talking. To your root gripe, here's the problem: If we portray 20A charging as a weak compromise they'd have to "settle for", then everyone reading that will think "Oh, well I better get 50A / service upgrade, or just not get an EV" and that would be a terrible message. That is the prevalent mindset already, and we are trying to stomp it out. Commented Apr 10, 2023 at 20:46
• @jay613 Clearly you have no experience with EVs. AC and heavy traffic with "aggressive driving" don't hurt range much - the AC is a heat pump, and regenerative braking recaptures a lot of energy. The killers are electric heat and high-speed highway driving (drag is the square of velocity). That said, a 50-mile round trip can be handled by even a 15A 240V charging circuit - 2880W for 12 hours can recharge 34KWh, which is 68 miles at an abysmal 500Wh/mi. My inefficient ID.4 isn't even that bad during the winter. Commented Apr 11, 2023 at 12:29