How to Calculate if Safe to Charge at 48 amps?

[joebruin77]

Hi, everyone,
I live in SoCal and in the mid to late summer, temps where I live can go above 100 degrees for several weeks in a row. I have a Tesla Wall Charger and I can charge up to 48 amps. My question is, how do I calculate if it is safe to charge at the full 48 amps during periods of high electricity use? My house has a relatively new 200 amp electrical panel. If I am using the central AC (which I think uses 50 amps), how do I know if it is safe for me to charge at the full 48 amps? One option is to avoid charging during the day and only charge the Tesla at night. Another option is to charge during the day but reduce the max amps to 20 or 30 amps.

Any advice to figure this out would be much appreciated.

Thank you,
joebruin77

 

[F14Scott]

Start with this:
Electrical Load Calculations for Residential Panel - Online Load Calculator

What's your number?

 

[gfunkdave]

If your wall connector was installed by a licensed electrician, he performed the required load calculations and you can use your wall connector without worrying about it. Your A/C doesn't use anywhere near 50A - it has a high startup current and then settles to much less than 50.

 

[joebruin77]

hey F14Scott and gfunkdave.

thank you to both of you for the helpful replies!

 

[COrocket]

Don’t SoCal utilities give you a discount to charge at night in order to reduce peak grid loads? Might be worth looking into and scheduling the charge at night. Starting the day with a full charge is usually sufficient unless you are going 200+ miles a day.

 

[joebruin77]

Thanks for the suggestion. They do offer a discount, but in order to get it, I have to install a time of use meter. I'm checking out all the costs and rebates currently available.

 

[linkster]

I would charge at a lower amperage during the cooler nighttime temps to reduce the load on the battery cooling system.

 

[nomis_nehc]

Encino so it's possible you're on LADWP. Not familiar with LADWP's time of use, but make sure you do your analysis first of course. If you fall under the Edison territory, it may be worthwhile to consider the TOU-Prime rate as well.

 

[linkster]

Opps, I forgot to mention that since I keep my cars forever and due to the expense of the traction packs in the S and M3, I insulated the garage and installed a Mitsubishi mini-split to keep temps more accommodating for the girls during the hot summer months.

 

[Lloyd]

You didn't need to air condition your garage! The cars have their own air conditioners to keep the battery at the proper temperature. A little ventilation is all you need even in the warmest days.

 

[Saghost]

The car is pretty smart. It tracks the voltage before it loads up and the voltage drop as it loads the line, and will cut back the charge rate if it sees an excessive drop or uneven voltage.

 

[GSP]

@joebruin77

Most overnight charging at home can be done at 24-32 Amps and easily be done before sunrise. If you don’t have a long commute even 16 Amps will be sufficient.

48 Amps is nice to have available if you want to quickly charge to 100% before taking a long trip. Also, some people have short off-peak windows to charge at night, as little as 4 hours. 48 Amp charging can be a big help to them.

I charge at 32 Amps, scheduled to start at 10 pm or later. That is much easier on the 25 kVA transformer that my utility uses to serve many homes. I don’t want to needlessly cause an outage that would cost my utility and inconvenience my neighbors.

For your SoCal situation, I would recommend installing the HPWC to supply 48 Amps (60 A breaker and wiring). Start out charging at 24 A during off-peak hours. If needed, turn it up so that charging completes in the off-peak window. You also will have the ability to crank it up to the full 48 A when you need a quick top up.

Enjoy your 3 ,

GSP

 

[lolder]

I would charge at a lower amperage during the cooler nighttime temps to reduce the load on the battery cooling system.

You don't have to worry about that. The car handles 150 kw supercharging. 48 amps at 240 v is only about 11 kw. Higher AC charge rates are more efficient.

 

[joebruin77]

Thank you to everyone who posted a reply. I appreciate your input.

I know there have been other posts on this question, but thought I would revisit it here. Some people say it is easier on the battery if you charge at a lower amp. But others say it is actually more wear and tear on the battery system because the total charging time is longer. To pick random numbers as an example, is it better to charge at 48 amps for 1 1/2 hours or at 30 amps for 2 1/2 hours? I was thinking of reducing my amp rate not so much for the car but more for the effect of the total amps available for the rest of the house. But come to think of it, which would be better for the car's charging system overall? Thank you.

 

[dhanson865]

You didn't need to air condition your garage! The cars have their own air conditioners to keep the battery at the proper temperature. A little ventilation is all you need even in the warmest days.

If the garage is attached (especially if it's below a living space) it's a great idea to insulate it even if you don't actively air condition it. The car might not care as much but it will affect your overall electric bill for conditioning the rest of the house.

It also means the driver doesn't have to worry about preconditioning as much before a trip.

 

[Saghost]

Thank you to everyone who posted a reply. I appreciate your input.

I know there have been other posts on this question, but thought I would revisit it here. Some people say it is easier on the battery if you charge at a lower amp. But others say it is actually more wear and tear on the battery system because the total charging time is longer. To pick random numbers as an example, is it better to charge at 48 amps for 1 1/2 hours or at 30 amps for 2 1/2 hours? I was thinking of reducing my amp rate not so much for the car but more for the effect of the total amps available for the rest of the house. But come to think of it, which would be better for the car's charging system overall? Thank you.

In principle, slower charging is better for the lithium cells in the pack. However, all AC charging in a Tesla is slow enough that I doubt you'll see a meaningful difference.

Other systems like the charger module and coolant pumps will technically benefit from a shorter running time, though again I doubt you'll see a measurable difference in normal operation, and it may be offset by increased thermal cycling in the charger module.

The efficiency of the charger seems to be higher at higher rates.

Overall, it's not really a problem. The car is designed to handle it, and the differences in durability/reliability between AC charge rates are unlikely to matter for the life of the car.

 

[posity]

You are way over-thinking this. Your home electrical panel and the Tesla were designed by experts that have considered these conditions and already have built-in protection.

Your modern 200A panel is designed so that if the total loads exceed 200A, your main breaker will trip. If you find that happening (meaning you have exceeded all the current that your home can safely handle), then you can start dialing down the Tesla charge rate. So just charge at your convenience: your panel will protect you from unsafe scenarios. There are some older split bus panels that do not have a main breaker: most electricians will strongly advise folks to replace those style panels. But you said your house is new and split bus panels were eliminated decades ago.

Regarding whether to run the Wall Charger at full bore, just rely on the considerable thoughtful engineering that Tesla has performed. Plug it in, set the max charge to 80%, and forget about it. There is a complicated relationship between rate of charge and frequency of charge that affects the life of your battery. All of us yahoos on the Internet (myself included, and I'm an electrical engineer that installs battery-based systems) are not privy to the mountains of research and development, and the complexity, that have gone into the Tesla battery design. I believe it is fair to say that Tesla knows more about battery designs, and battery management controllers, than anyone in the world. I think its safer to rely on Tesla than the google-informed (or worse) opinions about electrical operation you'll get from the Internet.

 

[joebruin77]

Again thank you to those of you who took the time to reply.

The bottom line is that I need to stop over thinking this, that the Tesla and the modern electrical system in my house have been designed by experts who know what they are doing, and that I should just charge my car and not sweat the small details. Great suggestions! Thank you.

 

[eprosenx]

You are way over-thinking this. Your home electrical panel and the Tesla were designed by experts that have considered these conditions and already have built-in protection.

Your modern 200A panel is designed so that if the total loads exceed 200A, your main breaker will trip. If you find that happening (meaning you have exceeded all the current that your home can safely handle), then you can start dialing down the Tesla charge rate. So just charge at your convenience: your panel will protect you from unsafe scenarios. There are some older split bus panels that do not have a main breaker: most electricians will strongly advise folks to replace those style panels. But you said your house is new and split bus panels were eliminated decades ago.

Regarding whether to run the Wall Charger at full bore, just rely on the considerable thoughtful engineering that Tesla has performed. Plug it in, set the max charge to 80%, and forget about it. There is a complicated relationship between rate of charge and frequency of charge that affects the life of your battery. All of us yahoos on the Internet (myself included, and I'm an electrical engineer that installs battery-based systems) are not privy to the mountains of research and development, and the complexity, that have gone into the Tesla battery design. I believe it is fair to say that Tesla knows more about battery designs, and battery management controllers, than anyone in the world. I think its safer to rely on Tesla than the google-informed (or worse) opinions about electrical operation you'll get from the Internet.

FWIW, you should never rely on your main breaker tripping due to an overload. Your primarily line of defense is “load calculations” which ensure you don’t hook too much to your electrical system. The breaker is just a backup and should not be relied on. They sometimes fail to operate.

The same holds true for a split bus panel but even moreso since there is no main breaker.

I do agree that you can just charge your Tesla at the max rate your EVSE can supply and not worry about it. My M3 LR charges at 48a daily. The charge rate is so vastly below the supercharging rate that I don’t worry about the batteries. It could be harder on the electronics though since it is running them at their design spec limits.

I charged my car all weekend on a 20a 120v circuit and I hated how long it caused the cooling systems to have to operate...

 

[posity]

eprosenx - just so you learn because you do a lot of posting here, this idea that the load calc is your protection is entirely wrong. Your breakers are ALWAYS your circuit protectors: the idea that they fail and a load calc is your savior is just not how it was ever intended to work, or how it works. A load calc is a statistical calculation that represents the likelihood of a mains overload occurring on a regular basis: we want to avoid the main circuit protector tripping, so we do a load calc. If we simply total up all the loads and make sure they don't exceed the panel capacity, most homes would fail miserably (and not be able to install near the number of breakers you will find installed in most panels). However, NFPA recognizes that statistically you just never run all your loads at once. So with a load calc, we are playing the odds that an 'all on at once' or 'most on at once' occurs rarely. Your main breaker is your mains circuit protection in case that statistical load calculation exceeds a sigma that NFPA has considered reasonably probable.

As I said in my post, I am an electrical engineer. These forums are tough to read when you do this for a living as there are so much misinformation spread on these forums and it is virtually impossible for most people to know which posts are correct. Bottom line is most folks should strongly consider relying on the services of a professional electrician, study up, and ask lots of questions of your electrician when they show up. Great to see that the OP's concerns were handled here.