Electrical advice - 100A home service

Electrical advice - 100A home service

I am looking for advice from people who know their electric service. I am NOT seeking "legal" or "permit" or "what happens if you don't follow protocol" advice. I'll take care of all of that locally. What I'm seeking is the physical practicality of using *what I have* which I will explain in detail, with a new Model 3 that will draw a continuous 40A.

How my house is laid out now:
~~ Underground service of 200A or more capable (confirmed with utility)
~~ METER rated for 200A (confirmed with utility)
~~ On the (right) side of the house, where the utility comes in, and where the meter is, we have our MAIN. We have confirmed that the cables coming off the METER and connected to the MAIN (about 2 feet or less of cable) is NOT 200A cable. It's rated for 100A and has a 100A MAIN breaker on it.
~~ The cable that runs from the MAIN BREAKER, up the wall on the right side of the house... through two attics somehow... and down into the garage into the panel (on the left side of the house) is 100A rated as well.

The panel itself is maxed out on physical space. And the panel will not accomodate "twins" so we cannot double-up any of the existing breakers. My local electrician friend believes that, if we were to swap out the panel itself, giving us more space... then add a 16-50 NEMA 240V... that I'm still never going to come close to the 100A level. And we have done several AMP-CLAMP tests, and with central air on, a window air unit in a bedroom, all lights, tvs, tv gear, computers, 2 fridges, and a handful of other stuff... I got one leg of my panel to 22A and the other to 31A. I realize that doesn't take into account each unit STARTING UP... I further realize this doesn't take into account several other things like hair dryers, curling irons, blenders, vacuums, power tools, the higher draw stuff... but those are also devices that are almost never on and can easily be controlled (unlike a fridge or AC condensor, for example). They are also devices that would likely pop their individual 15 or 20 A breakers before allowing the house to hit 100A anyway.

So, to make sure I understand how this works... for the cable running from meter to panel on the 100A MAIN... that should be 100A **PER LEG** - correct?
Meaning, 100A service really means 100 on each SIDE of the panel. So the Tesla will be 40A on BOTH of course, because it's a 240V. AC and Furnace obviously the same. Let's assume that CAR and AC (or furnace) use even 60A across 240 (so, each leg)... that leaves me with 40A on each leg for all the 120V breakers, right?
Do most people feel that 100A is more than sufficient, or would you, to be extra cautious, bump service to 200A, which in my case, may cost several thousand and take me out of the running for the car anyway.

Mozart | 20. September 2017

I recommend installing a tesla wall charger on a 40 amp breaker. Set the operating current at 32 amps (#6 on the rotary switch inside the unit) This will charge at about 23 miles per hour. Dialing down the charging rate from 40 amps to 32 amps won't make that much difference.

sv_18 | 20. September 2017

First: I'm not an electrician, so take my comments with a grain of salt! (which shouldn't matter since you have an electrician)

I believe many jurisdictions have required calculations to ensure you don't exceed max load... if your electrician did that and you are okay, then I wouldn't worry too much. The nice thing is that although you want to charge at max speed, you don't have to. If you find that your breaker trips every time you charge with the AC on, then just bump down the max current the car will draw in the summer! It's a setting on the car which is remembered based on charging location (at least on the S and X)

Coastal Cruiser. | 20. September 2017

Nice clear description of your question brian. Here are some limited notes for you. Not an answer, as I am not an electrician...

1) Second, you may want to move this post to either the General or the Model S forum. You may well get an experienced answer here from a Model S owner with a Model 3 reservation, but there is a higher concentration elsewhere.

2) First, is your electrician friend not able to answer your question? There's no substitute for boots-on-the -ground.

3) As best I can tell from my backyard electrician knowledge, you are on the right track. Keep in mind too that if you skip the upgrade and wire for the EV, you will find out soon enough if you need to upgrade if breakers start to trip. That's what they're designed to do.

4) Can you elucidate back there on the connection between the METER and the MAIN PANEL. You say the wiring is 100A rated. But as you discerned later, is that 100A per leg (assuming you have three wires making that connection)? In other words do each of the two hot legs terminate to two separate 100A breakers. If so that would portend you not only have 200A service from pole to meter, but perhaps to the MAIN as well.

Frank99 | 20. September 2017

You'll need to put in a double-pole breaker (two slots wide). If your current panel is full, you'll need to swap the panel, or add a subpanel. If your usage is as you've described, a 50A breaker would be fine - sounds like you've got plenty of margin, especially as you're likely to charge the car late at night. If you're worried about load, installing a 14-30 Dryer outlet on a 30A fuse and buying a 14-30 UMC adapter from Tesla would also work.

Adding a subpanel would be the cheapest upgrade. You'd install an appropriate sized breaker in the main panel to feed the sub-panel, and move one or more circuits out of the main panel into the subpanel to make space.

If you need to swap panels, and you have 200A service to the meter, then install a 200A panel and upgrade the wires from the meter to the panel. The panel is on the order of $200, and breakers are less than $10 apiece if you can't re-use your old ones. The biggest cost will be labor and permits.

PhillyGal | 20. September 2017

While you didn't ask this part specifically, I wanted to shed some light on the overall use of 100 amps for a household that has an EV.

When we got our Model S, we had a 100amp panel. There was physically room for the 40/50 amp NEMA 14-50 to be on its own breaker. Charging the Model S overnight, we it was unlikely we'd be running anything more than our central AC, dishwasher and single speed in ground pool pump - we never had a breaker trip or problem charging.

In other words, with room on the panel to have a proper breaker dedicated, the total household shoudln't be an issue.

We did swap to 200 amps later just because we were getting solar panels and lumped it all together for tax purposes.Not that I can tell a difference other than the massive and ugly panel size that now requires us to move our wall-hung ladder to open. Pain in the balls.

Coastal Cruiser. | 20. September 2017

"that now requires us to move our wall-hung ladder to open. Pain in the balls."

She said it.

brian | 20. September 2017

She's from Philly. It's how the girls talk there. Also, I like her videos, so she gets a pass.

brian | 20. September 2017

Oh wait... my bad... not who I thought it was. Though, the first comments stand...

Carl Thompson | 20. September 2017

"Oh wait... my bad... not who I thought it was."

I think she may be who you thought she was.


Coastal Cruiser. | 20. September 2017

This is getting confusing.

Hey brian! How's your electricity plan coming along?

daryl | 20. September 2017

Install a Quad pole breaker and you won;t need to upgrade your panel.

bp | 21. September 2017

I'd recommend installing a 14-50 outlet instead of an HPWC, if you're going to charge on a 50A circuit. That's what we're currently doing with our S P85, with the charging dialed down to 30A (which is enough to fully charge the car - to 90% - overnight).

The HPWC doesn't provide any additional capability when you're running on a 50A circuit. Tesla sells a 14-50 charging connector (it is about the same price as an HPWC). The big difference is that installing the 14-50 adapter is simple - just plugging it in. For the HPWC, you are supposed to connect it directly to the breaker box, which means running conduit to the breaker box and installing the HPWC, which may cost more for installation - and an additional cost if/when you ever need to replace it or uninstall it.

With the HPWC, you can dial down the charging inside the HPWC. Using the 14-50 charging cable, you dial down the charging inside the car. Using geolocation, the car will remember that you've dialed down the charging at that location, and will use that lower charging level whenever you're connected to the 14-50 - at that location.

My recommendation is to keep this simple - and look at the total cost for installation/removal when deciding whether to use an HPWC or a 14-50 outlet.

Carl Thompson | 21. September 2017

"Tesla sells a 14-50 charging connector ..."

You can buy an extra one but one will be included with the car purchase.


PhillyGal | 21. September 2017

@brian - Yes, Philly girls have potty mouths. And yes, I do make videos under the alias Electric Jen. Never had the heart to change my forum name.

Brian | 21. September 2017

It sounds like your house is set up for a 200 amp service, but you only have 100 at your main panel, which means it should be much cheaper to upgrade if you have a need or desire to do so. It also sounds like with the power that you are adding to the power that you are currently using, should be okay.
If you put in a 240 volt circuit, for charging and you find that you are tripping breakers, then you could always upgrade your service.
One thing that you didn't mention. How much driving do you think that you will be doing each day? This will determine the minimum charging that you require. If you don't pan on traveling far most days, then you could even get by on 120 volt charging until you can get the service upgraded.
I myself will probably just use 120v at first, until I determine which bay will ultimately be my 3s home. My round trip to work is less than 40 miles, so I shouldn't have any problems. I'll just have to plan ahead if I am going to be going on a longer trip. Who knows, maybe I'll never have to put in a 240v circuit.

Brian (with a big "B")
Not the OP

rxlawdude | 21. September 2017

Coastal_Cruiser | September 20, 2017
"that now requires us to move our wall-hung ladder to open. Pain in the balls."

She said it.

It would have really been funny if the ladder was "well-hung." ;-)

andy.connor.e | 21. September 2017

On a 40A breaker, you've only got about 80% load available due to losses and temperature contingency, (resistance increases as temperature increases leading to more losses). 80% of your 40A breaker is 32A. If you really need faster charging than that, i suggest visiting a Supercharger from time to time.

Frank99 | 21. September 2017

Sorry, Andy.
On a 40A breaker with NEC compliant wiring, you have 40A available with less than 5% voltage drop, over the entire temperature range that the circuit is expected to see. Increased resistance due to temperarure is taken into account by assuming worst-case temperatures when sizing wires and other elements.

If the load on the circuit is expected to be on for more than 3 hours out of 6, then the load is required to be limited to 80% of the breaker/wiring rating. Technically, as long as you limited your charging sessions to 3 hours out of 6, you could safely pull 40A from the circuit. Except, of course, that the NEC defines an EVSE as a continuous load requiring the 80% limit, regardless of how long you charge.

andy.connor.e | 21. September 2017

You basically just elaborated why im right, and then essentially stated that if you use the circuit for less than 3 hours, its ok to plan for 100% load. Thanks for clarifying.

Carl Thompson | 21. September 2017

"You basically just elaborated why im right, and then essentially stated that if you use the circuit for less than 3 hours, its ok to plan for 100% load. Thanks for clarifying."

@Frank99 said neither of those things.

What you said was:
"On a 40A breaker, you've only got about 80% load available due to losses and temperature contingency ..."

Which is wrong.

You also said:
"if you use the circuit for less than 3 hours, its ok to plan for 100% load."

Which is also wrong in the context of EV charging.


andy.connor.e | 21. September 2017

This is what happens when too many people get involved. I'll be wrong and choose not to continue.

Thrillion | 21. September 2017

Why not put a 60 Amp breaker in?

I did not see a mention of a clothes dryer, dishwasher, hotwater heater or stove.
If there is nature gas running these, I'd put in a 60 amp and move on. If not, I'd upgrade to 200 amp.
We run all of our appliances at night, the exact time you will be charging. I believe you can schedule a charge time like at 2:00 am though.

A better test of your usage will give you the answer. Kill your main breaker one morning, use your clamp meter after 2 hours of no power and flip the main back on. See what the in rush is on the meter with everything drawing power.

Model_D | 21. September 2017

We have a 100 amp panel.
The most important thing is we have a gas furnace, gas stove/oven and a gas clothes dryer. When we moved in the AC was on a 50 amp breaker and the dryer was on a 30 amp breaker. I noticed that the AC has a requirement to be hooked up to a 30 amp breaker. It also had fuses just before the unit. Of course they are 40 amps. Sometimes I wonder why professionals are called professionals. It made me wonder if I could run the 50 amp breaker for the charger. However, the easiest thing to do was to swap out the outlet and get the 30 amp adapter for the UMC. About $75 and I am charging at 5.76 kW which is faster than I need. We have about 10 hours of charging at a reduced cost per night. Way more than we need.

If I were you I would install a 30 amp dryer plug (you might already have one) and charge at 24 amps. Unless you put tons of miles per day and are home for only a very short time, 24 amp charging should be more than enough.

Model_D | 21. September 2017

And a gas water heater.

rjadams72 | 21. September 2017

As long as the loads calc out ok, just add a sub panel. The circuit for the EVSE (not a charger people, the charger is in the car) has to be dedicated, no other loads.... yes that means the AC condensing unit too..... nothing else. Simple and cheap and please have a licensed electrician do the work..... with permits!
Fyi.... they're called conductors and feeders not cables.

bp | 22. September 2017

Each car comes with a Universal Mobile Connector (UMC) which can use multiple adapters for different outlet types - from a standard 110 outlet to a 14-50.

Tesla also sells a 14-50 connector - that only works on 14-50 outlets - which is about the same price as an HPWC (which have come down in price considerably since 2013).

While you can use the UMC for charging at home, we bought the 14-50 adapter and installed in on our wall with the Tesla cable organizer, making it easy to store the charging cable when the car leaves the garage - and that allows us to keep the UMC in the car for emergency charging.

Though... I can recall us using the UMC only a couple times since we bought our 2012 S P85, and with so many superchargers, destination chargers and 3rd party chargers available - as long as the car as the J1772 adapter, there probably isn't much need to have the UMC in the car any more...

Iwantmy3 | 22. September 2017

Keep in mind the time of day that you will be charging the car. I have a similar situation. However, I expect to charge the car between midnight and 6am 99% of the time. Our dryer and oven are never in operation at that time. The dishwasher might be (on a timer), but it is 120V and less than 15A. The only items that could be in play are the air conditioner and the furnace fan motor. The air conditioner is on a 30A breaker. The fan motor is much less.

Certainly, if I turned on everything to maximum now, I would overload the 100A breaker. However, between 12am and 6am there will be no issue. In the rare event that the car may need to charge in a different time period, I will need to be aware of what is being used.