How Does Charger Know Circuit Limitations?

How Does Charger Know Circuit Limitations?

My brother's Volt has a manual switch for use on a 110 volt circuit: 15 amps for a dedicated circuit, 8 amps for a shared circuit. I don't see any kind of setting on the Model S to limit the current while charging. I understand that the charger could sense what type of adapter is in use, and regulate the current for higher voltage circuits, but I don't see how it could know what kind of fuse or breaker is in use, or whether that circuit is dedicated or shared. I don't want to blow a fuse while charging from a motel's standard outlet.

Any ideas?

ir | 15 décembre 2012

There is a setting on the screen where you can set the amperage in 1A increments. Dial it too high and you pop the circuit breaker / blow a fuse in the charging station / house. When that happens, pick a lower setting, reset and try again.

DouglasR | 15 décembre 2012

Hmm. I'll go out and check the screen again. I didn't see that setting. Can you tell me where it is?

ir | 15 décembre 2012

NOTE: I have not received delivery yet. However based on playing with the showroom model and RTFM here:

When you pop the charging door, the big charging screen shows up. That is where you can:
- See current status
- Set standard vs. long range
- Adjust the charging current

Chris25 | 15 décembre 2012

The outlet will determine what the max amps are. Ex 14-50 is max 40 amp flow. The car does sense which adapter you have connected and will adjust the max pull off that. If I recall, it will pull up to 80% of the rated amps for that outlet.

dtesla | 15 décembre 2012

I can't find the reference, but I want to say the car communicates with the charger to negotiate how charging will proceed.

tezco | 15 décembre 2012

My AV J1772 charger is rated at a maximum of 30 amps. The first time I plugged it in using that charger, the car automatically set it's maximum rate to 30. I'm assuming the charger negotiates with the car in this instance. I suppose the Tesla charging cord performs the same function, sensing which adapter plug is attached at the other end. I suppose that if you were to make your own NEMA 14-50 to NEMA 10-30 adapter, the cord might sense the NEMA 14-50, and initially set the max rate to 50 instead of 30. Fortunately Tesla lets you set your rate manually if needed.

DouglasR | 15 décembre 2012

Yes, I finally found the setting on the "Charging" tab. I don't know why I missed it before. I'm guessing that the J1772 unit communicates with the car to negotiate the correct amperage, whereas when using a standard outlet, the default amperage is based on the type of adapter that is connected.

This came up because I will be visiting my brother, who does his own wiring (in a Rube Goldberg sort of way). He was telling me he had his welder hooked up via a NEMA 14-50 outlet, but that he had fused the outlet at 30 amps. I don't want to blow his circuits.

sidney_wang | 26 janvier 2013

A quick follow-up question on @DouglasR scenario. So if he manually sets the amp intake of his car to 30 amps *before* he inserts the charging cable to the charge port, then the car charger will *not* sense the NEMA 14-50 adapter that is being used and thus will not try to re-sets the amperage back to 50 amps? And this feature is location-based? It remembers the max amp setting for this particular location?

pilotSteve | 26 janvier 2013

I've noticed several nice aspects of charging at limited amperage. I have a home-made 10-30 plug to 6-50 socket adapter (from a 10-30 dryer cord and a 6-50 socket) and this clearly requires special care and reduced amps.

(1) when the car begins charging it ramps up the current slowly; you have maybe 15-30 seconds before it draw full amps. So right after you plug in and the charging screen shows up (with its amp limiter control) you have time to set a lower limit. 24A in my case (80% of the 10-30's 30A breaker).

(2) the car geo references each place you charge and remembers current limits you set for that location. So next time I'm at my buddy's show with his 10-30 I will certainly double check everything but it should automatically limit to 24A again.

Tesla has done a great job implementing charging IMO.

sidney_wang | 26 janvier 2013

@pilotSteve - thanks! Your scenario is very similar to mine. I have a custom made L14-20 plug to 14-50 socket. The Tesla supplied L14-50 adapter will get plugged into this custom made socket. Based on your description of step (1), it sounds like the amperage can only be adjusted *after* you begin charging? There is no way to adjust before charging begin? 15 seconds (probably less for me since the outlet is 20 amps) should be enough time for me to tune down the amperage? I just don't want to cause the breaker to trip or burn.

Yes, the geo feature is definitely nice so that I don't need to stress myself out every time I need to do this.

awinters | 6 décembre 2015

I have a 40 amp 240V breaker going to my Eaton residential EV charger for my Nissan Leaf. I am getting a Tesla which utilizes 80% of the maximum breaker amperage (which is 80% of 40 = 32 amp). I was told that the Tesla will allow (80% of a 50 amp breaker = 40 amp). So if I swap the 40 amp breaker with a 50 amp breaker in the box, will I automatically get the higher amperage going through the Eaton EV residential charger, resulting in a higher amperage to charge my Tesla faster? Thank you.

Bighorn | 6 décembre 2015

Depends on the plug/charger, not the breaker. Tesla adapters will impart a current limit while home-builds will not and will need a manual override on the charging screen.

tes-s | 6 décembre 2015

No. The Eaton charger does not know how big the breaker is, and will therefore not put out higher current.

Not a good idea to put a larger breaker in anyway since the wire is probably not sized for a 50amp circuit.

You don't have to do anything. The Eaton charger will work fine with the Tesla. Just plug it in.

awinters | 7 décembre 2015

Thank you NOUVELLES. I am sure 35 amps of 240 will be fine, since I never anticipate depleting the Tesla to zero. I use the car as a town car and anticipate making some long distance trips because I now can.

Is there a way to calculate how many miles I can charge up my care at 35 amps 240V for a Tesla 90D?

Bighorn | 7 décembre 2015



AIA304 | 7 décembre 2015

Does car look at resistance of control circuit built into the handle/cable to determine the range of amperage to feed car ? Have seen this method used in past to determine what's plugged in, like earphones @8 ohms, versus line level output > 8 ohms when determining using which music device output level.

Anyone know if this is the case for MS ?

AIA304 | 7 décembre 2015

Aware one can adjust amperage on certain connections. Did verify Ted Energy system readings on lvl 2 HPWC from 10 amp to 80 and back down.

evsisson | 7 décembre 2015

If my home's heat pump kicks on while the car is charging at 40A, the car drops the setting to 30A. I guess the starting surge current of the heat pump causes a quick drop in voltage to the charging circuit. I'll have to see if I can set the charge current back up to 40A while the heat pump is running. Look like I'll be seeing this all winter.

Bighorn | 7 décembre 2015

The car reduces current to 75% if it sees any instability in the line. You should be able to boost it back to 40A, but it may not be necessary depending on how many miles are needed.

Rocky_H | 7 décembre 2015

@evisisson, Lots of people have seen that with an air conditioner or heat pump or whatever. If you turn your charging down a little bit, to maybe 37 or 38 amps, it lessens the load and the voltage drop some, so that most of the time it will make it immune to the heat pump triggering that safety lowering condition.