40AMP or 50AMP how does it know

40AMP or 50AMP how does it know

Brought home my MS last week. I have a 240 outlet on a 40 amp breaker in the garage from my failed Nissan Leaf experiment back in 2011. The outlet I had was the wrong plug configuration so I called my trusty sparky who has about 50 years of experience to come over and change the plug that I picked up at Home Depot for $8. He said I could pull 50 AMPS easily off my 40 AMP set up as the distance from the plug to the breaker box is so short (about 2 feet). He gave examples of using a 20 amp tool off a 15amp breaker- said it's done all the time etc. His question was how does the car know whether your set up is 40AMP or 50AMP? As I learned from this forum if I had a true 50AMP set up the car would charge slightly faster? Thanks!

J.T. | August 14, 2016

It's like how does a measuring cup know it's full even if you're pouring more water into it. Max charging on a single charger is 40 amps.

steveg1701 | August 14, 2016

Though the car only pulls 40 amps it is a continuous load, which requires a circuit at 120% (e.g. 50A). Further, your friend has installed a 50 amp connector on a 40 amp circuit which is a code violation. If the wiring is thick enough for 50 then just spend another roughly $8 for the proper breaker and do it right!

When I first got my car it arrived about a month earlier than my HPWC did. Our home has a plug outside with a configuration that will support either a 15 or 20 amp plug so I got the Tesla adaptor that would let the car get 20, it worked at first but popped the breaker after a while. Investigation revealed that the 20 amp connector was on a circuit with a 15 amp breaker.

skip8jones | August 14, 2016

@72echo, Your trusty sparky's example is not applicable in the case of an EV charging application. One can draw more amps than a breaker's rating for a short burst but not for an extended period of time. At the very least, the breaker will trip off. At worst, wiring will short and burn. You didn't specify what outlet you purchased but it best be rated for 240V/40 amps in order to consistent with the 40 amp breaker and wiring size.

An EV charger draw is continuois during charging and the draw in the case of your MS, will be 32 amps, 80% of 40 amps. This is all code. Check with a proper electrician.

72echo | August 14, 2016

Thanks for the responses. I'm still confused. We did not install a 50AMP on the existing 40AMP. If I had a 50AMP set up the car would pull the full 40AMP ... right but with a 40AMP the car pulls less.

In other words does the MS always pull 40AMP and 40 is too much for the circuit the breaker trips?

skip8jones | August 14, 2016

@72echo, Upon further thought, did you purchase a NEMA 14-50 outlet in order to be compatible with the Tesla supplied UMC? If that is what has been done here and the breaker is still a 40 amp breaker, your car will think that it can draw the full 40 amps which should eventually trip the breaker. This is a dangerous situation. Do you know if the wire from your breaker to the connector box is large enough for a 50 amp circuit?

If you are going to stay with a 40 amp circuit, consider wiring in a Tesla wall connector. That eliminates a plug-in and leaves your UMC in your trunk where it is always available.

skip8jones | August 14, 2016

@72echo, Upon further thought, did you purchase a NEMA 14-50 outlet in order to be compatible with the Tesla supplied UMC? If that is what has been done here and the breaker is still a 40 amp breaker, your car will think that it can draw the full 40 amps which should eventually trip the breaker. This is a dangerous situation. Do you know if the wire from your breaker to the connector box is large enough for a 50 amp circuit?

If you are going to stay with a 40 amp circuit, consider wiring in a Tesla wall connector. That eliminates a plug-in and leaves your UMC in your trunk where it is always available.

murphyS90D | August 14, 2016

If you have a 40 amp breaker, set the car to 80% of 40 = 32 amps.

skip8jones | August 14, 2016

We are crossing messages. If your wiring, breaker etc is all at 40 amps, the car should be set for and draw 32 amps. You can set the car's draw in the "charge " window. You do not want to draw the full 40 amps on that circuit.

If you had a 50amp circuit, the charging draw would be 40 amps, 80% of 50.

72echo | August 14, 2016

the wiring and breaker are all 40AMP. So every time I charge somewhere new - say at the mall or a hotel I need to tell the MS what to draw?

skip8jones | August 14, 2016

No, the plug configuration will tell the car what to draw.

Tropopause | August 14, 2016

First, your Leaf setup assumingly needed 30 amps to charge, hence the 40 amp breaker set-up.

Most Tesla Model S owners charge at 40 amps when using the UMC on a 50 amp breaker.

HPWC's can charge up to 80 amps on a 100 amp breaker.

The new Tesla Model S with facelift, now come standard with 48 amp charger which will only charge at 40 amps on UMC but can charge at full 48 amps from HPWC.

You put the right plug (NEMA 15-40) on the wrong wiring set-up (40 amp breaker). This cross-installation will confuse your car into thinking it is a 50 amp breaker and thus charge at 40 amp (with no buffer for protection.) Therefore you must manually program the car to charge at 32 amps to achieve the 80% rule 40x80%= 32.

Either put a 30 amp plug on the circuit or upgrade the circuit/breaker to 50 amps.

p.c.mcavoy | August 14, 2016

@72echo - Let me take a shot at answering your questions. My understanding is based upon reading information published by Tesla, as well as what I find on other sites. Here is what limits charging amps and how your car will know if it can charge at 40 amps vs. other settings.

1) The charging adapter and plug you are using.

If you're using the Tesla supplied UMC, then you will only get a max of 40 amps even if you had your NEMA 14-50 outlet wired on a 100 amp breaker with true 100 amp circuit capability. That is because the UMC is only rated to a max of 40 amp charging and will limit the situation. This is true whether you have a single charger or dual charger in the vehicle. If you use one of the plug adapters for the UMC other than the 14-50 adapter, the UMC recognizes this and will step the amperage down accordingly.

If you are using something like a HPWC, then you can get up to 80 amps based upon how it is configured. If you look at the installation instructions for the HPWC that you can find under the Support / Charging links, you will find that there are a series of internal switches that can be set.

Various public J1722 charging stations may be set up to provide different levels of charging. This is based upon the rating limit of the individual charger. I use a J1722 station periodically at my work location. Depending upon whether other vehicles are charging I have seen 30 amps while other times only 22 amps. That's all being limited largely upon the power that the charging station can supply.

2) The charging logic in the Tesla monitors supply voltage, and if it senses a higher level of voltage fluctuation, it may automatically step down the amperage it draws. This is a protection algorithm in case there might be a wiring issue on the charger supply. I've seen some posts suggest that if this protection logic kicks in there may be a pop-up message displayed in the car, but I have never experienced this and can not say based upon personal experience.

3) As for your "trusty sparky", I take his input with a major grain of salt. Many tools rated at 20 amps will not actually draw 20 amps continuous. That is a peak rating which may occur for only a very short period of time. An example of this is what it know as starter rush-in current. The current required to start an electric motor (for example, your home AC system first kicks on) can be significantly higher than what it will then draw after it is up and running. This in-rush current lasts for only a very short period. That is NOT the case of what your MS will do when it charges. If you watch it, the charging logic starts off and ramps up the currently very slowly, but once it reaches the 40 amp level, it STAYS at 40 amps continuous for most of the charging period. It is that reason that Tesla recommends (correctly in my view), that your 14-50 NEMA outlet should be on a 50 amp breaker and supporting wiring in order to sustain CONTINUOUS charging load of 40 amps.

GHammer | August 14, 2016

To answer your question in a general sense, you don't need to ever tell the car what current to charge at unless you are connecting in a non-standard configuration.

Using the UMC, the adapter plug has a resister in it that tells tha car what the maximun charge current is for that plug. If you have the 14-15 adapter on it will tell the car to charge at 40amps, if you have the 5-15 adapter it will set the car to 12 amps.

Using a Tesla wall connector or other J1772, the connector will tell the car what current it is capable of delivering based on how the dip switches inside the unit was set.

The only time you need to manually set the current is when your connected in a non-standard configuration like using an aftermarket adaper to go from the 14-50 to say a 10-30 or if your current setup doesn't have the sufficient size wire to support a 50 amp breaker and you need to dial it back to 32. In this case the car will remember the location this non-standard setting was used and will only set it there in that location.

BTW You need a new sparky if he doesn't know the difference between intermittent and continuous loads.

Solarwind | August 14, 2016

@OP It appears your question was never answered. How does the car know what to charge at? The car sense the voltage before it starts to charge. As the current increases the voltage will drop, if the drop is beyond a certain point it will lower the current setting so the voltage drop in the circuit will not cause excessive heat to the point of fire. You will get a warning on your dash and on your phone that current has been reduced.

ir | August 14, 2016

@Solarwind: While it is true the car will lower current of the voltage sags, this is an emergency feature not meant for normal use. If you "lie" to the charger, it is more likely to throw the breaker or drop to an extremely low current.

The real way the car "knows" is with the type of adapter head in the UMC as @hammer describes.

> "So every time I charge somewhere new - say at the mall or a hotel I need to tell the MS what to draw?"

If you plan to "lie" to your car regularly, then you need to manually adjust the current on the touch screen.

But at the mall, you are going to use the 5-15 plug which will tell the car to limit charging to 12A. If you use the J1772 plug, the car can directly ask the charging station how much power to draw.

AoneOne | August 14, 2016

Just one more note: once you manually set the charging current at a location, the car will remember that setting and use it whenever you charge there. You won't have to remember to set the current on future visits.

tes-s | August 14, 2016

Thank your friend, and consult a licensed electrician.

Anthony J. Parisio | August 15, 2016

My understanding is the car tests the line and adjusts to 20% less then what it is given. This is to insure against a fire. This was discussed so many times before on this forum. Check the owners manual. I have had my car for two years and have never set the Amps. once. The car dose that. For example: I have a HPWC set to 50 Amps. The car only uses 40 Amps. When I go to another HPWC set at 100 Amps car pulls 80 Amps. I did nothing to change the setting. Read the manual section about charging.

@72echo if you are giving the car only 40 Amps from the breaker even if you use the Nema 14-50 outlet rated for 50 Amps you will pull only 32 Amps. Even if you change the breaker to 50 Amps but the wires are some how only 40 Amps or less (this is a dangerous thing to do!) The car will sense the resistance in the wire and pull less than 50 Amps. You trying to force the car to do other wise should be corrected by the car. In any case Sparky is WRONG! Do it right. Put a 50 Amp breaker with 50 Amp wire and 50 Amp outlet. Then let the car do it's thing.

Rocky_H | August 15, 2016

There are some specific questions that are not being asked (or not clearly enough) about @72echo's setup.

When you say " I have a 240 outlet on a 40 amp breaker", that is no informative. Consult this chart to find out what kind of outlet you actually have:

Part of the thing that is weird with this is that there is not actually a 40 amp outlet type that directly matches a 40A breaker. So, there is a provision in the electric code that allows a 50A outlet on a 40A breaker in the very specific situation where the appliance being plugged into it is specifically marked as a 40A appliance.

Since the Tesla charging cord is expecting a 14-50 outlet on a 50A breaker, that would be a code violation. Also, since your setup was originally installed on a 40A breaker, the wire is probably sized for a 40A circuit too, so is not correct for a 50A circuit.

So, two good safe options where you will not need to worry about manually adjusting current in the car:

(1) If it's a 40A breaker and wire size, get rid of that outlet and mount a wall connector that is actually set for 40A circuit supplying 32A continuous current.

(2) Redo the whole thing as a 50A. That will need a 50A breaker, 6 gauge copper wire (specified by Tesla's installation instructions), and a 14-50 outlet. Then, you can safely use the Tesla's charging cable.

SomeJoe7777 | August 15, 2016

Tell your trusty sparky with 50 years of experience to stay retired, because he doesn't know what he's doing.

There are a few very specific, safe, and correct ways to do this per the National Electric Code, and a million different ways to violate it.

If you intend to charge your car with the UMC (Universal Mobile Charger), the proper setup is:

- NEMA 14-50 outlet.
- 50 A dual pole circuit breaker.
- 6 gauge Copper or 4 gauge Aluminum wire (Aluminum wire requires BOTH the breaker and the outlet to be rated for Aluminum wire connections).
- 14-50 adapter on the Tesla UMC.

If you do not have 6 gauge wire and you don't want to replace it, then you need a different setup.

You cannot use an outlet that is rated for more current than the breaker and/or wire (e.g. 50 A outlet on a 40 A breaker, or 50 A outlet on 8 gauge wire is a code violation).

You cannot use wire that is rated for less current than the breaker (e.g. 8 gauge wire on a 50 A breaker is a code violation).

If the wire is in conduit, there can be additional temperature and conduit fill restrictions that further limit the current and/or require thicker wire gauge.

You want an outlet that you can obtain the proper adapter for on the Tesla UMC, and only the 14-50 adapter is readily available.

Get a licensed electrician who is familiar with Tesla charging setups and let him look at your situation and recommend the lowest cost, but safe and within code spec setup.