charge at 48 amps with 50amp breaker?

charge at 48 amps with 50amp breaker?

At present, we charge at 40amps using the mobile charger; the breaker is a 50amp breaker. Soon, we will receive a wall charger. Will we be able to charge at 48 amps with a 50amp breaker?

PaulOutBox | August 1, 2018

You can only do 80% of breaker value. 40A.

parkerjs | August 1, 2018

To charge at 48 amps you will need to change the breaker to a 60A. If you already have a 50A in your panel it is a simple and easy change to make. I did this back in March.

tes-s | August 1, 2018

You may have to change the wiring if you increase the size of the breaker. You should also do a load analysis to make sure your panel can handle the additional load.

Any electrician can help yoy. If you do have to change the wiring, may as well put in as big a circuit as your panel can handle (up to 100amps).

murphyS90D | August 1, 2018

To charge at 48 amps you need a 60 amp breaker and the proper wire size that is rated for at least 60 amps.

kerryglittle | August 1, 2018

I have a 50 amp breaker and only charges at 32 amps. But I'm not in a hurry. LOL. | August 1, 2018

The Mobile Connector (MC), Gen 2 is limited to 32 amps. All Teslas can charge at least 40 amps with MC, Gen 1, and all new cars (in the last 2 years) can charge at 48 amps with the WC.

Note that the M3 with Standard battery will be limited to 32 amps, period, but none are available yet.

redacted | August 1, 2018

"To charge at 48 amps you will need to change the breaker to a 60A. If you already have a 50A in your panel it is a simple and easy change to make. I did this back in March." [And my house burned down]

Argh. To restate @tes-s, putting in a bigger breaker, by itself, will overload your wiring. You need wiring commensurate with the breaker. Which just goes to show, when it comes to advice you get what you pay for it.

greg | March 9, 2019

Late to the game here, but 50-amp and 60-amp breakers can take the same wire gauge. If you already have a 50 amp, you should have 6 gauge wire. Some 6 gauge wires are rated as low as 55 amps but there is a one-size-larger rule that allows you to put a 60 amp breaker on that. I talked to an electrician about it and suggest you do the same.

kevincwong | March 9, 2019

After using a Juicebox Pro 75 on a NEMA 14-50 to charge at Fiat 500e at 6.8kWh, I was excited to unleash the Juicebox to its potential with the 80A dual chargers in our Model S. However, the 14-50R and the charging plug is limited to not exceed 50A. In order to go beyond 40A, I would need to go with a hardwired HPWC and upgrade my breaker as well.

Funny thing is that I rarely charge at the 40A rate at home. Instead, I adjust the rate so that the car will finish charging just before 6:30am when it leaves the garage. Why? Because the slower rate of charge seems easier on the battery, and the battery is warm and ready to go. No more limited power due to temperature on a cold morning. Charging at 16-20A on average to get 220 range miles is plenty for my wife's roundtrip commute with plenty of overhead.

nothotpocket | March 10, 2019

@greg I'm pretty sure that's not correct for 6 gauge NM-B (Romex) wire. Because EVSE is considered a continuous load, you can not round up the wire size, like you would a circuit breaker. For most other 6 gauge wire (like in conduit) you'll be fine.

reed_lewis | March 11, 2019

I always refer to this chart for power capacity for different sizes of wire.

When it comes to continuous power, NEVER go to the next smaller sized wire. Go to the next larger sized wire. Your typical 50 amp load probably does not use 50 amps (or even 40 amps) for very long. For example an A/C compressor uses that much power to start up, but uses much less when running continuously.

But with an EV charging you are pulling that amount of power continuously.

milesbb | March 12, 2019

@nothotpocket I read through the 2017 NEC. I believe NEC 240.4B the "one-size-larger rule" does apply for NM-B (Romex) wire. The only extra requirement for Romex is using the 60 deg C current rating not the 75 Deg C rating, NEC 334.80 . The current rating for #6 copper at 60 deg C is 55 amp so if you have # 6 copper wire you should be able to go to a 60 amp breaker as stated by Greg above. That is assuming you do not exceed the maximum load calculation for the home and assuming you do not have some condition that requires derating of the cable such as running two parallel cables in an insulated area. It is possible to have a legal 50 amp circuit using #8 wire, in that case you would not be able to change the breaker to 60 amp.

richardturnock | March 28, 2019

The current in amps for charging a Tesla is not continuous. There is a ramp up, then a short peak and then a long ramp down.

AERODYNE | March 28, 2019

One reason for upgrading the wire to 6ga is reduced voltage drop. Over a long time, might pay for itself with greater efficiency.

nothotpocket | March 28, 2019

@milesbb That's a common source of confusion in the NEC. While it is allowed to round up for overcurrent protection (e.g. a fuse or circuit breaker) rounding up is not allowed for conductors.

As an EVSE is considered a continuous load by NEC rules, the highest amperage allowed for 6 gauge NM-B wire is 80% of its 55 amp rating, or 44 amps. Said a different way, a 48A EVSE will require a conductor sized no less than (48*1.25) 60 amps.

NEC 210.19(A)(1)(a) is the section that covers it for us.

finished | March 29, 2019

richardturnock | March 28, 2019
The current in amps for charging a Tesla is not continuous. There is a ramp up, then a short peak and then a long ramp down

Please stop. This is completely incorrect. Charging is continuous. Period.

reed_lewis | March 29, 2019

@richardturnock wrote: "The current in amps for charging a Tesla is not continuous. There is a ramp up, then a short peak and then a long ramp down."

This is COMPLETELY WRONG! When charging on A/C the power goes up within 5 seconds to whatever power is available on the plug, and stays at that level until the charge on the battery is somewhere about 96% or so.

So if you are charging from 20% to 90% using 40Amp available power at 240V that is 9.6 kW which means a 100 kWh battery will take at least 7 hours of continuous power flow at 40 Amps.

What you are describing is super charging which is a completely different thing. Yes, super charging ramps up quickly, then tapers down as the battery fills up.

redacted | March 29, 2019

Given the ... diversity ... of advice here, I'd suggest discussing with an electrician before doing anything unconventional. Although, if you think about it, what's the worst that could happen?

murphyS90D | March 29, 2019

The worst that could happen is your house and car could burn down.

The electrical code exists for a reason. Don't try to circumvent it.

And the reason is not to provide employment for the inspectors or income for the government.

Sailfast | March 29, 2019

I followed the instructions that come with a HPWC, used 6 gauge stranded copper Romex, about 33 feet from 60 amp breaker to the charger. . More than 2 years later, no problems.

nothotpocket | March 29, 2019

@Sailfast That's good to hear, but it's best to stick to the electrical code. If a problem does arise, there's the risk of denied claims by homeowner's insurance, civil liability, etc.

redacted | March 29, 2019


NKYTA | March 29, 2019

My ears! ;-) ;-)

tes-s | March 30, 2019

I just took a wire off an old lamp and used that to connect my HPWC to the breaker panel, with a little duct tape around the punchout to protect it. So far, so good.

redacted | March 30, 2019

@NKYTA Tesla's caps, not mine.

NKYTA | March 30, 2019

@redacted, but with a handle like yours, I would have expected... ;-p ;-)

ckcland2 | March 31, 2019

+1 tes-s, thanks for the laugh :)

reed_lewis | March 31, 2019

C'mon. One should be able to use a couple of bent coat hangers! Just make sure they do not come in contact with each other. :)

jjgunn | April 1, 2019

Borrowing grandma's extension cord from behind her nightstand is not advisable.

davelv | December 2, 2019

Can you run 48 amps on a 50 amp breaker- yes you can. Will the wiring be undersized - not if originally sized for 50 amps. Does it violate the NEC - yes for an EV but not for heaters, welders or any other equipment that runs less than 3 hours at a time. So if you charge less than 3 hours continuous at 48 amps, or about 60 driven miles, at a time your loading would be no different than what the NEC would allow. You decide if it is worth the risk if anything goes wrong and the insurance company doesn’t pay.

GHammer | December 2, 2019

@davelv No you cannot run 48 amps on a 50 amp breaker, the breaker will trip far sooner than 3 hours. More like 5-20 minutes. I know from experience.

Earl and Nagin ... | December 3, 2019

That all depends on the condition of the breaker. I've had old breakers at RV parks trip at 40 amps after a few minutes.
You're right that 3 hours is probably way longer than most will hold. I believe the 50 amp rating is just to handle motor startup surges which generally only last for a few seconds.

tes-s | December 3, 2019

If a 50a breaker trips at 48a, time for a new breaker.

GHammer | December 3, 2019

Sorry Tes, Breakers will trip if they are used above the 80% threshold for a length of time. As E&N noted, The length of time it takes will vary based on the condition of the breaker as well as now close to the rated value the load is. I used to set up portable power distribution systems for concerts and fairs and people were always plugging things in right up to the load rating thinking they were safe and then 15 minutes later the breaker would pop and they'd wonder why.

Have also encountered this in theater lighting systems as well.

GHammer | December 3, 2019

OK, Just reviewed the UL code for breakers and it does say they should hold at rated capacity. In practice and in my experience they dont, perhaps they should be replaced but in my experience we would have been replacing lots of breakers that in normal 80% operation work fine.

davelv | December 3, 2019

There are 80% standard breakers and also 100% versions. But even 80% breakers should handle 100% for up to three hours. Per Schneider Electric FA104355.

GHammer | December 3, 2019

In theory, practice and theory are the same.
In practice, they are not.

inconel | December 3, 2019

Does the 80% rule for continuous load apply to both breakers and wires?

For example the new Chargepoint Flex Charger can charge up to 50A but its connectors are designed to hold 6 gauge wire which is rated for 55A as mentioned here. Is it then not safe to draw more than 44A on those wires?

GHammer | December 3, 2019

@inconel 6 gauge thwn and thhn individual wires are rated at 65 amp and 75 amps respectively. Romex is only rated at 55a. To get the full output of that charger you need to use individual wires in conduit, not Romex.

nothotpocket | December 3, 2019

@davelv Summarized things pretty well. The electrical code for EVSE does not allow for use at over 80% rated capacity. If you want to try it, feel free to do so but keep in mind there's a risk that it will start a fire and your insurance company will deny the claim.

The 50A breaker should hold just fine at 48A, but the wiring can overheat inside the wall. That's the problem.

milesbb | December 3, 2019

If you really want to know. A standard home circuit breaker is covered by UL 1077. UL 1077 "19.1 An overcurrent protector shall be capable of carrying 100 percent of its rated current continuously." The UL standard also requires that a 50 amp breaker trip in less than 1 hr at 135% of rating (67.5amps). The UL standard also requires that a 50 amp breaker trip in less than 4 minutes at 200% of rating (100 amps). Of course the breaker is to be tested at a standard orientation and a standard ambient temperature, your breaker may fall outside these parameters so may operate outside these limits.

inconel | December 3, 2019

@GHammer that's clear. Thanks.

NKYTA | December 3, 2019

For those not EEs, I thought you needed a 50A breaker for 40A continuous draw.

We have a slightly malfunctioning charger at our office, 29A or 30A causes the breaker to trip. Not sure what the breaker is at, but should be 36A at least.

Which is 17-18 Rmph/hour for my old S.

And @GHammer is talking the Resistance of the wires.

The circuit breaker for that charger is likely “old, worn out, or damaged”. Totally plausible.

tes-s | December 4, 2019

"OK, Just reviewed the UL code for breakers and it does say they should hold at rated capacity."

Gee, imagine that. They actually do what they are supposed to do. :)

murphyS90D | December 4, 2019

Sometimes when a breaker is tripped it is no longer capable of holding its rated value in the future.

Breakers are selected to protect the wire. The selection has nothing to do with what the load will be.
The wire size is selected to handle the current that will be required by the load.

In the USA a continuous load should not exceed 80% of the breaker rating.
In Canada I think that was recently reduced to 70%. I'm not in Canada so don't know for sure.

barrykmd | December 4, 2019

^^^ Excellent summation of the facts.

finished | December 4, 2019

One thing not mentioned so far regarding tripping of breakers---ambient temperature plays a role in when the breaker trips, and at what current. UL standard and NEC current ampacities assume 25 degree C ambient temperature (77F?). Higher ambient temperatures require derating, which means you need a bigger branch circuit. AND, if your conductors run through a high ambient temperature (>86F) (i.e. Phoenix garage), the circuit has to be derated more. Its all in the NEC. Lastly, if your termination at the breaker is not tight, it can get hot and trip the breaker at less than the rated current. It can also burn your house down.

P.S. UL 489 is the standard, not UL 1077

bp | December 8, 2019

When installing our 2nd HPWC to replace a 14-50 wall outlet, the electrician reviewed the wiring and determined he could install a 60A breaker for that line so the HPWC could draw 48A instead of 40Aa;.

Anthony J. Parisio | December 8, 2019

Still the reason to stay at 80% of circuit’s ability is to keep the wires cool. No one wants to start a fire. So for me I want to stay under the ability of the circuit and not every make it get hot.

tes-s | December 8, 2019

+1 @AJP. I have a 100amp circuit to my HPWC but usually set the car to charge at 40a instead of 80a.