Boost mode for EVSE?

Since an EVSE is considered a continuous load, electrical code compliance limits it to 80% of the nominal rating of the branch circuit it is connected to. The definition of "continuous" appears to be three hours or more, so it should be acceptable to build an EVSE that will feed the car 100% of the nominal branch circuit rating if it automatically shuts down after three hours. An EVSE with such a "boost mode" could be useful for EV owners whose charging capabilities are limited by wiring that is cost-prohibitive to upgrade. For example, an EVSE for a 20 A circuit could have a normal mode where it would limit current to 16 A, and a boost mode where it would supply 20 A but automatically shut down after 3 hours. Normal mode would be used for usual overnight charging, while boost mode would be useful to get as much charge as possible in a short time, such as if you want to go out in the evening after the drive home from work has drained the battery.
Has this ever been implemented?

They don't make a EVSE with such a mode nor do I see they ever will but I do it all the time. My EVSE has a 20a and 30a setting and when I need to I use the 20a setting(actually pulls 19a which is good) and of course 30a is limited to 27.5a by the Leaf.
Note while you can generally pull full load amps for a short period of time after about a half-hour things will get very warm, things like the breaker(you can actually feel it) and even the blades of the plug, I'm sure the socket is very warm also but you can't really feel that. You also need to know your outlets and wiring are up to snuff, a weak or old outlet and things could go from very warm to HOT which is NOT good. You can't really feel things like wiring, especially behind the wall but you might be able to feel it near the breaker panel, warm is OK but NOT HOT.
If you want to play around with maxing your circuit I'd suggest an EVSE like Juicebox that lets you set the current draw down to the amp, start at 80% and gradually start increasing your draw by an amp, then keep a close on eye on things by feeling the plug and wiring if you can, also feel the breaker. Again warm is ok but not hot. Once you find a good max setting you'll have to manually stop charging, automating it would be rather complicated and again your not going to find an EVSE that does it automatically.

In my experience, I have seen a 20A breaker trip within seconds of running around 18A equipment load connected to its circuit. A high current draw will heat the breaker and cause it to trip. I expect that higher rated breakers will trip too when they are loaded close to their rating.

The heat build-up is greater when the breaker is in an enclosure or exposed to warmer ambient temperatures. And, don't forget about the heat build-up of the circuit's cables behind walls or within conduits/boxes/receptacles. The purpose of a breaker is to protect the cable, so the heat of both need to be considered.

The electric codes (NFPA and CSA) are setup for safety reasons under various conditions. Just because something may work under some conditions, even though it violates the code, that does not mean that it should be done.

The wires inside the EV_SE can get quite hot and crispy, as seen in a FLiR image here:

http://fsamw.myevblog.com/i-miev-technical-stuff/hot-evse-terminals/

According to NEC the term "continuous load" is fairly ambiguous.
It could mean anything from 20 minutes to 3 hours.
For example a clothes dryer is considered a continuous load even though I have never seen one with a timer that goes longer than 90 minutes and the heater element cycles on and off. Based on observations it is definitely not a continuous load, but NEC defines it as continuous.

A 20amp breaker that trips at 18 amps is worn or defective and needs to be replaced.
A good 20 amp breaker, for example a square d QO should be able to hold a 22 amp load just about indefinitely then trip from over heating at 25 amps any where between 50 seconds and about 8 minutes.
It's called a trip curve.

If you run a wire at its max amp rating it will start to overheat well before 3 hours.

NEC always figures worst case scenario. In this case using NEC logic the worst case scenario would be "what if the timer failed and it stays running at 20 amps"?
Then what if someone plugs this evse into a nearly dead tesla and it runs a 20 amps for 20 hours on a 12ga circuit?

NEC has a section dedicated to wiring in evse units, article 625. I suggest everyone adhere to it.

Oilpan4 said:

A 20amp breaker that trips at 18 amps is worn or defective and needs to be replaced.

A good 20 amp breaker, for example a square d QO should be able to hold a 22 amp load just about indefinitely then trip from over heating at 25 amps any where between 50 seconds and about 8 minutes.
It's called a trip curve.

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I agree on the worn breaker. I expect that most people's homes have old/aged breakers in their power panels, so there is a good chance that they are not operating under like-new conditions.

I have seen some Square D model QO series breaker time-current trip curves:
https://download.schneider-electric.com/files?p_enDocType=Tripping+curves&p_File_Name=910-03.pdf&p_Doc_Ref=910-3
Like most overcurrent protective devices, a breaker has a trip curve with a band (i.e., lower curve and upper curve), which is the ideal curve +/- margin for temperature and other deviations. The breaker may trip anywhere within this band. Based on the lower band (lower part of the curve) for a 20A QO series breaker, the breaker may start to trip with 20A (1.0 multiple) load after approx 600 s (10 min). The higher the current, the sooner the breaker will start to trip, so with a 25A (1.25 multiple) load, the lower band shows that this breaker may start to trip sooner, after approx 60 s (1 minute).

If it will shut down in 3 hours you are only gaining 36 minutes, but it shuts down. Leave it at normal and let it sit another 36 minutes or even longer.

The additional power is too small to gain advantage in all but the most narrow of circumstances.

smkettner said:

If it will shut down in 3 hours you are only gaining 36 minutes, but it shuts down. Leave it at normal and let it sit another 36 minutes or even longer.

The additional power is too small to gain advantage in all but the most narrow of circumstances.

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I agree. Charge rate is one of those things where it takes more than a nominal increase to make a noticeable difference.

It makes more sense, if possible, to reconfigure an existing 120 volt 20A circuit as a 240 volt 16A EVSE circuit with no Neutral. That gives about 4x the charge rate.

Going from the oem Panasonic 120v 12amp granny charger configuration to modded 240 12 amp was night and day difference.
Almost 20 hours to go from lbw to full, ain't not body got time for that. Cutting 20 hours down to 8 or 9 was a game changer.
Then going from my modded 240v 12 amp granny charger to a 240v 16 amp duosida was a nice improvement too.
Then their's the chademo, 395v at 20 amps is almost twice as nice for about 6x the cost of the duosida.

A little more research suggests that the limit is 1 hour or a 50% duty cycle for loads under 225 A, at least in Canada anyways:

8-104 Maximum circuit loading (see Appendix B)

(3) The calculated load in a consumer's service, feeder, or branch circuit shall be considered a continuous load unless it can be shown that in normal operation it will not persist for
(a) a total of more than 1 h in any two-hour period if the load does not exceed 225 A; or
(b) a total of more than 3 h in any six-hour period if the load exceeds 225 A.

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That does make it less useful. An extra 4 A over 3 hours is 10% on a 30 kWh car, but only 3% if the boost is limited to 1 hour.

Well I tried running my home chademo on a 30 amp circuit, just to see what would happen.
It ran for over an hour on 2 different 30 amp circuits using square D QO breakers and it didn't trip.
My home chademo typically draws 34 to 35 amps.
Normally I plug it into my 100 amp welder circuit on 4 gauge cable.

Oilpan4 said:

Well I tried running my home chademo on a 30 amp circuit, just to see what would happen.
It ran for over an hour on 2 different 30 amp circuits using square D QO breakers and it didn't trip.
My home chademo typically draws 34 to 35 amps.
Normally I plug it into my 100 amp welder circuit on 4 gauge cable.

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:shock: did you feel the breakers after that hour? My guess is they were quite toasty(feel the breaker itself near the toggle with the back of your finger). I've occasionally ran close to 30a(27.5) with a standard breaker but even that the breaker got almost hot after an hour and I have no idea how hot the wiring behind the wall got. Also running at 27.5a the prongs on my L14-30 plugs get quite warm after a couple of hours, can't imagine at 30+amps

It was marginally warm, not even enough to make me want to look at it with my flir i7.