208/240v EVSE charge percentage question

So my EVSE pulls 19a @ 240v. At home the voltage at the outlet is 237v NL(no load) and 233v under the 19a load. Using this setup I'm able to add 28% per hour to my '13 Leaf with the 6.6kw charger. At work I have access to a dedicated 30a outlet/circuit but at 208v(actually 203 at socket NL) because at work the power is 3 phase not single phase as at home but drops to 192v under the 19a load, at work I only add 18% per hour starting with the same state of charge on the battery. Now at work the outlet is quite some distance from the panel, probably half way across the building, still my percentage drop numbers don't work out and I'm wondering why.

Figuring the watts at home I get 4427w by multiplying 233v X 19a.
At work I get 3648w by multiplying 192v X 19a(I have no real way of measuring amps at work but I assume using the same EVSE it should be the same as at home, where I was able to measure it with a ammeter)....maybe this assumption is incorrect?
Now according to my calculations 4427w is 21.5% greater than 3648w but heres what I don't understand. 28%(how much I gain per hour at home) is 55% greater than 18%(how much I gain at work) :? Why doesn't the percentage gained in watts equal the percentage gained in charging percent per hour, comparing work to home?

Seeing how much voltage I drop at work I wonder if dropping the charge amps(something I can do as my EVSE is adjustable) would yield almost the same charge to the car. If I'm doing my math correct it looks like at work I'm losing 209 watts to wiring and such, which seems quite high. I don't know what the gauge of the wiring at work is but I'd assume 10G(probably no more) and I have felt components(outlet and 30a breaker) after charging for several hours and nothing seems abnormally warm. I can't really see or feel the actual wire because it's in metal conduit, which doesn't feel warm.

The whole thing is kind of a bummer as at work is where I'd really appreciate the ability to recover quickly, at home I have all the time in the world to charge, in fact I use the charging timer and I'm guessing most of the time the charging timer only starts hours before I go to work.

Are you limiting your amps at home? 19A is somewhat weird.

arnis said:

Are you limiting your amps at home? 19A is somewhat weird.

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Yes, my upgraded Leaf EVSE I believe goes up to 20a max @ 240v but I figure since the wire going to the vehicle is only 12G I didn't want to push it's maximum of 20a so I dialed it down a bit and settled on 19a

you are using EVSEupgrade.com? then max I think is 20 amps. why don't u take ammeter to work and measure current?

how do you measure your voltage drop/

DaveinOlyWA said:

you are using EVSEupgrade.com? then max I think is 20 amps. why don't u take ammeter to work and measure current?

how do you measure your voltage drop/

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Yes, EVSEupgrade.com and I have measured the amps at home, no practical way to do it at work. I measured the voltage at the outlet(NL) and then measured the voltage at the prongs going into the EVSE(left plug out 1/8" and was able to get my voltmeter leads to touch the prongs).

Most amp meter readings I see are off, I would trust your voltage if you have a decent meter but I even see Fluke meter measurements on amps 1-4 amps off at times. If the unit is 20A and the car is drawing 20A or more it will be 20A not 19.

EVDRIVER said:

Most amp meter readings I see are off, I would trust your voltage if you have a decent meter but I even see Fluke meter measurements on amps 1-4 amps off at times. If the unit is 20A and the car is drawing 20A or more it will be 20A not 19.

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Both ammeter and VM are commercial quality and I've setup my EVSE to limit to 19a(19 flashes of the light which agrees with my ammeter). What confuses me is why the percentage drop/increase doesn't carry forward from watts used to percentage gained on SOC :?

jjeff said:

EVDRIVER said:

Most amp meter readings I see are off, I would trust your voltage if you have a decent meter but I even see Fluke meter measurements on amps 1-4 amps off at times. If the unit is 20A and the car is drawing 20A or more it will be 20A not 19.

Click to expand...

Both ammeter and VM are commercial quality and I've setup my EVSE to limit to 19a(19 flashes of the light which agrees with my ammeter). What confuses me is why the percentage drop/increase doesn't carry forward from watts used to percentage gained on SOC :?

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curious as to why you are limiting current. my house has one of the most unreliable breaker panels ever made which is one of many reasons why I chose an adjustable EVSE and I feel that 20 amps is more than enough of a safety margin. I am surprised that there is that much of a voltage drop across the EVSE itself

DaveinOlyWA said:

jjeff said:

EVDRIVER said:

Most amp meter readings I see are off, I would trust your voltage if you have a decent meter but I even see Fluke meter measurements on amps 1-4 amps off at times. If the unit is 20A and the car is drawing 20A or more it will be 20A not 19.

Click to expand...

Both ammeter and VM are commercial quality and I've setup my EVSE to limit to 19a(19 flashes of the light which agrees with my ammeter). What confuses me is why the percentage drop/increase doesn't carry forward from watts used to percentage gained on SOC :?

Click to expand...

curious as to why you are limiting current. my house has one of the most unreliable breaker panels ever made which is one of many reasons why I chose an adjustable EVSE and I feel that 20 amps is more than enough of a safety margin. I am surprised that there is that much of a voltage drop across the EVSE itself

Click to expand...

I'm mainly limiting the current because I don't want to put a full 20a though my EVSE's 12 gauge wire and the voltage drops aren't across the EVSE but rather to the EVSE, in other words the behind the wall wiring and connections getting to the EVSE.

DaveinOlyWA said:

jjeff said:

EVDRIVER said:

Most amp meter readings I see are off, I would trust your voltage if you have a decent meter but I even see Fluke meter measurements on amps 1-4 amps off at times. If the unit is 20A and the car is drawing 20A or more it will be 20A not 19.

Click to expand...

Both ammeter and VM are commercial quality and I've setup my EVSE to limit to 19a(19 flashes of the light which agrees with my ammeter). What confuses me is why the percentage drop/increase doesn't carry forward from watts used to percentage gained on SOC :?

Click to expand...

curious as to why you are limiting current. my house has one of the most unreliable breaker panels ever made which is one of many reasons why I chose an adjustable EVSE and I feel that 20 amps is more than enough of a safety margin. I am surprised that there is that much of a voltage drop across the EVSE itself

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My unreliable FPE 100A panel caught on fire at 4:30AM with a minimal load. Charging at 40A certainly should not have stressed the panel but it did. I now have an Eaton 200A BR panel.

If the op is really charging on #12 wire the maximum continuous current is 16A (80% of 20A). An EVSE is considered a continuous load and the maximum current is 80% of the rating.

jjeff said:

DaveinOlyWA said:

jjeff said:

Both ammeter and VM are commercial quality and I've setup my EVSE to limit to 19a(19 flashes of the light which agrees with my ammeter). What confuses me is why the percentage drop/increase doesn't carry forward from watts used to percentage gained on SOC :?

Click to expand...

curious as to why you are limiting current. my house has one of the most unreliable breaker panels ever made which is one of many reasons why I chose an adjustable EVSE and I feel that 20 amps is more than enough of a safety margin. I am surprised that there is that much of a voltage drop across the EVSE itself

Click to expand...

I'm mainly limiting the current because I don't want to put a full 20a though my EVSE's 12 gauge wire and the voltage drops aren't across the EVSE but rather to the EVSE, in other words the behind the wall wiring and connections getting to the EVSE.

Click to expand...

oh my bad. just reread your explanation. your voltage drop is alarming...

jjeff said:

EVDRIVER said:

Most amp meter readings I see are off, I would trust your voltage if you have a decent meter but I even see Fluke meter measurements on amps 1-4 amps off at times. If the unit is 20A and the car is drawing 20A or more it will be 20A not 19.

Click to expand...

Both ammeter and VM are commercial quality and I've setup my EVSE to limit to 19a(19 flashes of the light which agrees with my ammeter). What confuses me is why the percentage drop/increase doesn't carry forward from watts used to percentage gained on SOC :?

Click to expand...

2 reasons.

A) You have a load when the car is on or charging that doesn't contribute to the charging process. The power used by the Leaf is a fixed amount no matter what EVSE you use or what the overall charging rate is. So in general the higher the KW the better your charge rate will improve that ratio on top of the expected change if you didn't know about the other loads. Think of this as base load or overhead, either way it is power that is used by the Leaf for some purpose other than charging.

B) The onboard charger has differing efficiencies. As you go to higher charge rates the onboard charger itself is wasting less power.

see http://avt.inel.gov/pdf/fsev/SteadyStateLoadCharacterization2012Leaf.pdf
and http://avt.inel.gov/pdf/fsev/SteadyStateLoadCharacterization2015Leaf.pdf

The 3.6KW charger is most efficient around 2KW charging rate even though max is much higher. (note the internal charger efficiency is better down around 8 or 10 amps but I'm choosing 12 amps due to the overhead you are concerned about).

Full charge rate for the 3.6KW charger was around 88.x% (maybe 88.3 or so). Less efficient at full amps

Max efficiency for the 3.6KW charger was around 89.x% (maybe 89.5 or so). But around 12a not 16a.
Max efficiency for the 6.6KW charger was around 90.x% (maybe 90.5 or so). More amps is better on this one.

Max efficiency for the 120V with 3.6KW charger was around 86.x% (closer to 86.25 or so)
Max efficiency for the 120V with 6.6KW charger was around 78% (closer to 77.9% or so)

say the overhead waste is 300W max

the efficiency difference of the 3.3 / 3.6 KW on board charger between charging at 3.6KW and 2KW isn't more than 2%.

To compare that if we imagine 2,000 W as 100% efficient and 3.6KW as 98% efficient it is like saying it is 3724W. 3724 / 2400 = 1.55. 300W is 8% of 3724W and 12.5% of 2000W. 12.5/8 = 1.56.

So if I didn't get lost in the math too far the overhead of the charge process gives us a practical wash vs the inefficiency of the charger at full speed for the 3.3 / 3.6 KW charger.

You want to charge faster than the halfway point of 8A at 240V or 10A at 208V to avoid letting the overhead waste dominate but between there and full charging rate the difference in efficacy vs the relative percentage of overhead waste almost negate each other.

If the efficiency difference is less (which it might be) or the overhead waste is more than 300W you'd want to charge as fast as possible even on the 3.3/3.6 KW charger.


On the other hand the 6.0/6.6 KW charger gets more efficient as the percentage of overhead waste decreases amplifying the desire to charge faster. No brainer on those cars to charge as fast as you can.

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GlennD said:

If the op is really charging on #12 wire the maximum continuous current is 16A (80% of 20A). An EVSE is considered a continuous load and the maximum current is 80% of the rating.

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Well that was my thought but my EVSEupgraded EVSE outputs 20a(I was incorrect in thinking it was 22 or even 21a, 20a max) at 240v using the same 12G 20' wires from the EVSE to car so iEngineer must feel the cable is sufficient for 20a continuous, otherwise he would have limited charging to 16a(something I'm glad he didn't do). Doesn't code say the 80% rule is more for permanent wiring and not so much for extension or portable cords? I must say that even at 19a my cord from the EVSE to vehicle never feels warm to the touch but again for a little de-rating I limit the EVSE to 19a.
With Leafspy-Lite does it display charging current and input voltage to the vehicle? That would be handy for me to know as I don't really know my charging amps at work nor do I know the actual voltage supplied to the cars charger, only the voltage supplied to the EVSE. I've purchased a bluetooth dongle but have yet to install it or download the program for my iPhone 4, assuming it works with my dinosaur phone.
I'm thinking cutting back charging amps at work may not reduce charging rate that much as with less amps my voltage should go up, whether or now it's enough to compensate for the drop in amps is unknown but it's sure to increase efficiency in my case.

12AWG is fine for 20Amps.
In Europe we have a really clear understanding what we need:
16Amp (3,6kW charger) 2,5mm2 (aka 14AWG)
32Amp (7,2kW charger) 6,0mm2 (aka 10AWG)

The real problem is not the cable itself. Usually problems start with bad installation.
For example it is very important to have excellent torque on the screw that secures the cable
in the terminal. There are even standards that define the correct torque. Pulling really hard
each wire after securing is a reasonable way to verify.

Also our circuit breakers are chosen exactly to the specified load or cable used.
So for 16Amp EVSE (or 14AWG) we only use 16Amp breakers. The number on the breaker
is the number that will never trip the breaker. Anything above that will
kill the power within specified time (depending on breaker sensitivity class).
Regular breakers are for protecting the cables not for people and not for devices!

Leaf will suck exactly as much as EVSE allows. So for example my Leaf will charge at
exactly 16.0 amps. If voltage drops the power will drop but current will still be exactly 16.0A.
And I do have small voltage fluctuations depending on other stuff at home. I can see those
fluctuations on LeafSpy.


There are some other variables. Like hot weather. If cable is in 50C/120F that is not ok and wires
must be thicker. Resistance goes up with temperature.
Also for really really long runs its better to use thicker wire. Not for safety but for minimizing losses.
Really long is something above 100 feet (between power meter and vehicle because this is what
we pay for ).

Well, here we go by the NEC. They consider an EVSE a continuous load. That means a 20A #12 circuit can have no more than 16A. Also a 20A breaker will eventually blow at 20A. Just because you can set your EVSE to 20A that does not make it right. 20A would require a 25A circuit.

I know that Chris had problems with his 30A EVSE. He now considers it a 24A EVSE. As that is 80%.

In any event even 16A will fully charge over night.

Find me some specs for any 20A fuse that will "eventually" blow at 20A :roll:
The whole point of that number is the maximum allowed rating.

You think Europe thinks that EVSEs are not continuous load :mrgreen:

Fuse is for the cable not for the appliances.

NEC doesn't decide will the fuse blow or cable burn. Physics do

If tens of thousands of EVSEs on 2,5mm2 cable at 16A 240V continuous never burn the cable and never blow the fuse
what is different in US? Frequency?

arnis said:

If tens of thousands of EVSEs on 2,5mm2 cable at 16A 240V continuous never burn the cable and never blow the fuse
what is different in US? Frequency?

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insurance companies

DaveinOlyWA said:

arnis said:

If tens of thousands of EVSEs on 2,5mm2 cable at 16A 240V continuous never burn the cable and never blow the fuse
what is different in US? Frequency?

Click to expand...

insurance companies

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:idea: :lol:
I have no doubt, totally correct!

arnis said:

Find me some specs for any 20A fuse that will "eventually" blow at 20A :roll:
The whole point of that number is the maximum allowed rating.

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1st in the US using the term "fuse" when you mean "breaker" would confuse many non technical types that think of old school screw in fuses from the early 1900s. You might want to avoid the use of the word "fuse" unless you qualify it with some other technical term or are actually talking about old school screw in fuses for home electrical panel.


as to a breaker tripping at 20A when it's rated for 20A see:

https://www.industry.usa.siemens.com/services/us/en/industry-services/training/self-study-courses/quick-step-courses/Documents/circuit_breakers.pdf

Time-Current Curves are used to show how long at what power level it'll take for a breaker to trip. Take a look at some of these curves and they approach 1 and have an error band that crosses 1. Thus it is entirely possible a mass manufactured breaker will trip at its rating if the load is constant and continues past 2 or 3 hours at the specified temperature. Should the temperature go higher the device may trip at lower power levels than rated.

Keep in mind that this trip curve was developed based upon predefined specifications, such as operation at a 40°C ambient temperature. Variations in actual operating conditions will result in variations in circuit breaker performance.

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Take http://static.schneider-electric.us/docs/Circuit%20Protection/Molded%20Case%20Circuit%20Breakers/0100-400%20A%20Frame%20FA-LA/FA-FC-FH/0600DB0105.pdf for example, the bottom of the error band crosses 1.0 rating at 600 seconds (10 minutes). Not hard to have a continuous load last more than 10 minutes.

Good post, dhanson. 28 years in designing power distribution, but I still get people that question proper design because they did an internet search.