Calculating OCPD

VIC1958

Senior Member
I am reviewing plans for a residential car charging station. 240v and the charger has the ability to charge at multiple output ratings from 16 amps up to 60 amps, the amperage can either be set within the charger or it can be changed with an app on your phone. The manufacturer makes mention in their spec sheet that the OCPD need to be a double pole breaker rated at 125% of the load, simple math tells me that the OCPD would need to be a minimum of 65 amp rated breaker since they do not make a 62.5 amp breaker. Am I missing something here?
Thanks.
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
Being that it is adjustable via an App, the OP should plan for for the highest charging current and use that in their calculations.
that would be my likely approach but it is possible that plugging 60 Amps into the load calculations might create havoc with the service size required.
 

Fred B

Senior Member
Location
Upstate, NY
Occupation
Electrician
that would be my likely approach but it is possible that plugging 60 Amps into the load calculations might create havoc with the service size required.
Likely might, but if I as an installer has no physical means to restrict user from "overloading" because of user available adjustments, I wouldn't install anything less than maximum available. Now thinking about it, I know on Generator installation I'll use load shedding to prevent overloading with larger loads, is it possible or allowable (Code) to use such a device so that even if an appliance can potentially overload, it will cut out if other loads would be causing an overload situation when charger is running?
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
Likely might, but if I as an installer has no physical means to restrict user from "overloading" because of user available adjustments, I wouldn't install anything less than maximum available. Now thinking about it, I know on Generator installation I'll use load shedding to prevent overloading with larger loads, is it possible or allowable (Code) to use such a device so that even if an appliance can potentially overload, it will cut out if other loads would be causing an overload situation when charger is running?
What is the worst that could happen? If the guy tries to run it at 60 amps and you have a 20 amp circuit the OCPD just opens. So the installer does indeed have an inherent means to restrict the user from overloading the circuit.

This is the old argument about what if someone down the road decides to violate the code. What if they do? It is not your job to prevent it and you can't no matter what you do now.
 

Fred B

Senior Member
Location
Upstate, NY
Occupation
Electrician
What is the worst that could happen? If the guy tries to run it at 60 amps and you have a 20 amp circuit the OCPD just opens. So the installer does indeed have an inherent means to restrict the user from overloading the circuit.

This is the old argument about what if someone down the road decides to violate the code. What if they do? It is not your job to prevent it and you can't no matter what you do now.
The OP is at plan review stage, at this point I would look at all potential, size for maximum, and layout possible alternatives. Otherwise could have to deal with a customer later complaining the breaker keeps tripping every time they try to use their charger (they won't tell you unless pressed that they are or have adjusted settings above your determined maximum total loads.) You give them the options and put in writing any limitations that doing less than full mfg capabilities installation would create, and what if any limitations current service might place on the installation. Even then you would likely get complaints but you have a recourse for "Here is what it will cost to get it up to full capacity".
 

GeorgeB

ElectroHydraulics engineer (retired)
Location
Greenville SC
Occupation
Retired
?? where did you and the OP learn math? 😅 😅 1.25*60 = 75.
he used the assignment != which is "not equal". Yes, 1.25*60 = 75. Yes, 1.25*60 != 62.5.

This isn't some "new math" unless it was new before 1965 when I had it in a math class.
 

GeorgeB

ElectroHydraulics engineer (retired)
Location
Greenville SC
Occupation
Retired
Being that it is adjustable via an App, the OP should plan for for the highest charging current and use that in their calculations.
OP didn't mention brand/model, but at least 1 Tesla has internal switches for maximum with 2 limits below that, coordinated with another charger and app limited.

As mentioned, it is the vehicle that contains (with common AC chargers) the charger. Hardware and software limits are communicated to the vehicle from the EVSE device.

Some of the new DC chargers may be different; they can supply far more than the ~12.5kW-14kW from a 60A 208/230V circuit.
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
The OP is at plan review stage, at this point I would look at all potential, size for maximum, and layout possible alternatives. Otherwise could have to deal with a customer later complaining the breaker keeps tripping every time they try to use their charger (they won't tell you unless pressed that they are or have adjusted settings above your determined maximum total loads.) You give them the options and put in writing any limitations that doing less than full mfg capabilities installation would create, and what if any limitations current service might place on the installation. Even then you would likely get complaints but you have a recourse for "Here is what it will cost to get it up to full capacity".
what if the guy had a 3 phase heater that could be wired either delta or wye. would you require the current be calculated for the worst case even if the lesser case was how the wiring started out?

I don't have an issue with telling the guy up front what he is getting. That just seems like a good idea anyway, but trying to force a solution to a problem on him that might never appear seems wrong.
 

Rock86

Senior Member
Location
new york
Occupation
Electrical Engineer / Electrician
what if the guy had a 3 phase heater that could be wired either delta or wye. would you require the current be calculated for the worst case even if the lesser case was how the wiring started out?

I don't have an issue with telling the guy up front what he is getting. That just seems like a good idea anyway, but trying to force a solution to a problem on him that might never appear seems wrong.
I would call out for the contractor to wire the heater as delta or wye depending on the needs. That is a hard wired connection which the operator has minimal access to and once the EC installs it that way, he is free and clear.

If the EC installs a system which is designed for the operator to adjust on the fly, why would the Engineer restrict the product from its full limitations? That's like installing a dimmer switch but putting a block plate to limit the level which they can dim the lights.
 

Rock86

Senior Member
Location
new york
Occupation
Electrical Engineer / Electrician
he used the assignment != which is "not equal". Yes, 1.25*60 = 75. Yes, 1.25*60 != 62.5.

This isn't some "new math" unless it was new before 1965 when I had it in a math class.
Well, i'm not that old, but I'm also not young enough for the new math which is out there hahaha. I didn't recognize the "!=" until you made it clear.
 

VIC1958

Senior Member
From what I am taking on the conversation is that the Code addresses the maximum size OCPD for equipment but is silent on the fact that a smaller sized OCPD can be installed as long as the the conductors are sized correctly?
 

myspark

Senior Member
Location
SCV Ca, USA
Occupation
Retired EE
I am reviewing plans for a residential car charging station. 240v and the charger has the ability to charge at multiple output ratings from 16 amps up to 60 amps, the amperage can either be set within the charger or it can be changed with an app on your phone. The manufacturer makes mention in their spec sheet that the OCPD need to be a double pole breaker rated at 125% of the load, simple math tells me that the OCPD would need to be a minimum of 65 amp rated breaker since they do not make a 62.5 amp breaker. Am I missing something here?
Thanks.

Yes!
You are right.
There are two popular models on the market. The regular small charger that you can plug into a regular 120 volt outlet --but will take the whole night --around eight to twelve hours to achieve a full charge from an almost depleted EV.

The fast charging model will take around four hours for a full charge. The OCPD required for this model is 40 Amps. The load (nameplate) is rated 32 Amps.
So, considering continuous load 32 x 1.25 = 40.

There is no higher rated yet on the market.. . .not even the Tesla X Sports Model as far as I know.

Most chargers are adjustable e.g. from low charge to high charge.. . . usually in three increments
I am installing one right now. . . and its a high charge model that requires a 40 A OCPD.
All high charge models require a minimum 40 Amps, 2 Pole 240 volts.

As you stated a smaller OCPD can be installed with appropriately sized conductors.

I am doing this for a friend Pro Bono.
This is the second one I’m installing for another friend.
The downside for being a sparky. :) Can’t say no. :(
 
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