208/240v EVSE charge percentage question

[EVDRIVER]

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

The internet is always correct, even more so when it gets repeated.

 

[arnis]

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.

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.

Have you ever disassembled high voltage fuse? Breakers suck in this cases. Don't underestimate fuses.
Fuse can be faster/more sensitive/more reliable than breakers. So when I said fuse it includes breakers.

Also Schneider's data shows exactly what the quote says.

Every circuit breaker has a continuous current rating, which is
the maximum continuous current a circuit breaker is designed
to carry without tripping.

Same thing for fuses.

PDF document is not the best for extracting graph data. Alignment can be off (and in this case it appears to be so).
If breakers trips at specified current (nominal temperatures) then it is defective.
Also its good that breaker trips if temperature gets above acceptable.
Fuses (and breakers) with above required rating make things less safe (even if cable is oversized).

Maybe 80% 100% rating confusion is the reason why US have tendency to choose more powerful breakers (no fuses this time).
Thank god EU did not mess up this time :lol:

Fun fact - did you know that after reusable breaker has tripped few times (especially short circuit) it is getting damaged in process.
It must be replaced eventually. This is why sometimes they start tripping even at nominal power (continuous load).