Why only 80%?

For 'cause the government said so.

That's a rule written into the national electric code - any load that exceeds a certain duration (15 minutes? Not sure of the number) cannot exceed 80 percent of the circuit rating.

The rule is there to address long term heating of the wiring/connectors - sometimes a load will be fine for several minutes, but would start a fire if left for hours.
Walter

I'm sure the electricians will pipe in here with the technical answer, but it has to do with the code for continuous loads (hours at a time).

It has to do with heat buildup on continuous loads. Intermittant loads are able to use the full rating amount.

From the US Regulations: A "continuous load" is defined as a load where the maximum current is expected to continue for 3 hours or more. Continuous loads .. must not exceed 80 percent of the branch-circuit rating. The exception provides for 100 percent rating of a circuit if supplied by an over current device and assembly listed by a qualified testing laboratory for continuous operation.

It has to do with the heat generated by running under a continuous load. And for reference a continuous load is defined as more than 3 hours at maximum. While the circuit breaker is designed to run at its rated value for indefinite periods of times, it will end up producing more heat as a result. The other parts of the system (wiring, equipment, the box, other circuit breakers in the box) may not be able to handle the heat. So we de-rate the circuit breaker to 80% to lower the heat generated by the continuous load.

Sizing a Circuit Breaker | Basics content from Electrical Construction M) Magazine

cgiGuy said:

I'm sure the electricians will pipe in here with the technical answer, but it has to do with the code for continuous loads (hours at a time).

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But as usual the code is based on science and reality.

Typical circuit breakers need to trip on over-current. The trip points near the breaker rating are based on thermal mechanisms. To ensure that the breaker will pop at over 100% of rating for moderate lengths of time, they need some margin to almost never pop at lower currents. For continuous loads, the accepted number to make it possible to design a cost-effective, reliable breaker is 80%.

There are new, sophisticated, expensive, electronic circuit breakers that are rated at 100% continuous load. Tesla uses them in those big Distribution Centers for Supercharger Cabinets. OTOH, we would much rather pay ~$50 for our 80% breakers than 1,000's of dollars for 100% breakers.

mesalum said:

I have seen referenced that we can only charge 80% of rated circuit amperage. But there was not any explanation as to why. Why can we only charge at 40A on a 50A circuit, or 24A on a 30A circuit, etc.

Thank you to those that answer.

To those of you rolling your eyes now and thinking "this has already been answered, use the search feature." I couldn't find anything. Now is your chance to stop reading and continue to the next thread. I apologize for wasting your time.

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Mesa, you got some good answers there. Essentially safety on several points.
I want to side with you on the second part of your post. Yes, it is difficult to search and many replies are derogatory and unhelpful. We have to keep on trying.
Your ordered P85 sounds beautiful, there aren't enough blue ones out there.

Wiring has to be limited to a current level that will not cause heat buildup to such an extent that the wire's insulation or the conduit it runs through is damaged. The National Electrical Code defines how to arrive at the maximal current for the conditions. Factors include the cross sectional area of the conductor, the metal the conductor is made of, the number of other current carrying conductors in close proximity, the maximal ambient temperature, the heat rating of the conductor's insulation, the heat rating of the conduit carrying the wires etc. Most wiring is assumed to carry current only part of the time; the rest of the time it can cool off. When a conductor doesn't cycle off to cool, it will overheat unless it is limited to a lesser current. Since we can't reduce the current to motors or other appliances, the solution is to use a larger wire which generates less heat since it's internal resistance is less. Thus the rule that requires a larger wire for the same current, or a limit to less than normal current if the wire size isn't increased.

mesalum said:

Thanks everybody for the excellent information. Now I have another question. Keep in mind I have not taken delivery yet.

From my research I have learned that if someone makes an adapter cable that bypasses the method for the charger to determine the rated amperage that you dial down the amps pulled on the cars charging screen. Can you also dial it up? What I am thinking is that if the NEMA code states that continuous draw is three hours can we tell it to charge at 50Am for 2.5 hours and then dial down? Does the charging software do this already behind the scenes?

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You might be able to rig something but using the Tesla charge cable and adapter it won't let you go over 80% of the circuit rating. So 40A on the 50A circuit for example.

mesalum said:

Thanks everybody for the excellent information. Now I have another question. Keep in mind I have not taken delivery yet.

From my research I have learned that if someone makes an adapter cable that bypasses the method for the charger to determine the rated amperage that you dial down the amps pulled on the cars charging screen. Can you also dial it up? What I am thinking is that if the NEMA code states that continuous draw is three hours can we tell it to charge at 50Am for 2.5 hours and then dial down? Does the charging software do this already behind the scenes?

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I don't think so. The car complies with J1772 standards, and J1772 requires the car respect the amperage limit that the EVSE passes to it by duty cycle on the pilot wire. For the car to allow you to override what the EVSE said was safely available would be a violation of the spec, and generally dangerous.
Walter